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Philadelphia exterior roof insulation with uninsulated brick walls?

ndrwstn | Posted in Energy Efficiency and Durability on

I am trying to determine how to best insulate the roof of an existing 1914 two-story house (former warehouse) in Philadelphia. The building is brick masonry with a flat roof pitched to a center drain. The building covers 100% of the 17′ x 76′ lot, faces south on the narrow end, and is bounded by an alley on the east and north walls while being attached along virtually the entire west wall to the adjoining row home. The interior and exterior of the brick was stuccoed, and an interior stud wall was insulated with fiberglass that likely wasn’t doing much (on account of water damage and a poor install). We have exposed the interior brick walls and plan to leave them exposed and uninsulated.

We need a new roof and are planning to have the roofing company tear off the existing in preparation for laying down a rigid foam insulation under 3/4″ ply with a new roof on that. We are not planning for any interior insulation in the rafters (the center-to-center is slightly inconsistent and is between 18-20″ wide and 2x4s are attached cross-wise to allow for drywall). The roofers (who we like) are unfamiliar with doing anything greater than 3″ polyiso (for around R-18.5?).

The Internet (this site included) seems to indicate that for Philadelphia, I should be targeting R-38(+?) for my roof insulation if this were new construction, and I feel as if I should be targeting R-30 minimum according to the conventional wisdom. However, with brick walls (with neither interior nor the possibility of exterior insulation) I am trying to determine the point of diminishing returns.

Surprisingly, we do quite well in the winter, as we have a lot of solar gain in the morning through most of the day, and have been able to keep the temperatures inside mostly comfortable with minimal heating. The summers are nearly unbearable, as that same solar gain required two air conditioners (12000 and 6000 BTU window units) running almost continuously to keep us comfortable.

Having said all of that, my primary question is: how much insulation should I target for the roof? Would it make sense to have them lay down a double-layer of 3″ polyiso or just put one and realize that with uninsulated brick walls that I’ve effectively chosen to pay more for heating/cooling?

Thanks very much and sorry for all of the possibly extraneous information.

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  1. STEPHEN SHEEHY | | #1

    I'd put down the double layer. I'd also look into reclaimed insulation.

    Someday you may decide to insulate the walls. This is the best opportunity to do a good job on the ceiling. The additional cost for 6" is mostly material expense and it isn't a huge roof. It should help with both heating and cooling.

    A white roof might help reduce the cooling load. I'm assuming the summer heat gain is mostly through the roof.

    Daughter just bought a condo in Fishtown. We love Philadelphia.

  2. GBA Editor
    Martin Holladay | | #2

    Like Stephen, I suggest that you install 6 inches of rigid foam, not 3 inches. It's important that you find a roofer who has experience with commercial roofs, and who is comfortable with the details required for a roof with 6 inches of rigid foam.

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