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Community and Q&A

Foam Strips – Magic Bullet?

9923yxA742 | Posted in Energy Efficiency and Durability on

Dear Community,

First off, I’m new to this community and new to building. But that being said, this has been an incredibly helpful place that has provided a wealth of information for me to devour for the past while.

I’m working on a development of build-to-suit homes, and we have decided to build small, super-energy efficient houses, starting with a spec-home we are building to help us market/sell people on the idea.

We are right now planning for this spec house which we will start building in a couple of months. Performance is a priority. But doing it as economically as possible is also pretty important. Thirdly, this is fairly new territory for the builder and crew, so coming up with a plan that is somewhat straightforward and achievable is also a priority. Fourthly we are based in Philomath, Oregon, in the valley about 90 minutes south of Portland, similar climate – coastal, mild.

Basically I have been reading and reading and talking with people and coming up with a few possible methods that would allow us to build a tight air barrier, eliminate thermal bridging, and avoid any moisture-issues. Suddenly I stumbled upon this picture/method in a thread in which it was not heavily discussed, but seemed like a brilliant solution to me – foam strips attached to the rafters. Apparently this house is owned by Jesse Lackman? Jesse i don’t know if you can hear me, but regardless I would be extremely grateful to hear opinions about this in general, as well as for my specific situation.

The full technique used in this house I couldn’t find. But it seems to me if I stick foam strips on the rafters, then I can put regular OSB sheathing on the foam-clad-rafters, tape up the OSB good for a air barrier, and continue as per usual with WRB and roofing. Spray a decent wollop of spray foam, probably shell out for closed cell, on the inside of our scissor trusses, and voila. No vapor barrier redundancies to require extra air gaps, no bridging, a process that’s simple and fairly traditional, minimal extra material, and a little extra cutting.

So what’s the catch?

Additionally, could this technique not be replicated in the walls as well? Foam strips along the studs, osb sheathing for a barrier that seals with the roof sheathing (roof and wall meet cleanly), siding as per standard construction. Spray wall cavity, and call it a day.

Being in our climate, huge R-values aren’t really necessary, we haven’t gotten plans to a point where we can really dive into energy analysis and calculate our specific target R-values, but I’m looking at around 40 for the walls, going towards 60 in the roof.

Again, forgive my ignorance if there’s something pretty significant I’m missing, I just haven’t been able to find anything outside that individual thread about this technique and in that thread it seemed to be casually agreed upon as a shrewd move on the builders part. A shrewd move that seems almost too perfect for my situation.

Thanks in advance,

Nandan Rao

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  1. 9923yxA742 | | #1

    Oh forgot to specify - I was thinking 1-2" Polyiso foam strips.

  2. kevin_in_denver | | #2


    I don't see anything easier or cheaper with that method as compared to foam sheet outsulation.
    You may be using less foam, but the full foam sheets are easier to air seal than the wood sheathing.

  3. Expert Member
    ARMANDO COBO | | #3

    I would question the structural integrity of your roof assembly with foam strips in between the rafters and the sheathing. Structural sheathing should be nailed directly to the rafters/trusses. I definitely would run this by an engineer.
    The idea of solving thermal bridging is good, but IMO is applied incorrectly. The better way to do it is to install rigid insulation outside of the sheathing; that goes for both the roof and wall assemblies. You should have someone do a WUFI or ASHRAE Fundamentals analysis for your wall and roof assemblies, then you will know what type of insulation you need and how much.
    Also, installing closed cell foam under the decking in a wet/humid climate requires you to have PERFECT moisture management detailing on your roof assembly, and pray that the installation and maintenance is good for as long as you plan those houses to last.

  4. 9923yxA742 | | #4

    Kevin - The easier cheaper, besides just the material (which isn't wholly insignificant by any means), is the fact that it means I wouldn't have to put wood sheathing under and over the foam, and any air gab/moisture control layer where necessary in that model, which saves a few passes around the roof and the associated material as well.

    Armando - I had assumed the wooden structural sheathing was necessary to attach to the rafters, not because I'm educated on the matter at all but rather just because it's common practice, which is why this picture/idea came out of left-field for me, but unless I am being mistaken by an optical illusion, the builders of this pictured house are indeed putting foam on rafters, and the first layer of sheathing on the foam. Thank you for alerting me to the fact that this might be a serious structural issue. I also appreciate the concern about the closed-cell foam, again I just heard about this, or what I understood as this, as a practice around here and assumed it was kosher, but I'm glad to realize this is a danger-area that needs to be thought-through.

  5. GBA Editor
    Martin Holladay | | #5

    I'm putting in another vote for attaching the sheathing directly to the rafters, and putting your rigid foam sheets -- full sheets, not strips -- on top of the sheathing. That way your sheathing stays warm and dry.

    If you want to avoid the expense of a second layer of sheathing, install 2x4 strapping over the foam, parallel to the ridge, followed by steel roofing.

  6. homedesign | | #6

    If you want to build AFFORDABLE high performance homes....
    I suggest that you resist the URGE to insulate at the underside of the roof deck.
    I realize that vaulted ceilings, attic storage and an HVAC octopus in a conditioned attic are very popular....actually I live in such a house.

    I think the path to AFFORDABLE high performance is to forget about vaulted ceilings and attic storage.
    Build homes with boring ;--) flat, airtight ceilings and Vented Attics... top it off with a very thick layer of very affordable blown cellulose at the attic floor.

  7. 9923yxA742 | | #7

    John - you might be right, the vaulted ceilings may end up being a big cost factor. It's tough though because we're trying to build and push small homes, so making them spacious is a biggie. I also get really nervous about relying on sealed drywall/inside of the house because after electrical and hvac I feel like it won't be too sealed anymore... and the number one for me is to reach the performance goal. If I don't have that I just have a slightly more expensive home.

    I didn't put this in the original question, which maybe I should've because you're a great group to put this out to, but my target goal is to get the home 80% more efficient than a house just built to code, and spend between $20-30,000 more on construction than built to code. I don't know if that kind of situation came across in my vague qualitative descriptors originally. I also don't know if that makes people say I have plenty of money or it's an impossible feat, but that's what my current goal is.

    Martin - I appreciate the vote. It seems like people feel confident that structural integrity is seriously compromised with the foam strips. The steel roofing is an interesting possibility, although it might be a tough sell in the neighborhood, literally every house in this whole region pretty much has shingles. And my understanding is that we need a nail-able surface for shingles.

  8. svghG2y3U9 | | #8

    Hi Nandan. That is my house. The sheathing went on over those foam strips then 4" of closed cell polyurathane foam was sprayed against it from the bottom. The strips were a thermal break between the wood I-Beam rafters and the sheathing. Spray foam has very good racking strength, the strips vs full sheets allowed the foam to "glue" the sheathing to the rafters. That would not have happened if full foam board sheets were used. Plus to "slide down" the sheathing would have to crush the foam, not likely -especially with the foam gluing it to the rafter sides. It would have to either rip the foam joint at the sheathing or the rafters. The roof is a hot roof - my own design, and was built in 1999. There haven't been any problems with the design. There is more at


  9. svghG2y3U9 | | #9

    Here's a shot of the foam being sprayed on the inside of the roof sheathing;

    In retrospect I wouldn't putz with the strips again, instead I would use foam board between two layers of sheathing as the thermal break for the rafters.

    The walls of our house were two 2x4 with staggered studs. 2"of polyurathane was sprayed against the wall sheathing, and encapsulated the studs;

    The remainder fo the roof and wall cavities was filled with fiberglass insulation.


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