Hello, and allow me to introduce myself. I’m a seasoned commercial construction superintendent about to design and build my own home. I haven’t done any residential construction in 3 decades, and frankly haven’t paid much attention. Further, for many years all I’ve done is retail TI’s all over the country so I’m not even in real ground up construction anymore. But I’ll get by, with a little help from my (new) friends!
On to my ?’s…I’m in climate zone 4A, (5 miles north of 3A) and I’m going to do the do’s of advanced framing with 7/16 ply, VB, and 1″ XPS as my WRB topped with Hardieplank siding for most walls. Is it a no brainer to add furring on top of the XPS, or just a choice? I also want to flash and batt the 2×6 stud bays with 1″ ccSPF and R19 batts. Am I creating a moisture trap? I REALLY like the idea of 1″ ccSPF for air sealing and added rigidity. And the continuous outsulation is also mandatory in my mind. I intend to do a similar set up with the roof insulation for an enclosed attic with the requisite ply, 30#felt, R15 XPS, ply, felt, and shingles. I’d like to also add 1″ of ccSPF on the underside of the roof ply and, again, I’m concerned with trapping moisture. Your considered analysis and suggestions are welcomed. I researched everywhere I could but could not find my detail anywhere, which is why I’m suspicious of it! Thank you.
Replies
You will have an area between the wall that can’t breath between the 1” XPS and the closed cell spray foam, this is an area that moisture will be trapped. You only want one vapor barrier in the wall and foam is considered a vapor barrier. Refer to IRC code for specifics on foam r value/thickness required in all assemblies. Without getting too specific and technical with really advanced framing methods, I’ll describe the two methods used most by me. Most of my work is in Zone 5, so our required foam depth will differ from yours. For very basic not too detailed description I use flash sprayed at 2” (my guys spray heavy, so it’s usually 2 1/2”) and batt after that. Or I apply rigid insulation on outside of stud w/ 2”. We like to use Zip system r sheathing b/c it’s an overall time saver. For the rain screen we opt for a drainage fabric like (Home Slicker) vs battens, again for time saving and ease of application.
Why the flash-n-batt in addition to the outsulation? Is that your air barrier? FHB has a pretty article on that: https://www.finehomebuilding.com/2019/03/15/air-barriers
Most high performance builders I know are going with outsulation, either xps or Rockwool, although Gutex is gaining.