Ultimate attic insulation

Icy roof deck, not a good sign!

In the first two posts of this overly wordy series, we saw a few ways to insulate an attic while avoiding some of the worst problems that can lead to moldy, rotten attics and roofs.

If you recall, the big problem is that moisture from the house rises up through the walls and all the little cracks around light fixtures, hatches, wiring, and the moisture condenses on cool surfaces. Over time, this will lead to mold growth and potentially, rotten roofs.

How do you know if you’ve got a problem? I’ll give you a hint – if you have ice forming under your roof like in this picture, you had better do something before you have to replace your roof!

The goal is to create a comfortable, efficient and safe home. One that is durable and requires minimum maintenance. Is this an achievable goal? There are durable homes, as evidenced by those that haven’t rotted out over hundreds of years, but they’re not comfortable or energy efficient. There are comfortable, efficient homes, but we’re increasingly seeing issues with mold and wood rot in these because some builders are neglecting to deal with the higher moisture levels. So how do we solve all the problems at once?

In this post, I’ll present one option. Keep in mind – this is my opinion, based on current understanding of building science. There are undoubtedly situations where this will not work. Consider this article my interpretation of the current building science literature and do your own research and consult other professionals before building your dream house. In other words, don’t blame me if you blindly follow what I write and subsequently have problems!

So what’s the solution? It’s actually simple – keep humid air away from cold surfaces and, in the eventuality that water does get in somehow, allow it to drain out and evaporate before it can do further damage. Allowing quick drying is key to avoiding rot.

Please note – the proposed solution is not easy to implement in existing homes unless the attic is very accessible. Unfortunately, many attics are under low-pitch roofs which are a real pain to work in. Or, they’re filled with HVAC equipment and ducts, again, a real hassle to work around. So this isn’t a panacea. But if you are one of the fortunate ones who has an accessible attic, or maybe you are about to replace your roof, this construction can produce a really efficient and durable attic space.

How NOT to insulate your attic ceiling

In the second post, you saw the benefits of turning the attic into part of the “conditioned space” of the house – you can put ductwork up there without worrying; you don’t have to sealing every little crack in the attic floor or the attic hatch; you can store things up there without them baking in the summer and freezing in the winter. You’re just creating an attic that is part of the living space of the house. Nice.

In spite of all of the advantages of spraying foam under to your roof deck, there are downsides – leaks can get trapped by the foam; roofs can be difficult to repair; and there’s still serious thermal bridging issues caused by the exposed wood from the rafters or wall framing.

Is there any hope? I think so!

Remember, the first big problem is moisture coming in contact with cool surfaces. How can you avoid this? You could spray foam over all the rafters and framing. And this can work very well. But it doesn’t address all of the concerns.

The approach I like to see is the use of a rigid board foam product, like foil-faced polyisocyanurate. These products have some of the best R-value of any insulation product. They’re also available at your local home center. You can even order Thermax White Finish board foam that has an interior that can be used as the ceiling finish. It won’t be beautiful, but hey, it’s an attic!

Another feature of this solution is the foil-facing is is a complete moisture barrier, so moisture cannot move through the insulation. That’s a plus and a minus – you have to use it wisely. In this case, it’s a plus because we don’t want moisture moving through it.

The critical step here is that the entire attic has to be encapsulated. Remember back in the first article where I talked about a leaky balloon? That is especially true here. If you have any holes, the balloon (your house) is going to leak. This really isn’t a trick. If you were finishing the attic to be a bedroom, for example, you’d put up drywall and seal all the seams. You wouldn’t leave big gaps to the outside, right? Same thing here, except instead of dealing with heavy sheets of drywall, you’re using light foam board, preferably 2″ thick, giving around R-14 of insulation.

The details are important, just as they would be in finishing a room. All seams have to be sealed. That prevents air and moisture from sneaking through between the panels. Larger gaps at seams should be filled with canned foam. And irregular areas should be sealed up using 2-part spray foam, and is better left to the professionals.

R-14 might not sound like much, but it’s much better than you’d think because it’s covering all the surfaces. In a normal insulation job, the insulation is between the rafters and wall studs which allows thermal bridging that greatly reduces the R-value and provides opportunities for moisture build-up.  So R-14 everywhere is actually almost as good as putting 6″ of insulation between the rafters. But it’s not to current building codes, so you should add more insulation.

