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talusscree | Posted in Energy Efficiency and Durability on

We are currently making repairs to a structure that was remodeled extensively about 10 years ago in coastal Maryland. Their were several issues with the work, particularly surrounding the details of 2 decks over living spaces. We are replacing the decks and will have limited access to the ceiling from below where there is currently fiberglass batts. The ceiling/deck is framed with 2×8 lumber. There is only one light in the ceiling that we should be able to air seal. Could we fill above the batts with cellulose?

We also had to make repairs in the floor that has a crawl space. There is duct work present. Currently the floor is insulated with fiberglass batts(poorly fit). There is a network of drainpipes leading to a sump well, a mediocre vapor barrier, and a heavy duty crawl space dehumidifier. My first inclination is to encapsulate but wasn’t sure if the open sump well would have any negative effects. There is also the danger of insects tunneling through the foam if we sprayed up the walls and the rim area. In the past we have saturated the lower framing of the walls and rim with Boracare before foaming so as long as it remains dry it should deter any insects. If the client insists on keeping the current insulation, would fiberglass or mineral wool batts be better?

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  1. GBA Editor
    Martin Holladay | | #1

    The type of roof assembly you are talking about (the one under your decks) is called an unvented low-slope roof. There are two ways to insulate this type of roof: either with spray foam insulation installed on the underside of the roof sheathing, or with rigid foam insulation installed above the roof sheathing. Neither fiberglass batts nor cellulose can be used to insulate this type of roof assembly.

    For more information on this topic, see Insulating Low-Slope Residential Roofs.

    For information on addressing the crawl space, see this article: Building an Unvented Crawl Space.

    For information on insect problems in your crawl space, I suggest that you seek local advice. Insect problems and their solutions are intensely local phenomena.

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