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Community and Q&A

Exterior foam decision — IRC table 601.3.1 — and housewrap over foam

dirkgently | Posted in Green Building Techniques on

I am building an attached 3-car irregular shaped heated garage and trying to make it energy efficient but not break the bank. Some last minute roof changes during framing have made the attic area of garage usable space which will become a bedroom some day

I did a 4′ ICF foundation because I want to heat the slab (partially by solar panels someday) there is 5″ of high density EPS under the slab (craigslist bargain)
2×6 walls were necessary so the exterior walls would bear more on concrete rather than ICF foam.
I have planned all along to install 3/4″ poly iso or 1″ xps on sheathing exterior which I now realize is not thick enough for a 2×6 wall construction because of table 601.3.1 guidelines.
Siding is to be vinyl
interior walls dense pack cellulouse
I dont like polyiso because it does not breath due to the metal skin on the panels. I also do not like the idea of nailing the vinyl siding thru 1.5″ xps (R7.5 needed to keep sheathing warm). Nailing furring strips onto the foam to attach the siding is not very appealing to me either because it will start to reduce my overhang depths and get me closer to my building line marker lines (yes, the lot is that tight).

So..Question Is. If I use 1″ polyiso (R6.5) for a garage which will be heated to about 50-60 degrees in winter how at risk am I of sheathing rot?
Also, I plan to use house wrap over the foam….I do not find much information on this detail…to me it makes sense as windows/doors can be flashed normally. In this case I would leave the foam panels UN-taped so to promote drying (in case of some error/failure).

Thanks for any input. If you can point me to other posts that is fine. If I posted in the wrong area let me know.
If someone can soothe my nailing vinyl thru 1.5″ of foam concern that will probably answer a lot for me.
any info on the untaped seams will also help much.

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  1. GBA Editor
    Martin Holladay | | #1

    If you intend to install foam that isn't thick enough to keep your sheathing above the dew point during the winter, you raise the risk that the sheathing will accumulate moisture. You can reduce that risk by keeping a low relative humidity in the interior of your garage.

    Only you can decide whether you can live with the higher risk that accompanies the decision to spec thin foam. If I was making the decision, I would go with thicker foam.

    There's nothing wrong with foil-faced polyisocyanurate in this application, as long as it's thick enough. Foam-sheathed walls aren't designed to dry to the exterior; they are designed to dry to the interior. Moreover, the entire point of choosing thick enough foam is to help keep your sheathing dry in the first place.

    For more information on housewrap and foam, see Where Does the Housewrap Go?

  2. dirkgently | | #2

    Thanks for link to house wrap info..very helpful as was the innie outtie windows article.
    Is there a list of manufacturer who makes Poly-iso in 1-1/8 or 1-1/4" thickness that anyone is aware of?
    innie windows with house wrap against sheathing and foam on outside is how I plan to go now.

    Does anyone have a suggestion to get an additional R-1 so I can use 1" polyiso on exterior? I only ask because there are so many new products out there.

  3. GBA Editor
    Martin Holladay | | #3

    If all you are worried about is the difference between R-6.5 (one inch of polyiso) and R-7.5 (the code minimum), I say stop worrying, as long as your local inspector has no objection. Close enough for a garage.

  4. dirkgently | | #4

    I am not sure if Riversong is still about the web site but I find his concerns about rigid foam on exterior/not drying echo my own fears. I have also read he is a big supporter of "interior" rigid foam. Could you direct me to some links on interior vs exterior?
    I realize the big drawback of interior foam is the band joist problem. If rigid foam is used on the interior of the band joists am I correct in guessing this would be about 80% effective as outside? or is it just not that simple.
    Since I live in a brick house I used rigid foam on interior prior to drywall back in 2004. I really knew little about the subject then (some may say I still don't) but it made a huge difference as the 2nd floor is very cozy even with fiberglass and my heat bill is low. My blower door test came came back surprisingly good for the main house and i was told if I do any furthur air sealing I will need to introduce makeup air (I have hydronic heat so this will be a major expense).
    So in short I think interior foam works pretty well.
    My fears have doubled because I am currently repairing a customers home who had leaks at a window which caused extensive sheathing rot across 2 floors. The 1st floor has 1/2# foam filling the stud cavities and was sopping wet. The second floor (where the window leaked) has fiberglass with poly vapor barrier inside and was relatively dry.
    If anyone cares to know....the 7/16 osb used above pulled off the foam fairly easily with 99% of the foam remaining in place in the stud bays. Even the bays which were not badly rotted or wet.

    On the exterior foam note: If I go that route I just may use the foam backed vinyl siding to get my additional r-value. The only advantage to me here is I can still avoid strapping the exterior. Has anyone out there used this technique?

  5. GBA Editor
    Martin Holladay | | #5

    Plenty of builders -- including me -- have successfully used interior rigid foam to improve the R-value of walls. It's not as good as exterior rigid foam, in my opinion, but it's a viable option. The keys to a good installation are good detailing at the rim joists and partition intersections, and attention to air sealing.

    You are correct that today's well-insulated walls with low levels of air leakage are much more susceptible to water damage from bad window flashing that the walls that were built 50 years ago. Because of this, impeccable flashing details are essential.

  6. dirkgently | | #6

    Hi Martin,
    I decided to go with 1.5" polyiso. I did a mock up assembly over the weekend by installing 1 sheet then 4' of vinyl was screwed thru the foam into sheathing/studs. While individual screws can flex as an assembly it is very rigid.
    I want to do "innie" windows and have some ideas on flashing but...would really like to see some drawings and more importantly pictures. I have been looking all over the site for an hour or so and cannot find any.... mostly details and videos for outtie type installations.
    Can you provide me with any links?
    I was planning on using simple 2-1/2" vinyl window trim (ie..wide j channel) around the windows.
    Thanks again

  7. GBA Editor
    Martin Holladay | | #7

    Links to GBA's details for innie windows can be found near the top of this page: Building Plans for a Deep Energy Retrofit.

    Here is one of them: Exterior Insulation Retrofit at Window Sill.

    You can also find good details on innie window installations in a JLC article by Thorsten Chlupp, Installing Exterior Insulation in Cold Climates.

    Below is a detail for innie windows; this comes from "Remote: A Manual," published by the Cold Climate Housing Research Center in Alaska. (You can click on the image to enlarge it.)

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