Is spraying 1 1/2″ of closed-cell foam to the interior of block walls an efficient form of insulation?
greener_is_better
| Posted in Energy Efficiency and Durability on
I have a structure located in central AR that is 8′ tall block walls. The wall cavities are NOT grouted. The walls are above ground level. The interior of the walls are lathed with 1 1/2″ thick strips. Im looking to spray 1 1/2″ of closed cell foam directly to the blocks. Will this be an efficient form of insulation?
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Zach,
I don't know what you mean by "efficient." If you could manage to get 1 1/2 inch of closed-cell spray foam installed, it would have an R-value between R-9 and R-10. That probably meets code requirements for insulating so-called "mass walls" in your climate zone, but it isn't much.
There's a problem, however. Closed-cell foam is very difficult to trim, so installers don't install it flush with the inside faces of studs or furring strips. If you have a 1 1/2-inch-deep cavity, you will be lucky to get 1 inch of spray foam installed.
R10 only qualify as code-min for mass walls if the foam was continuous (not broken up by furring), and the foam was on the exterior side of the thermal mass (the CMU block.)
With the foam on the interior side it would have to be R13, and not thermally broken by the furring.
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_11_sec002.htm
If it's possible to put 2" of Type-II (1.5lbs nominal density) EPS on the EXTERIOR, and finish it with a faux-stucco like Quickcrete Foam Coating it would would just edge over the code min at ~R8.4 (code min- R8).
After putting the EPS on the exterior you then installed split unfaced R13-R15 batts with a compression-fit between the CMU & furring you'd have a thermally-broken ~R6 on the interior side, and the wall would outperform code-min.
For a lot more money you could take it a step further and fill the cores with non-expanding injection foam, but the cost/benefit just isn't there compared to 2" of EPS.
Since the window flashing is likely set up to direct bulk water to the exterior of the CMU, it's probably best to air-seal the exterior of the CMU with a spray-applied weather resistant barrier (WRB) prior to adding the exterior foam.
The installed cost of 1.5" thick 2lbs closed cell foam is about $1.5-1.75 per square foot. The installed cost of 2" of EPS is under buck a square foot, plus whatever the spray-applied WRB and finish coat costs are. They might end up roughly equal, or the EPS + goop solution may be slightly more expensive, but it's at least comparable in cost, and (unlike the closed cell foam solution) would meet code min.
If you have the walls already opened up on the interior, by all means install the split batts (with a snug fit side-to-side, and top to bottom) before putting it back together, but don't waste the foam budget on any high R/inch foam that going to be thermally bridged by the furring- save it for the exterior. Here's why:
A split R13 compressed to 1.5" thickness will deliver something like R5.8 center cavity, R4 average after the thermal bridging of the furring.
A split R15 would deliver R6.5 center cavity, R4.25 after thermal bridging.
The hollow-block CMU delivers another ~R1, the interior gypsum another ~R0.5 for a total "whole wall" average in the R5.5-T5.75 range.
If instead you put a flash-inch of 2lb foam you'd only come in at about R4.8 whole-wall, which underperforms the full-depth fiber solutions.
Alternatively, if you cut'n'cobble 1.5" of foil faced polyiso between the furring (sealed with can-foam, then trimmed flush) you'd hit ~R6.5 whole-wall.
This all presumes a 20% "framing fraction" of thermally bridging furring & trim. But even if you drop that to 15% it only adds another R0.5 to those numbers.