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Does applying spray foam require removing sheetrock from rafters?

We are in climate zone 3 warm-humid coastal NC and are renovating our house, including air sealing and improving the roof insulation. We have a 1.5 story house with a 14:12 pitch vented roof structure and about R-19 roof insulation in 2x8 rafters. It has knee-walls, the floor joists are open at the rafter-joist-wall intersection and we have a mini-attic insulated to ~ R-30.

Spray foam has been suggested as a way to improve the insulation and air-seal, bringing the attic and knee-wall areas inside the envelope.

I'm not asking whether this is the best approach. I've read enough articles here (in particular, the 2011 article http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulat... ) to see that there are multiple approaches. I think I have a handle on the general pros and cons of fluffy insulation and rigid foam board. I don't think I understand spray foam as well.

Would the existing ceiling sheetrock and insulation need to be removed to apply spray foam?

What is your estimate of the cost of closed-cell foam per board- or cubic- foot installed?

What else should I know about closed-cell and open-cell foams used for roof insulation?

Thanks very much.

Asked by Bennett G.
Posted Jul 11, 2018 11:03 AM ET

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5 Answers

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1.

Bennett,
Q. "Would the existing ceiling sheetrock and insulation need to be removed to apply spray foam?"

A. It depends on where the spray foam is being installed. If the spray foam is being installed in the triangular attic behind the kneewall, the important issues are access -- is there a door? -- and how big the space is -- is it big enough for the spray foam contractor to maneuver and deploy the wand?

If the spray foam is being installed in the attic above the horizontal ceiling, the same rules apply. Is there an access hatch? Is there enough room in the attic to maneuver?

If the spray foam is being installed in the roof assembly that forms a sloped ceiling, the contractor will have to remove the drywall ceiling, or work from above after removing the roof sheathing.

Q. "What is your estimate of the cost of closed-cell foam per board- or cubic- foot installed?"

A. Prices vary widely from region to region. Call up a local contractor for an estimate.

Q. "What else should I know about closed-cell and open-cell foams used for roof insulation?"

A. You might want to read these two articles:

Open-Cell Spray Foam and Damp Roof Sheathing

High Humidity in Unvented Conditioned Attics

Answered by Martin Holladay
Posted Jul 11, 2018 11:18 AM ET

2.

Thanks for your note, Martin. With the steep roof pitch, I think there would be room in the attic and possibly behind the knee walls, but there is stil a fair bit of cathedral ceiling that would have to be demolished. Thanks for the links. I had forgotten about the potential moisture issues.

I was hoping for a ballpark cost vs going with some combination of fluffy & rigid foam, which I think I have a better handle on.

Answered by Bennett G.
Posted Jul 12, 2018 7:10 AM ET

3.

Bennett,
If you are willing to consider installing rigid foam above the existing roof sheathing, followed by a second layer of roof sheathing and new roofing, that is by far the best solution. For more information, see "How to Install Rigid Foam On Top of Roof Sheathing."

Answered by Martin Holladay
Posted Jul 12, 2018 7:21 AM ET

4.

We are re-roofing, so that's not an obstacle. Rigid foam above the roof deck does seem like a good solution, particularly if I can find reclaimed foam. I think 6" of polyiso would get us to about the desired R-38. Attaching the roof decking really well is really important here, given the hurricanes. We're in a 130mph zone. The one issue that seems to stand out with this approach is the difficulty in hitting the rafters with ~ 9" long screws.

Is there a known technique that a framing or roofing crew can use to assure the screws hit the mark?

I'm also considering a hybrid approach with maybe 1-2" of polyiso above and dense-packing fiberglass or cellulose into the air gap above the existing insulation in the rafter bays. This approach would use much shorter screws above the deck, but require more fussy work to add netting/osb/rigid foam below the rafter bays behind the kneewalls and in the attic to support the insulation going into the rafter bays there.

Both approaches avoid ripping down the drywall and limit insulation removal to the kneewall areas and attic, which is a plus, but do require the detail work of blocking and sealing the ventilation at the eaves and insulating the attic gables.

Thanks again, Martin. This discussion is going to help me work with my architect and builder.

Answered by Bennett G.
Posted Jul 12, 2018 11:21 AM ET

5.

Bennett,
Q. "Is there a known technique that a framing or roofing crew can use to assure the screws hit the mark?"

A. I would mark the rafters on the old roofing with a chalk line. (Using trial and error, you can find rafters by driving nails through the roof and feeling whether the nail hits air or solid wood. Rafters are usually either 16 inches on center or 24 inches on center.)

Transfer the marks to 20-penny nails at the ridge and eaves. Once the rigid foam is installed, transfer the rafter marks to the rigid foam, using the nails as a guide, and chalk the new lines.

Any error can be corrected by trying a new approach, either slightly to the right or slightly to the left, until you feel a rafter.

Answered by Martin Holladay
Posted Jul 12, 2018 11:30 AM ET

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