"R-19 batts are very air permeable, and on lengthy unobstructed rafter bays you may have fairly a stack outcome. It does not have a large number of air leakage to offer some drying there. Also, if the batts are unfaced or kraft-confronted, the drying towards the inside could be considerable.
Like most mansard roofs, your roof is unvented. Your mansard roof has distinct sections -- the steep sections on the perimeter, as well as small-slope area in the middle. All these sections must be insulated in a similar way, mainly because all of these roof assemblies are unvented.
It feels like you have an current roof with tile roofing. I'm guessing that you're creating from Europe. It appears like the insulation contains "tight-fitting Wooden fibre board" among your rafters -- thickness and R-worth aren't specified.
Although roof ventilation might help dry out moist roof sheathing, it’s necessary to limit the move of h2o vapor escaping through the dwelling so the roof sheathing in no way will get damp to begin with.
Your strategy to install a stack of rigid foam rectangles in Every rafter bay is known as the "Slash-and-cobble" technique of insulation. It truly is a method usually chosen by homeowners, but by no means preferred by insulation contractors.
I believe that Bill Rose is right: most of these failures require humidity that is carried by going air. These are not diffusion failures. When the rafter bays seriously are correctly dense-packed -- a huge if -- the chance of air motion is very small.
Q. "I here are the findings am wondering if I ought to furr out the rafters to receive the specified thickness of spray foam, or if installing several inches of polyiso foam to your underside of your rafters, after the spray foam, could perform?"
The for a longer period that you choose to wait around to install rigid foam previously mentioned your roof sheathing just after your rafter bays are filled with fluffy insulation, the bigger the risk that the roof sheathing will accumulate humidity. 1 or 2 many years need to be no trouble, but I wouldn't extend the workout much farther than that.
The perimeters of the roof are coated with asphalt shingled (GAF Slateline to be correct). It feels like I need to pull down the fiberglass and have it spray-foamed just before I drywall.
Though roof air flow can reduced the chance of ice damming, it’s essential for builders to limit the move of heat into roof ventilation channels by including a number of ceiling air barriers and by installing thick insulation, making sure that as small warmth as you can escapes from your dwelling.
OK, I was figuring that the whole with the rigid foam + cellulose needed to be a certain R-worth together and had been thinking about using considerably less rigid foam. click reference I've been looking through about different sorts of cellulose. Due to the fact I am setting up from over would anything like stabilized cellulose operate because it has a little drinking water included?
Your system will perform great, so long as the layer of rigid foam along with your roof sheathing has an R-price of no less than R-20. When you are applying XPS, you will need no less than 4 inches of foam. If you are employing polyisocyanurate, you require at the least 3 inches of froth.
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It truly is considerably more vital that you have an interior air barrier than an inside vapor retarder. Be sure that you've got created a fantastic air barrier by sealing all penetrations and cracks.