DIY Home Improvement, Remodeling & Repair Forum > DIY Home Improvement > Roofing and Siding > Radiant barrier combined with spray foam thoughts

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Old 01-08-2009, 09:46 AM  
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Default Radiant barrier combined with spray foam thoughts

Hello all,

This is my first post so I hope I've found the right forum for my question.

I am enclosing my 11x31 attached patio and have been researching insulation options. What do you think of the following and please feel free to make improvements or suggestions.

The roof is sloped and shingled and supported by 4x4 posts on 8' centers all around.
I am in Kentucky so the climate is both hot and cold.
The outside wall is South facing with a 20" eave.

The wall plan is pretty standard - Vinyl siding on Tyvek on OSB on 2x4 studs and then Drywall.

My question is how best to insulate. I would like to try radiant barrier and closed cell foam of some sort instead of fiberglass.

This is what I'ld like to consider:

On the inside of the OSB in the 2x4 cavity, glue 4 - 3/4" foam blocks at the four corners. On these, install 1/2" rigid foam insulation (foil on one side) with radiant barrier facing the outside (OSB). This gives the required air gap for the RB. Then seal the perimeter with can insulation. Install another piece of Rigid but facing in toward the conditioned room, leaving another gap between the foil and the drywall.

So from outside to inside would be -

Vinyl siding - Tyvek - OSB - air gap - foil - rigid insulation - foil - air gap - Dry wall (whew!).

My thoughts are:

Vapor barrier - Tyvek, and Rigid will both provide a vapor barrier but will the gaps (osb-rigid & rigid-drywall) allow moisture that permeates the 2x4's to condense in the gaps?

Would the outside air gap get too hot as there is no ventilation there?

2x4 thermal bridge - would it be better to wrap the interior with foil to cover the studs and use thicker rigid (instead of two pieces)?

Ideas, comments, concerns anyone?


Last edited by rbobbitt; 01-08-2009 at 09:58 AM.
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Old 01-08-2009, 01:01 PM  
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Welcome RBobbitt:
Someone must have convinced you that the foam insulation with the radiant barrier far exceeds the R-rating of fiberglass. It sounds like you are going to an awful lot of trouble and expense and, unless it will double the R-rating, it is not worth it. I agree with you, the air spaces will most likely be vapor traps and the vapor may be enough to run down and out on your floor.
I would stay with the fiberglass and use 1" foam on the outside rather than the OSB. The roof is standing now without the rigidity of the OSB and I doubt that will change with the foam.

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Old 01-09-2009, 02:38 PM  
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Thanks for the feedback and hi to a fellow Kentuckian!.

Glennjeanie, I'm not really convinced on the merits of foam and foil but I am always looking for a better mouse trap. The math suggests to me that together they may be a better use of the standard 2x4 space. Two inches of rigid (or Spray) is about R-13 plus two dead air spaces and two foil surfaces should be about R-16 and tighter. Isn't standard fiberglass R-13 but without any radiant barrier benefits? I've read that radiant foil needs at least 3/4" space to be effective. Two 3/4" gaps and two inches of rigid should beat fiberglass (in theory) in the same space. Also, I'm not too sure I like just rigid on the the outer wall. It seems to me that a good bounce from the kid's might break it.

Roofing God, can you explain why double foil is no good? In Kentucky we have both sub freezing and 80 + temps so I would think that foil in each direction would be beneficial. The faced rigid (Polyisocyanurate board, foil-faced) is cheaper and cleaner to install than DIY spray but with the same R value, about 6-8 per inch.

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Old 01-12-2009, 04:53 AM  
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the vapor barrier goes to the heated side of the insulation,any double layers will trap condensation,and can lead to mold problems
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Old 01-15-2009, 12:39 PM  
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You will do well to use the fiberglass on the inside cavity between the studs and use the foil faced on the outside nailed directly to the sheathing. The vapor barrier (the foil faced insulation) will be on the outside but that doesn't really matter any more because studies have shown that. For example, in Northern states such as Vermont and Minnesota they are now placing 2 inches of styro on the outside sheathing and removing the interior vapor barrier. A barrier must be placed on only one side because otherwise you will trap moisture in. Also, exterior foam provides a thermal break with the framing.

Josh Jaros (Jaros Bros. Construction)
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Old 01-23-2009, 08:03 AM  
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Thanks everyone.

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barrier, condensation, radiant, rigis, spray foam

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