The best way to insulate your attic – part 2

Wow, where's that hole go?

In the first installment on attic insulation, I discussed why it can be dangerous to add insulation to your attic without air sealing the attic floor first. Moisture can slip through tiny cracks in the attic floor and lead to rotten roofs. Given this information, we walked through the process of finding and sealing all those insidious air leaks in your attic, some easy, some difficult. But finally, after fixing all these problems, you could lay more insulation down on your attic floor, more confident that doing so wouldn’t lead to a humid, moldy attic.

But what if there’s an easier way?

Whether you’re building a new house or retrofitting an older one, you can make life much easier on yourself by using professionally applied spray foam insulation that air seals and insulates in one shot. There are two ways of doing this, each with their own benefits and disadvantages. We’re going to review both methods. One is spraying foam on the attic floor, instead of using loose fill or batt insulation. The other is spraying foam under the roof deck.

Spray foam on the attic floor

Recall all those little (and not so little) cracks shown in the photos from the last article?  Those cracks that you have to painstakingly caulk or use canned foam to seal? Using spray foam, you don’t have to worry about any of these, making them much more likely to be dealt with correctly. They just spray a nice thick layer of foam all over the attic floor and it air seals every area it covers. Plus, it gives you high quality insulation that isn’t going to get kicked aside every time you go up to the attic.

The great benefit of this method is that it eliminates the fine detail work. You spray foam and it seals. Case closed. Almost. You still have to worry about the details. For example, some recessed lights can’t have insulation within three inches of the housing. Others can be insulated, but shouldn’t be foamed over.

You also still have to properly seal your attic hatch or all that other air sealing will be wasted because the gaps at the attic hatch are usually so huge. And so on. In addition, there are two types of foam products: open cell and closed cell. They have very different properties that you should know about.

Consistency: Open cell foam is soft like a sponge or the foam in your sofa. Closed cell foam is hard. Open cell foam tends not to shrink over time. Closed cell foam can shrink as it “cures”, but a proper job shouldn’t shrink more than a tiny amount but you may see rips in it in some places where it’s torn itself apart.

R-value: Open cell foam has an R-value around R-3.5 to R-4 per inch, a little better than fiberglass. Closed cell foam has an R-value of R-6 to R-7 per inch. Cost: Open cell foam is typically somewhat less expensive to install for a given R-value than closed cell foam.

Moisture permeability: this is an important one, unfortunately, it’s complicated. Open cell foam lets much more water vapor through than closed cell foam. Sometimes this is a positive, other times its a negative. Recall that an important part of the insulation process is air sealing so that the moisture doesn’t flood out of the house and into the attic. If that’s the case, then wouldn’t open cell foam be bad? Not exactly. Most of the water is carried along with the air, so if you block the air movement, you dramatically reduce the amount of water movement. Be careful – I didn’t say it would stop water movement, I said dramatically reduce it.

Open cell foam dramatically reduces the amount of moisture that escapes from the house to the attic by blocking air movement, but some water still passes through because of the high “vapor permeability” of the open cell foam. We’ll come back to why this is so important later. In most cases, open cell foam on the attic floor is vastly better than fiberglass, cellulose or any “loose fill” product. Remember, you’ll still needed a ventilated attic, just like you do with any normal insulation product because you want to allow it to flush out any moisture that does get in there.

Spray foam on walls and ceiling

Spray foam under the roof deck (the ‘ceiling’ of the attic)

The second application of spray foam is directly under the roof deck and around the attic, encasing the entire attic in foam.

Why would you want to insulate the ceiling of the attic rather than the floor? Take a look at the accompanying photo. This is a bedroom with a cathedral ceiling. There is no attic in the conventional sense because the sheet rock screws right to the rafters. But really, this is no different than an attic.

Suppose the room had a conventional flat ceiling installed just above the windows. The space above would be the attic. Remember all the painstaking work required to seal the ceiling as discussed in the last article? If you insulate directly under the roof and on the walls like shown here, you don’t have to worry about any of that because the attic is part of the “conditioned space” of the house. That means that the attic is roughly the same temperature and humidity as the rest of the house.

This is a great solution because you’re not worrying about filling in every little crack on the attic floor. You don’t care about having your air conditioner and ductwork in the “hot” attic. You can store your boxes in the attic and you won’t sweat (or freeze) – it’s comfortable in there! There are so many advantages to doing it this way, so why isn’t every house built this way?

The problem with this method is that it can be done wrong and, even when it’s done right, people blame it for major home problems. Many builders are “once burned, twice shy.” And there’s lots of misinformation or poor understanding of how to do it properly. Let’s examine some common myths/complaints/worries about insulating directly under the roof:

  • If you don’t ventilate the roof, it will overheat and shorten the life of the shingles.
  • If you have to replace your roof, it will ruin your insulation and you’ll have to do it again.
  • If you don’t ventilate your attic, it will get too hot.
  • If you get a roof leak, the water will be trapped in the insulation and your roof will rot out.
  • If you don’t ventilate under the roof, you’ll get ice dams.
  • If you insulate your roof improperly, it will rot out.

