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BigJimmy 10-27-2007 09:57 PM

R-19 insulation: Minimum Cavity Depth
Hey there-

So, I've got the living room walls gutted to the studs. An architect with whom I work (who also lives in my village) told me that village code requires an R-19 insulation value for all new exterior walls (which applies to me since I've demo'd as far as I have). Based on this, I set out furring the existing wall studs with 2x2's. Since my house is balloon framed and was built back in the days of gas lighting, I set out today to start adding fire blocking/stops at the floors and ceilings. I had talked to a village inspector who told me that I could either drop 2x4's between the studs or use 2x6's which would overlap into/onto the floor space (and could be face nailed at the overlap). Well, I set out to do it with 2x4's and after a few, I started thinking about using 2x6's since I have a stack in the garage and figured it might make things somewhat easier.

Now I'm staring at one of the cavities that has been furred out and I immediately realize that 2x4 (3.5" depth) with a 2x2 (1.5" depth) creates a 5" overall cavity depth, not the 5.5" inches that one would get with 2x6 framing (DUH!). Feeling somewhat embarrassed that I didn't notice until this point, I run downstairs and start looking online for the thickness associated with R-19 fiberglass insulation. According to Owens Corning (and a few other sites), they indicate the use of a 6.25" thick batt in a 2x6 (5.5" deep) cavity. From what I've read about insulation, it looses its R-value as it is compressed. While I'm guessing that the .75 compression based on above is acceptable (the manufacturer at least should know what he's talking about!), what about my 5" deep walls? Do I need to add an additional .5" furring strip to create the true 5.5" cavity for an R-19 value or can I get away using the wall as-is, i.e. 5" deep with R-19 batts compressed an additional .5"?

I'd call the inspector but unfortunately, it's the weekend and he's going to be out for the next 9 days. Basically, I'm trying to do whatever is code-legal.


AtlanticWBConst. 10-28-2007 08:10 AM

You want 5-1/2" to have proper R-19 cavity depth dimensions.

Any time you "squash" insulation, it looses it's rated R-value.

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