R-13 in 2x4 walls
I have a project in north Georgia (climate zone 4), where the minimum code-prescribed R-value is R-13 for walls. I'm stumped on a couple of issues:
1. Several fiberglass insulation manufacturers have products that claim an R-13 or R-15 in a 3-1/2" wood stud wall. How do they achieve a R-15 in the same depth as they would an R-13? Do they just jam more stuff in the same volume? It seems that if it's the same basic product in either case, it's almost counterproductive to jam more stuff in the same volume, as the dead air is really doing the insulation work.
2. If I go with an open cell spray foam, it looks like I can only assume an R per inch of 3.5 or 3.6. At R-3.6 per inch, a 3-1/2" studwall only get me up to R-12.6. The code minimum for my building is R-13. I realize I could always add a rigid insulation board to the exterior of the wall sheathing to make up the difference, but that's not necessarily common practice in this area. Does this essentially rule out open cell spray insulation as a feasible option in this case? Do people just call R-12.6 good enough and move on?
3. At least in the residential portion of the IECC, the code prohibits computed R-values from including contributions from "other building materials or air films" (IECC 402.1.2). It only permits calculated R-value to come from what might be described as "manufactured insulation" products. Air films, plywood sheathing, etc don't have a lot of insulative contribution, but they could theoretically get me over the hump from R-12.6 to R-13. But I'm not permitted to do that. Of course, by the time I account for the periodic wood studs in the cavity, I'm not even really getting the full R-12.6.
I feel like open cell spray foam is inherently a better product than fiberglass. I realize there's plenty of debate out there regarding the relative "healthiness" of foam, fiberglass, cellulose, etc. I'm just having a hard time justifying the insulation values I can expect.
Posted Jun 18, 2012 10:22 AM ET
Edited Jun 18, 2012 10:31 AM ET
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