Let’s get the easy ones out of the way first…

  • Shingle life: studies have definitively shown that a fully ventilated attic will only cool the shingles slightly compared to an insulated roof. Research shows that the color of the shingles and the direction which the roof faces has a far greater effect on shingle temperature. Next.
  • If you replace your roof…: Why do you replace your roof? Because it has rotted out! You only replace your roof decking when something has gone seriously wrong. Insulating under the roof properly significantly reduces the chance of your roof rotting out. Next!
  • Your attic will get too hot: um, how hot is your attic now? Probably 130 degrees. How hot will it be after insulating under the roof properly? It should be about the same temperature as the rest of your house, maybe 10 degrees warmer. Next.

The other issues get a bit more complicated.

Roof leaks It is true that roof leaks can be problematic. With closed cell foam applied directly under the roof deck, it is possible that water from a leak could repeatedly wet the roof deck, rotting out a section over time. Additionally, such leaks can be difficult to find. With open-cell foam, it is said that the water soaks through the foam then drips on your ceiling or the attic floor so the source of the leak would be visible. However, open cell foam lets interior moisture through and can lead to a rotten roof, so it shouldn’t be used in this application.

Ice dams Ice dams occur when the bottom layer of snow melts. The water then drips down the roof and freezes closer to the edge of the roof where it is colder. Most ice dams are caused by warm air escaping from the house that heats the roof deck, melting the snow. You often see this above recessed lights both due to air leaks and heat from the light.

With sufficient spray foam under the roof deck, it is very unlikely to have significant warming of the roof deck from below. However in cold, snowy climates, where snow sits on the roof for months, conditions can permit ice dams, even with considerable insulation. However, in these climates, conventional insulation would be vastly worse.

It can be very beneficial to have an air channel between the roof deck and the foam combined with proper soffit and ridge venting. This allows cold air to flow, keeping the roof deck cold which minimizes the chance of ice dams. A detailed description of ice dams can be found here. The key thing is that you have to do it in a way that’s right for your climate. What works in Philadelphia might not be appropriate for Minneapolis or Atlanta.

Rotten roof deck caused by excess moisture

Rotten roof – I wrote about this before, but lets look at the common cause of rotten roofs – moisture from inside the house reaching the cold roof deck and condensing. How does this happen? Recall from the last article, warm, humid air from the house moves up into the attic through any little crack. If the roof and attic spaces are insulated with closed cell foam as shown in the photo above, most of the attic is warm enough that condensation cannot occur – except at the wooden parts which provide bridges for the cold.

In moderate climates, this isn’t a problem, however if the humidity is high enough, the wood can get cold enough for condensation to form. If you use open cell foam, moisture passes through much more easily than closed cell foam. So it is possible for enough moisture to move through the foam to cause condensation problems at the cold wood of the roof deck. It is a very bad idea in most climates to use open cell foam directly under the roof deck. See – Unvented roof assemblies – Ice dams Green Building Advisor – Can spray foam rot your roof?

In the next post, I’m going to show you a practical roof design that eliminates virtually all of these problems….


45 thoughts on “The best way to insulate your attic – part 2

  1. Thanks so much for talking about how spray foam is great for air-sealing the area it’s put in. My sister has been talking about how her house heating bills have been going up over the past few months and she’s not sure why since her heater is pretty new. We’ve been looking into insulation and seeing if she might need to get her attic insulation replaced with something more efficient.

  2. Hello,
    We resently bought a home and needed to take out all the attic insulation due to raccoon damage. Got a few quotes for 6in of open cell spray foam for the attic floor in the $2000-2500 range. But there is very little info on people doing it this way. Your article is one of the only one that even mentions it. My question is… Is 6in enough as related to r value? And is it worth the extra money? Thank you

    • A lot of “green” builders use open cell on the attic floor. It allows moisture to very slowly move through and out into the attic, where the ventilation from soffit and ridge vents can flush it away.
      Regarding R-value – you have to check the charts for your area and local building codes. For example, here’s the map from Energy Star showing recommended insulation levels. Your insulation contractor should also be well aware of the requirements for your specific township/county etc.

      Here’s a chart comparing open cell and closed cell spray foam. Open cell is listed as R3.7 per inch while closed cell is R6 per inch. So 6″ of open cell would be about R22 – far less than typically recommended for most attics. However, it’s tough to compare R-values for foam versus fiberglass or other loose insulation because foam is such an effective air blocker and therefore gives much better protection from drafts and moisture problems that arise with other insulation methods. There’s a lot of debate about this, so I’d check requirements with your local building official. You wouldn’t want to get “dinged” during an inspection for having inadequate insulation levels.

      As for being worth the cost? I would definitely use foam if I had to insulate my attic again. However, if budget is an issue, you can sometimes get away with spraying less foam on the attic floor then topping it with blown in cellulose or fiberglass. Not my favorite method but it can work.

      • Ok thank you,
        Would you know what deprment i should call? Not really sure my self. I live it Pittsburgh, which would mean I need an r 38-49. 13in of foam seems crazy. Just want to do the right thing…

