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This is a very inexpensive way to add to the efficiency of a structure---radiant barrier material. True, you can buy the OSB with a mylar layer, but you waste an awful lot of it in cut-offs. This stuff is two layers of reflective material laminated onto bubblewrap. It's astounding---on a hot summer day, just standing under the sheathed roof was way cooler. It came in 48"x25' rolls. I used it on all the rafters, the gable walls (anything higher than the living space), even on the porch rafters to help reflect heat away from the porch.
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After framing, just roll it out and staple it to the rafters and studs---even out over the eaves. Do it kind of loose so the material sags a little. Make sure the space between the radiant barrier and the living space is ventilated so moisture can escape--it won't go through this stuff.
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The Cuyamaca Cottage has some tricky steep angles in the roof---17:12 pitch, curved in places, 28' off the ground. Cutting the sheathing in place was not possible---here's a trick to cut it safely on the ground and have it come out perfect.
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Climb up with a piece of the radiant barrier. Roll it out over the section you need to sheath. Staple it just enough to hold it in place (I like the hammer staplers, and I just tack it hard enough so the staple still sticks out some) then just cut the radiant barrier with a box cutter to the exact shape of the section. Now you can pull the staples and use the radiant barrier as a template for the sheathing, then go tack the radiant barrier in for real, and drop in the cut sheathing over that.
Oh and I learned the hard way to always wear sunglasses when using this highly reflective material, unless you build at night....