To keep things in perspective, a perfect R-14 insulation stops 93% of the heat loss during the winter, so it’s a great start. If you just did this much insulation in your attic, you could keep your attic at 70 degrees on a 15 degree night with the heat produced by two hair dryers. That’s not bad, but we’re going to see how you can cut that even more…

Sources of moisture

We have to pause for a moment and discuss the sources of moisture that can damage the house. They are:

  • Interior moisture in the air when it’s cold outside
  • Exterior moisture in the air when it’s hot and humid outside
  • Exterior water (rain), caused by leaks or bad construction (flashing etc.)

Any solution needs to deal with these issues. Not all climates have all the problems. For example, if you live in the dessert, you’re not usually concerned about high outdoor humidity. Likewise, if you live in the deep south, you usually aren’t worried about it being freezing cold out. The point is, you have to take into account your local climate conditions. This solution works in all climates, though the amount of insulation may need to be adjusted up a bit for extreme climates.

Finishing the job

At this point, you’ve encapsulated the entire attic with 2″ of poly-iso board foam under the rafters and inside the wall studs. It’s the same as being inside a foam ice chest and just as effective at keeping the temperature even. Keep in mind that you have to insulate every surface that is next to the outside. The obvious ones are the ceiling and the walls. But don’t forget overhangs, like over a porch. it’s just as important to insulate those.

The next part completes the job. You get a foam installer to spray a few inches – R-20 to R-25, directly to the back side of the foam you just installed. Of course, this can only be done if you’ve ripped the roof and outer siding and sheathing off or are in the process of building a new house. But if you are, this works great. R-20 on the back of two inches of foam board gives a total R-value of 32. If did what code calls for (in most regions) and put R-40 between the rafters, you’d end up with R-25. And, if you did what most people ACTUALLY do, and installed R-29 fiberglass between the rafters, you’d only have an R-21 roof AND you’d run the risk of rotting out the roof.

Building an attic this way reduces the winter heat loss on a 15 degree day so much that an entire attic could be heated with a hair dryer – on low! And summertime air conditioning in the attic could be done with a small window unit. That’s efficient!

Some important features

Why am I so adamant about encapsulating the attic with foam board then spraying foam directly onto the back side of the foam board, from the outside? Because it solves virtually every problem that building scientists and builders fight about.

  • It completely isolates cold surfaces from inner humidity.
  • It provides an air gap directly behind the roof and wall sheathing allowing a ventilated structure.
  • It provides excellent R-value / minimizes energy loss.
  • If the roof or walls leak, the water drains out  so the wood can dry quickly.
  • It should be done in combination with a ridge vent and matching soffit vents.
  • It is compatible with different roof types – slate, metal, rubber, asphalt because of the air channel between the foam and the roof materials.
  • It keeps the attic at the same conditions as the rest of the house.
  • It doesn’t reduce the strength of the roof or walls, like installing foam board between sheathing and the studs/rafters does.
  • Closed cell spray foam between the studs and rafters increases the strength of the house.
  • It works in cold snowy climates as well as hot humid climates.
  • It’s more tolerant of minor building mistakes than other techniques that require “perfect” construction because moist moisture that gets through will flush out before it can do damage.

Given all these advantages, why doesn’t everybody build this way? Because there are generations of builders, architects, code officers and engineers who have done it the old fashioned way and change is hard. Even though they’re in the process of building new homes every day that run the risk of rotting out. Even though they have an opportunity to build a new generation of homes that should never require a roof replacement or suffer from another ice dam. Even though they could make homes that require vastly less energy to heat and cool. They’re not going to do it because it’s “not the way things are normally done.”

In a future post, I’ll show how this construction technique can be modified for all the walls of the house and also show how you can create a variation of this construction that can be used in retrofit situations.

Until then, I hope this series helps some of you avoid moldy, rotten roofs or if you already have one, I hope it helps you find the cause and fix it for good! If you’ve got these problems, send in your pictures and post your thoughts to the comments.

61 thoughts on “Ultimate attic insulation

  1. I have an L style ranch house that has the original insulation in the attic. I was looking to upgrade but I only have 5.5 inches of height and about 16 inches in width for insulation. I know r-30 is generally used for my area (long Island, NY). I do plan on putting in floor boards so I can have storage, what would be the best option? Use an r-19? r-30 and compress the insulation down with floor boards? spray foam? foam board?