  3. Hello,

    I have a few questions for you on making my decision on how to insulate my new home. I’ve learned a lot from reading your articles, just a few things unclear. I am in upstate ny, we get a lot of lake effect snow in the winter time, and stays on roof.The home is simple gable end, and has full ridge vent and soffit vents. I have air handler in attic, but for heat I will mostly be using my radiant system I have in my lower floor (slab on grade) and under the second floor. It has come time to get someone to do insulation, and of course different opinions coming in. I see you are a fan of the spray foam, I am too, want to make most efficient home I can without going overboard. In walls I believe I am going to go with 3.5 inches of closed cell, that seems to be pretty common idea I am getting from contractors. The attic though, some different opinions there. One is suggesting 1″ of closed cell on attic floor, then 15 inches of cellulose blown in over that. I like this idea, because I feel the 1″ would supply the air barrier you wrote about being very important. However, I have a friend that is a contractor (not biased by trying to bid the job or anything) saying No to spray foam in attic, because moisture has to be able to get out somewhere? I understand that you dont want moisture to build up in attic and create problems. Does the moisture have to get out somewhere else could their be moisture problems on the inside of the conditioned space? Another contractor suggested again the 3.5 inches of spray foam in walls, but then fiberglass batting on attic floor, then blown in cellulose on top of the batting. Not sure of what the purpose is of the batting instead of just straight cellulose there, but this method does seem like it would breathe some more. So, what I am wondering is do you feel that putting spray foam on attic floor and walls box in the conditioned space too tightly? I want to be efficient but I dont want to be boxed up so tight that I have condensation/ mold problems inside. Let me know your opinion on this if you would thank you! I did not install an air exchange system, quite a few windows in the space though. Thanks!

    • Great questions Joe. You’ve been presented with a few options that sound logical but in your climate, some might not work for various reasons.
      One of the biggest concerns in high snow areas is ice damming. I’m sure you’re familiar with it but I’m going to quickly describe it for others who might read this. Ice dams are when the snow on your roof melts then refreezes down by the gutters. This can cause extensive damage because the ice tends to work its way under the shingles and often leads to water damage in your walls.
      What many people don’t understand is that ice dams are cause by the snow melting from underneath due to heat escaping from the house. Inadequate insulation and air sealing near the junction between the wall and roofline make this very common.
      Because of this, and because foam provides great air sealing, I like your option one, far and away above the others. Neither of the other two insulation options provides air sealing and you’d almost certainly end up with severe ice dam issues.
      As for the moisture buildup – the foam layer on the attic floor will minimize the moisture getting into the attic from the house, but you’d still want to ensure that the attic space is vented (as you have). That will let any moisture escape, reducing the chance of any attic moisture issues. The other two insulation methods would allow an order of magnitude more moisture to escape into the attic. Even with venting, there’s a good chance that you’d end up with condensation on the roof sheathing and all the negative issues associated with that.
      That brings us to your living space. Yes – your house will be very tight if properly constructed and insulated with foam all around. I’m surprised that your builder didn’t tell you that a fresh air ventilation system is basically mandatory under those conditions. This is a much better solution than building a leaky house that “breathes” as old timers like to call it. But a breathing house is a leaky, inefficient house. It’s an outdated concept because, frankly, it makes no sense. You want controlled ventilation, not random air moving through the house which causes energy loss, poor air quality and moisture problems elsewhere like in the attic.
      Given what you’re trying to do, I strongly encourage you to add a fresh air ventilator. Once you’ve lived in a home with one, you’ll wonder how you lived before! The air will always be fresh. Even in Pennsylvania, the air in my house gets stale in winter since the house is closed up for months. When I turn on the ventilator, it’s like night and day.
      Without a ventilator, in a tight house, you’ll have a hard time controlling odors and humidity levels. The air quality will suffer and life inside will be unpleasant.
      Hope that helps.

  4. The pros on This Old House (Massachusetts) are recommending closed cell insulation for the ceiling in addition to attic floor insulation (no type specified), mostly to prevent ice dams–this is for non-living space use of the attic.
    That’s opposite to what I have read everywhere else: you do one or the other, but not both due to temperature and moisture issues.
    I am interested in your thoughts on this.
    Thank you

    • There may have been special considerations in the attic that they featured. But as you note, pretty much all the building scientists avoid doing double insulation that creates a dead air space. There is validity to doing the insulation under the roof deck to reduce ice dams, but I’m not sure why they would recommend the attic floor insulation as well. If you have a link to the show I’d be interested in seeing how they’re doing it and what they say about the job. Thanks for bringing this to my attention

  5. Don’t know if you are still responding to this thread, but I’ll give it a shot…

    I’m in Massachusetts and had pretty bad ice dams last winter.

    I have a ~1000 sqft attic with a 5/12 pitch room (the crawlspace is about 5.5′ high).

    Ceiling joists are 2×6, and about half the attic is covered with plywood and used for storage. Roof rafters are 2×8. There are old R-19 fiberglass batts under the plywood. Areas that aren’t covered with floor have an extra layer of batts.

    I have a leaky whole house fan and leaky pull down stairs. 2 small gable vents, a ridge vent, and marginal soffit vents based on venting guidelines (the eaves overhang is only 4 inches).

    The state and local utilities have a program called Mass Save where they have done an energy audit and recommended “cost effective” improvements, some of which are subsidized. They will perform “targetted air sealing” of the attic floor including the pull down stairs and whole house fan. Also, they will cover 75% of the cost to add more blown in insulation up to R-38.

    The problems with this are:

    1.) I have a 25+ year old AC system that might fail in the near future. The ducts are old and not very well insulated, so when its time to replace, I’ll probably need new ducts. Doing that work AFTER adding a layer of cellulose is going to be very difficult, so it is probably best to replace the AC first, but it’s expensive!

    2.) As I said, my soffit vents are kind of inadequate, so to really make this solution work well i need to do something about that. I might be able to get my roofer to come in an totally open up the soffit and replace the panels with highly perforated aluminum. Right now, i have a total of 100 sq inch of net-free-area in the soffits, which is probably half of what I should have. The problem is the roofer might not want to do this unless he is also replacing the roof which is going to need to be done within a few years.

