Cost effectiveness of one thick layer of rigid foam vs. two thinner layers?
Hi all,
We live in Central KY, zone 4a. We are shooting for a “pretty good house” standard for approximately r-30 walls. We are leaning toward a 2×6 frame construction, with blown-in cellulose, plywood or osb sheathing, a couple of inches of rigid foam, and brick veneer.
I see that it is most energy efficient to use multiple layers of exterior rigid foam so that you can stagger the seams. However, this obviously doubles the labor cost of adding the foam.
A very brief look at prices at the big box stores shows that the prices for one 2″ foam panel is very close to two 1″ panels. It looks like the 1″ panels are slightly less than half, but I’m sure that savings is not near enough to make up for the additional labor.
So, has anyone does a cost benefit analysis of whether the extra expense of installing multiple layers is worth the energy savings?
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Replies
Clay,
That's a tough analysis to make, mainly because the energy savings (if any) attributable to the use of two layers of rigid foam with staggered seams rather than one layer of rigid foam is unknowable. Foam panels may shrink as they age, but it's hard to predict the rate of shrinkage.
The other variable, of course, is that labor costs vary widely from place to place.
If I were you, I wouldn't try to justify two layers of rigid foam with an economic analysis. If you really want to save money, go with one layer of rigid foam. Either butt the panels of foam tightly together, or use canned spray foam at the seams. Over the next forty years, try to avoid reading any articles about rigid foam shrinkage.
If you want a very high quality job, install two layers of rigid foam with staggered seams.
Martin,
As always, thanks for the reply, and patience.
Wasn't aware of shrinkage. Two layers it is, then.
Clay,
For more information on foam shrinkage, see this article: Using Rigid Foam As a Water-Resistive Barrier.
Scroll down the page to find the relevant paragraphs about rigid foam shrinkage -- look under the bold heading, "Do rigid foam panels shrink?"
Thanks again Martin. I would definitely want a wrap around the foam for protection. Is the recommendation for two layers of foam due to the water concerns, or the loss of R value through the seams of the shrunken foam?
Clay,
Most people don't use rigid foam as a WRB -- they use housewrap -- so water entry is rarely the chief concern.
The major worry is heat flow. Some building assemblies (walls and roofs) show "striping" under certain weather conditions, due to condensation or frost patterns that reveal that heat is leaking more quickly through the foam seams than the center of the foam. The striping can be seen through the roofing and some types of siding.