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.


148 thoughts on “How NOT to insulate your house

  1. live in a basic construction, “cookie cutter”, home in Nampa, Id. It was built in 2001. If you aren’t familiar with the climate, it’s cold and dry in the winter, and hot and dry in the summers. Humidity isn’t something I ever thought we would have a problem with in a desert climate. But, we have one, or so it seems. Four years ago we replaced all our windows. But, we now have mold build up on several of them. It’s not a minor mold problem either. I’ve cleaned it up several times and still it grows. I have also noticed condensation on the inside of the windows. Other issues we are experiencing are, mold in our bathroom. (No window, but we do have a vent fan that is original to the home) The exterior walls in our bedroom are freezing in the winter and warm in the summer, with occasional dampness. Our master bedroom is always a completly different temperature than the rest of the home. There is also a noticably cold section in the floor of our bay window. Our master closet also exhibits temperature fluctuations significantly different than the rest of the home. It has one small exterior wall and a large interior wall it shares with the garage. On of the other rooms, that also has mold issues in the window, is always hotter than the rest of the home, and I think there is always a “scent” inside. My husband differs in opinion with that. We have no idea where to start. Is it possibly an insulation problem? It’s incredibly frustrating, and I feel like it doesn’t matter what I do the mold is always there.

    • Given that you live in a “dry” climate, the moisture causing the mold has to be coming from inside your home. Do you use humidifiers? If you do, then you need to keep the humidity lower, maybe much lower, in your home. During the winter, if you have a humidity of more than around 40%, you’re going to have moisture problems on colder things like windows and exterior doors.
      If you’re not using humidifiers, then you need to be diligent about using the bath fan during and AFTER taking showers. Bath fans need to be run for about 1/2 hour after taking a shower in order to flush out the moisture, assuming the fan is working properly. You can test the bath fan by holding a sheet of paper up to the fan. It should suck strongly onto the fan. If it doesn’t, you need to get it working and venting properly.
      As for the cold walls, they clearly aren’t properly insulated. If you can find someone who does blown in insulation, they can inject insulation right into those walls. That can help a lot if done properly.

      So, for mold, the solution is to get rid of the extra humidity in your home. Make sure the dryer is venting properly. Make sure the bath fan is venting properly. If you have a gas heater, make sure it’s venting properly. Get yourself a cheap (<$30) humidity monitor and keep the humidity lower until most of the condensation issues go away. This might be 40%. It might be 30%. It might even be lower.
      here’s a humidity meter that would work:

      If you keep the bath fan running all the time, it will pull in more fresh, dry air from outside which will help keep humidity levels down. With that, and the humidity meter, you can figure out how much you need to run it to avoid condensation. Plus, the added blown in insulation for the cold walls and you should be much better.

  2. I renovated a 1957 Cape-style hose in 2009, doubling its size into a Center Hall Colonial. The entire second floor is new construction as is the hip roof. The stand-up attic space is fully vented on all four sides by soffits and a ridge vent runs along the full length of the peak. I was present during the construction to check that insulation was being done according to specifications. Hollow box corners were created and filled with foam, insulated lighting cans used, caulk and foam were used to seal gaps as well as along sill, floor and top plates. Gaps between sheetrock and bath ventilation fans were also caulked. Finally an energy audit company was used to do a blower door test and examine the entire house from basement to attic to address any deficiencies.

    The attic has R-30 fiberglass insulation covered with a full plywood floor. A second layer of R-30 batts were laid perpendicularly across the plywood flooring and all soffit vents were kept clear of the additional batts. The pull-down stairs hatch was covered on its top side with foil-backed foam insulation and the attic stair opening has a hatch covering of dense foam board with high R factor.

    In early January this region experienced a prolonged, bitterly cold period of temperatures in the single digits before wind chill factor. The cold snap broke with two days above freezing- the second day reaching 61°F with high humidity and fog from the melting snow. This was followed by a very rapid deep freeze. A day later was sunny that began warming the east, south, and west exposed sides of the roof. I happened to go up to the attic to notice the entire roof sheathing of the north side from soffit to ridge was very wet. There was also evidence water had condensed on roof nails and dripped. I used a large fan and a dehumidifier to dry the sheathing over two days.

