GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Open-cell foam sprayed on the underside of roof sheathing

BobHr | Posted in Energy Efficiency and Durability on

I was at a house recently. It was a 2-story built around 1920, so it probably had 2 x 4 rafters. In speaking with the owner he said that he open-cell foam sprayed on the under side of the roof deck. I asked if I could see it.

What I found was that the open-cell foam was left uncovered, with no fire barrier. I do not know how thick it was, but doubt it was much over 4.inches. I am in Nebraska to give you an idea of the climate.

He was told open-cell was used as it will allow any roof leaks to show up.

The electrical has been replaced — expect for knob-and-tube that goes to the attic light. The wiring was mostly under the open-cell.

The existing insulation was left in place. It was vermiculite with fiberglass batts over some but not all. It was a walk-up attic with planks in the middle.

I advised the owner that the foam needs a fire/flame barrier and that the foam should not have been installed over knob-and-tube. I also told them that the vermiculite may contain asbestos and to have it tested before disturbing it.

To my questions:

1. What is going to happen to the insulation when the shingles need to be replaced?

2. Should the existing insulation have been left in the attic or should it be removed? If removed, why?

3. What is your opinion on using spray foam on the underside of the roof without providing a more durable and weather resistant roof?

4. How much of a fire hazard is the wiring for a single light?

5. For this house, what is your recommendation for bringing the attic into the conditioned space? Or would you have left the thermal and air barrier at the attic floor?


  1. GBA Editor
    Martin Holladay | | #1

    1. Nothing. Re-roofing won't disturb the foam. If the existing roof is stripped, there will be nail-holes penetrating the spray foam, but these won't cause any problems.

    2. I would probably have removed it, but there really is no need to. The fact that there is vermiculite that may contain asbestos obviously increases the cost of disposal. Leaving the insulation in place will lower the homeowner's energy bills compared to removing it.

    3. The type of roofing is irrelevant.

    4. A big enough hazard to be concerned. Knob-and-tube wiring should never be buried in insulation -- even cellulose insulation. Clearly, the insulation contractor was poorly trained.

    5. See answer to 2. above.

    Two additional points: The insulation should be covered with a vapor retarder (to prevent problems due to diffusion of moisture from the interior to the cold roof sheathing) and a thermal barrier (for example, 1/2-in. drywall).

  2. tpbPhMwNHx | | #2

    1. These should be exclusive and properly applied insulation shouldn't pose an issue for replacing the shingles.

    2. Removal of existing materials is dependant on lots of things, including the type of existing insulation, the type of spray foam insulation being added, and budget. If the builder is on a budget, and the existing insulation is modern, with compatible spray foam being added, then no.

    3. This seems like some short-cutting to me. A rush job perhaps? I'd be tempted to leave it until there's a patch of good weather coming up, tear out anything sub-par, and do the insulation/roof at the same time. Both components are crucial to a house.

    4. Done properly, an attic light should not be a high safety/fire risk., but 'done properly' is the key :)

    Most of these things come down to what you can afford, so try to think ahead and do it right if you can.

    Hope that helps!

  3. Riversong | | #3

    Open-cell foam under a roof deck may or may not show a roof leak. Open-cell, because if its matrix structure, can hold water like a sponge. So, rather than revealing a leak, it can hold moisture against a roof deck long enough to allow it to mold or rot.

    Also, open-cell foam is not a vapor barrier so, left unenclosed or unsealed, it will allow water vapor to diffuse through to the cold roof deck where it will condense in and saturate the deck, leading the the same condition described above with a roof leak. If sealed with vapor retarder paint directly on the foam, then it no longer allows sufficient downward drying and the roof above it should be vented.

    Foaming the roof decking does not necessarily move the thermal/air barrier to the roof unless all penetrations and joints, including any at the eaves where roof meets wall, are sealed. But open-cell still requires a vapor barrier and fire barrier.

    As per Martin, knob-and-tube wiring is designed for exposed locations only. There is a fire hazard and electrical code violation if it is encased in insulation.

    It would have been best to remove the existing attic floor insulation. The fiberglass is almost certainly filled with mouse leavings, and the vermiculite will present an ongoing hazmat issue for any future renovations, including simply installing an additional ceiling light. While it might require a certified asbestos mitigator to do the removal, once done is presents no further hazard or additional cost. If it's desired to maintain the insulative value of the attic floor, then blown cellulose would be a far better option, because it is fire, insect, rodent and convection resistant.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |