Foil-faced polyiso and construction adhesive
I live in the San Francisco Bay Area and my application is as follows:
Plywood – sealing all plywood seams with Siga Wigluv – 2 layers of ¾” foil faced polyiso – WRB – rainscreen – fiber cement siding.
Most articles I have read recommend using a construction adhesive between the foam and plywood to be applied at the bottom/top of the wall assembly and at all outside corners. In my assembly, the polyiso will be cap nailed to the studs, thus the construction adhesive is solely to stop air movement. I have been told by a particular manufacturer rep. that I should stay away from a construction adhesive as it will not retain movement and will crack over time, thus providing poor air sealing over the long term (I live in earthquake country and thus get the occasional jolt). This particular rep recommended using a sealant instead.
The manufacturer of the polyiso I am using (Rmax thermasheath -3) recommended Liquid Nails heavy duty construction adhesive (LN 903/LNP-903 – latex base). I contacted Loctitie (they makePL-300 for foamboard) but their representative stated that PL-300 does not work well with foil faced polyiso, nor do any of their other products. Siga Primur has been recommended as a superior product for my application, but this product costs an arm and a leg.
Am I making a mountain out of a molehill here – should I just use the construction adhesive and be done with it or spend a small fortune on a high end sealant such as Siga Primur?
Any comments are appreciated.
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If your design is so sensitive (both from a thermal efficiency & moisture migration point of view) that minscule air leaks forming in construction adhesive over then next 50 years presents a problem worth solving you're in the rong biz. :-)
Definitely a mole hill, not a mountain.
Using rigid foam as the PRIMARY air barrier in an assembly would be questionable for a number of reasons, but it sounds like you have defined the structural sheathing as part of the primary air barrier. Putting some effort into making other layers air tight is still a good idea, but the magnitude of air leakage that will occur when the construction adhesive gets a bit brittle is truly "in the noise".
I have a related question-- i am planning on using two layers of 2" polyiso on my northern-california roof-- one layer of just polyiso and another of polyiso with attached nailbase. Would like to use metal fasteners for the first layer and and glue down second to minimize heat loss through the fasteners. Would like to have a sturdy wind-proof (under 90 mph) system.
Given all this, what glue do you recommend
At least on the nailbase my roofer bought, the OSB was barely attached to the foam, a bead of adhesive on roughly a 1 foot pitch. Not structural for anything more than moving it around.
PL premium would probably work here, and it would be easy enough to test on a sample piece of polyiso. You don’t need foam compatible adhesive with foil faced polyiso since the adhesive is bonding to the foil facing and not the foam itself. The purpose of foam compatible adhesives is to not chemically melt the foam which is not an issue with a non-permeable facet where only the facer will be contacting the adhesive.
From what I have seen on my house 27 year old foil covered foam will rip off and leave a Tenacious clump of foil and caulk on the stud. The insulation board will likely delaminate before the adhesive lets go. No science, just anecdotal.