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How to add insulation to the inside of a flat roof?

zrec | Posted in Energy Efficiency and Durability on

I have a 1948 building in South Central PA with a flat, rubber membrane roof. There is a no attic space. The roof sheathing is directly on top of the joists. The joists are 2×12 with existing fiberglass batts installed between them. I am going to be lathing out the ceilings and hanging new drywall to cover up existing plaster damage and provide easy access for electrical contractors to rewriting the unit. I want to add some insulation while we are doing this work. I thought adding 1/2″ of rigid foam board directly to the plaster, then 2×4 lathing, then 1/2″ drywall would be the easiest method. However, I’m worried about creating a condensation issue between the rigid foam board and rubber membrane. What is the appropriate way to handle this situation? Can I/Should I add the rigid Polyisostyrene? This is a rental property. I don’t want to sink a ton of money into it. I simply want to help out the tenant’s on their energy bills, but not create problems for myself down the road.

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  1. GBA Editor
    Martin Holladay | | #1

    The fiberglass batts are a mistake. The right way to proceed is to remove the fiberglass batts and put them in a dumpster, and to install spray foam insulation on the underside of the roof sheathing.

    An even better approach would be to install a thick layer of rigid foam above the roof sheathing the next time that you re-roof the building.

    Here is a link to an article that explains everything you need to know: Insulating Low-Slope Residential Roofs.

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