    • High density spray foam would give you the most R-value and air tightness. That would be my first choice. Plus it adds considerable strength which is good if you’re going to use the attic for storage since invariably there will be accidents and that can save someone from having something fall through the ceiling.
      Even 3″ of foam Will be vastly better than any amount of fiberglass.

    • It certainly sounds like it. That space should be quite comfortable and trouble free as long as the workmanship is good.
      The trick is being thorough everywhere so that moisture can’t sneak into the cavity.

  2. I have a finished upstairs with storage along the rooline, the fiberglass batts need replacement and I was wondering if I can place new batts in then cover it with foam sheets?
    the soffits and ridge is vented

    • I’m interpreting your description in two possible ways. I’m gong to describe each and comment on them so others can try to follow the logic in two different cases.

      First case: fiberglass on the floor of the attic space, directly above the ceiling of the living space
      In most climates, it’s not recommended to put vapor retarders (like board foam) in above loose insulation like fiberglass batts. The problem is that the board foam can trap moisture in the cavity where the fiberglass is since the moisture migrates from the living space, up through the ceiling, and into the space with the fiberglass.

      Second case: fiberglass is between the rafters, directly under the roof. In this case, the board foam would be going across the rafters. Air would flow from the soffits, up behind the fiberglass and out the ridge vent. The board foam in this case is good because it reduces the amount of moisture that could get into the cold space under the roof and above the fiberglass.

      There’s actually a third case, that’s something of a hybrid. When you finish attic spaces, you get short knee-walls that run down from the roof to the floor. Behind the knee-wall is a space that people often convert to storage space. Sometimes they insulate directly under the roof in this space. Other times people insulate the back of the knee-wall, and across flat section of attic “floor” that is directly above the ceiling of the room below. If this is what you have, then you would follow case 1, and insulate between the sloped section rafters with foam board underneath the rafters. OR you would insulate the “floor” and up the back of the knee-wall.

  3. Hello again — I am now reading about all the negative health effects closed cell spray foam is being linked to and I am wondering if I could pin rocwool between the joists and close it in with rigid foam or if there is a healthier alternative to the spray foam? Any advice would be appreciated

    • Unfortunately, if you search the web long enough you will come up with horror stories for virtually every product. In spite of what you read properly installed spray foam is an excellent product. I’ll respond to your other question separately.

      • I would prefer a well to have good air circulation under the roof so I would not spray foam into the rafter spaces. It is flammable as is the foam board though some foam boards have a thermal barrier of sorts (see Thermax by Dow). You could put an additional layer of foam board between the bays with air space next to the roof of a couple of inches.

      • Quick answer – using Roxul between the rafters, an air gap between that and the roof with good ventilation from soffit to ridge, and a layer of faced board foam, like aluminum faced PolyIso under the Roxul (facing the interior of the attic) – all would work quite well and you’d end up with a much more comfortable attic while minimizing the risk of condensation.
        I would recommend removing the insulation on the attic floor. An alternative is to condition the attic space by providing small supply and return vents from the HVAC. The main thing is to prevent the attic from being a totally dead air space that is allowed to get cold and humid.
        This has been a popular question this winter so I’m going to attack it in a full article shortly.

      • Ted,
        You stated below that you were going to address Roxul between rafters with layer of faced board foam below the rafters / conditioning an attic in a full article soon. Did you publish this article? Could you please provide a link? Thanks!

  4. Hello! This is exactly the article I needed to see! We recently installed a very pricey and brand new HVAC system and its currently sitting in our unconditioned Pennsylvania attic — subject to hot summers and cold winters. I knew we needed new insulation, and I had people out for quotes, but it just didn’t feel right to leave that new equipment up there subject to the elements. I have been reading endlessly about the pros and cons and different solutions and I think I almost have it lol. My new plan of attack is a lot like TJ’s above — I plan to secure my baffles and vents and then install rigid foam board over the entire attic roof. I plan on cutting holes in each bay and using spray foam to fill in behind the rigid board. My question is two fold — will this work? and do I have to remove the old insulation on the floor or can I just throw some plywood walkways down and leave it as is?

