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Community and Q&A

Insulating ceiling under a deck

MeriMeri | Posted in Energy Efficiency and Durability on

Climate Zone 5

We have opened up the ceiling of a small section of a kitchen that is under a roof deck with membrane roofing. This is in a 1910 house that has NO insulation in the walls. There is currently no insulation in this 2×8 framed ceiling cavity. There are recessed down-lights. (The main roof of the house is receiving new R60 blown in)

Back to the little section of ceiling under the deck.

The insulation subcontractor wants to install 2″ closed cell spray foam and 5 1/4″ BIBS. I worry about the double vapor barrier around the deck sub floor.

The other option is to install only BIBS and pay close attention to the interior vapor barrier sheet. My worry is that with R30 fiber the dewpoint will occur on the sheathing and there could be dripping into the insulation that would stain around the lights

From a vapor standpoint it’s obviously best to keep the sheathing warm. I could also propose R30 spray foam but I doubt this will pass due to cost.


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  1. MeriMeri | | #1

    Another idea is to suggest that he redo the membrane roofing and add a ventilation cavity above the existing deck subfloor.

  2. GBA Editor
    Martin Holladay | | #2

    In Climate Zone 5, an unvented roof assembly that combines spray foam with fluffy insulation (as your insulation contractor suggests) requires a spray foam layer with a minimum R-value of R-20. So 2 inches of closed-cell spray foam (R-12 to R-13) isn't enough -- you really need at least 3 inches of closed-cell spray foam.

    For more information, see these articles:

    Insulating Low-Slope Residential Roofs

    How to Build an Insulated Cathedral Ceiling

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