    3.) I’ll lose my attic storage unless I spend even more money to build a higher floor for storage by sistering in new 2×6 on top of the existing joists.

    I am seriously considering the alternative of a “hot roof” with closed-cell spray foam. I know it is expensive, but then I would be able to deal with the AC at my leisure because there wouldn’t be blown-in cellulose to deal with (the existing AC would probably perform better too). I could even board over the entire attic floor and double my existing storage without compromising my insulation. I wouldn’t have to worry at all about air-sealing the attic floor (obviously I could no longer use the whole house fan, but I don’t really use it now anyway with the AC). The catch here, other than the expense of the foam itself, is that I DEFINITELY would have to replace the roof first. Parts of the roof decking look like they may have accumulated mold in the past and may have rotted some (probably from condensation). I ducted the bathroom exhaust fans when I moved in and it seems to have prevented the damage from getting worse. Regardless, I don’t want to foam over a compromised roof deck. Also, there are roof vents that would need to be removed anyway if I switch to a hot roof.

    I am also a bit concerned about off-gassing from polyurethane foam.

    I don’t know what to do. I’m concerned that passing up the conventional solution (i.e. vented attic) is giving up a lot of value because of the subsidies. The utility company won’t help me out if I want to do spray foam.

    • Thanks for posting your question. Still pretty active, fielding questions almost every day.
      Your situation sounds a bit like mine was. Attic based ducts, some flooring, mediocre insulation. As you noted, blown in cellulose makes it a nightmare to do other work up there. In my case the idiot insulation company filled my attic with cellulose insulation after I told them not to.
      A hot roof / spray foam roof assembly would give a much nicer situation up there and you wouldn’t have to worry about the attic stairs and whole house fan. Of course, the whole house fan would be useless after the hot roof since there’d be nowhere for the air to go.

      Sounds like you’ve done your homework on the pros/cons. The subsidies are really nice BUT you have to live with the house and given the situation you described, you’d probably be unhappy with the result in the long term.

      As far as off-gassing, I’ve mostly seen good results (minimally offensive odor) but there was one nightmare scenario with a client who was chemically sensitive AND had a bad contractor. After seeing a number of jobs, I think it depends on the installer and how well they mix the components. If they get the ratios wrong, then the foam doesn’t set right and the off-gassing is much worse. What I told my client (and they ignored) was to insist on visiting a recently sprayed job so you can smell it yourself and ask the homeowner for their impressions. You really want to see the work of the insulation contractor before hiring them. It’s too expensive a job to take on faith. I can’t emphasize this enough. Don’t trust friends’ recommendations. Don’t trust reviews. Look at their other work. Is the foam neatly applied? Does the contractor know what they’re talking about?

      Good luck. Hope it all works out.

      • Thanks for the reply.

        I haven’t pulled the trigger yet on this.

        I have a quote from a regional company (Icynene dealer) that has been in business for 30 years. They claim that the members of the foam crew have a minimum of 5 years. One of their employees is a SPFA-certified project manager and field examiner, and has written articles about worker safety and quality control inspection for Sprayfoam Professional magazine. Everything SOUNDS good, but as you said, the best way is to actually SEE their work.

        The problem is, I don’t know how practical or reasonable this would be. I have email addresses for 2 references, but I would feel awkward asking to enter their homes. I COULD ask to see a freshly sprayed job, but again I’m not sure how practical this would be, presumably with a homeowner anxiously awaiting the end of the 24-hour waiting period to re-enter their home.

        On a different note, I have a question about thermal bridging. How much foam should be covering the rafters in order to prevent thermal bridging contributing to ice dams? I’m also concerned that if it is any less than 2″, the lumber itself could absorb moisture from the air because the foam is not vapor impermeable at less than 2″. If so, could this moisture then “condense” inside the lumber where it reaches the dew point?

        I also found the following article from Building Science Corporation where an additional 6″ of rigid foam was added OVER roof sheathing in order to control ice daming (total R-70) 17 years AFTER filling the rafters with closed-cell foam. In this case, the attic was finished, so there was no thermal break between the sheetrock ceiling and the rafters. This house is actually very near where I live.

  6. I have a new home (built in 2012). We have about 12 inches of batt fiberglass in our attic. We’ve got two gables and air vents throughout. We have one room that is cold during the winter. I’m looking to increase the R-value and thought about doing blown-in insulation using Owens Corning’s rental. Half of the attic has no flooring. The other half has flooring for storage. I’d like to reduce the amount of storage that I don’t need. My question is, do I need to pull up the plywood/particle board flooring that is currently over the fiberglass batts or can I simply do the blown insulation over it.

    Thanks for your help.

    • Before I reply to your question, I wanted to comment on your note about a cold room. This is a common issue that has a few possible causes.
      – does the room get adequate heating? If it has hot air heating, compare it to other rooms.
      – does the room have more windows or more northern exposure than other rooms?
      – Are there any unexplained drafts in the room?
      If the house already has a foot of insulation, you’re near the point of diminishing returns. I’d personally look to optimize your air sealing and heat distribution before adding insulation.

