How NOT to insulate your house

Sometimes insulation hides bigger problems.

The other day, I was talking with a friend of mine – an energy auditor who was a builder for decades. The topic came around to bad advice that “experts” give about insulating. It’s something that we both feel passionate about because homes get ruined and people get sick when innocent people follow this bad advice. We both adhere to a similar do no harm philosophy of  “if it’s worked for decades as-is, don’t mess it up!”

There’s a science to building and the tighter and more energy efficient you make a home, the more important it is that you do things “right.” It’s like the difference between making a log raft and a submarine. A log raft is leaky, but it’s forgiving because it floats by virtue of the logs. It doesn’t have to be water-tight. A submarine had better be water tight and structurally sound or you’re going to drown and get crushed by the intense pressures of the ocean.

Unfortunately, unlike boats and submarines, homes today are often built in the cheapest way possible, with little regard to physics. Renovations are even worse because people often hire unqualified “low-bid” contractors to do the work without realizing that the few thousand dollars that they save on construction may cost them tens of thousands to fix or even send them to the hospital due to mold or poor indoor air quality.

The problem is, people familiar with building science are extremely rare, as are the chances of finding a builder who knows how to make a healthy, energy efficient home. That’s why you’re here reading this now – you want to learn what not to do when insulating and how to do it right.

#1 – Insulate the attic after air sealing and reducing moisture

Adding insulation is one of those jobs that people think can be done blindly. Just throw some pink stuff up in the attic and you’re good to go, right? Wrong.

If you’re lucky, you’ll get away with this. If you’re not, you could end up with an unhealthy, moldy attic. Why?

Many homes are leaky and have insufficient insulation in the attic. This allows a lot of heat up to the attic. Warm spaces have less condensation risk than colder space so when you add insulation to the attic floor, you’re intentionally blocking some of that heat from keeping the attic warm. This makes the attic colder and a colder attic with moisture leaking from the house can result in condensation, mold and wood rot.

Bath fan ducted into attic

This is one of the main reasons that energy efficiency experts tell you to air-seal the attic where the insulation will be installed and ensure bath fans are properly vented before insulating. Air sealing reduces the amount of moisture that that can get into the attic, reducing the chance that you’ll end up with a mold problem. But, you want extra protection in the system because some moisture is inevitable. Maybe you got away without proper attic ventilation in the old house, but as you tighten things up and insulate, it becomes increasingly important to have good attic ventilation.

I personally don’t care whether you use gable vents or properly matched soffit and ridge vents, just do it right. If you use gable vents, it’s best to have two of them – one on each side of the attic to provide a good cross flow of air. If you use a ridge vent, you MUST have an equal or greater volume of air coming in to the soffits.

And that picture at the top of this post? That’s a wet mess of insulation caused by the builder neglecting to put an air and moisture barrier behind the bath tub. Instead, they were lazy and there’s a big hole with fiberglass draped over it – that’s worse than useless!

#1a – Turn off the humidifier

Humidifiers can destroy a home and make you sick. The more you tighten and insulate your home, the less you need to use humidifiers. And yet, I see very tight, new homes, being built with whole house humidifiers that dump gallons of water into the air. Moisture finds the tiniest cracks and holes and moves into spaces like the walls and ceilings. Much of it ends up in the attic where it can rot out your roof.

My advice to people as they add insulation is always “turn off the humidifier.” If the house is still too dry, find out where the air leaks are and plug those because that’s what causes dry air.”

I know this personally. Before I weatherized my own home, we’d have a humidifier going all winter. The air was so dry that my lips chapped and lightning would fly every time I touched the dog. But ever since we dealt with all the big air leaks, the humidifier hasn’t run once. It’s comfortable in the house all year round.

#2 – Choose your insulation carefully

Insulation is not a “one-size fits all” product. Insulation, like cellulose, that works great in the attic and the walls should not be used in many basements and crawlspaces where it can get wet. Likewise, fiberglass, which is ubiquitous, shouldn’t be used for the band joists in the basement or any other place where air sealing is required since fiberglass is does nothing to stop air movement.

You have to think about why you are using insulation in each location. For example, the band joist is often leaky, letting cold air in. It is also probably very cold because it may be exposed to the outside temperatures, making it a likely candidate for condensation. This tells us that you want an insulation that air seals and keeps interior moisture away. The perfect product for this is closed cell spray foam (as in the link above).

What about wall insulation? Walls are *usually* fairly air tight on the inside and leaky to the outside, with a few holes for electrical outlets and such. In general, walls are pretty forgiving about the type of insulation that’s used, but you can still screw them up. For example, a lot of building codes call for vapor barriers on walls, but using a vapor barrier on the inner wall AND using a tight outer wall can lead to horrendous moisture problems inside walls. This is an entire article itself, so I won’t go into more detail here. Just know that a conventionally built wall is usually forgiving, but if you start doing anything different, you need to be really careful.

Another insulation location is the attic. As noted in #1, you need to air seal before you insulate. But what if you could air seal and insulate in one shot? In fact, spray foam does exactly this. I love the stuff. Rather than having to spend days or weeks in a dirty attic, carefully caulking every crack and filling all the holes with cans of foam, and then carefully laying out insulation, you can instead just remove the old insulation and have a foam contractor come in and in a day or less, your attic will be completely air sealed and insulated. It’s really a superior way to insulate. I used it in all my own renovations and the results are awesome.

There are other places that are critical to consider. For example, cathedral ceilings. I’ve seen a lot of problems with insulated cathedral ceilings, especially those with tongue and groove planks instead of sheet rock. Because of potential moisture issues, I would personally never use a loose insulation like fiberglass in these ceilings. Moisture goes right through fiberglass and will condense on the cold underside of your roof. Even if you use vent chutes to provide an air space above the insulation, you can still have problems. So in choosing insulation for a cathedral ceiling, you really want a material that stops air and moisture from reaching that sensitive roof deck. Again, the ideal material is usually closed cell spray foam.

