Radiant Barrier Insulation
I am finishing an attic room on the canadian prairies (very cold winters, hot summers), and am considering the use of a newer product offered here which is a bubble pack sandwiched on either side by foil. The application is for the underside of a hot (unvented) roof sprayed with 4″ of icynene ridge to eaves (2×4 rough sawn rafters). I am above all concerned about heat gain in this assembly during the summer. I am strapping the interior of this roof anyway for drywall and thought that a radiant barrier (in theory) would do a good job of reducing summer heat gain (and increasing winter heat loss). Strapping would ensure an air space towards the roof and another layer of strapping would provide me with an air space towards the drywall for the radiant barrier to take effect. Because this is a unvented assembly, however, I’m worried about cooking the shingles and having extreme premature shingle failure. Does anyone have any opinions/experience with these installations? The product is expensive so I want to be sure that the investment will have a return in comfort and heating/cooling costs. The owner is not sure if he wants to spend the money, and the manufacturers info I suppose is biased. Any thoughts?
Gio
prairieHOUSE Restoration and Development
Replies
You're right, the manufacturer's info is biased. Very much so. Look at the small print and the very high R-values they claim incorporate the R-values of all the air films and building materials in a typical wall. Kind of like claiming I've got $2000 bucks on me (if I claim the money in everyone else's pockets as well).
I've examined the stuff and would estimate that it gives an honest R-4 or so. In theory radiant barriers are great, but in practice, once they develop a dust layer on them, they don't reflect so well anymore. I've used the bubble-wrap foil stuff as retrofit for ease of installation. And I could imagine it for speciality application where volume was limited (backpacking gear, in an airplane, etc.). But for all the drawback of fiberglass batting, it gives you more bang for the buck and it is readily available.