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Community and Q&A

Insulating a Low-Slope Commercial Roof

KeeKeeK | Posted in Green Building Techniques on

We are installing TPO on a flat roof in Zone 5.  The building is old with a deteriorating parapet wall that will be repaired. There have been moisture problems, mold, etc. When we demoed part of the 5/8″ rocked ceiling, we found there were faced fiberglass batts that were holding water.  We are removing the sheathing when we install the TPO. I believe we need to remove the fiberglass batts. Should we install rigid foam before we install the new sheathing to help dissipate moisture? The plan is to then install a 3″ layer of rigid foam on the exterior of the sheathing. The contractor wants to install cellulose but I’m not sure if that will just add to the moisture issue and we should go with rigid to help dry out the roof.

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  1. Expert Member
    ARMANDO COBO | | #1

    1. Fix the structural components and take care of moisture problems first.
    2. Air and moisture seal, seal, and seal.
    3. Install all or most of the insulation on top of the roof decking. If any insulation is to be installed under the roof decking, it depends on roof framing and depth of framing members: a) If you have dimensional lumber or TJIs, I would use dense packed cellulose, but any permeable insulation will work. b) If you have wood trusses, I would install ocSPF against the rood decking.
    Most large commercial roof decking are steel deck with steel trusses. Lots of old buildings and small commercial buildings have wood frame. You can Google for details of the type of building you have and get some good ideas.

  2. KeeKeeK | | #2

    Thank you, Armando. The plan is to make sure to seal, seal and seal some more!

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