#3 – Insulate continuously: like a jacket for your home

One of the biggest mistakes people make is ignoring the big insulation picture and how it all connects together. You can do everything I’ve written about above perfectly and not have the insulation work at all. How?

Imagine an ice chest. This is just a box of insulation, sealed on all sides. Ideally, this is how a house is made. But what would happen if you took the top of the ice chest and moved it a couple of inches away from the cooler? It’s not air tight any more and whatever is in the chest would end up at room temperature. Same thing with a refrigerator – what happens if you leave the door open? The fridge has to run continuously to try to get cold while all the heat from the room rushes in. Any child would look at these examples and say “duh!” If this is so obvious, why do highly paid builders often construct houses that are no different than an ice cooler without a lid? Or to use the jacket analogy, would you wear a jacket with the zipper wide open?

Unfortunately, the more complex the home’s design, the harder it is to ensure continuous insulation. Modern homes, with a dozen different roof lines and a hundred walls can be difficult to deal with properly. I’ve seen contractors start by insulating under the roof in one place then switch to insulating the walls in another, rendering the roof insulation meaningless.

One trick to use is the “pencil trick.” Take the architectural drawing of the house and, without lifting the pencil, draw a single line representing the insulation around the house. Wherever you lift the pencil would be a discontinuity where outside air could short-circuit the insulation, rendering it useless.

#4 – Be smart – Don’t break the original home’s design

Going back to the “do no harm” principle, you have to be really careful not to “break” the working design of the house. I’ve seen insulation contractors do really stupid things that lead to moldy, rotten homes. If a house was built 50 or 100 years ago, and it’s never had a mold problem, there’s probably a good reason why. For example, there are a lot of older homes build with plaster and lath. These have an air gap between the exterior stone wall and the plaster interior wall. That air gap allows water to drain and dry out. But yet we see people putting cellulose and other non-water resistant insulation into that gap. That’s like sticking a sponge in there. It will soak up the water and likely end up in a moldy, rotten mess and useless insulation.

There are other stupid things you can do. For example, pretty much every roof that goes on gets a ridge vent. But why add a ridge vent to a 50 year old home that already has gable vents? Worse, that 50 year old house won’t have matching soffit vents. The result is a house that is less energy efficient and more prone to mold and rotten roofs because the ridge vent sucks the humid air out of the house and into the attic.

If something doesn’t make sense to you, don’t let the contractor do it. Sometimes they’ll be offended and say something like “I’ve been doing this 30 years, I know what I’m doing.” Well, maybe they’ve been doing it wrong for 30 years! If you have any doubts, get another opinion or two. Or post a question on my Q&A section. There’s a science to this and there’s a good chance that your contractor doesn’t know the science. I’m not saying that they’re not a good contractor, but they may simply not be knowledgeable about the current best practices. Regardless, if they’re not willing to listen or back up what they say with facts, you should find another contractor.

#5 – The home is a complex system

I touched on this in #4 with the discussion of the ridge vents. A building is a lot like a human body – what you do in one place affects things in another. Just like adding a ridge vent can lead to increased energy bills and a rotten roof, you can do other things that have unintended consequences.

For example, many utility rooms with furnaces or boilers have large vents that provide fresh air into the room. Often, people look at these as sources of cold drafts (which they are) and close them up. The problem is, they were put there for a reason – they supply fresh air for the combustion that the heater is doing. Closing the vents and sealing the utility room can lead to improper heater operation or even carbon monoxide problems which can lead to illness or death. Unintended consequences can be deadly.

Another example of unwanted side-effects of insulation – spray foaming a house without ensuring adequate air supply.

I’ve been known to say “a house can never be too tight.” Unfortunately, you can screw things up by making a house very tight without considering fresh air and humidity requirements. Using spray foam, you can make a house that has extremely low air leakage, so you have to consider introducing fresh air using an HRV (heat recovery ventilator). There are standards for this (See ASHRAE 62.2 and related). Basically, you want a certain amount of fresh air per occupant in the house. For typical families, this amounts to about the amount of air introduced by a bath fan. Not much! But often, people adding spray foam insulation either don’t consider or don’t know about the fresh air requirements, and the results can be horrible.

Concrete example: you make a house super tight at the same time as you remodel the kitchen and add a powerful new range hood. When you run the range hood, it creates a suction so powerful that it can sucks the exhaust fumes from the furnace and water heater back into the house. Again, this can kill you. No joke.

Less extreme example: you tighten up your house and then you find that every time you make a fire in the fireplace, you end up with a huge amount of smoke in the house because there’s just not enough air going up the chimney. To compensate for this, you have to open a window whenever you have a fire.

There are numerous situations like this that can occur if you “just insulate” without considering the complex system that is your home. Please, go into these renovations with open eyes. If you have any concerns, call in a trained energy auditor/building scientist. I’m also happy to answer questions posted here. But be aware that in order to do this right requires a series of precise measurements that take into account a variety of factors about your house and various appliances in the house. In some areas, these measurements are required if you are adding a certain amount of insulation or doing significant renovations.

35 thoughts on “How NOT to insulate your house

  1. I HAVE to reinsulate my attic. Because of some remodeling I have had to replace some ceiling sheetrock. We had a new roof put on and at that time installed a ridge vent. We had the usual roof vents as well as gable vents prior to the new roofing. By default I have had to get into the attic and have installed some baffles and have had to “unplug” some of the soffit vents because the junk insulation that was dumped over the 60 year old bats was plugging them. I am considering putting in Solar Guard reflective insulation under new battings as a vapor barrier where there was/is none after removing the old insulation. To much? Again there is no vapor barrier in the ceiling now.

