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Community and Q&A

Vented flat roof

tina81 | Posted in GBA Pro Help on

I’m trying to find the best way to insulate my vented flat roof. I’m in Wisconsin, zone 6b. I’ve demoed my living room down to the studs. I’m planning to replace the roof in the next 5 or so years, with a non vented roof with insulation on the top of the roof decking. However, in the mean time, I’m trying to come up with a cost effective solution.

I have a modified bituman membrane and shiplap roof decking. The membrane is in good condition. I foolishly coated the roof with a elastomeric sealant before I knew this was a bad idea. The roof joists are 2×6. The roof has sofit vents around the entire perimeter of the house, but no gable vents in the center.

My plan is to keep the roof vented and ether install roxul R-15 or dense pack cellulose with a vent baffle in the roof joists and a smart vapor retarder over drywall, keeping a vent gap of 1-2″. I know this is not an ideal solution but I’m trying to increase insulation in the living room until I have funds to replace the roof.

1) Would this situation create more issues with my roof assembly, is there a better alternative?

2) Is it always necessary to have an air sealed baffle in a vented roof?

3) Would making these changes in only one room of the house be a problem? I plan to remodel the other rooms over time. The other areas of the roof have plaster ceilings with about 1 inch of rock wool insulation in the roof joists.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Betina,
    It sounds like you are working from the inside, and that there is no insulation in your ceiling joists and rafters at the moment. Is that correct?

    It also sounds as if you can see your roof sheathing from inside your house. Is that correct?

    You haven't described your roof assembly. Do you have ceiling joists, and then a tiny cramped attic, and then rafters? If so, how many vertical inches do you have between the top of your ceiling and the underside of your roof sheathing?

    Or do you simply have rafters, with the finish ceiling attached to the underside of your rafters? How deep are your framing members (rafters or rafters and joists)?

    If you plan to add rigid foam on the exterior side of your roof sheathing in the future, then you don't want an air space between the top of your mineral wool insulation and the underside of your roof sheathing. If you need to insulate this roof assembly from the interior, the safest way to proceed is to create an unvented roof assembly using closed-cell spray foam.

    For more information on this type of roof, see Insulating Low-Slope Residential Roofs.

  2. tina81 | | #2

    Martin,

    I'm working from the inside of the house and there's currently no insulation in the wall. I can see the roof sheathing. This is only in one room in my house, the rest is not accessible right now and has insulation in the rafter space.

    I have no attic space in the house, just rafters which are 2x6 with plaster walls directly underneath.

    I can only see the roof sheathing in the living room of my house, so I'm not sure what condition the rest of the sheathing is in. Some of the sheathing around a chimney may need to be replaced because of a leaky chimney cap. To do the spray foam does the roof sheathing need to be in good shape? I'm assuming if I go with the closed-cell spray foam, I should insulate the underside of the whole roof assembly at the same time? Would it be possible to install the spray foam in the rest of the house without removing the ceiling?

    Thanks for your input!

  3. GBA Editor
    Martin Holladay | | #3

    Betina,
    Q. "Some of the sheathing around a chimney may need to be replaced because of a leaky chimney cap. To do the spray foam, does the roof sheathing need to be in good shape?"

    A. If some of the roof sheathing is soft, spongy, or rotten, it needs to be replaced before installing any insulation. (It's also important to fix the defective roof flashing.) This is true regardless of what type of insulation you decide to install -- spray foam, fiberglass, mineral wool, or cellulose.

    Q. "I'm assuming if I go with the closed-cell spray foam, I should insulate the underside of the whole roof assembly at the same time?"

    A. That would be best, but it isn't strictly necessary. If you are only remodeling one room at a time, it's possible to insulate the roof of just one room.

    Q. "Would it be possible to install the spray foam in the rest of the house without removing the ceiling?"

    A. No.

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