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Community and Q&A

Fixing Unvented Roof

Mmmtin | Posted in Energy Efficiency and Durability on

Hello All,

I have a 7 year old flat roof home in zone 6 that was designed with an unvented roof. Only insulation under the TPO membrane and sheathing is 6 inch batt insulation right above the drywall ceiling, and starting to see “ghosting” on our ceiling. Considering multiple approaches to remediate the issue, would thick, 7.5 inch poly iso boards installed above existing sheathing and roof address my issue? This is probably the least intrusive approach without ripping apart all of our ceiling. Thinking we’ll probably have to replace the moist air trapped in the 3ft “attic” space after this is done. 

Thank you!

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  1. Expert Member
    Akos | | #1

    The problem is the gap between the fluffy and the rigid insulation. That 3' space is an in-between space that is neither indoors or outdoors and can build up moisture over time. The proper solution is to move the batt insulation directly under the roof deck.

    There is no easy fix for this. You can try and vent the existing assembly and see how that holds up. I'm in the north edge of zone 5 and vented flat roofs are pretty common and mostly hold up.

    1. Mmmtin | | #2

      Thank you for the reassurance! Good to hear vented flat roofs do well since they’re uncommon here but was also a proposed option.

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