  5. When you are using PIR AND are willing to rip out the outer roof then why not put the pir plates on the outside? That way you create a perfectly closed space and protect the roof structure from the outside water and temperature changes, and also without that messy foam spraying?

  6. Pingback: TOURING THE GREAT BENEFITS OF ATTIC INSULATION | Milwaukee Insulation Pros' Blog

  7. Pingback: Proper Attic Insulation Can Help Save You Energy Year Round | Las Vegas Insulation Pros' Blog

  8. We have a none-heated, walk in-attic space above the three car garage that is use for storage exclusively. It is bare rafters with one little light bulb. We want to have our general contractor insolate, dry wall, add lighting and put a vinyl flooring (self-stick tiles) but no heating. Is it Ok to finish an attic and add heating to it? We are in the Mid-West, Chicago nasty winters. A finish storage attic with good lighting is all we need to use the space for storage of holiday ornaments, suitcases, old files, etc.. It will help to keep the attic cleaner (gets very dusty) and free of mice. Any suggestions or comments? Love your articles ! Thank you!
    Magda J

  9. What about fire codes and use of polyiso panels? If the attic is to be used for storage, can a polyiso system be used without a thermal barrier? The only product I have seen so far that explicitly states that it meets requirements w/o a thermal barrier such as 3/8 inch drywall is Thermax by Dow.

    • You would have to check with your local jurisdiction and find out what they approve. Pennsylvania and New Jersey can be very different from one another and other states can be different as well. In some places the foils faced Thermax bored is acceptable. While I believe other places are so strict that they require drywall on any foam product.

  10. I did something very similar, but with some twists. I wanted to retain as much headroom as I could, so I began the insulation between the rafters. When we replaced the roof of this 1950 house, I had the roofer use foil-faced sheathing and install ridge vents. I had to cut the soffit to install the continuous venting there (previously we had gable vents). Then I encapsulated the attic but retained the deck venting.
    Between each rafter by I installed 1/2″ foil-faced polyiso board, with the foil facing up. I used small square chunks of polyiso board as spacers between the foam board and the roof deck to create a 1″ gap, so that the deck would still be vented, and this also created a double radiant barrier. The rafters were not perfectly spaced, and were not perfectly parallel, so I had to measure at the top and bottom of each bay, then cut the polyiso board to fit. With the polyiso board propped up against the roof deck and between the joists, I used a foam gun (pro gun with cans of Great Stuff, essentially) to run a bead on each edge of the board, to seal the cavity and hold the foam board in place.
    Then, I tacked another layer of 1/2″ polyiso board to the face of the rafters. With 2×6 rafters, this left a 4″ deep cavity between the upper and lower polyiso boards. I had bought a lot of fiberglass insulation off craigslist (as I had the polyiso board), and I packed this cavity densely with it. The more densely packed the fiberglass is, the higher the r-value per inch of final thickness, btw.
    At the ridge, I was unable to stuff fiberglass in after putting the lower foam board on the rafters, so I filled the void with layers of thick commercial polyiso board (also from craigslist) then capped it with the 1/2″ polyiso board.
    Then, I sprayed about 1″ of high density polyurethane foam all over the polyiso board. The attic gables were treated similarly, using precisely cut thick polyiso board between the studs, 1/2″ polyiso covering, then spray foam.
    All this was relatively straightforward, and is easy to comprehend. But there is something complicated about this approach that you don’t mention in your article: how to provide ventilation to all of the rafter bays. If you have a simple gable roof with a single ridge, no problem. Every rafter bay continues all the way to the soffit. But most roofs aren’t like this. Mine has three ridges, and thus four valleys, so many rafter bays stop before reaching the soffit and would not have access to the outside air. Also, we have a chimney that blocks off 4 rafter bays.
    I had to build plenums to provide air to the isolated rafter bays, made with polyiso “pony walls” between three and five feet in from the outside wall. Because I have a dominant ridge that is substantially higher than the other two intersecting ridges, I could provide airflow to all but two small cavities. Had the ridges been at the same height, I would have had to use an underslung box duct to provide ventilation. Is this how you would have done it?
    The ceiling on the outside of the pony wall is more traditionally insulated. I made press-fit baffles, about 30″ long with a 3″ wall at the end, out of polyiso board and installed them on the top plate of the outside wall, to allow me to stuff the existing mineral wool insulation right up to the edge of the house without blocking airflow. The baffles come out far enough that I was able to put 11″ of fairly well compacted mineral wool insulation on top of the ceiling, all the way to the pony wall. As it is an old house and the AC vents are not next to the outside walls, all ductwork is within the encapsulated space.
    At the ridge, I have about 5′ 7″ of standing room. I removed the collar ties and installed a ridge beam, so the attic space would be usable. Then I re-ran the electrical wires to pass through the joists so I could install an osb floor (with screws so everything is still accessible).
    Now, our electric bill has been cut by 2/3 during the hot Dallas summers, the house feels much more comfortable year round, and we have a huge, clean, conditioned storage space.