      To your question about blowing insulation over the flooring, that’s ok. You can often fill gaps under the flooring by blowing it in before blowing insulation over it.
      That said, you may want to leave some of the floor uncovered to provide a safe walkway around the attic. I did this in my attic and am glad I did! Walking around an attic can be dangerous and having that flooring can really help.

  7. Would love some input. We just bought a 1963 colonial in suburban NYC area. The roof is shot (27 yrs old and two layers) and we plan to replace it soon. We’ve been told the house had considerable ice damming last year during the rough winter. There are two attic spaces – the larger space has drop down stairs and the a/c air handler, the smaller space is accessed through a small door in the knee-wall. Both attic spaces have plywood flooring with old batt insulation underneath. Ideally, we would like to use the attic space for storage. This has lead us to the idea of using spray foam under the roof (after we put on new roof).

    We’ve had two foam companies out thus far to give us an assessment (including an energy audit). Both recommended open-cell foam for under the roof deck. However, in reading your posts, you recommend against open-cell due to the potential of moisture issues. I know open-cell is less expensive and that may be why they are pushing open-cell to get the business, but when pressed on this, they insist it is the proper type of foam to use. I also had a roof contractor warn me off of foam as he said that he often is called in to deal with rotting roofs caused by foam and that it is a mess to deal with from a roofing standpoint. And then there is the question of ridge vent or no ridge vent, airflow above the insulation or no? I’m quite confused. I’ve read a ton online about spray foam and seem to get a lot of conflicting information. It is a big investment (both new roof and insulation) and I want to make sure we are making smart, informed decisions. I sometimes wonder if we would just be better off putting more batts on the attic floor and not using the attics for storage – maybe that would be a better return on overall investment.

    • A lot of people are in the same boat. It’s tough with all the conflicting information and every contractor has some anecdote about how the “other” method has led to some issue. And I believe them all. I’m sure they’ve all seen problems at one time or another. However, an anecdote is not science and, as they say in science “correlation is not causation.”
      Here’s a question for them – “how many roofs have you seen rotted out that don’t have spray foam?” I could just as easily say that most of those roofs would be fine today if they had been foamed.

      In our climate, cold in the winter, hot in the summer, roof problems are usually related to wintertime condensation. Warm, humid air from the house comes in contact with the cold roof deck. All winter long, that roof stays moist. First you get mold then the deck rots out. This is especially true in homes with attic access and air handlers up there because there’s a steady supply of humid air in the attic with no place to go.

      Sealing the house from the roof deck down has a bunch of advantages, especially in your configuration. The tricky part is when you have a complicated roof line because you have to do a thorough job. As long as the foam installer is competent and they foam over any area that could get cold in the attic (roof deck, gable end walls and overhanging soffits) you’re usually in good shape.

      As for open vs. closed cell, both are vastly better than no foam at all. Closed cell becomes increasingly more important as the climates get colder. In NYC, less so. The advantage of open cell is that it’s more forgiving of less competent installers because it’s so springy and expansive. Because of this, they’re more likely to get a good coverage. With closed cell, the installer has to know their stuff. If they mix it wrong, it won’t cure properly and may shrink and pull away, leaving gaps which indeed could lead to a rotten roof. When done properly, it’s a thing of beauty. Plus, closed cell is structural – it literally glues your roof on. So the choice of closed vs. open often has as much to do with the quality/experience of the installer as it does the other properties.

      Here’s a pretty good, relatively short article:

      By the way, if any of the contractors make a claim like you mentioned, tell them to put it in writing as part of the contract if they want your business. That will test their convictions. All too often, contractors will state things definitively when they really are just trying to close the sale.

      p.s. it’s counter-intuitive, but adding batts to the floor will accelerate potential roof rot issues in the future. Why? Because the colder the attic gets, the easier condensation forms. Old houses with minimal insulation rarely had roof rot due to condensation because the attics were so warm! Beware of unintended consequences.

  8. Townhouse in palm desert California looking for best way to insulate attic because of heat. Plenty of room up there. Do you recommend any in the area. Is there a general price they run like per square foot. Just starting my search so any so information is appreciated.

    • Prices are hard to pin down because they vary so much place-to-place. Even in my area, if I get a contractor from the north part of the county, they’ll often be a fraction of the cost from central county because of the relative affluence of the customer. That said, if you go spray foam, I’ve often heard $1 per square foot per inch of thickness. For a hot attic, you’d use 4″-6″ so it can add up fast.
      On the other hand, you can get blown in insulation much cheaper for a given R-value. So you might get 15″ of cellulose (which I highly recommend instead of fiberglass) to give your attic roughly R-50.

      For overall effectiveness, the hard spray foam is king, even though you pay a lot more for a given R-value. But it seals everything up nicely and doesn’t make a mess – you can still store stuff in the attic if you want. Plus, it actually strengthens the house because it essentially glues it together! Great stuff. I did it in my addition wherever I could and would never go back to fiberglass, regardless of cost.

  9. Ted, I have to say, awesome info! Just what I’ve been searching for in regards to my attic, basement & crawl. I have an early 70s ranch with a 4/12 pitch roof in Michigan, so we experience 80-90 degree temperature swings from summer to winter. Amongst all my other projects I’m in the process of insulating the attic. I have standard 2×4 web trusses and only 4.5-5″ of insulation currently (r-19?). The insulation travels all the way out over the top plate with no baffles, so no ventilation. I am installing baffles for air flow & have continuous soffit vent & ridge vent. The attic space is not usable & I only venture up there for wiring issues, adding lights, etc. After air sealing, is adding additional insulation to the floor my best bet? I currently have a garage full of r-30 unfaced fg batts. I had thought about addition foil-faced polyiso to the bottom of the rafters to help condition the space more & possibly keep any radiant heat/cool in the space, but I questioned how any moisture would escape & you mentioned dead air spaces, so it doesnt sound like it’d be a good idea. Would it be at all beneficial to install partially up the rafters but leave the peak open so any moisture could escape through the ridge vent?

