Radiant barrier in 2x6 rafters


Old 06-01-01, 07:31 AM
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I would like to include a radiant barrier (5/16" foil-covered 'bubble-wrap') as part of my insulation plan for a cathedral ceiling. My rafters are 2x6, so I only have about 5 inches of space to work with. If I use the following scheme, will I end up trapping moisture between my radiant barrier and my drywall?...

(order of layers from shingles to bedroom):
1/2" air space
5/16" Astro-Foil
3" fiberglass blankets or solid foam

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Old 06-02-01, 10:37 PM
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Is this a new addition? I' am not sure about the radiant barrier products, but have seen them at Home Depot...

You might consider installing 2" x 2"s on the 2" x 6"s, and then you would have enough room to install a 5 1/2" thick
high density R-21 batt into the ceiling area..

In any case you should have ventilation running up from the soffits to a ridge vent in each rafter space. I would install styrofoam vents from the bottom to top in every cavity to insure that the ventalition is functional..

The biggest thing about insualtion is that once the drywall is up, not much can be done to rememdy the situation..

Good Luck

Old 06-04-01, 11:24 AM
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To answer your question, yes, this is an addition (actually finishing an existing attic space). It did have 2x4 rafters, so my "2x6's" are actually 2x4's with 2x2's already nailed on. That probably only leaves me with 4 inches to play with, if I include an air gap under the sheathing.

If I skip the radiant barrier, that leaves me with 3 choices, as far as I know:

Option 1: The most conventional would probably be 3.5" high-density R-15.

Option 2: I could go with a 5.5" R-21 and use some "duro-vent" styrofoam spacers to keep my air gap. I assume that squeezing the 5.5" rolls into a 4" space would end up dropping that R value (not sure how low, maybe 18?)

Option 3: Take the road less travelled and install "polyiso" sheets normally intended for exteriors. I would cut them to fit snugly. They are R-5 per inch, so I could probably put in 4 inches for a total of R-20.

Which would you choose, if cost is not a factor? Is option 3 even possible/allowed?

Old 06-04-01, 01:22 PM
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I guess all things considered, I would pick option 1.

However this still may not be the right choice..

But I dont think option 2 is even possible. I think you would find it tough to install the 5 1/2" batt in the cavity. I know from past experience where I worked as an insulation estimator, the installers didnt even like putting in high density R-15 batts because they were actually a little thick for the 3.5 inch space. If they didnt install them immediately after opening the bag it made it a tough fit. So I think you would ahve trouble compressing a 5 1/2" batt into a 4 inch space. It might end up putting alot of pressure on the drywall fasteners.

Not sure if Option 3 would be acceptable by code, I would call your local building dept.

However in my area (S.E. Michigan) two types of insulation claim it is unnecessary to have venting if you fill the complete cavity. One is a wet spray cellulose which is blown in and the other would be icynene which is an expandable foam product which is sprayed in. Both these products claim to create an R-value equal to fiberglass insulation. I would only try this method if they guaranteed it in writing, so that if there was a problem you would have recourse..

hope this helps


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