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Vapor and Air Control in Hot Roof

SamGU | Posted in General Questions on

I live in Zone 4.  I have a “Hot Roof” – Sloped ceiling.  (Soffits yes, but no exit ventilation is possible due to design).  Sheetrock and old insulation is down and I believe the key to reinstalling the insulation to prevent wet/moldy conditions is to ensure the cold roof and cool fiberglass insulated rafter cavities do not get exposed to the warm humid air from inside the house.  I plan to use a spray foam gasket along the top plates and other areas prior to installing sheetrock.  Will the faced R38 paper be adequate to provide moisture barrier between conditioned space and rafter cavities above?  Also, is there any type of tape that can be used to tape seams that need to be sealed up as well?  One last thing…since I have no exit ventiation should I also seal up the soffits to stop air from coming into the rafter cavities because there is no where for it to escape or leave then open to allow some air to come into the space?  Thanks, SG

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Replies

  1. user-2310254 | | #1

    Sam,

    Do you have a low slope (mono-pitch) roof? If so, this article should be helpful: https://www.greenbuildingadvisor.com/article/insulating-low-slope-residential-roofs.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Sam,

    There are basically two types of roofs: vented and un-vented. Roofs with ventilation channels work fine with batt insulation, but unfortunately, even with diligent air-sealing, un-vented ones usually don't.

    Your best bet is to choose between one of these five roof assemblies we know work, and use the one you think best suits your situation.
    https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work

    1. SamGU | | #3

      Malcolm, Thanks. Would this work.....Open the soffits to ensure I have good air flow coming into rafter cavities, install plastic baffles to allow a 1" air channel between underside of roof sheeting and Install faced insulation? The only thing I wouldn't have are exit vents. I've had 4 or 5 roofers here and all say I'm opening a can of worms by trying to add exit vents during a remodel. Effectively, cutting holes in the roof and installing Lomanco Omni vents of SmartVents is inviting a leak. Here are a couple of pictures as well.

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