Any downside to glass-faced polyiso?
RobInNorCal
| Posted in General Questions on
Local roofing supply companies offer Glass-faced polyiso from mainstream companies (Dow, Manville etc.) for ~25% less than the foil-faced sheets sold at HD and the pro contractor yards.
I will be air-sealing the CDX/sheathing layer, then installing the foam board under the (well taped seams) water resistant barrier. Is there any downside to using glass-faced vs foil-faced insulation boards ?
Thx,
Rob
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Replies
Rob,
There is no real downside to using glass-faced polyiso -- most of which is destined for roofing applications -- on walls.
Note that many manufacturers of roofing polyiso label their products with a lower R-value per inch than manufacturers of foil-faced polyiso. The labeled R-value on the roofing polyiso is more realistic, and is more likely to reflect the R-value of the polyiso after the blowing agents have begun to dissipate in a couple of decades.
If instead of new roofing polyiso you use reclaimed roofing polyiso the discount in material cost relative to virgin-stock foil faced goods will be more like 75%-80% not, 25%.
Re-use is always going to be greener than using new product, and used roofing polyiso has already undergone any shrinkage due to age, etc. When using multiple layers there are often noticeable but small differences in material thickness to adjust for, and the length/width dimensions may not be exactly 4.00' x 8.00' anymore, but unless the foam thickness is going to be 5" or more those issues are negligible.
Some of those reclaimed goods blown with CFHCs are still performaning at R6+ per inch even decades later, but discount any post Montreal Protocol material to R5/inch for design and performance expectations.
In a cold climate (and perhaps elsewhere), the higher perms of glass faced is an advantage - it will provide additional resistance to moisture accumulation.
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
warm climate - zone 3 - and Dana, I have read your suggestion and tried to find reclaimed in the California North Bay without success. Seems like it's the one thing Google can't find easily (or, more accurately, I am not searching for it correctly), and I've checked craiglist multiple times with few hits over the past 6 months. Also guessing that 1" boards are not great candidates for recycle due to scarcity as roof deck material and breakage.
Anyone have any experience with/recommendations for rolling a liquid WRB over glass faced boards?
I use regular house wrap over felt faced plyiso, much quicker and simpler than a liquid WRB. You can use the strapping for the siding to hold the housewrap down to the foam.
I am attaching a nice guide from PIMA on Classification of PI board insulation, including facers.
Peter
Related question: I'm planning to screw the 1x4 horizontal furring strips (needed as nailbase for vertical board siding) through the foam to the studs. I've worked with XPS "blueboard" in the past, and it doesn't compress much, but polyiso seems much softer; do any of you have tricks or tips for attaching corner boards and other direct-attached materials through these soft boards?
It takes a lot to compress polyiso, you would probably split the 1x4 before compressing it much.
For corners it is best to use wider plywood strips to reach back to the corner studs.