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Community and Q&A

Perlite in wall cavity recommendations.

Khkiley | Posted in GBA Pro Help on

I have a 2×4 wall cavity that is only about 5″ wide. The cavity contains a steam pipe which is uninsulated. The cavity is obscured by another wall that makes access to it very difficult.

I would like to fill the cavity with perlite to insulate the steam pipe. I can close up the wall to prevent the perlite from leaking out, and have access to the top.

The house was built in 1928. From the outside: brick, air gap, roofing felt, 3/4″ tongue and groove.

Are the any drawbacks to loose file perlite in this configuration? Is perlite for insulation treated differently than perlite for gardens which is readily available at the HD?

Thanks in advance for your assistance.


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  1. GBA Editor
    Martin Holladay | | #1

    Kurt Kiley,
    As far as I know, perlite performs well at high temperatures and should work in this application. I see no reason why you couldn't buy your perlite from a garden supply store.

  2. Expert Member
    Dana Dorsett | | #2

    While there will be some benefit from insulating that steam pipe, it's still going to be a tragically low efficiency way to heat your house. Even with all the steam plumbing fully inside the building envelope with a brand new 82% AFUE steam boiler and all new tuned up venting the system would struggle to hit 75% as-used system efficiency.

    With steam distribution lines between floors running in the exterior wall cavities, even after insulating you're probably not going to make 65% system-efficiency even with a brand new boiler and insulated distribution plumbing.

    If cash grew on the trees out back you could yard the whole heating system out and start over, but there are many other things of lesser investment that still have reasonable short-term return on investment. A perlite cavity-fill falls into that category, but thinking about & planning for the big picture/longer-term is still worthwhile.

  3. Khkiley | | #3

    Thank you very much for the information Martin.

    Dana, the perlite filled cavity is happening because it's an opportunity that presented itself while the wall was open for a bathroom renovation.

    The steam system will probably still be in the home long after I'm gone. It works well, low maintenance, and it's very high on the comfort scale. There are some kinks to work out, (venting, and some radiators need to be pitched) but it works well. Insulating and air sealing the old house are some of the projects on the horizon.

    Thanks again,


  4. Expert Member
    Dana Dorsett | | #4

    If you have the wall wide open you'll get substantially higher-R and better air-retardency by sculping rock wool batts to fit!

    Perlite would make sense if the wall was closed and you had to pour it in. Seize the opportunity to hit R4.2/inch with rock wool instead of R2.5-R3/inch with perlite. The odds are you'll be able to put R6 or better between the pipe and the great outdoors if you use rock wool, but probably less than R5 with perlite.

  5. Khkiley | | #5

    Dana, I would be all over the rockwool if I could, another wall Tees into it, and the cavity is only about 5inches wide with about an inch or less exposed on one side. Without taking down a wall the poured perlite is about the best I can do. I might be able to wedge some in from the side and then use the perlite to fill what I can't reach.


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