Insulation choices
user-483478
| Posted in Energy Efficiency and Durability on
Hi I am building a house in Michigan zone 5 and am deciding on either 2×4 walls 16″ oc with r13+5 or 2×6 walls 24 oc with r-23 mineral wool. Does the extra r value with the 2×6 wall make up for not having a continuous thermal break. Pricing wise they end up pretty close. If the 2×6 wall has the same performance as r-13+5 I would go that route to save time installing exterior foam. What are your thoughts. Thanks
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A simplistic calculation would be-if my math is correct-studs are r=1.25/in wood-at the studs 2x6 inch wall has 5.5x 1.25= 6.8, the four inch wall has 3.5x1.25=4.48, plus 5=9.48, but in a 24in oc wall studs are only ( 2.5/24=10.4 per cent of wall, whereas 16 ioc is 2.5/16=15.65%
2x6 wall 24 ioc sans ext insulation effective wall r value then is 6.8 x 0.104 =0.7 +23 x 0.896= 20.6;meffective r=21.3
2x4 wall is 9.48x0.156=1.47 + 13 x 0.944=12.27or effective wall r of 13.53
Ceteris parabis ie similar sheathing well sealed which would add say 7/16x 1.25=0.54 trivial and to both anyway
Also assume you will be same foundation dimensions and cantilevering the 2x6 _if not you either lose 8inches inside the house dimensionally !
Am assuming rock wool friction fit batts OR dense pack cellulose on so no sagging /ain’t voids even on the 24 ioc wall-
If I’ve done the math right from pure r value 2x6 without ext seems compelling choice, however these are average wall value- the 2x6 wall varies between 7-23, whereas the 2x4 with ext sheathing has less variability-r 9.3- 18 ;a range from also have to perhaps consider how how bad is risk of condensation on that 2.6 stud inside-ie is humidity rel high at any time fo year? Vapor barrier inside ?climate- I’d reckon most will vote for the wall without ext sheathing. And what about costs ? A 10 foot 2x4 stud runs , $5, a 2x6 $6; a strip of r23 mineral wool 2x10 would be $3 @15 cents sq inch, when add in cost of foam
You are using 33% more lumber in the 2x4 wall, and the price of stud is only 20 per cent less AND you have to add the cost of the foam.
Consider another alternative:
2x6 top & bottom plates, with 2x4's at 12" oc alternating every other one flush to outside/inside.
This gives you a complete thermal break between the top & bottom plates, and gives you the 6" of insulation. I found to be the same cost as 2x6 @24" oc because although it is slightly more board feet of lumber, the cost of 2x4's in my area is so much less per/bf than the cost of 2x6's, so the material cost was less. The upcharge in the labor was about equal to the material savings.