Polyiso vs. EPS Foam Insulation Over Stucco
I have a stucco house in high desert New Mexico (zone 5B). The stucco is in bad shape after having been terribly abused by the previous owners, so I’m taking the opportunity to have the whole house wrapped in continuous rigid insulation as part of the re-stucco project. The boards would be applied over the existing deteriorating stucco, fastened through it and the sheathing and to the studs, and then a new 1/2″ two-coat cementitious stucco cladding will be attached over it.
They say they can only use a maximum of 1.5″ of foam because they’ll need to use masonry nails to attach it to the studs because the nails have to penetrate the stucco, and the thickness of the stucco and the Celotex sheathing plus 1.5 inches of foam (2.75″ total) is the most they can do to allow the nails to penetrate the required once inch into the studs given that the longest masonry nails they have are 4″.
I have two options for the foam: Polyiso or EPS. The bid is about $11k ($9.50/square foot) for Polyiso and $10k ($8.60/square foot) for EPS. Here are my estimates of the foams’ long-term R-values in my climate (taking into account Polyiso’s R-value reduction over time and the performance characteristics of Polyiso and EPS in different temperatures as documented in the “Polyiso Penalty” article:
Polyiso R-7.5 in the heat; R-6.7 in the cold
EPS R-5.7 in the heat; R-6.8 in the cold
Either choice would exceed the code-minimum of R-5 for continuous insulation applied over 2×4 studs in climate zone 5.
I’m considering going with the EPS option for the following reasons:
1. Lower upfront price.
2. My manual J modeling shows that it’s basically a wash: compared to EPS, Polyiso yields a lower summer peak cooling load by 200 BTUs, but the peak heating load is increased by 80. Big whoop.
3. My peak heating is much more expensive than my peak cooling (horrible oversized 80% efficient gas furnace @ $1.25/hour for 2 hours a day vs a wonderful swamp cooler @ $0.06/hour for 12 hours a day); the better cooling performance is likely to produce few financial gains.
What do folks think? Is there anything I’ve overlooked? Any reason why I should go with polyiso instead of EPS? Is only 1.5″ inches of EPS likely to make much of a difference?
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