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Community and Q&A

Dense packing a flat roof

Beach13 | Posted in GBA Pro Help on

Hi Recently I’ve been wanted to upgrade my insulation as my house was built in 1975 and I’m sure the insulation is not pulling its weight.. I got an estimate from an insulation installer who said that they can take of the fascia board vent screens and put in and blow in cellulose and pack it in in the flat part of the roof. I asked him about moisture and vapor issue and he said that it wouldn’t be an issue. Although I am a bit hesitant on this as I’m sure it will be an issue down the road. I was wondering if some of the experts can lead more info on this situation. I live in Southern California, region 3

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  1. GBA Editor
    Martin Holladay | | #1

    Beach 13,
    The insulation method suggested by your insulation contractor is a violation of most building codes. (Cellulose is defined as an "air-permeable insulation," a type of insulation that can't be installed between rafters unless the builder provides a ventilation space between the top of the insulation layer and the underside of the roof sheathing.)

    While it's true that moisture accumulation problems are worse in very cold climates than mild climates, I still don't recommend the approach you are describing.

    For more information on this issue, see Insulating Low-Slope Residential Roofs.

  2. charlie_sullivan | | #2

    Martin's answer is a good conservative one. But in Southern California, it is less risky than in somewhere that gets colder. How risky it is depends on the specific of your local climate, which we could comment on better if we knew your location more specifically, e.g. a zip code, and also depends on how air-tight your ceiling plane is. Unfortunately, a house built in 1975 is unlikely to have good air tightness ... and it sounds like it would be hard to get access to all the places you'd want to do air sealing.

  3. user-2310254 | | #3

    If your home is close to needing a new roof (and the roof line is relatively simple), installing rigid foam above the sheathing would be the best solution.

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