Is a thick, unvented hot roof a good candidate for spray foam?

Question

Upon removing some drywall from an unvented hot roof I noticed two serious issues:

1) The roof sheathing is not a taped or sealed ply but a 2x8 T&G
2) There is no vapor barrier and the cavity is discolored and moist

This particular hot roof is framed with 4x10s, so the cavity is pretty deep. I have decided that I need to pull all the sheetrock and address it. I cannot add a vented attic space here so I'm thinking that I will have closed-cell foam sprayed in.

The question is whether or not this is an appropriate solution and how much foam should I use?

The cavity is 9.5 inches deep... I don't need that much foam but if I spray less do I need to fill the space? With what?

Is there a better solution than spray foam for this?

Answer 1

Polyurethane foam is an excellent insulator, and it needs no added vapor barrier. Depending on your climate and local building authority requirements, you will need anywhere from 4 to 7 inches of foam. The remaining airspace does not need to be filled. It will help prevent your new ceiling from getting wet from inevitable condensation. Alternately, you could install baffles against which to spray the foam, creating an air gap below the roof sheathing and creating a sort of cold roof, if that has any benefit in your climate.

The foam is such a good vapor barrier, you will need a plan to remove excess moisture from your house. Frequent use of bath and kitchen fans may be adequate, depending on climate and remaining construction. You might consider whole house ventilation through an air to air heat exchanger or similar.

If you are removing the ceiling, you can pretty much use any type of insulation that makes economic sense in your area, but in very cold climates, 9.5 in of fiberglass is not really adequate. In warmer climates, you could avoid stripping the ceiling off by blowing in fiberglass through holes in the ceiling, then patching or covering the holes. You can also apply foam through holes, but you may have to fill the whole cavity, which would be expensive overkill.

I don't think it's possible to accurately place foam through holes such that the cavity is not totally filled, but it might be worth questioning an experienced applicator. If you blow fiberglass, then you will need to add a vapor barrier somehow. Some heavy solids paints meet the vapor barrier spec of 1 perm, so could be considered to be a leaky but adequate barrier. Or you could add a thin layer of foil backed sheetrock. Or just sandwich polyethylene sheeting between layers.

Without knowing your climate or local prices, I couldn't say what the best option might be, but I hope I've given you enough ideas so you can decide on your own. Cheers.

Answer 2

You need to make the foam your vapor barrier, which means using closed-cell foam. Secondly, you need sufficient foam in order to prevent condensation inside the wall. Thirdly, you do not need to fill in the space, but you need to provide enough insulation to bring the total to the required minimum for insulating ceilings. Lastly, foam is nasty when it burns, so you'll need to put an ignition barrier (such as cellulose, hardboard, or drywall) directly in contact with the foam.

The required minimum amount of foam for the various climate zones of the United States/Canada are:

• Zones 1-3 — R-5
• Zone 4C — R-10
• Zones 4A, 4B — R-15
• Zone 5 — R-20
• Zone 6 — R-25
• Zone 7 — R-30
• Zone 8 — R-35

Since you are in zone 4 or 5, you'll need a minimum of R-20, which is about five inches of foam.

I would fill the ceiling to slightly more than this minimum with foam, install drywall, and then fill the gap with blown cellulose insulation. You would need to remove all of the drywall.

You could also add ventilation to this knee-wall/cathedral ceiling by using mid-roof vents such as The Edge Vent or The Smart Vent installing a baffle to prevent insulation from blocking the air flow, and filling the walls with blown insulation or faced batts (with the paper against the drywall). This would likely be much cheaper than foam insulation, but not insulate as wall.

Insects (such as ants and termites) can tunnel through foam insulation.