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Is there an inexpensive approach to insulate a “no attic” roof?

MarkInMD | Posted in Energy Efficiency and Durability on

Hello, all. My home is in Maryland, and was built in 1963. The style is “mid-century modern”, or what we used to call a “split level”. The main living area has a rolled roof (which is light grey in color to reflect the sun) with very little slope; that roof rests directly on sheathing that is in turn nailed to the ceiling rafters. There’s nothing even vaguely resembling an attic. The rafters are, I think, 2×8, and they span the living room and kitchen area – a distance of about 25 feet, with a width of around 30 feet.

The ceiling is relatively high – 13 feet at one end of the room, down to 10 feet at the other end – and that directly reflects the slope of the roofline.

There is no insulation in that ceiling, and in the summer, that room heats up and forces the A/C to run constantly in what seems like a losing battle to cool that space. When I called a contractor to inquire about having insulation blown or sprayed in, he quoted me a price of about $5000 (for blown cellulose insulation only, holes in the drywall plugged, but no finishing work)

In thinking about this, I’m wondering how crazy it would be to simply drill a series of small (1/4″ or so) holes every 18 inches or so on a line between the rafters and spray expanding foam (maybe the fire-stop variety, or the extra-expanding flavor) through those holes. My goal is to get some R-value in there without having to rip my ceiling to shreds. I know it’s not ideal, but it would seem efficient and at least marginally effective.

To me it seems crazy that there isn’t a long, thin tube that could be threaded through a hole at one end of the ceiling and then pulled out, spraying a layer of insulating foam behind it. I’ve been told that no such thing exists. I get that it would be a specialty market, but I don’t see a reason why it couldn’t be done relatively easily with existing equipment. Is there something I’m missing? Many thanks in advance your collective wisdom.

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Replies

  1. user-2310254 | | #1

    Mark,

    Your best option is to install insulation on top of the sheathing. See this article for details: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

  2. GBA Editor
    Martin Holladay | | #2

    Mark,
    There are several issues here. First, you may wish to read this article: Insulating Low-Slope Residential Roofs.

    I don't recommend that you install an air-permeable insulation like cellulose or fiberglass, unless you are able also (simultaneously) to install an adequately thick layer of rigid foam on the exterior side of your roof sheathing.

    The method suggested by the cellulose contractor violates the building code.

    It's possible to insulate joist bays or rafter bays with just a few holes -- as long as you are blowing cellulose (which you shouldn't do unless you are also installing rigid foam and new roofing). If you want to install spray foam, you need lots of big holes -- or better yet, you need to demolish the drywall or plaster ceiling for full access.

    There exists a product called "injection foam" that is designed for installation in closed cavities -- but the product is controversial. It's tricky to install, and often results in drywall blow-outs. Many contractors who used to offer the product no longer do because of installation disasters.

  3. MarkInMD | | #3

    Thank you both very much, gentlemen. I greatly appreciate you sharing your knowledge. These were both most helpful answers.

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