Vapor permience and wall drying potential
Hi Everyone,
Long time reader but first time posting. I am currently working on a new house for myself in an area near Edmonton Alberta Canada. For simplicity for my framer we are doing a standard 2×6 wall construction with 1/2″ plywood sheathing and then I am doing a SA WRB, 2″ of comfortbatt 80, and a rain screen.Â
I am a little bit torn on my WRB selection when it comes to vapor permience. With this wall assembly in my climate because I am not going 50% of my insulation value to the outside I want to ensure as much drying potential as I can from within the wall cavity outwards should there be a need for it. I am stuck between using Henry VP100 (33 perm) or SIGA Majvest 500 SA (16 perm). Will the higher perm rating of the VP100 even make any difference due to the vapor permience of the plywood behind it? I am also considering whether it is worth going with one of the SA products or am I better off just using something like a SIGA Majvest 200 instead?
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Replies
Anything over 10 perms is considered vapor-open. Obviously 33 perms is more vapor-open than 16 perms, but as long as you're limiting how much air and moisture are flowing through your walls, by making them airtight and providing some vapor control on the interior, Siga Majvest will work just fine.
The main advantage of self-adhered WRBs is that they provide air-sealing in addition to stormwater resistance. You can use a mechanically-adhered WRB but if you do, you'll need another way to make your walls airtight. The most common approach on high-performance buildings is currently taping the sheathing seams. That can work well if your plywood is high quality; if it has a lot of voids, it can take extra time to make sure the seams are fully sealed.
Siga products are a little tougher than Henry Blueskin, which is a benefit, but how important that is depends on the project.
Thank you for your reply Michael.
The air sealing benefit of a SA WRB was my initial reason for wanting to go that route. As you had mentioned the toughness of the SIGA product is definitely one benefit, the other for myself was the available tape selections from SIGA that appear to be far superior to what Henry offers and I am not risking using a different product on the Henry that may or may not be fully compatible and lead to issues down the road.
The only benefit the Henry products have for myself was the price and higher perm rating, which price I am less concerned with if I am getting a superior product. As for the perm rating, if the difference between the two will not net me any considerable difference in drying potential for my assembly I am leaning towards the SIGA Majrex. I am already intending on using the SIGA Majvest inside the house so maybe keeping the same manufacturer throughout is not such a bad idea as well.
MadMattS,
The difference in permeance of the two won't have much effect on the wall assembly, as it's the sheathing that will act as a throttle on outward vapour movement - but there will be a difference in how readily the sheathing itself can dry. However as Michael said, with adequate air sealing and an interior vapour-retarder that shouldn't matter.