    1. Depends upon your climate. As a general rule, the colder it gets, the more important a vapor barrier is due to the risk of condensation. In more moderate climates, the current thinking is that no vapor barrier is good because it allows small amounts of moisture to easily escape through normal ventilation of the attic.

      Studies have shown that small holes in the sheetrock, like from electrical boxes and such let much more moisture through than the painted sheetrock, so I’d personally want to ensure that any holes through the sheetrock are sealed tight. BTW – if you use/install recessed lights, don’t believe the label “air tight” – they’re anything but! The only recessed light fixtures that I’ve seen that approach air-tightness are the newer LED lights that seal tightly against the sheetrock with a gasket and have no air-holes.

      Anyway, as for the reflective insulation – another important point is that if you put it under the insulation, then that shiny surface does nothing to help. You’d be just as well off with a piece of plastic dropcloth. In order for radiant barriers to work, they have to have a gap of at least 1″ or so between the shiny surface and any other building material.

  2. I’m getting bids to insulate the exteroir walls of my mater bdrm and bath which faces the north of my 73 model house. Its usually 5 to 6 degrees cooler than rest of house during the winter. I have already had new dp, low e argon filled windows installed and apprx 15 in of insulation blown up in attic. All that helped for the summer months here in N. Texas but not so much for the winter months. One contractor bid to spray foam the stud cavities from either the exterior or interior by drilling 3 holes top,middle and bottom and injecting tripolymer 105 foam. The other wants to foam the air gap between the brick and exterior sheathing using the same method as above using cfi insulsmart foam, this just doesnt sound right to me. What about the weep holes? Do you think either of these insulation practices work? Or is there a better solution to my issues. Im thinking of hiring an energy audit that you mentioned with the blower an ir test to see where my problem realy lies. Any suggestions or comments will be appreciated. Thank you for your help in advance. Jason

    1. Jason,

      Given your issues, I think a professional auditor would be the way to go before investing in more upgrades, especially if they’re talking about foam filling the gap between the brick and sheathing – like you noted, there’s a reason that they’ve built with those gaps forever!
      If you do decide to go with an injected foam solution in the wall, then I strongly advise you to have the walls thermally scanned before and after to ensure that they actually filled the wall cavities. Without a test like this, you have no guarantee that they did a good job. I would further advise that you get in writing a guarantee that the wall cavities will be fully filled and that it is their responsibility to cover all costs for return trips. Ideally, the foam contractor would have their own thermal imager so they could inspect the job as they go, but you’d be really lucky for this to be the case.

      Back to your original issue – if the rest of the rooms on the north side of the house are comfortable and just master bed and bath are problems, there’s a chance that you’ve simply got inadequate heat going there. I have a friend with exactly this problem and we’ve had to work hard to try to get more hot air supplied up there from the furnace. If you have baseboard heat, that’s another story. Sometimes, you’re best off getting a mini-split heat pump just to service the master BR – I did this in my own home and don’t think I could live in the house without it! Being able to adjust the BR to exactly the temperature you want is an amazing luxury.

      Hope these tips help.

      1. Thank you for the quick response, the mstr bdrm and bath are the only rooms on the north end of house and we have centeral heat and air electric heat strips 15 kw. Planning on heatpump on next change out. Thank you again Ted, I will be getting an audit scheduled soon. What should I expect to pay for such a service?

      2. A quality energy audit would run $350-$500 for a whole-house review, assuming an average sized home.
        When I took on clients with issues like yours, I would offer a troubleshooting visit at an hourly rate and that would usually run $200-$300.

  3. I totally agree with you, public education is crucial but I would add not only in your field but in many others too. Can you imagine when it’s a health issue? Do you know that many doctors don’t take in account the dynamic of the whole organism when they act on it ? Same thing in psychology and I bet in many other domains. The thing is if we don’t take in our hands our own file chances are nobody will. And this is very disappointing because the only thing I want is TRUST someone who’s responsibility is to do a job I can’t do without having before hand to spend hours like I did reading about it because obviously all those guys are missing obvious points I SEE and I know nothing about their work. Reading your articles make me realize how much I am ignorant. Now I am going to have to make an inventory of the past renovation I made and the one I was intending to do.

    VENTILATION of a roof made 3 years ago.

    In reference to an above comment made 2 days ago by Jon who says:
    “They ….. overlooked the fact that our house doesn’t have soffits. I had no idea about this kind of construction/venting and how the venting needed to be 50% soffit 50% ridge vent. When I told the roofer that the warranty for the singles would be voided without proper ventilation, he claimed that he thought we already had soffits? The entire job was complete and no one on the crew questioned our lack of soffits? They had to come out and add edge vents to the entire perimeter of the roof ”

    The question of the roof scares me because I also trusted a well-known company who I am sure didn’t do their job correctly if it’s true that when you don’t have soffits it’s important to put edge vents to protect the integrity of the roof. And from what I can see they didn’t : ( And it did also cost me
    $ 10,000.00 for a security I didn’t get.

    Any recommendation to ventilate my roof properly? Should I approach the roofing company about this?

    On the other hand the attic isn’t YET, thank God!!, isolated. I don’t know Ted if there’s a relation between what I just said and the ventilation of the roof but maybe it does. The condition of the attic is okay because it’s an old house (1953) and air seems to circulate in it because of the cracks. Also we still have the equivalent in wood of soffits with opening in them (like in the old time). I know we’re suppose to replace wood by appropriate soffits. We didn’t yet.

    In any case I made an evaluation of the energetic efficiency of my house and the independant company recommended to isolate the attic. This company will come after to check if it was properly done. That said you raised my level of awareness and now I am going to make sure that the company who will isolate my attic with this foam product will do it in a proper manner. That said I feel now insecure because I am afraid to miss something obvious but a least now, thanks to you, I am less ignorant.