  10. this is such useful info!

    we’ve recently moved into a 72 year old two story house that currently has batt insulation in the attic floor.

    Shortly after moving, we reshingled the roof, installed a ridge vent and a solar powered fan vent (in addition to the two tiny vents that were originally part of the roof… Needlesstosay, now that its winter, the house seems quite drafty, and we’re thinking that it mighg have to do with the attic insulation.

    I am pretty sure there are a LOT of air leaks through the attic floor (lighting, ductwork, etc), and I have to wonder if you think Insulating the floor would make sense?!? We’d like to remove the old batt insulation, close up whatever holes are found in the floor, and then insulate it…

    I should point out that we’re in toronto (heat waves in the summer and snowstorms in the winter), and that we plan to one day convert the attic to living space with a few windows.


    • I’m glad that you mentioned that you’re planning on converting the attic to living space in the future – that one bit of information completely changes the recommended approach!
      Because you plan to convert this to living space, you’re going to want to insulate directly under the roof, rather than the attic floor. You’ll want to treat the attic as if you’re converting it to living space now, except that you won’t be adding the windows.

      Imagine that you’re standing in the attic. Any part of the house that separates your body from the outside should be insulated and air sealed. That means the vertical walls and under the roof. It should end up being just like the rest of your living space.

      The vents that you’ve already installed should not be visible after this work since that would defeat the purpose of the insulation. Depending on the approach you take, you might leave the ridge vent and add soffit vents at the eaves to allow air to flow under the roof and out the ridge. Or you might have the entire attic spray foamed, sealing everything up with air and moisture-proof foam. You would end up with an attic that looks like the photo in my article. Done right, this is really a fantastic way to insulate that could save you thousands and lead to a much more comfortable living space in your entire house since it will eliminate much of the air flow that causes drafty conditions *everywhere* in the house.

      If you can’t afford the foam method, it becomes trickier. If you haven’t done so, take a look at this article:
      as well as this one that references canadian building code:
      Please note that I’m not endorsing the Icynene product, just the information presented. I personally do not like Icynene for this application because it allow much more moisture through than “close cell” type foams which are virtually moisture impermeable. Icynene is a soft, permeable foam, that should not be used in application under the roof because it may allow the moisture to rot your roof.

      Here’s a discussion of the topic:

      And here’s a Building Science report on unvented roof assemblies that would work in your climate:

      Whomever you have do the work should be someone who has done this type of insulation job before and understands the science behind the process. I’ve seen a fair number of incorrectly done jobs that had to be expensively redone because the method used to insulate actually led to the roof rotting out. If this sounds scary, it should! A few dollars saved today often means thousands wasted in the future.

  11. hi quick question we bought our house 4 years ago and it already had spray foam in attic for the last 14 years the lady before us had in put in been great been trying to sell our house and now no surveyor will take our house on now because of this foam in attic they are saying our house un morageable have you had anyone with this problem help as we are stuck with no were to go for help

    • John, I’ve never heard of such a thing. Usually spray foam insulation is seen as the top-of-the-line insulation product.

      Where do you live? It seems odd that people there consider this a negative unless there is something dramatically wrong with the installation. There are numerous papers discussing all the benefits of spray foam so it should be possible to prove to whomever you need that foam is accepted by both local and international building codes.

  12. Ted, What a great article that covers all the bases! I have a ranch built in 1951 in Richmond, Va. The a/c and ductwork was installed in 1973 in the attic. I hate that! In the last year and a half, I have replaced the roof, chimney, windows (Andersen), gutters, and had the house sided with vinyl. And I might be saving $40 a month in the summer. Big whoop!!

    And you’re right, in the summer I am so frustrated trying to keep this house cool. The master bedroom is especially a problem because that area of the attic is floored and I’m sure there is almost no insulation under that flooring. I also replaced the a/c with a 12 sear Lennox unit in 2004. In 2012, I had to replace the air handler because it had rusted out. That cost me about 40% of the original replacement system.

    I believe your recommendation to spray the roof deck would be my best option. The other issues with this attic area is I have a whole-house fan that has a 3’x4′ grill opening and two feet from that, a drop-stair access door. The a/c return duct is in the ceiling of that hallway as well.

    Here’s my question: What kind of certifications should I look for in an installer? What questions should I ask? Do you recommend getting rid of the old floor insulation? What can I do about these openings (attic door, whole-house fan, a/c return) or do I have to worry about them if I get the roof deck insulated?

    I’ve been researching this issue for over a year and your info is the BEST I’ve seen. Thanks so much. Your site’s been added to my favorites!!

