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What is the best application of 1/2″ rigid foam insulation AND what is the best way to waterproof?

Bryan Henson | Posted in Green Building Techniques on

I am doing a project in Southern California where we have 1/2″ rigid foam called out on the exterior of the building to cut down on thermal bridging. I have done a good bit of research on my own but was not able to draw a conclusion about how to proceed due to the fact that there was so much argument between one type of foam and another. There was also a considerable amount of opinions on the location of the Water Resistive Barrier. Any Comments?


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  1. Riversong | | #1

    ½" foam is going to offer next to nothing in terms of a thermal break. What are the issues you're concerned about, what types of foam are you contemplating, and what are the "considerable" options for locating the WRB (it seems there are two)?

    It's not clear what you're asking for.


    By "called out" I'm assuming you mean mandated by local code. If so is it both walls and roof that get this treatment? In Southern Cal you are facing a brush fire hazard so an ignition resistant siding and roof would be logical. Give us a little more detail and we'll collectively come up with some recommendations.

    I was just in San Diego for the Custom Builder Symposium and got to tour some of the homes Terry Wardell has been building out there. You have some amazing craftsmen working in that area.

  3. Bryan Henson | | #3

    Thanks for the response Robert. I will do my best to answer your questions. First, we have not specified the rigid foam yet. I know Dow makes a rigid foam that has vertical channels built in to the back, however with the recent press (feature in GBA) regarding EPS we are considering switching to Polyisocyanurate. 1/2" rigid provides an R value of 3.3. For my climate (coastal southern California) it would seem that this would be sufficient though I am open to your opinion. Though there are many conflicting opinions on where to put the wrb, you are right in that there are only two options. We will be using Tyvek stocco wrap and they suggest the wrb goes over the rigid which is placed directly over the sheathing.

    Thanks for your input.


  4. Riversong | | #4

    Polyiso foam will certainly offer more of a thermal break than others. Whether it's sufficient depends on the rest of the thermal barrier assembly. What is your wall system composed of? And are you trying to keep winter heat in or keep summer heat out?

    If you're planning to use StuccoWrap, I assume you're applying a stucco exterior finish. The best practice with stucco is to use a 2-layer WRB with a grade D 60-minute paper or paper-backed lath on the outside.

  5. GBA Editor
    Martin Holladay | | #5

    In climate zone 3, where you are located, the International Residential Code requires a minimum of R-13 wall insulation. Your plan to install R-3.3 insulation will only work if you also include additional insulation between your wall studs.

  6. Bryan Henson | | #6

    Thank you for your response Michael. The roof in this application is composed of SIP panels with a red tile roof which is quite fire resistant. The exterior cladding is a three coat stucco. What I am looking for here is your recommendation/thoughts on what type of 1/2" foam to use (EPS, Rigid, Polyisocyanurate) and where to locate the WRB....Taking into consideration R-value, drainage plane, and permeability in order to let the wall system breathe. We have 2 X 6 walls on the exterior with icynene.
    Due to the fact that the exterior will have a stucco finish we are concerned about a high inward vapor drive due to solar radiation. For this reason a semi-impermeable foam OR WRB shoud be used? Then comes the question of where to locate the WRB; definitely outside the foam since stucco will go over it...but how about behind the foam in between the structural sheathing and the foam itself? It seems this would give the ultimate waterproofing and semi-impermeable results.

  7. Riversong | | #7

    EPS and Polyisocyanurate, as well as XPS are all types of rigid foam board. EPS is the most vapor permeable, then XPS, while foil-faced polyiso is a vapor barrier.

    A WRB, such as polymeric housewraps, are highly vapor permeable and will do nothing to stop or slow inward vapor drive. And, in your climate, you should be concerened about inward solar-driven vapor drive during the rainy season.

    In your case, an exterior vapor barrier of polyiso foam board would protect against radiant vapor drive. But if you've applied an inside polyethylene vapor barrer so that the wall assembly cannot dry to the interior, then you should use XPS foam board on the exterior to allow some drying to the outside, and I would recommend 1" so that you have a semi-impermeable layer and a decent thermal break. On top of that, the industry standard for succo applications is a double layer of grade-D 60-minute building paper to create a drainage plane behind the stucco.

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