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Community and Q&A

Insulation retrofit at folded roof

Scott Schaub | Posted in Energy Efficiency and Durability on

As part of a renovation of a mid century modern home at the border of zone 4A and 5A in central NJ I’m trying to figure out how to most practically add insulation to a portion of the house that has a folded (imagine a flattened out “W”) roof. The existing is an engineered assembly of 2×4 rafters between layers of 1/2″ CDX plywood with foil faced fiberglass batt insulation of an undetermined R value or condition. We re-roofed with 1/2″ recovery board and fully adhered .060 EPDM. Adding insulation on the exterior was not acceptable architecturally as it would have added thickness to the roof edge. The architect is, however, open to adding thickness at the interior/underside. How do people feel about adding 2″ of XPS like // to the underside. I have not confirmed that my drywall contractor is okay with installing through 1×3 furring at 24″ O.C. but that’s another question. Alternatively or additionally I have thought of cutting a band of the interior sheathing to remove the existing insulation in order to dense pack the cavity. Thanks for your thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    In climate zones 4 and 5, the 2006 IRC calls for a minimum of R-38 ceiling insulation. Assuming your decades-old fiberglass sandwich is worth about R-10 -- I'm being generous -- then you need at least R-28 of foam.

    If you like XPS, that means you need at least 6 inches of XPS. Polyiso is more friendly to our planet's atmosphere than XPS; if you choose polyiso, you'll need at least 4.5 inches.

    Forget the Wallmate, which has a low R-value. Tack up the polyiso, and then secure the polyiso with 1x4 furring strips, 16" o.c., attached to the old ceiling with long screws. Your drywall contractor will be happy with this assembly.

  2. Scott Schaub | | #2

    Martin- Thanks for your quick reply. I was concerned with foil faced polyiso being a vapor barrier and not allowing the roof to dry to the inside since the EPDM will not allow it to dry to the exterior (I should have noted that the roof is un-vented). Are there concerns about leaving the old foil faced insulation in the roof cavity? What do you think about the option of replacing the cavity insulation with dense pack cellulose? Am I sweating the vapor barrier question too much where I should be keeping in mind all the good info on GBA that reminds us that vapor through diffusion is minor compared to vapor through air movement? Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Foam-insulated roofs (for example, SIP roofs) don't allow vapor transmission and therefore don't need to be vented. The foam is a vapor retarder; your roof assembly should stay dry -- at least until you get a roof leak.

    Your 2x4 roof cavity is too shallow to be worth the trouble of figuring out a way to install dense-packed cellulose. A 3.5 inch cavity isn't much.

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