Vapor Permeability of Single-Component Spray Foam
To all the master gurus on this site, I’ve noticed that lately there are single-component kits to spray into cavities/walls/etc. Vega Bond and Kraken to name a couple.
Out of curiosity, I called one of them and asked if the product was truly closed foam. In Amazon’s Q&A, someone had asked the question and received a USER response that it was a closed cell product.
I was told by the company that because it’s a single component product, it’s about 75ish percent closed and 25ish percent open.
If I live in NH, right at the border of 5 & 6, if I use such a product to spray the rim area between floor joists at 2 inches, would the fact that the product is actually 75/25 be a problem as far as vapor?
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IMO, this is a technical question for the manufacturer to confirm the perm rating for different thickness of their product. Not what you want to hear.
If there is 25% open cell, then it is logical to conclude that it allows moisture to migrate through. If the manufacturer were to suggest that 25% open cell can form a vapour barrier, there are likely to be a few people who would like to hear that explanation,
I look at the website for both products and they have no information related to perm rating. If they can’t provide it when you call them, it tells you everything you need to know.
Yeah, I'll probably stick with cut/cobble. I was thinking IF this would work, it would be a LOT easier.
I know it's different, and 2" XPS has a perm of .55, but was thinking if Great Stuff (which is also single component and therefore likely the same-ish) was good enough for the perimeter, MAYBE such a product was good for the entire cavity.
When I spoke to the rep, he didn't answer what the perm was, but did say "it would work if I sprayed it at 4 inch" - which is a LOT and makes a two component more worth it.
IF these one component worked, my plan was to do 2-inches an cover it with 3.5" rock wool batts.
You can read also about how spray foam can come unglued from the surface it is attached to. In addition, there are articles that talk about the installers not seeing the tiny holes that remain after install. It is not necessarily the installer’s fault.
If the product is paintable, you could spay paint it with vapour barrier paint once it is cured. More work than using the two part foam.
Lots of people swear by spray foam. In theory it has some amazing qualities. In our case we put a significant amount of continuous insulation on the exterior and did not worry as much about the rim joist. That was planned ahead of time because of past experiences.
I tried cut & cobble on a small closet roof in my home with 2x8 rafters, using 6" of XPS scraps and cans of one part spray foam, being careful to use lots of foam to seal everything tight. Then three years later I took down a section and found that the inside of the sheathing was wet. Took everything down; let it dry and had it spray foamed with two part foam. So I'm not convinced the c&c method works well on roofs. If you do it, find a place you can check in a few years.
Cut'n'cobble is never recommended for roofs, because the air seal is not reliable over time which leads to exactly the moisture issues you encountered. Cut'n'cobble is usually OK for walls, although it's a lot of labor to install, so usually not recommended just because of the time involved to insulate that way compared to other methods.
Bill
Bill,
In my work I have never recommended spray foam, as I was concerned about material toxicity and high GWP blowing agents. My understanding is that some of these concerns have been mitigated over the last few years. Suprisingly, I can't find any articles here at GBA about upgraded or improved spray foam formulations. I'm not a major proponent of spray foam in general, but understand it has its place in retrofits of stick framed structures. Do you have any knowledge about the new formulations and their safety/efficacy?
Thanks!
Ethan