Poor man’s continuous insulation (CI)
I recently saw an interesting approach to continuous insulation (CI) that might make sustainability more affordable. Simply nail 2×2 horizontal strapping to the inside of the studs.
I’m in Georgia so condensation is not a big issue. I attached a sketch showing polyiso bands at the top and bottom of the interior wall strapping to support the gyp board. Also offset the rim board 1/2″ to accommodate a little polyiso CI.
This probably doesn’t meet IBC’s definition of CI but it could be an affordable way to improve insulation. If you are trying to reduce petroleum based insulation, it could make batt or loose fill alternatives more effective. Still have to make sure the envelope is well sealed.
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Steven,
What you are describing is called a Mooney wall. There is some information on it here on GBA, and a lot more on the web.
The performance and some construction details of that type of assembly can be found in the discussions of Case 3 in this document:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf
Thank you so much. This is great information. I would like to explore using it in moderately priced homes in moderate climates like Georgia.
...and for new construction, the horizontal strapping could go on the exterior to reduce thermal bridging at the rim joists. It might be too much bridging to be considered CI by IBC.
Steven,
I'm not sure there is any advantage to moving the insulation to the exterior. usually it's easier to provide continuous insulation and strap on top of it.
I agree but in this case I'm looking for CI without the use of foam plastics. I don't think it is possible except with using mineral wool which is too expensive.
>"It might be too much bridging to be considered CI by IBC."
As long as the insulation portion hits R20 or higher it will meet code. That might not be the case if using 2x2s unless using 1.8lbs fiberglass, but would if 2x3s using R13-R15 batts for the 2x4 section and R8 econobatts on the 2x3s.
Alternatively one could use R20-R23 batts in a 2x6 wall and use 2x2s.
I attached a sketch of a Mooney wall with exterior strapping.
IIRC two years ago GA code was in the process of getting updated and the builders balked at the requirement for exterior insulation however there was a sketch of a proposed wall which used 2x6 top/bottom plates with 2x4 studs. The studs were flush with the interior edge of the plates and rigid insulation was attached to the exterior edge of the studs which made the foam coplaner with the rim joist and therefore sheathing easier. This mock-up was supposedly the "affordable" option for production builders.
May want to contact Allison Bailes at Energy Vanguard because I believe he mentioned it on his blog.
Thanks for the great information. Here in Georgia where energy prices are relatively low and the climate is relatively mild, high efficiency exterior walls are not often used. High-end homes use them but it is desired for comfort rather than energy savings. Most people are aware that the typical 2x4 exterior wall is extremely inefficient so I'm looking for cost effective options for mid-range homes.
Steven,
" I'm looking for cost effective options for mid-range homes."
That's where I'm not sure exterior strapping makes sense over just upgrading to 2'"x6"s @ 24" oc. Once you move the strapping to the exterior it has to be run from foundation to roof. It also pretty well excludes using batt insulation too, as the thin layer still represents just as much labour as the full batts. The only benefit is a very small amount of insulation over the rim joist. To be clear, that's just my take on strapping the outside, not doing Mooney wall as it is commonly done on the interior.
Just out of curiosity: What's going on with the foundation in that section? it looks like nothing I've ever seen.
I’m with Malcolm here: please provide some more detail about that foundation wall. From that basic drawing, it looks like a detail that will take any water running down the exterior and collect it at the base of the wall where it will soak the rim joist.
Bill
The siding will have a brick base with typical rigid insulation CI at the slab with brick veneer and weeps. There is a patio/walk on the other side not shown. It's a fairly common detail down there but I shaded the section to show how the insulation/structure worked so it's not so clear on other issues.
Ah- it makes sense now!
Thanks for that info!
Bill
I'm not sure if you can put the sheathing on the outside and maintain shear resistance. You might need the plywood directly over the studs or go with some other type of bracing for the wall.
The thinner insulation like Rockwool AFB comes in dimensions for steel studs, you would have to trim each one to get it to fit between your strapping. That is a lot of trimming. Dense packing might be better.
I think the suggestion of standard 2x6 at 24OC or if you want higher performance 2x4 with Bonfiglioli strips on the inside out of some R6 Zip R would be simpler.
Thanks Akos. I had never heard of Bonfiglioni strips. Using something like Zip R-sheathing would make it easy. Still get some bridging at the floor sheathing but that's tolerable. Also makes installing insulation easier than a mooney wall since this can use 2x6 high density batts.
Lots of great options. It seems that simply putting an inch or two of EPS on the outside of the sheathing is just as easy and cost effective as the other options especially if it is installed and taped so it can act as a WRB.
Foil faced polyiso will be much easier to properly tape if you want it to act as a WRB.
Bill