In order to use only one sort of insulation in unvented rafter bays, that you are restricted to spray polyurethane foam. A different likelihood, certainly, is to create your roof with structural insulated panels (SIPs).
Based on section R806.five in the 2012 IRC, “Unvented attic assemblies (Areas involving the highest-Tale ceiling joists as well as the roof rafters) and unvented enclosed rafter assemblies (spaces involving ceilings which can be applied straight to the underside of roof framing customers/rafters and the structural roof sheathing at the best with the roof framing users/rafters) shall be permitted†so long as many disorders are met.
one. You'll want to get an engineer to evaluate the security of your respective 2x4 rafters. "Furring down with 2x2s" isn't going to Increase the capacity of your respective 2x4s to bear the snow load. It truly is probable that an engineer will suggest you to sister 2x8s or 2x10s on to your existing 2x4 rafters.
Is it all right to add peel-and-adhere membrane to pick out parts of the roof and hold the rest breathable with tar paper?
From the absence of an airtight ceiling, roof air flow can perform a lot more damage than fantastic, considering that air motion in rafter bays can really encourage indoor air to leak by means of ceiling cracks.
2. Install strong site-developed ventilation baffles in Every single rafter bay prior to deciding to spray the foam. Like that you end up having a vented cathedral ceiling.
Appears like you've got the framing and roofing in place. Your choices are described inside the short article earlier mentioned--you are able to include venting by installing baffles under the sheathing to carry again the insulation, or you could put in at the least R-twenty of air impermeable insulation.
Thanks yet again for your answers. Now I just need to seek out an installer On this spot who is aware what they're a knockout post undertaking (up to now it's been iffy)!
Q. "Really should I Chunk the bullet and hazard of spray foam insulation, or ship the builders back up around the roof in December to chop ridge and soffit vents?"
"Each individual residence is exclusive. The factors which could have saved the roof sheathing of your home you describe could involve:
Nonetheless, two other areas of side-roof have presently been dry walled about with fiberglass as well as a plastic sheet -- the lavatory closet along with the hallway. I am frightened to ask but ought to I open up These up yet again?
Rafters are 2x4, 12/twelve slope, plus the roof is difficult with intersecting ridge lines, valleys and dormers. (Your house initially experienced a cedar roof. The deck is the standard spaced fir planks.) On account of the construction, there is sort of no probability of receiving ample air intake by using soffit vents. (Along with the structure troubles, you'll find troubles mainly because every one of the Areas amongst rafters where they fulfill the very best plate were being bricked in, apparently at time of development). Proper common movement-as a result of underdeck roof ventilation is going to be difficult. Once we first looked into this job, the building inspector asked us to nevertheless use vent channels (affordable polystrene was Okay) to seize what ever ventilation we could.
The code limits on the usage of air-permeable insulation concerning rafters were being produced to circumvent the roof sheathing from rotting. When fiberglass batts are set up in unvented rafter bays, the batts make it possible for moist indoor air to reach the cold roof sheathing.
Environmentally friendly builders typically try to meet or exceed these minimal R-values. Having said that, the code supplies a number of loopholes permitting builders (occasionally) for getting absent with reduced ceiling R-values than essential within the prescriptive table.