    The house is single occupant. Short showers and dual ventilation fans are used in a large Master Bath. Heating is hydronic radiant floor heat. Humidifiers are attached to the air handlers on each level and humidity levels kept around 25% when the temp is 20°F or lower and between 30-35% when above. This is to prevent Maple harwood floors from drying out as the radiant heat lines are beneath the flooring. Humidified air by evaporative pad method comes out the AC vents at ceiling level on each floor. The air tends to be slightly cooled going through R-8 insulated flex ducts. I have been monitoring the attic humidity and it has remained low during the current cold weather following the last thaw creating the problem. The sheathing has now remained dry along with the weather.

    If I condition the attic space by attaching sheets of 8″ foil-faced foam board to the common rafters creating vented spaces from soffits to ridge vent, is there a high probability of outside humid air condensing on the underside of the roof sheathing in similar situations of a thaw, followed by a flash freeze followed by a thaw? These situations have been rare but I suspect the water vapor condensed in the past without being observed. I’m concerned about the roof decking eventually rotting from condensation in the colder months. Hence the idea of conditioning the attic space to mitigate further condensation. Is this viable or are there other options to consider?

    • So, first I’ll say, congratulations on working hard to do everything right with the construction. It can be very difficult to have renovations done this way.
      The humidity you experienced during the rapid change from cold to warm/humid then back to cold is very common with a vented space. When you had the warm, humid weather, all the ventilation did its job, allowing that warm, moist air to fill the attic. Unfortunately, when you then had the rapid freeze again, the roof cooled quickly and that moisture all condensed on the inside roof deck. This type of wetting cycle won’t do any long term damage as long as it’s relatively infrequent and has an opportunity to dry out quickly.
      Your suggested solution would probably help a bit as the amount of humid air under the roof deck would be limited to just that small amount in the rafter bays as opposed to the entire attic volume.
      Just as an FYI, I have a ventilated attic and we experienced exactly the same conditions during that weather change. While I didn’t check our attic, I’m sure the same thing happened. But I’m really not concerned since that occurs so infrequently.
      If I were you, I’d monitor the conditions up there over time, especially after these types of weather events and make sure that the roof deck dries out in a few days and you don’t start getting signs of mold growth. If things look good, then you’re not going to have your roof rotting out. However, if the wood stays wet for long periods (more than several days at a time) and frequently, there’s a good chance you’ll have long term problems.

      • Thanks very much for your quick reply. I’m rechecking the integrity of original caulking around ceiling AC vents, bath ventilation fans and return registers to make sure it has not cracked or shrunk from drying or opened from the house settling. I have a remote temperature humidity monitor in the attic so those parameters can be closely monitored. I reduce the humidistat setting at night to suspend the humidification. Humidity usually remains at a comfortable level overnight.

        In considering foam board to condition the space, all the ducts and the air handler will be in an environment closer to the internal temperature of the home. Besides reducing interior humidity that can come in contact with roof sheathing, cooling requirements should be lower in the summer. When interior humidity is required on coldest, dry winter days, the output from the AC vents will be warmer.

        I’m sure with your suggestions and my increased diligence, condensation can be greatly minimized to those rare acts of nature beyond anyones control.

        Kind regards,


  3., I am much less educated on the terminology of these things, but I will try to describe my issue the best I can. On my home, when I go up into the attic against the side walls – they do not attach to the roof. There is about a 6-8” gap that I can stick my hand in and cold/hot air blows into the attic freely. This is the same on both sides of the house. However, at the ends of the roof angles there are the normal baffles for the ventilation. So my question is, are these gaps between the roof and wall intended to be there for ventilation purposes? Or would I be ok sealing them to stop the airflow that is coming into the house? Thank you very much for your help in advance!

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