Best Wall Assembly for High R-Value
Hi. I’ve been reading and learning over the last year as we progress our building plans, but only recently joined the site.
I’m hoping to get some constructive feedback on what seems to be the last major decision we face; whether to go with an effective R24 Rigid Foam (2×6 with R20+7.5ci) wall or an effective R28 Double wall (2×6 R-20 + 2×4 R12). I’m using the Canadian Wood Council Wall assembly calculator as my basis for the effective R-values.
Coastal eastern Canada.
HDD18C is 3,847 – Using Env. Canada 20 yr rec for specific location
HDD18C is 4,050 – using last 50 years.
HDD18C is 4,087 – using last 75 years.
Slab On Grade with 4′ frost wall
2,300 sqft + 500 sqft bonus room over garage
Most windows face east for view
Only one small N. facing window
Attic – R60
Under Slab – 12.5R
R32 under Bonus room
540 sqft of windows in total (13% of exterior wall)
4,100 sqft of exterior wall (3,560 without windows)
Aiming to be 0.5-1.0 air changes
Heat Pump will be sized once final plan is locked down
Heat-Pump Water Heater
10.7 Kw Solar with plan to be NetZero.
Adding more ci becomes difficult, as local rules require wood siding, and going with more ci would increase the siding effort/cost significantly in comparison to the gains.
Using an online calculator with above (R24 vs R28 walls), it suggested BTU heat-loss is 61k-63k/hour with air exchange and windows being the largest losers by far (22k and 27k BTU/hr respectively). A wall increase of R-4 makes virtually no difference. In fact, going up to R35 only reduced total BTU loss to 59k/hr. Maybe I’m off on a few items, or entering something wrong, but proportionally, the wall R-value is not having dramatic swings in total BTU/hr loss. However, changing the Windows from R1.8 to an R3.5 drops the BTU loss by 16k/hr.
My questions, which I may already be answering myself;
1. How is a double wall justifiable, when it costs more (~$18k), for what appears to be only a gain of R-4. Even changing from batts to blown-in and getting R35 doesn’t seem to generate the gains needed from a ROI perspective. Any comfort gains recognizable between the two walls to justify the investment?
2. It seems that I’m better served spending the $18k on window upgrades?
3. The 10.7 kW Solar installed is $18k, which I estimate should be close to NZ without double wall. So little to offset here with double wall?
3. Am I understanding the rigid foam should not be applied over the double wall? Or does it depend whether or where an interior air barrier is? Leaving 1″ of ci would then make it R33, so a little more benefit, however I keep seeing suggestions that ci is not good for Double walls.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part