    I’ll have to move now to the heating system section to post a note since when I started reading your articles It was to read only about replacing a heating system and adding, while we’re at it, a heat pomp. And then I migrated to this topic.

    Thank you Ted !


    1. Thank you for your discussions. When I got started in this, I was doing one-on-one consultations and soon realized that the problems kept repeating themselves. So I’m glad my posts are helping people all over to avoid or remedy their issues. Sharing your stories helps to show others that these problems are universal and, like you said, unfortunately, we have to take matters into our own hands and ensure that the work being done is appropriate.
      If you take good care about sealing the attic floor, the attic ventilation issues become far less important. I’ve seen perfectly ventilated roofs with rotten sheathing because of lack of bath ventilation or similar issues that allow lots of moisture from the house build up in the attic. I’ve also seen very poorly ventilated roofs last for decades! The key is to know what the big factors are and address those.
      In your case, as you noted, the key is to isolate the attic. The easiest way is to get a coating of foam over the entire attic floor combined with ensuring that the baths are ventilated right up through the roof and the attic access has a good seal. If you do this then there’s very little chance of moisture buildup if you have any attic ventilation.
      If you have the budget for it, then you’ll want to use several inches of foam on the attic floor, to bring it up to code levels, but it gets expensive fast. An alternative is to use a relatively thin coating of foam – 1/2″ to 1″, coating the entire floor and joists (this air seals and greatly reduces thermal loss through the beams). Once you’re sure the foam job is good and is really air tight, then you insulate the rest of the way with another insulation product, like blown in cellulose. This is assuming that you’re in a coldish climate. Things turn around if you’re in a hot, humid climate like Florida.
      If you have a heating/cooling system with ducts in the attic, you have to take considerable care to seal that up. Since it’s under pressure, it can blow out lots of humid air and ruin your roof. These days, many areas require duct testing to ensure the system is tight. The company that did your energy evaluation should be able to do those tests as well.
      Good luck with your projects!

      1. So awesome how you take in account many variables at the same time, including the financial’s one to make sure you are really helping me look into fundamentals.

        The thing I retained from you is the importance of taking my time because anything I do without having a minimum of a good comprehension of the way things are linked together dynamical speaking (= big picture) can end up in hurting my old house instead of serving it.

        The thing is I haven’t done anything for decades but thank God the integrity of my house is still okay. Still I have now to address everything at the same time with a limited budget. And I find it hard to prioritize.

        I also retained from you that when something works we have to be careful when we touch it to “improve it” because by doing so we might introduce a critical problem instead of helping positively the house. And the attic in this regard is a perfect illustration of what can happened if we’re not careful with the way we will handle it’s issues.

        Every one of your recommendations regarding the isolation of the attic, on top of being greatly appreciated, makes totally sense for me.

        By the way the heating system with ducts is located in the basement, which basement isn’t finished (but this is another topic I won’t be able to address before a while but I already read some of the things you had to say about it. ONE issue at a time :)

        Last question in this department if you allow me :)

        Like earlier mentioned my roof was redone about 3 years ago. I understand that the fact that I don’t have soffits (beside old wood’s one with a couple of opening in them) should have been take into account in some way by the Installation of edge vents to protect the new roof ? I am a little bet confused because I don’t have a clear picture between the dynamic of the roof (top) and the attic and the house. Should I take care of this (hoping my roof wasn’t affected by the lack of edge vents).

        Should I prioritize the installation of modern soffits rather than keeping the old wood’s ones?
        I went to the attic this morning and you could see in the dark light where there’s opening in the wood soffits.

        Looking at my atttic wasn’t the same experience than before. Now I feel more in control thanks to your explanations.

        Thank you Ted,


        P.s. I live in the North Pole not far from this very known neighbor of mine Mr. Claus :) So yes this weather’s variable need to be taken into consideration when it comes to the isolation of the attic’s floor.

      2. Yes, big picture is truly important. A lot of harm has been done with good intents by forgetting about unintended consequences.
        As you note, prioritization is truly difficult because everything seems important. Your health is #1. Then your home’s health. Then efficiency and comfort. Often you get benefits together which is good! But sometimes, things people do in the name of efficiency can hurt. Like tightening the house air-tight without adding supplemental ventilation for fresh air.
        I’m glad your ducts are in the basement – that simplifies things in the attic.
        On to your question!
        I need to ask you a question or two before making suggestions. Sorry if you already answered these in a previous message, I just want to get it all in one place. This will help others to understand the thought process.
        – When you replaced your roof, did you replace all the wood under the shingles? If so, what happened? Did the roof rot out from underneath (the attic side) or was something else wrong with it?
        – Was the wood black or moldy when you looked from the attic?
        – When you redid the roof, did they add a ridge vent or was there one there before?
        – Is there any other venting in the attic, like louvered gable vents at the end walls?

        Let’s say for example, that your roof sheathing had rotted out before due to moisture in the attic and that you had gable vents and no ridge vents. This was normal construction a few decades ago and it worked fine for most homes. But if moisture caused your roof to rot, something is wrong. In a cold climate, that moisture is almost certain to have come from inside the house, though sometimes it starts at a wet basement, goes up the walls, and into the attic.

        So let’s go from here and answer the questions about your roof.

      3. Thanks so much for the info!

        This is a question regarding materials. Through all of the horror stories and research I have ran across in having our roof re-shingled and our insulation upgraded I have wondered why batting insulation AND wood or osb roof sheathing are used at all. I realize money is an issue. I too do not have much. However, seems like “roofs rotting out” is a common problem. Why is wood the standard roof sheathing if it has a tendency to “rot out” or why isn’t it standard practice for roof sheathing to at least have mold and moisture resistant paint underneath to prevent this from happening in the first place? Are you aware of a roof decking material that will not rot? There are materials for the outside of the house such as Hardy plank (not a fan of this look however) that are rot resistant. Why not roof decking?

