What is the best type of rigid foam to use and how do I ensure I am not trapping moisture in the wall?
I read your article on your website “Calculating Minimum Thickness of Rigid Foam Sheating” and viewed the rigid foam installation video on YouTube with David Joyce. Thank you for having a great website and videos with lots of information.
My question is regarding installing rigid foam on an older home (2×4 walls) that already has a vapor barrier on the inside, I read the table and understand that. I live near Edmonton, Alberta, Canada so I think I am in Zone 6 or 7? The map didn’t go that far north? Does the type of rigid foam make a difference as well? EPS, XPS, or polyisocyanurate? I have read that some types of rigid foam act as a vapor barrier so my concern is would I be trapping moisture in the wall?
Also from video it looked like the best way to strap this on the wall was using 1×6 lumber? Would that work on 2inch rigid foam, would it be strong enough to hold the hardi plank siding? Is house wrap (Tyvek) also required? The house has Shiplap for sheeting (3/4 inch boards).
Thanks in advance for your help and please forgive me if these answers are already somewhere on your blogs/ website. I am under quite a bit of pressure from the contractor to get this done ASAP but we aren’t agreeing on the methods. Any help you can provide is greatly appreciated.
Thank you,
Owen
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Replies
Owen,
Q. "My question is regarding installing rigid foam on an older home (2x4 walls) that already has a vapor barrier on the inside. Does the type of rigid foam make a difference as well?"
A. When you say "vapor barrier," I assume you are talking about interior polyethylene. According to building scientist John Straube, the jury is still out on whether it is risky to install rigid foam on the exterior of walls with existing polyethylene. However, Straube says that all indications show that these retrofits are "not so risky as most people think." Thousands of houses with interior polyethylene in Ontario and Quebec (and northern New England, for that matter) have had retrofit exterior foam installed, and we are not seeing large numbers of failures associated with this retrofit work. Struabe says, "These homes will probably be fine."
Q. "Does the type of rigid foam make a difference?"
A. Not really. Once you install enough foam to provide significant R-value (say, R-10 or more), the vapor permeance of all of these foams is very low. You won't get any significant drying through the foam.
Q. "From the video it looked like the best way to strap this on the wall was using 1x6 lumber?"
A. Many builders use 1x4s for furring strips.
Q. "Would it be strong enough to hold the Hardie plank siding? "
A. Yes. See this article: Fastening Furring Strips to a Foam-Sheathed Wall.
Q. "Is housewrap (Tyvek) also required?"
A. See these articles:
All About Water-Resistive Barriers
Using Rigid Foam As a Water-Resistive Barrier
Where Does the Housewrap Go?
You may also want to check out the following link if you are adding insulation do to (some) concern about environmental impact . There are huge differences in the global warming impact between various insulation products. Avoid XPS and spray foams that are not "water blown" whenever possible.
https://www.greenbuildingadvisor.com/blogs/dept/energy-solutions/avoiding-global-warming-impact-insulation