Re: Where does the housewrap go?
pnwbuilder
| Posted in General Questions on
After reading Martin’s article of a few years back https://www.greenbuildingadvisor.com/article/where-does-the-housewrap-go and associated comments I thought I had it figured out: since I’ve settled on installing outie windows, my WRB (Tyvek Commercial Wrap D that has 20 perms to reduce inward vapor drive) would go over exterior Rockwool insulation. What bugs me about this is that it seems counter productive to use vapor open mineral wool and then cover it with Tyvek that is much less vapor permeable. Is my concern misplaced or should I reconsider location of the WRB?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Vlad,
You have lots of latitude here, and can design your wall many different ways.
If you want to reduce inward solar vapor drive, I suppose you can do so. The fact that your goal to reduce inward solar vapor drive may conflict with your goal to have the highest possible permeance for your exterior insulation is due to the way you describe your goals. You have to choose what you want.
Tyvek's perm rating of 20 perms is quite permeable. I wouldn't worry. But if you are in search of a WRB with a higher perm rating, they exist. Cosella-Dörken makes a WRB called Delta-Foxx that has a permeance of 214 perms. (For more information, see "New Green Building Products — September 2010.")
If you prefer to install your Tyvek in a different location -- for example, sandwiched between your wall sheathing and your continuous mineral wool -- you can do that, too. But it won't make the layers any more permeable than if the Tyvek is on the exterior of the mineral wool. If you choose to put the Tyvek directly against the sheathing, you may have more of a challenge when it comes to integrating the WRB with your window flashings -- so think these details through carefully.
Thanks Martin,
After doing some more research and giving it more consideration I think in my case simplicity of construction would be the driving factor. With 3" of exterior insulation there shouldn't be any issues with condensation related moisture at the sheathing, and with a good rain screen inward moisture drive will probably be limited as well. Installing Tyvek on top of MW would make integration with window flashings a bit easier.
Vlad -
Just want to add that inward vapor drive is typically only a problem with reservoir claddings, primarily masonry.
Peter
Thanks Peter, that's a very good point!