        Thank you for all of your advice and tips!

      4. As for roofing material – it’s most likely price and availability. The switch from plywood to OSB or flake-board is a good example. If they can save a few dollars per sheet, that goes right to the builder’s bottom line. This is in spite of the fact that OSB is a horrible material for use in wet conditions! Plywood is actually a much more moisture tolerant building material.
        There are millions of homes with wood roofs that don’t rot out, so it’s not that they’re prone to it. It’s just that a variety of factors are conspiring to make moisture problems worse than ever.
        I talk about unintended consequences a lot. Here’s one. Older homes were much leakier and had minimal attic insulation so they were drafty and cost a lot to heat. But those leaks and drafts helped to flush out moisture from the home. Remember when we were young and we’d get nose bleeds every winter from the dry air? The air was so dry in our homes that there was little moisture going up to the attic to rot the roof.
        In addition, with the little bit of insulation homes had in the attic floor, a lot of heat escaped up to the attic. This heat reduced the risk of condensation in the attic. Another factor that helped prevent roof rot.
        Today, we have tight houses, which are much more comfortable and energy efficient. But that means there is a lot more moisture trapped in the house. When that moisture goes up to the well insulated attic, it finds a very cold space and condenses on the surfaces up there.
        Worse, because of the dry-air syndrome, builders installed a lot of high output humidifiers that dumped gallons of moisture into the home every day. This really speeds up mold growth and rot problems. Tight homes don’t need added humidity. Put all these factors together and you end up with the problems we see today.
        I should have mentioned the whole home humidifiers before. If anybody reading this has moisture problems and is using a humidifier, turn it off immediately! They are absolutely destructive in a modern home and truly shouldn’t be allowed.

    2. Hi Miche–FYI, our house doesn’t have soffit vents because we don’t have any eves at all. The roof is a very steep pitch and meets the bottom walls of the house right at the gutter. Many roofing companies are installing ridge vents but in order to have a ridge vent work properly, you need to have soffit vents at the bottom of the eves or in our case, have edge vents installed at the bottom of the roof line. Here is a link to what I am talking about.

      I really hope this system works. According to everything I have read, all the people that I have talked to, and many many hours of research (bitter bitter research) along with the thick r31 thermasheath rigid foam insulation (foil backed on both sides as a vapor barrier) installed with a 2″ gap in between the roof decking and fingers crossed that working in the wet humid Seattle weather over the next couple of months on this will not be too late to create a problem. I feel like if we did this job in the hot summer, I wouldn’t be facing possible issues of moisture problems right now. I am trying to be preventative in any issues happening as we continue to finish the job.

      To Ted (again thanks for you expertise!). To clarify a bit more about our attic space, it is actually a converted attic space. We are trying to make it a properly conditioned attic space with cathedral ceilings. The walls and ceilings that were ripped out were made of a thin plywood with wood wool insulation that was falling apart and not evenly covering everything anyway, The roofing shingles that we just had replaced continually grew moss and was the only house on the block that never had frost or snow on it in the winter. I know that older homes need to breath but I feel like the attic not being properly insulated (with a moisture resistant insulation) nor properly dry walled (plaster and lath on the lower floor) with leaky knee walls that led to the moss formation and the roof needing to be replaced in a shorter amount of time than desired. The new ridge vent that was installed (completely closed dead space before this vent was installed) along with the later added edge vents are the reasons that I am trying to do every single thing that I can to make sure that this job is done properly as to not have someone approach us later saying “Well, if you would have done “THIS”, you wouldn’t have that problems that you have now!” What is killing me is, only time will tell if this system is going to work and not have any problems in the future.

      I am still wondering if the best protocol in the partially insulated space would be to keep a dehumidifier running to take any possible accumulation of condensation (from inevitable leakage from downstairs heat or moisture) out of the air in the top rooms/converted attic until we can finish the job. We have fans running and are blocking off the stairwell to the best of our ability.

      Thank you for your time and I hope that the space description made sense. I would attach pictures but I can’t figure out how to do this.


      1. Tracy – thanks for sharing that Certainteed link – that looks like a great system!
        For photos, I think you’d have to make an album somewhere else (like Google photos) and share link to that. Like this for example:

        The dehumidifier might not be a bad thing to use since you know it worked before. And if you kept the humidity down to a moderate level, it would certainly help keep the exposed roof sheathing in better shape while you can’t work on it. They’re somewhat expensive to run because they use a fair amount of energy, but it’s a lot cheaper than replacing a rotten roof!

        BTW – I’m not convinced the moss on the roof is due to your construction / lack of insulation etc. Usually mossy roofs are those that stay humid and cool due to shading. Is it possible that your roof is more shaded than the others in the neighborhood that don’t have moss? Are there other conditions that might contribute to your shingles being damp? Do they all have the same roofing materials as you? Wood shingles grow good moss around here, especially down by the Delaware river where they’re in the shade and get morning fog often – our closest simulation to Seattle weather :-)

  4. Coming back to this – regarding unintended consequences of tight houses, ours got to the point where when we lit a fire, the basement filled with smoke. What?
    Well .. the fireplace had to draw from somewhere, and one of the biggest remaining holes in the house was the boiler flue. Where did it exit the house? Right at the top of the chimney, right next to the fireplace flue. So the fireplace was sending smoke up the chimney, and the boiler flue was drawing it back down in the the basement. (never mind the backdrafting, etc… we now have a sealed combustion boiler… solved that problem the expensive way!)

