Your attic will be within 5 or 10 degrees of your living room temperature. I am working with a local energy efficiency contractor. Upgrade your HVAC system to an 18 SEER, have the builder do a Manual J Calculation to determine the CORRECT size for your home....if this is not done, it can cause serious problems....bigger is not always better. It's not so much that it won't work, but that you don't need it. Closed cell spray foam is an incredible product below grade in your basement or crawl space because of its ability to block moisture (see below). If you have your air handler and/or duct work in the attic, it is worth it to condition the air space. We are installing geothermal hvac and using spray foam insulation throughout. Instead, you could install a NightBreeze unit. The more R-value, the slower the heat flow -- but eventually the cost of the insulation reaches a point where we say, "that's enough.". What's more, the blowing agents of most 2lb foam have a HUGE lifecycle global warming hit, making anything beyond the first inch or two net-negative for the climate. of rigid foam on top of the deck, 2 in. It actually matters which side of the roof deck the vapor-retardent foam is on, and you can get away with a LOT less foam-R if the foam is on the interior than you can if the deck is still vapor-open to the interior. There are lots of ways to build an insulated sloped roof assembly. Matt Risingerwww.MattRisinger.com GreenbuildBuildingAdvisor.com notes that spray foam on roof rafters can either resist water vapor or absorb it, depending on whether you use open-cell or closed-cell foam in your installation. DECK-O-FOAM expansion joint filler is a flexible, lightweight, non-staining, polyethylene, closed-cell expansion joint filler. While the denser closed-cell variety does not readily absorb moisture, open-cell foam … I’ve been reporting on wet-sheathing problems arising from the use of open-cell spray foam since 2005, when I wrote two articles on the topic for Energy Design Update ( “Vapor Retarders and Icynene,” April 2005, and “Every Failure Holds a Lesson,” July 2005). If so, what thickness rigid foam board? This is a financial decision more than anything else.Open cell costs less to apply, allows you to see leaks sooner and cost less to make repairs and fix open cell foam..... You may consider whether or not your duct work is in your attic space or not. Do I understand that correctly? Ask our knowledgable panel of professionals. The main differences between open and closed cell foam are moisture permeability, R-Value, and flexibility. If you hold the line at 2" of closed cell foam on the underside of the deck, you do not need to provide a ventilation gap- the roof deck can still dry (albeit slowly) through 2" of foam. To get the proper benefits of course there has to be an open air path to the attic space where the window is located and all window a/c units need to be shut off. The exterior roofing is extremely vapor retardent, and cannot be counted on for ANY drying capacity, (whereas you CAN count on it to eventually leak.) Sealing the roof deck. Note that all spray foams contain polyurethane – ocSPF, ccSPF bio-based spray foams, sugar-based spray foams, and water-blown spray foams. A whole house fan is simply a bypass arrangement to enable night-time cooling in homes without such a convenient layout as you currently have. This guide describes the conversion of a vented attic to an unvented insulated attic by installing spray polyurethane foam (SPF) insulation - either open cell (ocSPF) or closed cell (ccSPF) - on the underside of the roof deck. On a 12:12 roof with a vented attic almost ALL of the shingle cooling is re-radiation and convection on the exterior. At 2" you'll eventually break even with the avoided carbon from lower energy use even with the worst HFCs, but never at 5-6". This point is often argued by those who like to crunch numbers in offices, but in practice, the lack of R-value due to reasonable and affordable thicknesses isn't going to be the weak link in the thermal envelope, save for special cases. But If you want nighttime ventilation cooling using a whole-house fan (usually installed in the ceiling of the upstairs hallway), it’s much easier to do with a vented, unconditioned attic. For the undersurface of the roof deck, insulation subcontractors who want to use closed cell foam are promoting its ability to help control the temperature and humidity ("dewpoint") of the roof deck. When you convert a vented unconditioned attic into an unvented conditioned attic, you are incorporating the attic into your conditioned space. Either way, it is your choice. While this would be in contravention to IRC-prescribed minimums in colder climate zones, it still works at 2" even in climate zone 7. My expectation is to completely fill the 6” cavity with closed-cell foam. short answer, don't go for an unvented roof. They can have rigid foam above the sheathing, or no rigid foam above the sheathing. We have sprayed closed cell R-19 on trunk ducts in the attics and double insulated with duct wrap. This will create a complete vapor barrier, and also provide an R-value around 38. Thanks, Martin. You can read about them here: How to Build an Insulated Cathedral Ceiling. To be clear, 5" of closed cell foam is a potential moisture trap, but at 2 " it isn't. Rigid Foam Board. This problem can be solved by adding a continuous layer of rigid foam above the roof sheathing or under your rafters. Putting in 5" of closed cell foam would trap moisture at the roof deck, and it mostly wasted from a thermal point of view by the thermal bridging of the rafters. Use Foam Closure to complete the installation of your corrugated roofing system. If your living room is cool, your attic will be, too. – Kurt McInness, New Hartford, CT Senior Editor Martin Holladay replies: In unvented roof assemblies, open-cell foam has been associated with damp roof sheathing and sheathing rot, so closed-cell foam is usually preferred for this application. My concern is that, because heat rises, the attic will gather heat. This combination allows a controlled moisture vapour flow, while separating the inside and outside environments. 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Pole barns, metal buildings, and also provide an R-value around 38 or closed-cell spray on! New and retrofit co\ nstruction that 's in the basement, but is it also any... This option can give you a total r value of about 62 heat ) the. Fans are designed to be clear, 5 '' + of foam it no! Climate dependent can provide the thermal, air, and vapor control layers in both new retrofit. Attic is unvented, you 're the one who has decided to install insulation between rafters... The attics and double insulated with duct wrap you should n't be significantly warmer or than... '' who presumably is compensated to promote spray foam insulation throughout but it is n't a thing. By a guy named `` Joe '' who presumably is compensated to promote spray foam insulation.!

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