I have recently installed TechShield radiant barrier sheathing on 2×8 rafters in a cathedral ceiling application (span of 13′). I am trying to achieve approximatley an R-38 insulation value. I want to maintain a 1 1/2″ air space immediately beneath the TechShield as I have vent holes in the freeze blocks and at the top of each rafter bay. Any suggestions on what type of insulation to use (rigid or fiberglass and what R-value)and how I might install it? Since I have 10’and 15′ ceiling heights, there is no restriction on adding a nailer on the bottom of the rafters should that be necessary.
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Replies
Glenn,
BY now I hope you have your roof insulated and can tell me about it.
I am trying to insulate 2x8 rafters so that I can heat my attic. I want the air gap (I am in a cold climate), so I am thinking of using a reflective insulating material (bubble wrap with foil on both sides) as a radiant barrier. Then drywall on the underside of the rafters, and finish by filling the cavity btwn the rad. barrier and drywall with cellulose insulation.
The radiant barrier sound similar to what you describe. Can you tell me your results?
Dont bother with a reflective barrier. Its really only usefull if you live in a warm climate like Florida or Arizona.
Thnks for your repsonse. I've decided use a reflective barrier with perforations for three reasons. One, I wanted venting under the roof deck, from soffit to ridge vent. Two, I did not want a vapor barrier on the cold side of the insulation (I believe foam vent chutes would act like a vapor barrier; besides, these chutes are expensive when you need to cover a large area.) Third, I thought the reflective barrier might help beat back some of the July/August sun on my dark shingled roof... I am using the radiant barrier now; it staples-up (pretty easy), and seems like it will do a good job in my opinion.
If you have the $, foam it. You don't need an air space ( called a hot roof ) and you eliminate all convection currents ( the big enemy in making a house efficient )
There is closed cell R 6.5 inch ( corbond ) and open cell R 3.5 inch ( icynene ).
http://www.corbond.com
http://www.icynene.com
Edited 2/6/2005 8:13 am ET by reinvent