  5. Thank you so much for such a wonderful site!

    I have to say, as an artist and homeowner, I feel completely bitter that I have had to start to think like a contractor. I have learned more about what not to do in our 1930’s 1,800 sqft plaster and lath two story home than what “to do”. No one seems to know the right answer and money spent on “professionals” don’t usually go hand in hand. We just spent $10,300.00 on re-shingling our roof. The roof decking is osb, in decent shape so they didn’t replace it. New under layment was placed underneath high end designer shingles. They added a ridge vent, removed all of the vents at the mid roof line and overlooked the fact that our house doesn’t have soffits. I had no idea about this kind of construction/venting and how the venting needed to be 50% soffit 50% ridge vent. When I told the roofer that the warranty for the singles would be voided without proper ventilation, he claimed that he thought we already had soffits? The entire job was complete and no one on the crew questioned our lack of soffits? They had to come out and add edge vents to the entire perimeter of the roof. $10,000.00 doesn’t buy professionalism and I still had to act like a sleuth!

    Onto my question…We have a livable attic space that we are “upgrading”. We tore out the ceiling and walls, took out the wood wool that was falling apart and plan insulating our cathedral ceiling with thermasheath r31 rigid foam insulation (4-1/2″) with foil on both sides in between each rafter bay, extending all the way down, pass the knee wall to the floor. (there is a 2″ gap allowance in between the insulation and the roof decking that should allow the air from the retrofitted “soffits” (edge vent) to the ridge vent to escape, HOPEFULLY. My husband added wood to the rafters to extend them longer to allow for the thick foam insulation) Because we spent all of our money on the roof, our plans and time were delayed until the colder months. We live in Seattle, WA. I tried to block the stairwell going up to our two upstairs rooms(attic space) where the insulation is being installed but we waited too late into the cold months and warm air from the downstairs ended up condensing on the roof decking (on exposed nails and a metal patch). We rented a heavy duty de-humidifier and for two days and ran fans and a heater on low inside of the rooms to dry out the roof decking. The job is not finished, only 1/2 of the space is insulated. We had to return the dehumidifier to Home depot, it was getting expensive. Do you have a good idea for us in how to continue this project on a budget, doing it in our spare time, maybe waiting until summer without ruining the wood or causing moisture/mold damage? Should we be constantly running a dehumidifier until we can finish the insulation and drywall? Is there a temperature consistency/dewpoint that we should monitor in the space until the work is complete and done? PLEASE HELP! I don’t want to ruin our home and I don’t know what to do nor does anyone else I have talked to. We simply don’t have the money to hire someone to do the work right now and we have very little spare time because we run our own business. Any advise on this epic reply? Thank you so much for your time!

    1. I feel your pain. It’s largely the reason I got into the home energy/building science consulting business years ago. Too many unqualified contractors forcing homeowners to learn how to do things right or get stuck with sub-par or even dangerous workmanship. Frankly, it’s very disheartening.
      It does sound like you’ve done your research well. A good gap from soffit to ridge behind the foam should do a good job of ventilating that space – do your best to air seal the edges where the foam board gets installed so that the moist air can’t sneak in and condense when it hits a cold surface. You should be good once the ceiling is installed as long as you keep it intact (no recessed lights or other holes in the ceiling other than maybe power wires going to fixtures).

      As for your problem at hand – have you tried taping plastic sheeting over the opening up to the attic? You do have to be careful to seal it completely to minimize the amount of air from the house that can get up there. And if there’s any other sources of interior warm air that might get up there, do your best to seal that off. Since you’ve got the soffit and ridge vents, that should take care of most of the remaining moisture that does get in there.

      Regarding nails getting condensation on them – I wouldn’t worry to much about them. They’re conducting heat/cold from the outside very effectively so it’s almost impossible to prevent. And the metal patch – what is that? Is that like sheet metal that replaced some of the roof wood? if so, that too will likely be hard to prevent from condensation unless you can prevent any moisture from coming in contact with it. Depending on the size, you could temporarily mount a sheet of foam board over the patch -it doesn’t have to be the really thick stuff, maybe 1″ thick should do. You could run a bead of canned foam around the perimeter and squish the foam board right over it. This will insulate the patch and minimize the moisture that can reach it.
      If the attic is left open for ventilation (soffit and ridge vents) and you seal it off from the rest of the house and insulate the really cold patches, you have a good chance of combating adverse moisture buildup. You shouldn’t have to run a dehumidifier unless you missed a big hole where warm, humid house air is getting up there. A little humidity won’t hurt things as long as it gets a chance to dry and it isn’t soaking wet. Usually, when the sun shines on the roof, it should heat up enough to dry it out. Granted, Seattle in the winter doesn’t get great sun, and the natural humidity doesn’t help.
      I would check it out after a few days of being sealed up to ensure that it’s staying reasonably dry. If it isn’t, look for other sources of moisture that could be contributing to the dampness.
      Feel free to drop more questions as you progress.

      1. Wow! Thank you so much for getting back to us so quickly:) My father in law and husband (I am actually Jon’s wife Tracy commenting!) are quite impressed with how quick and and in depth you were with answering my questions. This has all been such a guessing game and to have a professional give us some advise, well…it’s worth it’s weight in gold. The internet has really helped but boy, there is so much information out there and it takes reading some of the same tips, do’s and don’ts over and over to find out what even qualifies as a valid question. Regarding condensation, like me, if anyone is wondering how long it can take to accumulate on the roof decking if you delay your insulation project, well…it can happen immediately, with the first cold night and warm house. The metal patch I was speaking of that gathered quite a bit of condensation was a cover over one of the former box vents. The only reason we noticed the condensation on the metal was due to the underlayment that was ripped around the edge exposing the patch above the cut wood decking. Not sure why the under layment was ripped, maybe someone forgot to mark the hole and accidentally stepped through it while laying the shingles? I am glad that we are re-insulating because this could have been a potential problem. Thank you for explaining how to patch it to lessen the potential for condensation to form.

