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Adding thermal mass to an old wood-frame house

Noel1 | Posted in Green Building Techniques on

Hi. I’m wondering if there’s a way to add thermal mass to the floors and internal walls of an old wood frame house without adding so much weight that it compromises the structure. Ideally, I’d like to add more low SHGC glass on the south side wall of a Southwest oriented bedroom, with sufficient overhang to block direct summer sun, but realize that doing so without adding sufficient thermal mass won’t result in good heat capture in the Winter.

We have a wooden subfloor over an unfinished basement. I’ve thought about adding tile or brick floors, but from what I’ve read, a thickness of 4 inches for thermal mass is recommended, and short of using full-size bricks, which would probably put too much strain on the structure, I don’t see how this can be accomplished. Any suggestions are welcome. If it’s something I can’t do, then it’s something I can’t do, but I thought I’d see what people had to say.

Some background on the house. It’s a 900 square foot, 110 year old wooden house, probably once railroad worker housing, about 45 miles North of Denver, Colorado. It has an unfinished basement with a stone foundation. The long side of the house faces East and West (just the wrong orientation for passive solar). The house gets too hot in the Summer and too cold in the Winter. In the Summer the temps can get into the 100’s. In the Winter the average temp is 29.

Please note that I plan to better insulate the attic and basement, whether I’m able to implement passive solar design or not. Thanks!

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  1. GBA Editor
    Martin Holladay | | #1

    Anything is possible. If you consult a structural engineer, I'm sure the engineer could design a beefed-up flooring system, or new beams and posts for your basement, to support almost any load you want, including a 4-inch-thick concrete floor.

    Here's the thing, though: the investment won't make any sense. There are far better ways to retrofit an old house for better energy performance. It makes no sense to spend hundreds (or thousands) of dollars on a new, thicker floor, since the energy payback of this improvement will be tiny or nil.

    It's better to spend your money elsewhere.

  2. Noel1 | | #2

    Thanks Martin. I guess that's kind of the conclusion I was coming to, but wanted to make sure. I'll concentrate on other improvements. Any suggestions on what I could do to minimize heat capture in summer and maximize in winter, aside from better insulation? We've got two large west facing windows in our living room that bring in too much heat in the summer time. Should I replace these with heat blocking windows, or would that minimize heat gain in the Winter too much (the sun shines nearly year round here)?

  3. GBA Editor
    Martin Holladay | | #3

    West-facing windows are problematic. If you can afford the cost of the work, you can build a porch on the west side of your house to shade your west windows, or you can replace your old west windows with new windows equipped with low-SHGC glazing.

    If you want more solar gain during the winter, add more glazing on the south side of your house.

  4. Expert Member
    Dana Dorsett | | #4

    Insulating & air sealing by far outweigh the benefits of more thermal mass. But along those lines, insulating the exterior walls with dense-packed cellulose (over 3/cubic foot) adds as much thermal mass as adding another half-inch of gypsum board- more than with most other insulation methods.

    Air sealing and insulating the rim joist and basement walls is usually on the order of a 15-20% reduction in space heating energy for most 1-story homes in US climate zone 5 (which includes Denver.) An inch or two of EPS held to the foundation wall with a non-structural studwall w/unfaced batts would make sense in your location. Foam seal the seams in the foam and the studs. A cut'n'cobble of 2-3" EPS into the rim joist & foundation sill similarly air-sealed would also be huge. (Foundation sill & band joist air leakage is usually well above all of the window & door leaks even in newer houses with foamy sill gaskets.)

    Getting a significant fraction of your space heating from enhanced passive solar & high thermal mass would normally require whole-wall R values (with thermal bridging of the framing factored in) of R30 or so. An example of an R30 wall would be a 2x4 wall insulated with cellulose in the stud bays with 2" of rigid foil faced iso on the exterior. Then with carefully tuned window U- & SHGC values for the orientation & shading-factor specifics you can reap a good fraction of space heating from the sun, provided you optimize the balance between thermal mass and solar gains.

    For a typical balloon framed 1900 stick-frame you won't get there with just windows & thermal mass alone, but it's possible to cut energy use dramatically with insulation & air sealing.

  5. Noel1 | | #5

    Martin and Dana, thank you very much for your advice. I will definitely look at replacing windows with appropriate SHGC glazing for West and South. I'll also look at air sealing and insulating the basement walls and rim joist. Those are two projects I can do during the Winter. In the Summer, I'll look at removing the siding and adding external insulation. You've been really helpful. Thanks again.

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