Conditioned attic conundrum
Conditioned Attic Conundrum
I am building a house in Climate Zone 4A (in a storm-prone area) with a conditioned crawl insulated with Roxul Drainboard and 2×6 walls of 1″ Zip-R that I plan to flash with closed cell foam and back with blown fiberglass. Unfortunately, the roof insulation system has me scratching my head and second guessing myself at every turn. I have read all of the articles on this site and elsewhere, and have designed the system accordingly – but am seeking a bit of additional advice.
What I have decided for sure is that it will be a conditioned/unvented attic with a 9/12 pitch standing seam metal roof over ¾” plywood. I have chosen to make the attic conditioned since I will have HVAC equipment and ductwork up there that I cannot relocate. There will be no combustion appliances in the attic.
My initial thought was to spray closed cell foam on the underside of the roof deck, furr the 2×10 rafters down with 2×2’s, and then fill that with blown fiberglass to meet the required R-38. I have, however, always been concerned about the possibility spray foam hiding roof leaks until a catastrophic failure and have also read a couple of articles about failures of roof decks with open cell foam due to vapor migration and condensation. While closed cell foam seems to be the better choice, I still have concerns that roof leaks will be hidden (I am in a storm prone where wind-driven rain often finds its way into every nook and cranny). In my search for another alternative, I have been reading in this forum about using foam board on top of the roof sheathing and have approached my roofing contractor with that idea. He likes the idea and has done exactly that on countless commercial jobs, but he would prefer to put another layer of plywood over the foam to have a good substrate for the metal panels. Assuming that does not create any moisture problems, I would still need to air seal the attic to complete the insulation system. To that end I have investigated the Knauf EcoSeal spray caulk and think it could provide the air seal I need for this assembly. My plan would be to install the unvented metal standing seam roof over ½” plywood over 2½” of polyiso foam board on top of ¾” plywood (2½” of polyiso is the minimum required on a roof in my climate zone per the excellent article on this site). I would then install blocking of some sort between the rafters at the top plate so that I could seal it by spraying EcoSeal caulk in the corners and then in all corners and gaps on the underside of the rest of the roof sheathing to create the required air sealing. Finally I would install blown fiberglass over netting between the rafters. So the assembly would look like this:
– Standing seam metal roof with no venting
– High temperature ice and water shield roof underlayment (roofer has spec’d Tamko TW Metal and Tile Underlayment, which is rated at 0.05 perms)
– ½” plywood
– 2½” polyiso foam board
– ¾” plywood
– Knauf EcoSeal air sealing spray caulk
– 9¼” blown fiberglass
I calculate this system to have an R-Value of 16 for 2½” polyiso foam + 32.375 for the 9¼” of blown fiberglass = 48.375, which is well in excess of code, not to mention I have short-circuited the thermal bridging of the rafters. If I do experience a roof leak, then the water should make its way through the blown fiberglass to a place where you would see it, find the source, and fix it.
If anyone would be so kind as to weigh in, my specific questions are:
1. Does the Knauf EcoSeal actually seal well enough for this purpose, or is spray foam the only way to get a truly tight seal? I will be certifying the house to EarthCraft standards, which requires an envelope leakage ratio < 0.50 or 7 ACH50, whichever is more stringent. 2. Am I going to trap moisture and/or create a condensing surface in the plywood and foam sandwich assembly? My concern is the ice and water shield under the metal roof will be a water and vapor barrier, and that the Knauf EcoSeal may create a second vapor barrier that could trap moisture. Would the ¾” plywood be permeable enough, however, to allow drying to the interior? Many thanks, Steve
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