        There is a door and stairwell that go up to our attic which was a conditioned living space. It seems more like a second story rather than attic due to the narrow stairwell with a window at the bottom. There is a hallway with a room on either side and a window in each room. One of the rooms has the ceiling and walls ripped out, our new insulation on most of the ceiling, all the way to the floor (like an A frame, no dormer) and no insulation on the end wall that has a window. The wall on the other side of the insulated room (same wall that shares the hallway) is open at the top exposing the dead space above the hall and other 1/2 of the upstairs room that is yet to be freshly insulated. The other room still has walls and a low ceiling but the knee wall has a door that is quite air permeable and the walls are in bad shape with cracks, holes, and poor construction. (like a high school student found a stack of plywood and made “walls” and a “ceiling”.) The disintegrating wood wool is still in the ceiling on this side but it isn’t covering everything very well from what we can see. So, it is quite open to warm air hitting the roof decking and exposed wood wool insulation. We have the door at the bottom of the stairs blocked with a piece of 2 inch foam but our ceilings and walls are plaster and lath, no insulation in between them. So, hopefully my epic description will help answer if you think we should run a little heater to keep it dry (seems like a bad idea), fans, or a dehumidifier upstairs with the probability of some warm air most likely forming onto the roof decking before we can finish the project? The dehumidifier really seemed to dry things out overnight before we insulated the one 1/2 of the attic. It will probably be another month before we can do the rest. It is a busy time of year with our home business!

        Thanks so much for your expertise, time, and attention to detail. I will make sure to “pay it forward”!
        Take care and happy holidays,

      2. So if I’m understanding correctly, this is the entire upstairs of the house? I’m confused about the roof ventilation. You noted the soffit and ridge vents – so I was assuming the upstairs was directly exposed to that ventilation. Is that true?
        And, since there’s so much demolition up there, a lot of the warm air from inside the house can get up there?
        As for the condensation – the sheet metal section will have condensation immediately but it won’t be a real concern. It might rust a bit but in the next couple of months, it’s not going to crumble to dust. Plus, hopefully they used some metal that’s water resistant. I’d be more concerned about the liquid water that drips off of it. So anyway, take a few minutes and put that insulated patch on it.
        The bigger concern is the wood which can rot and also provides a base for mold to grow. Can you tell how wet it’s getting? Ignoring the nails, which will get drops of condensation on them.
        In Seattle, you may be dealing with a Sissiphean task. The outdoor air during the day may be very humid. At night, the upstairs space will cool down very quickly leading to the condensation. Even out here in PA, some days my garage gets filled with condensation when we get a temporary warm front that brings humid air in. Everything in the garage is cold and the humid air condenses immediately, rusting all my tools. So you’re dealing with a real conundrum. If it were dry and cold, you probably would have a lot less of a problem. Not knowing your exact situation, I can’t tell whether it’s moisture from inside the house or outside air that’s the bigger problem. The best you can do is seal up the biggest holes, then monitor the situation. Big fans will help to move air and allow moisture to evaporate as conditions allow. If the space really is open to the outside due to venting, then a heater will be a huge waste of money and it will cause the mold to grow much faster, so don’t do that.
        If you keep a handle on it, minimize extra moisture load in the space, towel down any areas that are actually wet and run fans, you’ll probably be able to limp through the next couple of months. But it’s a dicey game to play – you really want to keep an eye on it and if it seems things are getting out of control (i.e. wet dripping from the roof deck or signs of mold growth) you’ll be forced to jump on it immediately. It’s still possible that you’ll get some slight mold growth in that period. Keep your fingers crossed and finish the project as soon as possible!

  6. Excellent post! My parents are looking at insulating their turn on the (last) century farm house. It has 3m walls, cement ceiling and corrugated tin roof with 1 working chimney, multiple vents and open door way entrance (and as a result lots of “wildlife” up there). The house gets quite chilly in the winter when the temperature drops to 0ºC. They are favouring the spray foam option and I was wondering what you think about that?

    1. Wow, that’s a tough one. Honestly, it sounds too complicated for a recommendation without being there to evaluate the nuances of the design. I’d also warn that if you ask a professional insulator, many will sell you on their solution whether it’s good for the house or not. Your parents could waste a lot of money on foam and have it make a minimal improvement or worse, cause a problem elsewhere in the house.

      Your best bet would be to get a comprehensive overview and recommendations from an independent home energy consultant. For about $500US, it’s a great investment. You want someone who doesn’t have a stake in selling you a specific solution and will be honest and direct with you.

      Good luck! Sorry I can’t give you more advice.

  7. I ran into something like this. Sealed & insulated the attic, and then had a new roof put on. The roof plan was to do a ridge vent and “retrofitted’ soffit vents, something called edge vent. Well… the attic insulators dense-packed the cellulose down into the crannies where the ridge vents were supposed to be venting. Needless to say they didn’t draw air very well. :( I went up with a stick and a shop vac and tried to get it out of there, but it’s nigh impossible.

    Is there any way to actually measure attic ventilation?

    1. I can’t think of a good way to measure ventilation that is reasonable given the large areas involved. If you had an attic that just had gable vents, you could measure pressures at each vent and combine that with the opening area to compute air flow but the measurement would vary tremendously with wind and temperature so I don’t think it would be terribly useful.
      In the situation you ran into, you might be able to tape plastic under the attic hatch (inside the conditioned space of the house), then use a manometer to measure the pressure there. With a properly sealed hatch, you should get zero but chances are, you’d have a negative pressure relative to the house, indicating air flow from the house to the attic. There are techniques you could use to quantify the actual air flow at that location, but again, it’s imperfect since there are usually so many places that air can move from the house to the attic.