      • BTW, after I had been working on mine for about a year, I found that the ORNL has done some research on subvented encapsulated roofs. They are a LOT easier to do during the initial construction, for sure.

      • Thanks, that would be great. I’m sure others would want to see also. Lots of people are interested in attic insulation and there are few case studies of people who have taken the time to do it properly.

      • I don’t know if this will go through, but here it is right before I applied the 1.5″ of hdpu foam. I should have taken pics of the whole process, I know, but here is a pdf I made of the cross section, and a pic of one area before I sprayed the foam. This project took me 4 years, on and off. Having to build the plenums, and baffle boards added a LOT of time to the project, as did the fact that every piece of foam between the rafters, including the plenums, had to be custom cut to the right width.
        On Fri, Jul 4, 2014 at 4:15 PM, Ted’s Energy Tips wrote:

        T.D. Inoue commented: “Thanks, that would be great. I’m sure others > would want to see also. Lots of people are interested in attic insulation > and there are few case studies of people who have taken the time to do it > properly.” >

      • Thanks for clarifying. I’m sure some people will misunderstand so it’s good to remind them not to compress their fiberglass batts or to they will lose actual R-value.

      • Well, I don’t know a simpler way to explain it, except by adding the clarification that you did. It’s a good way to use up a bunch of reclaimed insulation.

  11. This sentence – can you explain? “But it’s not to current building codes, so you should add more insulation.” The sentenced directly following that one don’t address the concern.

    • Thanks for the question – I understand the confusion.
      You can simply add additional insulation as required by building codes. The point is, even if you don’t, you’re going to end up with an attic that has more effective insulation than one that is built to code due to the inadequacy of typically installed insulation that lets air through and is compromised by thermal bridging. I’d take 2-3″ of foam any day compared with 12″ of fiberglass.

  12. “At this point, you’ve encapsulated the entire attic with 2″ of poly-iso board foam under the rafters and inside the wall studs” – a photo of this would be helpful. Unless nobody has ever really done this.

    • It’s far more common just to spray foam everything. The use of board foam is illustrated as a way that a person doing some DIY insulation work can create an even better solution. Sorry, no pictures as I haven’t seen it done.

  13. Ted,

    I live in Atlanta. We get hot, humid summers. My house was built in 1995. Both HVACs are in the attic. The attic is insulated with blown-in fiberglass insulation (I absolutely hate it, btw).

    In your blog above, you indicate that “In a future post, I’ll … also show how you can create a variation of this construction that can be used in retrofit situations.” I don’t know if you ever posted that, but I haven’t been able to find it, if you did.

    So, what is the best way to insulate my attic (I know you can’t give specifics). But, is there a way that I can use the technique you noted in your post? I would love to remove the blown-in fiberglass and create a “conditioned” attic. I like the thought of spraying the foam under the roof deck, but I like even better your idea of using the rigid foam board and then spraying foam on that. Can this be done (sort of reverse?) by first putting up the rigid foam (leaving a gap (say, “x” inches) between it and the roof deck) and then spraying the *underside* of the rigid foam board with spray-on foam on? In other words, it would be layered as follows: shingles, roof deck, gap, rigid foam board, spray on foam. Also, I purchased some rolls of Armafoil radiant barrier to put up in my attic, but now I am thinking of doing the above, instead. Or, can I do the above AND use the radiant barrier?


    • Thanks for the reminder. I’ve done so many of those types of writeups that I may have neglected to do the follow-up for this article.

