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Community and Q&A

Log walls and foil facing

Thomas Gardner | Posted in GBA Pro Help on


We are planing to add 3 inches of rigid foam to the outside of our 6 inch log home, giving the walls R 28, better then the R9 we get with just the pine logs. We will also have a ¾ rain screen under cedar siding. I have been reading that if you had a polyethylene vapor barrier in the house, to not use foam with aluminum-foil facing.

As we have logs and there is no poly vapor barrier, should I have aluminum-foil facing? Will the house dry to the inside through 6 inches of logs? Could I end up rotting the logs? We also plan to cover some of the outside walls interiors with plaster, to lighten the rooms. The home is in Vermont.

Tom g

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  1. GBA Editor
    Martin Holladay | | #1

    There is no reason to worry if your foam layer has one or more aluminum foil layers. The wall will dry readily in both directions. The cedar siding and furring strips will dry to the exterior, and the logs will dry to the interior.

    When you write that 3 inches of rigid foam will give you R-28, I assume that you are calculating R-9 for the logs and R-19 for the rigid foam.

    In your climate zone, EPS will perform better than polyiso. (It's possible to buy foil-faced EPS if you want; the foil facing makes the foam easier to tape.) Polyiso doesn't perform well at cold temperatures. For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6.

    Remember: It's important to pay attention to airtightness when performing the retrofit work you describe. Good luck with your project.

  2. Thomas Gardner | | #2

    Thanks for your help. This site has been fascinating to read, learning so much it makes my head ache at times :)

    Tom g

  3. Expert Member
    Dana Dorsett | | #3

    With R9 logs you can put 1.5"/R9 polyiso (seams taped with a temperature rated high quality foil tape) as the first layer, and 2" / R8 of EPS on the exterior and do well even in US climate zone 7. The nominal R would be about R17 for the combined foam, R9 for the pine for a whole-wall R of about R26. That's 3.5"- a bit thicker than discussed, but hang with me for a bit.

    As long as the average temp through the polyiso layer in winter is above 7C/45F (say, 30F on the cold side, 60F on the warm side polyiso will beat EPS performance. When the average temp through the layer is 15C/ 60F (say, 68F on the warm side, 52 F on the cold side), it's performance will be even higher than the labeled performance.

    Study these curves carefully:

    With a roughly 1/3, 1/3, 1/3 split between the layers, when it's +10F outside (a typical winter-average temp in a zone 7 climate) and 70F inside the average temp through a 2" exterior EPS layer will be about +20F, with the warm side at 30F, and it's performance will be a bit over R9.5. The average temp through the 1.5" polyiso layer will be 40F (50F on the warm side of the layer), and it'll be running about R5.5-R6. Assuming the logs will still be R9 (it might be a bit higher, due to fuzzy heat of fusion characteristics of the lignite content), For R24-R25-ish wintertime whole wall performance. That's a hit from the labeled R26, but it's less than a 10% hit. During the shoulder seasons both the EPS and polyiso would be outperforming the labeled values. During the shoulder seasons the polyiso would be in the R10-R11 zone, and the EPS in the ~R9 zone.

    But if that outer layer of foam was polyiso the wintertime hit would be much deeper.

    If your average winter temp is +20F, a typical zone average over 12 weeks of winter in a zone 6 climate (eyeball the coldest 3-month average for Montpelier VT with the cursor:!dashboard;a=USA/VT/Montpelier ) it'll do even better, since the polyiso layer would average about 45F, with the warm edge at about 45F, and always performing at least as well as if it were EPS, and the outer EPS would be averaging about R9, punching R1 above it's labeled weight. Again, during the shoulder seasons the foam would be performing better than labeled, for a whole-season average closed to the labeled values. If you cut back to 1.5" EPS (instead of 2") and 1.5" polyiso it would still be OK, if 3" is the thickest that works.

    In zone 4 (or even the warm edge of zone 5 going with 2" of polyiso under 1-1.5" of EPS might be more optimal, and in zone 3 or lower all-polyiso would deliver better average performance than EPS for any given foam thickness.

  4. Expert Member
    Dana Dorsett | | #4

    Most species of log used for this would run about R1.2/inch, and thus R7+ at the fatter part. If they're squared off a bit and only 4" at the narrower part where chinked it would be R5-ish there, but average performance would be about R6-ish.

    When the exterior foam goes up you get the benefit of about R0.5 for the trapped air films there in the thinner parts, adding the plaster on the interior you get about R0.5 for the blueboard & plaster, over the whole thing, plus another R0.the trapped air films at the interior side of the chinking. It'll be at LEAST R8 on average, and R9 is not crazy and might even be a tad low when you factor in the thermal mass benefits of wood + chinking.

    The D.O.E. estimate is:

    " Ignoring the benefits of the thermal mass, a 6-inch (15.24 cm) softwood log wall has a clear-wall (a wall without windows or doors) R-value of just over 8."

    R9 is a pretty good estimate for this wall, in the absence of more information about the species, true dimensions, chinking, etc.

    (Mind you, a typical 2x4 wall with crummy low density R11s, half-inch wallboard, half inch CDX sheathing and vinyl siding is also about R9.)

  5. Aj Builder, Upstate NY Zone 6a | | #5

    6" D logs might be R ?, can someone post more info. Rescheck is one source and is used to get a building permit in NYS.

    Rescheck says 7.52

    Another site says with thermal mass in Tennessee figure R-19.5, wow

    using MECcheck I find no program on the web called meccheck...

  6. Thomas Gardner | | #6

    I tried to find the site that told me soft pine logs where about 1.5 R per inch, but it and its chart of various woods seem to have fallen off the net.

    In any case we have square logs. I can tell you never buy or build a 6 inch log home and expect to heat it easily. The argument about their thermal mass is all marketing, as far as I'm concerned, although in the summer we do feel the heat poor out of the walls at night..

    Dana thanks for the info, you've given us a lot to think about.

  7. Expert Member
    Dana Dorsett | | #7

    BTW: Correcting myself, it's the lignin, and not lignite (which is the technical term for brown coal) in log homes that give it unusually large thermal mass characteristics in some temperature ranges.

    A 6" log home would do pretty well in US climate zone 2, maybe even zone 3, but it's a lousy performer in a Vermont (zone 6) climate. My brother lives in a 12" log home in zone 4C, which is OK, despite the fact that it's impossible to really air seal to current standards. Putting even an air-tight R5 on the exterior of a 6" log home would provide a noticeable improvement, but it's cost effective to do more.

    IRC 2012 code for zone 6 is a max U-factor of U0.048 which is R21 whole-wall, but for a mass wall it's U0.060 (R16.7 whole-wall). See Table :N1102.1.3:

    While the mass effect of logs doesn't quite meet the IRC definition of a mass wall, there are still significant thermal benefits there when you put insulation on the exterior. You're definitely taking the right approach, and in reasonable proportion in cost/benefit terms. The Building Science Corp folks posit R35 whole wall as a reasonable target on a cost/benefit basis in zone 6, but that's assuming lower cost insulation, and doesn't factor in the mass effects. An R25-ish whole wall with significant thermal mass is about right for an all-foam insulation approach, since retrofit foam is more expensive than other wall-R methods, and from an energy use point of view it'll do better than it's steady-state R. See Table 2, p.10:

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