Feedback on Wall Assembly
Greetings,
My wife and I are building a home in upstate NY, Zone 6A. I have gone around the merry-go-round of wall assemblies and want to confirm I have arrived at an assembly plan that follows best practices. Here it is:
Vinyl Siding > 2″ foil faced PolyIso such as RMax Thermasheath R-13 (taped/sealed) > WRB such as Henry BlueSkin > CDX Plywood sheathing > R-23 Rockwool between 2×6 studs > drywall > Paint
Some questions:
– My understanding is that by using PolyIso in the exterior of the assembly, I do NOT want a polyethylene vapor barrier on between the studs and the drywall because the assembly will dry to the interior. Is this correct?
– If I am correct regarding the inward drying, I assume then that I should NOT opt for a vapor retardant paint. Is this correct?
– I understand that with 2×6 studs in Zone 6A I should be hitting R-11.25. I also know PolyIso diminishes in cold weather. So is 2″ of foil faced PolyIso enough?
– I haven’t found any comprehensive reviews of the RMax Thermasheath, but it seems to be comparable to Dupont Thermax and is less costly. Are there legitimate differences between these products?
– Is Henry BlueSkin a good WRB for this assembly? Is there one that is more appropriate? Is there an equivalent quality WRB that is more cost efficient?
– Is the benefit of CDX plywood worth the cost over OSB with this assembly?
– Is there any reason to consider another option for cavity insulation other than Rockwool with this assembly?
Many thanks to anyone who responds!
Chris
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Interested in any responses you get. I'm thinking a similar wall but with a retrofit.
"NOT want a polyethylene vapor barrier on between the studs and the drywall"
With enough exterior rigid, you don't need anything more than painted drywall as a vapor retarder. Skipping any type of vapor barrier does help with drying in the summer time, so it is the better way to go in your case.
" PolyIso diminishes in cold weather"
The exact ratio of exterior rigid to cavity insulation is a bit of a squishy number. You can always stretch the ratio by making sure your wall is air tight. Keeping air leaks out of your walls will make a bigger difference in moisture accumulation. Even if the polyiso does de-rate in very cold weather, it will still work well the rest of the year. I wouldn't worry about it.
To me plyiso is a commodity, I buy the one that is the most cost effective. If available in your area, you can look for permeable polyiso (ie Ener-air) as this will allow for the wall to dry to the exterior as well. Since you have a lot of foam, this is less of an issue, but if cost is about the same, I would take the permeable foam.
"Henry BlueSkin a good WRB"
Be careful with Blueskin. You want either VP100 or VP160 as these are permeable, either one works great. A full peel and stick on a new build is a bit overkill. You can get similar air sealing performance by taping the seams of your sheathing with a quality tape and using standard house wrap.
CDX/OSB is a personal choice. In this case, the extra permeability of CDX doesn't matter so I would be hard pressed to spend extra on it.
"cavity insulation other than Rockwool"
Any type of cavity insulation will work. If you have a local dense pack installer, cellulose is the greenest option, it is also the best insulation for an attic. You can also look at high density fiberglass batts as budget option.
How are you installing the windows?
I don't know how far along you are in the design process nor what your educational level might be but here is a suggestion - take a look at the house plans and design process available from PASSIVE DESIGN SOLUTIONS in Nova Scotia (Natalie Leonard, P.E. is the owner). I have never met her nor had any contact with this firm but they have impressed me with their innovative designs and they will definitely be able to solve the type of questions you posed. A great deal of their work is shown on the internet both on YouTube and elsewhere, here a couple links - https://www.passivedesign.ca/pages/plans
https://www.youtube.com/watch?v=9WLhkoO97Fs&t=967s
Best of Luck!
You'll have to check specs on the vinyl siding. I don't know any that can be attached through 2" of foam. You'll have to install strapping for most siding, and many vinyls are not approved to go over strapping. I used the guidance from http://cchrc.org/remote-walls/ for my exterior insulation house in central NY zone 5.
I think you should get in touch with Simple Life Homes. They will do a free consultation on your required needs and guide you best, especially if you're looking to build a passive house: https://simplelifehomes.ca/