Are two layers of exterior foam better than one when the total thickness is the same?
Ry38men8ZA
| Posted in Green Building Techniques on
I plan to install 2″ of foam. I can get 1″ tongue and groove XPS for about the half of the price as 2″ flat edge XPS, so the cost for 2″ thickness is about the same, except additional tape and my labor to install.
Are two layers better than one? If so, why and how significant is the difference?
Thanks.
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Replies
The reason I plan to do that (2 layers) is so I can overlap them. Martin has a blog which describes the shrinkage of rigid foam so the joints open. A second layer of foam (overlapped) eliminates that possibility in breaking down the barrier. Unfortunately, it does double the labor.
The blog is interesting. Check it out.
Joe W
T&G foam board is better than square edge for most applications (though 2" also comes in T&G). You should have to tape only the outer layer (for weather protection), but the double layer will offer advantage only if the seams are significantly offset.
The labor does not have to be double, since the first layer can be lightly tacked in place, as well as the second, and then both layers held in place with strapping. The one situation in which I recommend a rain/drainscreen is with exterior foam (which I think is generally a bad idea with too many unintended consequences).
Here is one reason for two layers:
http://www.buildingscience.com/documents/insights/bsi-036-complex-three-dimensional-air-flow-networks?topic=doctypes/insights
Daniel,
That BSC article is about roofs, not walls. Though it makes sense to stagger joints of multiple layers on walls as well, it's hardly as important (as long as horizontal joints are T&G and there is an independent WRB).
What the article highlights, but does not directly say, is that an unvented cathedral ceiling/roof assembly with non-breathable layers is so problematic that extreme measures must be take to avoid problems. To me, this is a perfect argument against such vulnerable roof systems (and there are similar arguments against exterior foam board on walls).
Robert,
Problematic is a good word.
I'm not so sure that extreme measures have "avoided problems" on the Roof shown in the BSC Article.
The project is Not-So-Old and many of the shingles are now "toast"
Some parts(west facing slope) look more like 50 years old shingles.