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Old 03-06-2012, 01:54 PM   #1
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underfloor insulation w/non-standard joists?


Hey guys, I need to insulate under the floor in my 1950's ranch. The floor joists are 4x12's that vary from 4' to 8' apart. I'm not sure how I should go about hanging/holding the batts to the underside of the floor. It looks pretty simple if you have 16" or 24" oc joist but how is it typically done on a house like mine? Thanks, 1
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Old 03-06-2012, 01:56 PM   #2
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Rigid foam cut to fit would be ideal as it will yield a higher R-Value per inch.

After that, cutting batts (Roxul or FG) is somewhat easy (easier with Roxul) and covering them with a tyvek or insulation webbing will help support them.
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Old 03-06-2012, 03:05 PM   #3
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Old 03-06-2012, 11:16 PM   #4
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Oregon Energy Code requires R-30 for 10" joists, which is probably what the Inspector wants rather than the R-25 (8" joists), seems they always want more....http://ecodes.biz/ecodes_support/fre...Efficiency.pdf

I would use some unfaced R-19 batts sitting on 2" (R-10) foil-faced foam board (Thermax) with the air space above: http://www.buildingscience.com/docum...n-crawlspaces/ Note: no vinyl floors.

This would stop radiant, stop air movement so you can use f.g. and lower the dew point temperature in the cavity = no condensation worries, even here in the Pacific Northwest. Add a 2x2 nailer at beam level at exterior perimeter. Add some utility 1x4 strapping under the longer spans, tape all joints against air, use the scraps with canned foam on the concrete wall from decking to nailer. Be sure the dirt is covered with poly.

Gary
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