Anyone tried rigid foam over the rafters?
colmroge
| Posted in Energy, Heating & Insulation on
Hello all,
I came across this system as explained in this youtube video:
https://www.youtube.com/watch?v=6F_IkZJ8NOY
They have basically fastened foil faced rigid foam to the rafters on the inside of the attic. The foam was installed to preserve the soffit and ridge vents, essentially making each bay behind the insulation an air chase from eave to ridge.
Anyone have any experience with this method?
Replies
The problem with this method is that foam is extraordinarily flammable. You can't just leave it uncovered like that in most jurisdictions - it normally needs to be covered with drywall.
You can put the foam board between the rafters, with an air gap that lets the soffit to ridge vent still work, or you can put the foam on top of the deck, with sleepers and a new deck OVER that layer.
Covered foam
I believe the requirement to cover foam is for habitable spaces only. It is acceptable to use it between rafter bays and as avent chute and left exposed in and unfinished attic, so why not just nailed to the bottom of the rafters?
I certainly wouldn't do this in a brush fire area. Sparks drawn into the eave vents would get the foam going in a perfect chimney (or possibly blast furnace) configuration.
That is a stretch.
Foam vent packs are sold at every big box store and lumber store around here. They are used as insulation baffels and provide an air channel from the soffit vents to the roof or ridge vent. I've got them in two building that have passed inspections. The ceilings below the attic provide the nessecery cover. The only differance between them and ridge foam is thickness.
As far as a brush fire,... well there is an ax in the the shed and if little Johnny get it,......
Out west grass/brush fires set homes on fire while still a quarter mile away, due to flaming embers being drawn into the attic.
The1/8" thick foam in those
The1/8" thick foam in those chutes provides considerably less fuel that 4" of it covering the entire inside of the attic! It's not just the combustion, but the toxic gasses they make while they burn. If they are part of a ventilation pathway and it exits out a ridge vent, that's fine. Not so fine if it is inside the conditioned space envelope, which if you are putting it on the underside of the rafters, you are making the space under that into conditioned space.
Hot roofs
Have you ever seen a hot roof installation?
No ventilation and the underside of the sheating and rafter are sprayed with foam. The joist spaces are then blown with cells of f/g. The whole shebang is isolated from the habitable space by the dw ceiling beneath the joist. It is not part of the conditioned space. It meets code requirements.
What's the difference between 3-4" of foam sprayed in that applications and fastening the same amount of foam in rigid sheets to the bottom of the rafters ?
What's the difference between 3-4" of foam sprayed in that applications and fastening the same amount of foam in rigid sheets to the bottom of the rafters ?
1/2" of drywall and a chimney effect.
Huh?
The drywall is there in both cases so that is not an issue. The chimney effect is desirable in a vented roof.
Thus far I have found nothing in my copy of the IRC that make this type of installation non compliant.
What is in you copy ? Or you just shooting from the hip to be argumentive ?
The chimney effect is what turns a minor fire into a blast furnace.
I'm actually doing a modified hot roof installation now. Instead of having foam directly on the underside of the deck, I've spaced it back 1" for air flow to ventalate the underside of the deck, and to provide an additional layer of protection in case of water penetration (can flow down the air channel to outside the top plate). All the cellulose insulation is in the 2x10 rafter bays, giving me an R30 roof.
The case you mention - a flash and fill installation - actually covers the foam with a non- ignition covering. That's key. The drywall doesn't protect the foam, the fiberglass does.
Foam is normally covered in a hot roof installation as it is normally for a cathedral ceiling. There may be jurisdictions out there that allow uncovered foam inside the envelope, but that doesn't make it a good nor safe practice.
An unventilated attic inside a ventilated roof. What could go wrong? Water, air, vapor, heat. Water from a roof leak cannot dry to the inside; it must dry to the ouside via a vented rafter bay. So, too, with vapor in the rafter bay. On a typically humid Cleveland summer day, warm, moist air is drawn in at the eaves, moves through the rafter bay toward the ridge vent, passes along the outside surface of the rigid insulation which is kept warm (not cool) by the rigid insulation which prevents the air-conditioned attic from turning that outside surface into a condensing surface. Maybe.