Can I Use Foam Board Beneath My Radiant Barrier On Rafters - Insulation - DIY Chatroom Home Improvement Forum

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Old 01-30-2013, 04:59 PM   #1
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Can I Use Foam Board Beneath My Radiant Barrier On Rafters

I want to apply a radiant barrier to a 70`s ranch style with a conventional built roof frame...(no trusses). I live on the Gulf Coast area 2 and it is 30-100 here at times of the year. It is very humid about 8 months a year and from past reading I have learned that insulation between the ceiling joists is recommended to be installed without backing. So okay.

The roof was originally wood shingles. This means on top of the rafters there are 1x4 purlins about every 12" on center. I had my roof reshingled several years ago and at that time the wood shingles were removed, the purlins left in place and decking applied before asphalt shingles were laid down. My attic is fairly easy to access except to about 3' from the eaves but I can get there.

What I would like to do is apply the radiant barrier in strips between the 2x6 rafters, then say leaving a couple inches between the radiant and a foam board insulation, I would like to apply foam board between the rafters. Does anyone have an idea whether I will substantially reduce heat coming through to the attic if I do this? I have a roof vent that runs along the peak all the way across the roof so ventilation won`t be a problem I don`t think. I don`t intend to make everthing air-tight but was thinking adding foam board will help reduce heat transfer coming from the radiant barrier to attic. Any thoughts? Thanks all for your replies.



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Old 01-30-2013, 06:17 PM   #2
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If this a finished attic, or just an empty space?
I'd be far more concerned with air sealing, making sure there's soffit vents that are not blocked with insulation.
And enough insulation in the joist area.


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Old 02-01-2013, 06:37 PM   #3
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Sorry joecaption I`ve been at work the past few days. Yes, it is an unfinished area. Roof peaks are almost 6' from the ceiling joists. Looking out from the inside first rafters, then purlins, then plywood deck, then tar paper and shingles. I have been putting pieces of foam board out near the eaves to prevent the insulation from blocking the eave vents to make sure I`m getting plenty of air to the ridge vent. I am doing a little at a time as the heat up there allows.

I have friends in newer homes with radiant barrier who say it is helping keep their attics a lot cooler so I am going to do that but thought the foam board between rafters would help as well.

Thanks for your reply.
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Old 02-03-2013, 09:33 PM   #4
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Radiant would work for your location.

Is the HVAC in the attic?

Is the attic floor insulated already? How/what much?

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Old 02-14-2013, 01:17 PM   #5
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Gary in WA, Thanks for the reply. Yes my A/C evaporator/furnace is in the attic and the attic has insulation blown to six inch depth.

So to recap

My roof underside is unfinished and purlins are on top of rafters. Here is where I will put a radiant barrier.
6" blown insulation throughout.
Ridge vents.
A/C -furnace in attic.

I am home for a few days now and will check back. Thanks.
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