      The quick answer is exactly what you wrote. I would put up nailers in each area, along side the rafter (something like a 1×3 would be perfect). That could be tacked to the rafter, giving a nice space for the air gap under the roof deck. Then you could just cut the foam board to size and tack that in. Afterwards, spray seal the entire thing from the underside, remembering to coat the rafters to minimize the thermal heat conduction (thermal bridging) through the uninsulated wood.

      As for the radiant barrier, you could save time by using foil faced poly-iso, with the foil facing the roof. Or, since you already bought barrier, you could kind of wrap the foam board with a ‘U’ shaped piece of radiant barrier (that is, it would cover one big surface and the two sides of the foam, then shove that all into place).

      The fundamentals you have: 1) air gap under roof deck; 2) inexpensive insulation barrier; 3) spray foam to seal/insulate the rest.

      • Ted,

        Thanks for the quick reply. Sounds great! And, just so I am clear. I have soffit vents (of course), and I have ridge vents. So, at the foot of the rafters, do I make sure that the rigid foam board goes all the way to the top of the wall (plate?) so that the air coming in from the soffit vents does not enter the attic, but instead enters the “gap” between the roof deck and the rigid foam board (and I would have the contractor seal where the rigid foam board meets the wall (plate – whatever that thingy is that is the floor of my attic)? And, by extension, at ridge vents, I would make sure that the rigid foam board reaches up from each side of the roof line and on the inside (attic side) make sure they join (but not block the ridge vent) and are closed and have the contractor seal that joint with the spray on foam (in this way, making sure the air flows from the soffit vents through the gap and up out the ridge vents)?

        And, on one side of my roof (facing the back of my property), there are a number of passive roof vents (I think that is what they are called, anyway). So, for those, I would again make sure that they are not closed-off or blocked by the rigid foam board so that they can vent the air that is coming from the soffit vents through the gap that I have created. Is that correct?

        Also, I don’t have any gable vents. What do I do about the “end walls” of the attic? Do I do the exact same thing (i.e., create the gap)? If so, wouldn’t those “gaps” be completely sealed-off b/c they are not vented to the ridge vent and (obviously) there are not any soffit vents?

        Finally, what is the best/most efficient/cost effective (not necessarily the same, single solution, I would imagine) method to remove the existing blown-in fiberglass insulation?


      • TJ – exactly. Pretend you’re building a cathedral ceiling inside your living space – you want the attic to become a new room that is completely sealed from the outside.

        Good question on the gable end walls – Treat them like a wall inside your living space. No venting and insulated to “code levels”. For those, they can just spray foam if you want to save the effort of cutting the foam board. It’s a tradeoff between labor and expense. You’d have to get it quoted, or figure it out if you’re DIY’ing it. Often people will DIY, and you could get a lot of R-value with rigid foam board at maybe 1/2 the price of sprayed foam. Then you spray a couple inches of foam over that and you’re golden.

        For removal of blown in stuff, a spray foam contractor should have access to a giant vacuum cleaner like machine. I once did this manually with a helper and she said it was the nastiest job she’s ever done (and she has done a lot of nasty jobs!) So I’d leave that to the pros.

        Finally, the strong preference is for “closed cell” spray foam, not “open cell”. The closed cell is the rigid stuff. Open cell is (roughly speaking) like the foam in car seats.

  14. Ted – I’m looking to add more insulation to our attic in our house we recently bought. We’re in central NJ. The house is from the late 60s and it only has about 5-inches of rolled fiberglass now and we’d like to get it to R-60 with sprayed insulation. My questions are:

    1) do we need to rip up the plywood flooring that covers the 1st half of the attic or can we spray insulation right on top of it? If so, why (just curious to understand)? There is existing rolled insulation under the flooring in the joist. Ripping up the flooring is a lot of work.

    2) We’re thinking sprayed fiberglass and we would do it ourselves. Does that make sense or do you recommend cellulose or something else?

    3) Before spraying insulation, do I need to add soffit vent baffles in every bay or is there a rule to how many I need to add? There are none existing, but I noticed the existing insulation is pushed right up against the soffits and there is poor ventilation in the attic…gets extremely hot in the summer.


  15. Are you saying that there is an air gap between the top of the spray foam and the bottom of the roof deck? That’s the only way I can make sense of your statement to use soffit and ridge vents. A normally sealed attic would not need either of those.

