Builders’ Familiarity With Staggered-Stud Framing
My builder, or apparently almost all builders in west michigan (Zone 5) think exterior insulation is more difficult than splitting the atom. My hope was an r-11 or so (3-4”) exterior with standard 2×6 filled with rockwool. This would keep the exterior/interior ratio at the 1/3 to 2/3 levels. The alternative i was given was to:
staggered 2×4 studs on 2×6 top/bottom plate, and cavity insulation using 3” closed (r-22). This would provide a thermal break, and air sealing. They said it would be easier for the trades as well but i cant see how? They don’t get to route behind the inner studs due to the 3” foam covering 1” of the the inner studs, still requiring boring through all the studs as in standard framing.
i just cant wrap my head around this being any better/easier (considering the extra framing and the need for closed cell compared to ie rockwool on interior and exterior (or foam on exterior). Window bucks aren’t difficult, and this house is a basic ranch style with no crazy corners, bump outs, etc. so i don’t get it why exterior Insulation is so damn difficult to achieve. Im using vinyl siding so cladding shouldn’t be difficult. Considering im near zone 6, you’d think there were at least some builders (other than the most high end builders who dont build 1100sqft main level ranch homes) who had some experience doing this.
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Replies
> 3” closed (r-22)
Don't believe this R value for the long term. Even more so when you include thermal bridging and cold weather performance (which is what counts when it comes to condensation/sorption risk).
It is possible to build good high R walls without foam. Or who knows, maybe they would be more comfortable with Zip-R or rigid foam (strips or continuous) on the interior side.
Staggered stud is pretty simple to build, I could see why your builder would recommend it. I've done it where noise is an issue. It is a PIA to insulate with batts, dense pack or SPF does work much better.
The R value of the assembly is actually not all that much better than 2x6 24 OC framing and the cost with the extra labor plus SPF will definitely add a lot.
If you want cheap, your best bet is to go with 2x8 24OC walls. This is only slightly more expense in lumber, same labor cost and much cheaper than SPF for insulation. Since vinyl siding is self vented and allows for drying of the sheathing, it is a pretty good assembly in colder climates.
If you do go with 2x4 staggered stud, go for open cell foam. The closed cell does slightly increase the R value, but adds way more cost. The ROI on the cc SPF is probably never.
You only need 2” of exterior rigid foam to get to R13 if you use polyiso. Going over 2” does get more complex for trim details and anything mounted on the exterior of the home. It’s not super difficult, but requires some different techniques so might scare off builders who aren't familiar with thick exterior foam.
3” of closed cell spray foam is closer to R18 to maybe R19.5. Don’t believe the R7+ per inch numbers — those might be valid immediately after installation, but they’ll drop after that as the blowing agents gradually diffuse out of the material.
There is nothing wrong with a double stud wall. I agree with the other posters about going with open cell spray foam here if you go that route, or try dense pack cellulose. You might also try asking for 2” of polyiso exterior foam too, which will be less intimidating to your builder than 3-4” of something like EPS.
Bill
Hi Patrick.
I think a lot of builders are hesitant to install exterior continuous foam because of the flashing details that they'll have to learn and get right. I worked on an article with a BC architect who was a believer in staggered stud walls for exactly this reason, their buildability by average builders.
As you think more about how to build your walls, you may find this helpful: Walls that Work