  13. How would you insulation my attic? An energy audit three months ago found they couldn’t do the blower test because the mold was so severe they didn’t want it traveling thru the house. They also found my dryer vent was not attached and hot, humid air was venting directly into the attic instead of the soffits. I have a hip roof with two gable vents. I realized I had no ventilation of fresh air into the attic. I have a whole new roof. I have new soffits that had to be cut due to only 1 inch circles cut into them. I added two powered fans. No ridge vent due to roofing company said they would compete for air with the fans. I had mold remediation done and all insulation was removed. The dryer vent use to vent thru the attic out a soffit with the vent pipe ajar in the attic it is now moved and vents out an exterior side wall of the house, no longer thru the attic. The house had a whole house fan that I removed too. It is 27 years old home. I purchased it 6 months ago and didn’t know of the problem. I have only return air vents and no finished space. The energy company came back to complete the audit and used open sealed foam to seal cracks in the floor of the attic. What type of insulation would you use for my situation? Would you foam the roof or the floor of attic considering I have powered fans and soffits? Do you cover soffits/gable vents if you closed foam spray the attic roof? Or considering all the cost I have occurred would you spray in cellulose treated with borate?

    • Wow, this really fell through the crack. Sorry for the long delay in my reply…

      That mold problem sounds horrible. Unfortunately, things like this are all too common. It’s a shame that you had to deal with all those things shortly after buying the house. The inspector should have caught that (rolling of the eyes…).

      Insulation of that area now that you’ve had all that done – typically, we of the green persuasion recommend insulating directly under the roof. That would indeed require sealing up all those vents and fans that you’ve conscientiously installed. But if you can’t bring yourself to that, then you could certainly have the floor of the attic foamed, leaving you with a well vented attic with minimal chances of problems like you had ever arising again.

      Cellulose is definitely an option. If your energy auditor did a good job sealing holes in the attic “floor”, then cellulose is a quick, cheap and dirty solution. Only go this route if you plan on NEVER using that space for storage and you don’t need to access the area except on rare occasions. It’s such a mess if you ever have to wade through mountains of cellulose.

      Hope that helps!

    • My biggest problem with your post is that you claim you have a “hip roof” with two “gable” vents. By definition, “hip roofs” do NOT have gables. Hip Roof: A hip roof, hip-roof or hipped roof, is a type of roof where all sides slope downwards to the walls, usually with a fairly gentle slope (although a tented roof by definition is a hipped roof with steeply pitched slopes rising to a peak). Thus a hipped roof house has no gables or other vertical sides to the roof, unlike a tented roof with gables.

  14. I have a problem. I put in two feet of insulation that was blown onto the floor of the attic(recommended by the contractor). This house is 120 years old and there was no insulation at all in the attic. The problem is that there is a tenant in an apartment where the attic is located and he just called to say that it is 85 degrees in his apartment. It was hot before the insulation went in and they said it would be much more comfortable with the insulation.
    What should I do?

    • Is it not possible to adjust the heat in the apartment? Adding insulation in the winter would certainly warm up the space since all that energy loss should be reduced. But the problem you’re dealing with now is different.
      Here’s something to think about. Suppose it was 75 in the apartment before the insulation and now it’s 85. That 10 degrees warmth means the apartment is better at holding in the heat than it was. If the tenant opened the windows and reduced the temperature back down to 75 degrees, then the attic would be using the same energy as it was before the insulation.
      There should be a way for them to adjust the temperature of the apartment. It may not even be a legal apartment without that capability.

      • Thank you for your response. I turned all of the radiators down, so we will see if that does the trick.



    • Thanks Mario. Questions like yours help improve the site, so keep ’em coming!

      If you spray under the roof deck, which is usually the most convenient place to foam in existing homes because of access, you do have to seal the entire attic – close up gable vents etc. Think of it like a winter hat – you want the insulation all around between you and the outside cold air. So you’d foam the gable ends and the roof. This will make the attic MUCH cooler during the summer than would any normal venting so the AC will run more efficiently too.

      As for the double layer of shingles, that shouldn’t be an issue – as long as the roof is in good shape (it doesn’t leak), it should be ok.

      You can also do the attic floor like you say. Cracks usually aren’t a problem as long as they’re small. I haven’t heard of problems with foam under those circumstances, but there’s always a first. As long as the foam has room to expand up (into the attic), it won’t push down on the ceiling. But this is where having a professional foam installer can really help. They will have seen it all and will be able to see if they think the cracks are a problem based on their experience. I’ve done some foaming myself and honestly wouldn’t recommend it as a DIY job for most people because the foam kits you buy can be pretty finicky. Also, keep in mind that building codes may require treating foam with anti-fire agents and if you skip this step and you have a fire, it could be a big problem. Insurance companies have been known to reject claims making a bad situation even worse.

      Good luck with your project.

  16. Ted, we have a 1950’s house in Denver, CO. We also have an unfinished attic that has old blown in insulation on the floor of the attic. The problem we are having is that house itself gets super hot in the summer and very cold in the winter. Our thinking is too at rigid foam to the roof to create an extra barrier against the heat in the summer and cold in the winter. It also has to electric fans in the roof that are constantly sucking air out of the attic. The fans were already there in the house when we bought the house. Do you have a suggestion for us on how to approach this issue. Thank you for your help.