    I often contemplated building small plywood “ducts” about an inch out from the bottom of the roof deck, then spraying in foam underneath that, encasing the rafters. This would achieve essentially the same end product as your system. However, I’m concerned about detecting roof leaks – we have two mortar chimneys and several vents exiting the roof. Any kind of sealed attic system hides those leaks potentially for years, making future repairs much more expensive.

    My solution is to retain our vented attic and put 3″ built-up polyiso boards on the attic floor between the 2×4 joists, using spray foam to air-seal the holes during installation, then cross-roll an R-25 fiberglass batt. This puts maximum insulation next to the ceiling and wall edges (always a weak spot) while meeting code for covering the iso with a mineral fire barrier. It also limits the thermal shorts caused by the attic joists. The combination should be good for roughly a “real” R50 in a standard-construction attic and eliminate the problem with fiberglass leakage near the edges by the vented soffits.

    I haven’t actually started this yet…I’m working on how to get around the attic ductwork at the moment, but now thinking about just wrapping it in extra layers of plastic-wrapped batt blankets (it already has 1″ foil wrap). As much as I’d like, I can’t afford spray foam installed everywhere – the cost is just too high and the payback time is too long (it doesn’t even qualify for energy loans around here).

    Thanks for the good article.

    • The approach outlined in the article is one option that eliminates most of the standard complaints of the “hot roof” approach by doing exactly what you say – leaving an air gap under the roof deck and above the foam. I personally have no problem with the common approach taken by foam installers which is to spray right to the bottom of the roof deck, sealing the entire cavity. However, so many roofers complain about this approach and insist on soffit->ridge vents, so I’m outlining an approach that gives them their vents but keeps it sealed away from the inner living space so the attic is still fully sealed.

      The roof leak issue concern is a common one that IMHO is overrated. 99% of the time, roof leaks are at the type of penetrations you talk about, due to bad flashing. In the version of the foam sealed roof detailed, any leaks will drip onto the foam, run down it and out the soffits. Then, when it stops raining, the air flowing up from the soffit to the ridge will quickly dry out the cavity. Wood rots when allowed to stay wet for longer periods, which is what happens using most under-roof insulation techniques that trap the water and don’t provide a drying mechanism.

      Your proposed approach is good for a more conventional, vented attic. As you note, existing ductwork and other obstructions can be a real pain to work around, usually leading to compromised air sealing and insulation in those areas. But you do the best you can in the real world! That’s where a pressure test and infra-red thermography really helps because it lets you do your best then verify the quality of work and find the worst leakage areas so they can be sealed. Without this last step, you’re just guessing and a few hundred dollars spent for an afternoon with a thermographer is well worth the investment. I’ve found a lot of big problems that went unnoticed using normal visual inspections.

      Good luck on your project.

  16. Ted,

    Thank you so much for the rapid response. I’m in San Jose, CA, so if you can recommend anyone, I’d really appreciate it.

    If I go with floor insulation, to you recommend a specific type? From the sounds of it, I would love to do that foam board with foam sprayed on top, but I’m not sure if that will cause problems in the future if I do new plumbing or electrical.

    Thanks again,

    • Brian,

      San Jose, beautiful! I was just out in Palo Alto last weekend for lectures at Stanford.
      You’re lucky. That’s a pretty mild climate. Looking at your weather data (see this link) you’ve got less to worry about than we do out here in the east.

      You bring up good points about doing electrical or plumbing later. If you foam things, you can imagine that it’s going to cover all the wiring. If you don’t have wiring up there now, or have minimal wiring, then spray foam on the attic floor would work really well. For that situation, foam board is less attractive because it’s labor intensive. So by the time you’re done cutting and fitting it into place, you will have paid as much as just doing the entire job using spray in foam.

      Budget is also a consideration for most people. In most cases, foam will cost at least double what cellulose or fiberglass would. Plus, you can do cellulose or fiberglass yourself while foam is always a professional job. Personally, I would use spray foam in my own house almost every time if given the choice.

      Your choice of insulation also depends upon your use of the attic. Does it already have a floor, which would have to be ripped up to add insulation? Do you plan on using it for storage? Are there other considerations that might affect the choice of insulation?

      I’ve had pretty good luck with Angie’s List for finding contractors.

