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9 Foot walls with 8 foot XPS

johnreid88 | Posted in Energy Efficiency and Durability on

Hopefully an easy question, but one that has been racking my brain. We are in Salt Lake City, Utah, zone 5B. 9 ft walls in my unfinished basement, poured concrete. Only about 1 foot is above ground level all around. I want to install 1″ or 2″ XPS on the wall, then stud wall in front of that with batt insulation. I can only find 8 ft tall XPS in any thickness. So far no one is willing to order in 9 ft.

Should I tape/seal the 1 foot section on the top or the bottom of the wall?

Will it make any difference in the end?

After the walls are done, I was going to have the rim joists spray foamed 2 inches, and then bat insulation.

Thanks! so far I can only see doing the top would save my back.

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  1. charlie_sullivan | | #1

    I don't see that it matters to do the top vs. the bottom.

    I'd look at the option of buying EPS foam. It's sometimes harder to find a source, but when you do, it's easier to get them to make custom sizes, since it's more locally produced by smaller operations. And the environmental impact and cost are both significantly lower.

  2. GBA Editor
    Martin Holladay | | #2

    You can install the 1-foot-wide strip at the top or the bottom of the wall -- it makes no difference as long as the installation is conscientious. (If for some reason the seam is sloppy, with a wide, ragged gap, I suppose it would be better to have the ragged gap near the floor rather than near the top of the wall, where the delta-T is greater.)

    Pay attention to airtightness, and seal the seam with canned spray foam or high-quality tape, and you will be fine.

  3. charlie_sullivan | | #3

    To extend Martin's speculation about which location would be a better place for a sloppy seam--for winter, the concrete behind the top seam is colder, and so more prone to condensation. But in the summer, if the basement air is allowed to get humid, the concrete behind the bottom location would be colder and more prone to condensation. So which is a bigger danger depends on what your basement humidities are summer and winter.

    But if you do it well there's no problem with either location.

  4. Dana1 | | #4

    The energy-use economics of that extra foot of XPS at the bottom of the foundation wall are somewhat academic if you are also insulating with a studwall.

    Unless you are near the water table or have unusually damp subsoil on your site for drainage reasons you can just skip the XPS at the bottom.

    Or, you could be even greener and use 2" EPS instead of XPS, which uses a much more benign blowing agent. Even the cheap Type-I EPS (R3.9/inch, or R7.7-7.8 @ 2" ) with facers sold at box stores would be fine in this application. At your average outdoor dew points and annual rainfall layers the 12" of above grade exposed foundation is ample drying protection for the foundation sill, even if there is no capillary break between the foundation sill and concrete, and even if the footing below the foundation wall is below the water table, so an impermeable facer on the foam presents no problem. The facers on that cheap Type-I EPS stick pretty well to housewrap tape, which makes sealing it simpler than with XPS too.

    Alternatively 1-1.5" of foil-faced polyiso works too, and is easily sealed for the long term with exterior grade HVAC aluminum duct tape (eg. Nashua 324a, also sold in box stores.)

    Both EPS & polyiso have a fraction of the environmental impact of XPS, largely due to the drastically lower greenhouse gas potential of the blowing agents used (pentane, at 7x CO2 instead of HFC134a at ~1400x CO2). Type-I EPS even has less polystyrene polymer than XPS at any given R-value too.

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