  8. Have you ever worked with “Retrofoam”? We have a house built in 1979 with a master bedroom upstairs loft area and it only had an inch of styrofoam in the cathedral ceiling for insulation and batting in the walls. We’ve had all sorts of problems with the AC maintaining temperature and high humidity, so we had Retrofoam injected into our walls and ceiling, but the problem didn’t resolve. After we had the AC unit fixed, as it was also part of the problem, it’s maintaining temp, but we still have a humidity problem. Prior to the foam we had humidity around the same % and were running a dehumidifier which increased the temp of the space. I’m just baffled that we can’t figure out how to make the AC unit do it’s job and keep the humidity level down. And now, I’m not so sure we didn’t just make the situation worse. Any advice?

    1. Unfortunately, I haven’t. I investigated it and talked with an installer a number of years ago and it looked quite interesting but opted not to go with it because the installers in our area didn’t seem reliable/experienced with it.
      The one big problem I have with any injected product, whether it be foam or other product is that you’re at the mercy of the installer to do a good job unless you validate the job with an infrared camera. A friend of mine had a foam job done in his walls that was a total dud – when I looked at it using IR, you could see that the foam just dribbled down the wall and didn’t fill the cavities.

      I mention this because it’s very hard to know what’s going on inside of the walls without a test like combining a blower door and IR camera. It costs a few hundred dollars, but it’s well worth the expense because otherwise you’re just guessing and can spend thousands of dollars trying things that may or may not work.

      Humidity problems are a dead giveaway that you have a leaky house. A blower door test with IR scan should show you where the leaks are. It might be something systemic in the way the house is built or it could be something as simple as a bad duct in your attic which is the cause of the worst problems I have seen.

      So my advice, is find a good energy auditor locally who has experience in troubleshooting using IR and a blower door. You may have to interview a few people to find someone who isn’t a “production” energy auditor. These days, there are a lot of people just doing cut-rate energy audits but who really don’t understand building science and troubleshooting. You want a trouble-shooter. Explain your problem and listen carefully to their reply. If they seem to have a generic cook-book, they’re probably not a troubleshooter. If they can clearly explain what causes your type of problem, it probably means they’ve got some experience. Make sure they use a blower door and an IR camera. If they say they don’t need one or the other, send them packing!

      Good luck. Please let me know how it goes. I think your problem is very solvable.

  9. Thanks Ted!
    Your tips on attic insulation are very helpful. I’ve been researching a variety of things that can be done to increase energy efficiency while designing an addition. One topic is geothermal energy for the home. After speaking to several geothermal professionals/installers, one thing they all recommended was to first take a hard look at the energy efficiency of the existing home. And, consider a conditioned attic space.

    It appears to be an effective method of reducing the costs of geothermal installation; better insulated attic and home overall calls for a smaller and less expensive geothermal system. Further research suggests the attic is a great place to start with respect to energy efficiency. Your tips are extremely helpful in that regard. Again, thanks for the detailed info and photos.

    1. Yep, energy efficiency is definitely the first thing to look at if you’re looking into geothermal. One thing that is often overlooked is that a more efficient home is also more comfortable because the temperature is more even. Also, as you noted, it helps reduce the cost of geothermal. More comfortable and less expensive is a good thing!

      Good luck with your projects :-)

  10. Hi, I would be interested in your opinion regarding adding exterior foam to walls. I worked as a carpenter up to15 years ago and am a certified R-2000 builder. My own home (20yrs) is r-28 walls, strapped with 2×3 to protect the vb and provide a chase with a hrv the home has performed perfectly even at -55f temperatures. I am planing to build a smaller home for retirement and because there are more windows in smaller walls, the many headers, jacks and corners, I am considering adding exterior foam. Because of my background, I am having trouble with the idea of omitting thr VB. We use to omit the tape at the top of the walls to allow the typecast to breath into the vented soffit. Can the interior walls be build both foam and VB and the top exterior be vented with 1″ vent plugs to vent into the soffit. Thank you

    1. With your background, you should be writing these articles! :-)
      It sounds like you’re working in extreme environments up there. In such cases, the risk of condensation is much higher than down here where things are milder.
      The fundamental theory that guides recommendations is to ensure that warmer, moist air from inside the house can’t come in contact with any surface cold enough to lead to condensation. With enough exterior foam, you should be able to maintain inner surfaces at warm enough temperatures (roughly 50F), but you clearly have to be very careful in your construction. With your background, I’m sure there’s nothing new here.
      The “gotcha” that sometimes bites even experienced builders is adding too much insulation inside of the exterior insulation because that insulates the wall cavity from the warmth of the house. In doing so, it can allow the surface between the exterior foam and the interior insulation (usually the exterior sheathing and the 2x studs) to get too cold. That’s where the serious VB comes in to minimize the moisture that gets into the wall cavity. Problem is, with a tight exterior AND a tight interior, you’ve got the dreaded double VB situation that can lead to real problems. If any moisture gets in there, it can get trapped and build up over time.
      Given the severe temperatures you’re dealing with I’d be really careful. Read through for their info on cold climate considerations. And be very skeptical about any solution that needs to be ‘perfect’ in order to work.

      Sorry that this isn’t a more definitive answer to your question. I hope there are a few kernels of information that you can use.

  11. Fireplaces or stoves should have OUTSIDE AIR for COMBUSTION. Many stoves offer this as an “optional” kit and fireplaces can be retrofitted (good luck finding someone who understands it much less do it).

    1. Great point. In the “old days” with leaky homes, people didn’t worry about it. Now that we’re tightening up millions of those leaky old houses, outside combustion air becomes necessary. Fortunately, new home building codes require this but as you noted, there seem to be a lot of contractors inexperienced with this. Our best bet is educating the consumer so they can insist on proper combustion air.

  12. Great post, many people don’t quite know what goes in to insulating a home properly, and its a real shame that some companies out there are more than happy to charge people for a poor quality job – look out for these people!

    1. Thanks for the comment. If there are ever any warnings you have for people, please pass them along. I’m doing my best to help educate the public but it helps to hear from those of you doing the work because you guys are often fixing problems that others have caused. Send photos of the big disasters you see :-)

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