If you would like use just one sort of insulation in unvented rafter bays, that you are limited to spray polyurethane foam. A further possibility, obviously, is to construct your roof with structural insulated panels (SIPs).
If you put in 2x4s (or 2x3s on edge, mounted with prolonged screws) perpendicular towards the rafters, you are going to tackle many of the thermal bridging issue without the should include a layer of rigid foam.
Needless to say, at times you might want to prepare in advance to be sure that your rafters are sized adequately to permit for insulation and venting. To learn more on satisfactory approaches to insulate very low-slope roofs, see this information: Insulating Small-Slope Residential Roofs.
The most effective method is to perform each of the do the job at once: rigid foam earlier mentioned the roof sheathing (as recommended inside the article), and fluffy insulation underneath.
Q. "I am thinking if I ought to furr out the rafters for getting the specified thickness of spray foam, or if setting up some inches of polyiso foam on the underside with the rafters, following the spray foam, could get the job done?"
Install a layer of closed-mobile spray foam versus the underside on the roof sheathing, and fill the remainder of the rafter cavity having an air-permeable insulation. All over again, this kind of assembly is created to dry to the interior, so the assembly really should by no means include things like an interior polyethylene vapor barrier.
Superb write-up and discussion. I've a matter about an current unvented attic assembly. The rafters are 2x4's as well as the bays in between them are totally filled with shut cell spray foam. I would want to insert two 1in layers of polyiso in direction of the inside of attic after which go over having a vapor open up membrane to airseal the assembly.
Even though the GBA Site now includes a lot of posts on the topic, we continue on to acquire frequent questions on The easiest way to insulate a cathedral ceiling.
U.S. constructing codes will not need the use of inside vapor boundaries, even in chilly climates. Whatever they do require can be an interior vapor retarder. This requirement is often satisfied with kraft going through on fiberglass batts or vapor-retarder paint.
"Just about every household is exclusive. The factors that could have saved the roof sheathing of your house you explain could include things like:
We really Do not know if these roofs have proper vent channels, or what sort of vent baffles (if any) have been utilized to maintain the air flow hole among the best on the fiberglass insulation and also the underside on the roof sheathing.
I've loads of walls to re-insulate, and once the roof is handled I will have rafter bays to insulate, and It will be practical to acquire some insight into which sort of fiberglass provides what benefit for its cost.
As I explained from the short article on this webpage, you can't use fluffy (air-permeable) insulation within an unvented roof assembly Except you contain rigid foam or spray foam earlier mentioned the fluffy insulation.
I am a homeowner in Seattle. Possess a freshly-framed, vented cathedral ceiling to insulate. Straight bays, 2x12, 24" o/c. The current system is to utilize batts of some sort from the bays, get treatment to air seal the interior surface, stay clear of penetrations, air seal the vent channel way too (bays totally sealed), double primary The within area for vapor resistance. I've a pair questions about baffling the recommended you read vent channel: 1. Several of the developing industry experts I am working with experience the jury is still out on baffles and therefore are recommending leaving the batts open to your channel apart from close to the vents, assuming the one-two" air channel can be maintained.