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Insulation and vapor barrier question..

fres skead's Avatar

Join Date: Mar 2006
Posts: 20

09-27-09, 01:43 PM   #1  
Insulation and vapor barrier question..

I have a unheated attached garage 26'x26' , living room above, with radiant in floor heating. It currently has r20 fiberglass in between the floor joists( 2''x12'') with the radiant heat pipes above and a half asses vapor barrier holding the insulation from falling into the garage.

I was considering adding some ridged Styrofoam 2''x2'x8' and new vapor barrier to the ceiling in the garage. Along with 5/8 drywall. I will also be leaving the r20 fiberglass in place..

Where do i put the vapor barrier, next to the drywall or the floor joists?The garage temperature this winter will be around -20 Celsius -5 F .. Or if u have any other ideas how to make my floor a little more efficient please drop me a line.

Thanks in advance..

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shane21's Avatar

Join Date: Oct 2009
Posts: 231

10-22-09, 03:28 PM   #2  
well the response is a month after the post but most of the questions you have are addressed in the "sticky" post. Always install a vapor barrier facing the conditioned space. Never install a second vapor barrier that "sandwiches" your insulation.

If you have fiberglass insulation packed full into a 2" x 12" cavity the R-value should be an R-38.

fres skead's Avatar

Join Date: Mar 2006
Posts: 20

10-27-09, 06:34 PM   #3  
Looks like im going to be putting up vapour barrier next to the floor heating pipes in 16" strips running long down the joists and sealing with tape and acoustical sealant. Then up goes the old insulation and then the drywall.

Anybody see any problems with this solution?


Bud9051's Avatar

Join Date: Oct 2008
Posts: 9,771

10-28-09, 05:22 AM   #4  
Hi fres,
If you have radiant heat in the floor, I would use a layer of foil faced rigid insulation just below the pipes, foil facing up, then fill the rest with fiberglass or other insulation, then the drywall. The foil facing will act as your vapor barrier as well, although not being a continuous layer it isn't that effective. But it also isn't that necessary. The cavity will get warm, so be sure to insulate the ends well.
Some reading: Energy Savers: Vapor Barriers or Vapor Diffusion Retarders


fres skead's Avatar

Join Date: Mar 2006
Posts: 20

11-15-09, 08:55 AM   #5  
thanks i will give it a read

GBR in WA's Avatar

Join Date: Mar 2009
Posts: 1,167

11-16-09, 01:15 PM   #6  
I would use: vapor barrier paint, drywall, foil-faced rigid insulation board, fiberglass batts, radiant barrier (permeable), heating pipes. Radiant heat will warm the tops and middles of the joists, with the bottoms cooler causing: BSI-009: New Light In Crawlspaces —
The radiant barrier will reflect the heat from the joists and batt insulation, yet allow moisture through so there are not two vapor barriers or retarders.
The garage is similar to outdoors, but with more water vapor present after parking a hot vehicle. Where does that gallon of water go?
Be safe, Gary

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