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Community and Q&A

How to deal with a void (cricket space) above an unvented flat roof?

Rachel Kim | Posted in GBA Pro Help on

Hi,
we’re in the process of building a new house and originally had a non vented roof assembly detailed for the flat roof portion of the house where we had a single ply membrane over sloped rigid insulation over a vapor barrier over roof sheathing, with batt insulation placed tight against underside of roof sheathing.

During the construction process, the decision to slope the flat roof (about 1/2″ per ft.) by using stripped 2xs instead of using sloped rigid insulation was made. After the single ply went up, I found out that there was no rigid insulation placed between the ripped 2xs, and the contractor has suggested changing the roof assembly to a vented assembly for this reason. One of his suggestions has been to provide a ~2″ air gap above batt insulation in the ceiling, and provide air circulation with air intake along the perimeter, and by drilling vent holes in the TJIs and also in the sheathing between the ceiling cavity and the cricket cavity formed by the ripped 2xs above to let air circulate in there also.

Long story short, I have a lot of doubts about the effectiveness of this solution and want to still keep the whole assembly as a non vented assembly. I have suggested using ~3″ of cc SPF directly to the underside of roof sheathing and along the perimeter and filling the rest with batt.

The question is how we should deal with the air cavity between the ripped 2x above. We’re in the San Francisco bay area. Does it make sense to drill holes in the roof sheathing and shooting cc SPF in that cavity before we do the work below? Or since the ~3″ of cc foam will be creating a air /moisture barrier below and the single ply membrane is creating a moisture barrier above, is it ok to leave the cavity as is?

I’m scratching my head as to how to solve this problem effectively, would love any advice on this topic! Thank you,

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Rachel,
    First of all, you are correct about the wisdom of trying to make this into a vented assembly. Your contractor's suggestion won't work. To learn why, see Insulating Low-Slope Residential Roofs.

    Your suggested solution (installing closed-cell spray foam on the underside of the roof sheathing) is better than your contractor's -- although not quite as good as a continuous layer of rigid foam above the roof sheathing would have been. (When you install rigid foam above the roof sheathing, you address thermal bridging through the rafters -- something that spray foam can't do from the underside.)

    I wouldn't worry about the air spaces between the 2x sleepers. The air spaces don't need to be vented.

  2. Rachel Kim | | #2

    Hi Martin,
    Thank you for your feedback, I really appreciate it!
    rachel

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