      Here’s the list of energy auditors in your area who are ResNet certified


      • Thanks Ted,

        I contacted one contractor off that list. He mentioned that we could foam the rafters and completely close the gable vent, since we would no longer need the vent with the attic being conditioned space. He thought that would also allow me to get in there in the future and add electrical or other things. He also said we could go in there and air seal everything by putting some foam around individual things like pipes and fans. I must say though, I’m a bit concerned about completely closing off that gable vent. Does that sound reasonable to you?

      • This is a reasonable approach, especially in your climate. The single gable vent you have (you did say a single one, correct?) isn’t doing much for you anyway because you really need two to provide flow-through air.
        There are many advantages to sealing up the attic like that. It will be much more comfortable up there if you have to go up in the summer. Plus, as you noted, if you add wiring, lights and so on, you don’t have to worry about disturbing insulation or having issues with inefficiency because of air movement through the fixtures.
        So, in general, that’s a good way to go as long as the foam contractor knows what he’s doing.

  17. Ted,

    I have a few questions. I have a lifetime roof on my house already, so I don’t think I can add the ridge vents. In addition, I have one of those vents on the wall of the attic that goes to the outside. Would insulating the rafters instead of the floor of the attic be a waste since there is basically an open vent through the wall of the attic? If so, what can I do?

    Also, are you recommending I put the foam board on the floor of the attic as well as the rafters? Do I spray the foam over both if that’s the case?

    I have an old home that has no lighting in the ceiling. All I have to use is lamps? What lighting source do you recommend? Are there any canned lights that I can install?

    If I install insulation on the floor of the attic, will that cause problems with installing canned lights or ceiling fans & light combos?

    How can I find an insulation contractor that I can trust?

    Thanks for all of the help,

    • Hi Brian,

      Specifics are really difficult to pin down without knowing the exact configuration of your attic and home and that’s impossible to do remotely. I don’t make any generic suggestions. What I’ve tried to do is provide a foundation of principles that you can use.

      Most existing homes are best insulated by attic floor insulation. As you noted, with a gable vent, insulating in a way that lets the cold air bypass the insulation does no good.

      If you don’t have any recessed lights now, it’s typically a bad idea to add them for all the reasons mentioned in the article – they reduce energy efficiency and allow moisture to pass up through them and into the attic where it can cause problems.

      Ceiling fans are less of a problem because after they’re installed, if you have access from the attic, you can air seal the mounting holes pretty easily.

      Your best bet is to find a knowledgeable, local energy auditor or building-science person to help you through. They usually know the trustworthy local contractors. I have a strong preference for using energy auditors who are not associated with any single contractor due to problems of conflict of interest.

      You can look up local energy professionals on ResNet.

      Hope that helps you get started.

    • Thanks for the feedback. Just trying to get people on the right track. It’s always good when there are excellent solutions available but there’s so much misinformation that people don’t know what to do!

  18. You were right. I was being to vague. This is a 2 story house with conventional insulation in the attic and outside walls. The roof has ridge vents and perforated soffits. The insulation is on the floor of the attic. The attic is too hot. Our inspector suggested that we add turbines but I am hoping that you might have a better solution to lower the temperature.

    • I’m going to refer you to an articles I’ve written on these exact topics:

      To summarize: if the attic floor is properly air sealed and insulated, it barely matters how hot the attic gets. Adding more ventilation to “suck hot air out of the attic” can waste more energy than it saves. You money is (almost always) best spent carefully air sealing the attic floor and adding a ton of insulation.
      Turbines are a waste of money because: a) the wind barely blows in the summer; b) you’re poking more holes in the roof and those holes may eventually lead to problems (leaks, rot, etc.)

  19. Our last house which we built had an enclosed attic. We sadly had to relocate and are buying a conventionally built house. I know that it is not going to be as efficient as our previous house and am wondering what we can do to increase the efficiency. Any help you can give would be appreciated.

  20. Thanks for sharing. wandering if you can help with my attic. I live in 80 years old house with flat slop roof in NY. The ceiling is drop so i have around 1ft to 6 inch of space for the drop ceiling. I’m looking for the best way to insulate it. I’m currently looking at spraying 8″ open cell foam R30 into the joist. Is this the best options?

    Thanks you.

    • Keith, thanks for visiting.
      Just to make sure I’m understanding, it’s a flat roof, so probably rubber membrane of some sort on top? And you’d open up the drop ceiling and spray from below to the bottom of the roof decking?

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