    • It’s usually not recommended to have insulation in both the ceiling and floor of an attic because this creates a “dead air space” which can be prone to moisture problems. What happens is that moisture from the house can move up into the attic. The attic is somewhat insulated from the warmth of the house by the insulation on the floor, so the attic is colder than the house. Next the moist air moves up and comes in contact with the underside of the roof and the rafters which are colder so the moisture will tend to condense on these cold surfaces. As your house is now, that moisture would usually be diluted by the fresh, dry outside air that circulates in the attic.
      You can use rigid foam board under the rafters with conventional insulation above it (between the rafters) to turn the attic into a “conditioned space”. You would then close off the fans and also insulate the gable ends and remove the attic floor insulation. You’d be creating a room with a cathedral ceiling in the attic. See the diagram on the This Old House site.
      You’d still want to remove the insulation on the floor of the attic but if you fully insulate under the roof like this, it’s probably less important because there won’t be exposed cold surfaces. But I would still recommend having it removed.
      Alternatively, just leave the roof alone and blow in a lot more insulation (something like 14″ deep in total).

      Please be aware that studies have shown that those roof fans have a tendency to make your home LESS energy efficient because they’ll suck the air from your house. That air is replaced by outside air coming in through leaky windows, doors etc.

  17. I have a 1956 ranch style home, crawl space attic, hvac installed in there, Niagara Falls NY as a location. Trying to decide on best way to insulate my attic, I’m so confused as well. Contractors all have different views. Spray foam on roof deck, on floor, just using blown in cellulose to 10.5″ on top of original existing 6″ batts. What to do? Any help appreciated. With the foam on roof deck, I’m concerned about roof rot, moisture, finding a future leak? What’s best? Thanks in advance!

    • Sounds like my house!
      What I did in my house was add batts and then the cellulose company blew cellulose on top of that by accident. So now I’ve got like 2′ of insulation!
      On the plus side, it was cost effective and that section of my house is much more comfortable than the section that has cathedral ceilings and much less insulation. So blowing lots of cellulose in there is certainly an approach that can work well. HOWEVER, if I were to do it again, I would NOT do that. Here’s the problem – once you have all that cellulose in there, it’s a real pain in the butt to do any work up there. Simple things, like running a power wire to a wall outlet (through the attic) or working on the HVAC become nightmares. I would *much* rather have foam blown under the roof deck.
      Imagine a clean attic, where the HVAC equipment and ducts are easily accessible. Where you don’t have to worry about the electrician poking holes in the ceiling or pushing the insulation out of the way since the insulation is out of the way of harm. Where you don’t create a snow-storm of cellulose or fiberglass dust any time you have to access the attic. Foamed attics are a real dream.
      A recent article by a leader in the building science field talked about his experience with foaming the roof. Many years later and the roof deck is in perfect condition – no rot. While it’s possible for the roof to rot from above, I’ve seen far more roofs rotted out from moisture coming up from the interior than I have from water dripping in from above.

  18. Hi, I love all of your straight forward information! I am in the process of trying to figure out how to properly insulate my attic. So far, I have had four “insulation experts” come into my home to give me estimates, and each of them has proposed a completely different product. Some say to remove old insulation from floor and replace, some say to insulate over the old, some say spray foam the roof, some say spray foam the floor. I am so incredibly confused that I have absolutely no idea what I should do!

    Let me give you some background. The house is a “twin”, built in 1928. Philadelphia suburb. We have a peaked roof. The attic is unrenovated and “raw” There is loose insulation on the floor which needs to be removed because I don’t like it, and we need to have some old knob and tube removed. Down the road, I may or may not refinish the attic. In which case, I would sheet rock the ceiling in a cathedral manner. I may never have the money to do this, but I am hoping to. Also, I want to put in some windows on either side of the attic to get some fresh air, and perhaps move the heat out in the summer because the bedrooms underneath get so hot.

    So, can you please help me to figure out what to do. I would be so very appreciative.


    • You’re in good company – everybody who does their homework and gets a variety of quotes for this type of job comes away confused!
      Thanks for giving some background on the current state and possible plans because that really plays into the solution you choose. For example, if you might finish the attic as you say, it wouldn’t make much sense to do an expensive insulation job on the floor now.
      Personally, if I had the budget for it, I’d opt to spray foam directly under the roof – assuming the foam people say that your roof is a good candidate for this.
      Doing this will make the rooms underneath much more comfortable – you’d be amazed at how much cooler the attic will be when insulated this way.
      Feel free to shoot over any other questions you may have. These particular homes can be challenging to do correctly so you’ll want to work with an experienced insulation contractor.

      • Hi Ted,
        Thank you for your timely response. The problem now, is finding an insulation “expert”. And what if I don’t convert the attic into a living space? Would it be overkill to spray foam the roof? And what about changing the whole ecology of the house if I change the original type of attic insulation? Would it be okay to put windows in the attic with the spray foam roof? Skylights? Whole house fan? How should the attic be ventilated with spray foam roof? Would the attic then need to be heated and cooled?

        Can you recommend an “expert”?

      • I’m out of town for a bit but when I get back I’ll give you some recommendations on insulation contractors.
        Spray foaming the roof is usually the “best” approach if there’s good access to the underside of the roof so they can spray. The reason is that the roof is usually “cleaner” – that is, no insulation, floor boards, electrical wires, etc. So, if you spray the roof, it’s easier to do a good job that’s highly effective. If you reinsulate the floor, in order to do a proper job, they have to vacuum up all the existing insulation, fill in the holes etc. And then, it’s also more likely to get disturbed in the future – i.e. mice, electricians and so forth can mess with the insulation, rendering it much less effective.

  19. Foam Roof insulation makes our homes safe from poor climatic conditions. You said it right that if we don’t apply proper ventilation, we will have to face many difficulties. Thanks for the shared post.

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