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Community and Q&A

Log wall spray foam insulation: closed or open cell?

davebeller | Posted in Energy Efficiency and Durability on

Hi – Man, I’m glad I found your site because I’m torn and need advice for this unusual situation.

I have a house in Nashville, TN built in ’51 from what appear to be reclaimed RR ties (which means, they could be close to 100 years old by now). We’re the 2nd owner and have been in it for about 7 years. Why I bought this place, I’ll never know, but it could be worse. Anyway, we’re finally on our renovation and all interior log walls have been gutted so that I can access them to seal . By seal, I mean block the awful creosote odor that exists (none of the logs were exposed on the inside and we’re totally happy with that). Actually, the odor was only in the room with paneling (you know, more permeable then painted wallboard) and was only on hot summer days but in an effort to keep the peace with the wife (who is convinced we’re harming the kids), I’ve exposed all log walls so that I can deal with this.

No need to discuss creosote here as I’m not too worried. I’ve consulted with everyone from the Creosote Council, RR Tie Assoc, EPA, Permachink, TN Dept of Health, I could go on, but the overall consensus is, as old as these ties are, it’s probably not that big of a deal so I’m not worried about it. There’s a big chance any air quality test (which are quite expensive) could come back inconclusive so it’s been decided to just apply that money to mitigate what’s probably a small problem. Hell, the original owner lived to be over 90 in this place!

Anyway, for psychological reasons, a spray foam applied to all interior log walls would really make me sleep better. I’m not really doing this for the insulating purposes and there’s quite a lot of room between the logs that should be filled as the chinking is only on the exterior (currently, it’s filled with acorns from all the critters running through there. I’ve attached a few photos for your perusal).

Everyone on the web has differing views on open- or closed-cell. I certainly don’t want to harbor any moisture where the foam would meet the logs (which is the concern I’m reading about by using closed-cell). Any/all advise would be greatly appreciated!!


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  1. davebeller | | #1

    Oh, one more thing......the products I'm using on the exterior to restore (from Permachink) are all breathable, I believe.

  2. GBA Editor
    Martin Holladay | | #2

    As long as the logs are exposed on the exterior, any moisture in the logs will be able to dry to the exterior.

    To create an air barrier, you need either a minimum of 1.5 inch of closed-cell spray foam or a minimum of about 3 inches of open-cell spray foam. For more information, see Air Leakage Through Spray Polyurethane Foam.

    Some molecules (for example, water vapor molecules) can diffuse through open-cell spray foam. If your wife is worried about vapor diffusion through the foam -- that is, the diffusion of unspecified but very small molecules through the foam -- you might want to choose to install closed-cell spray foam, which is vapor-impermeable -- if only to maintain marital harmony.

  3. charlie_sullivan | | #3

    If you want to be even more thorough about blocking any possible vapors coming through the foam, I would follow the spray foam with foil-faced foam boards, taped with foil tape. A thin metal layer blocks vapors better than almost anything, which is why it's used in potato chip bags.

  4. davebeller | | #4


    I figured they could dry to the exterior but many of these blogs mentioned it would take 'more months than there are each year' for that to happen so I wasn't sure.

    They had some sort of silver paper that I'm guessing was used for vapor barrier but it didn't prevent the creosote odors from coming through. I'm planning to fur the walls out in order to get the recommended 2" for closed cell and I don't think I'll have enough room for the foam boards, however, I'll look into it.

    Thanks, again!

  5. charlie_sullivan | | #5

    I would guess that the silver paper was not well air sealed, and that negated any odor blocking capability it would have. Foil faced polyiso is available as thin as 1/2", and can be air sealed easily between panels with foil tape. Air sealing the edges is also important in order to have it be effective. You might be able to wait until after the foam cures and then see if you are still getting odor coming through. It might be hard to tell because the smell might be absorbed into furnishings in the house so it might take some time before you can be sure.

  6. davebeller | | #6

    Thanks, Charlie! Any thoughts on filling those long, sometimes wide, gaps between logs before I spray foam?

  7. GBA Editor
    Martin Holladay | | #7

    There is nothing wrong with filling the gaps between the logs with spray foam. But if you want to save money (that is, reduce the volume of expensive spray foam), I suppose that you could stuff the cracks with fiberglass insulation before you start spraying the foam.

  8. Expert Member
    Dana Dorsett | | #8

    Assuming the ties are at least 7" thick (RR ties are nominally 7" x 9" ) you can hit IRC code-min thermal performance with as little as 1.5" of closed cell foam if it's continuous (get rid of the furring first), and it would already be substantially less permeable to water (a pretty small molecule) than latex on wallboard.

    An even less vapor permeable solution would be 1.0-1.5" of foil faced rigid polyisocyanurate foam cap-nailed to the ties, seams taped with a high quality foil tape, and foam-sealed at the edges. Furring for hanging the wallboard can be 1x4s through-screwed to the ties. That's significantly cheaper than closed cell polyurethane too. The foil facers are impermeable to all volatile compounds, and the 3/4" gap between the interior foil facer and the wallboard adds another ~R1 to the thermal performance, which can squeak it by code minimum performance at somewhat less than 1.5" if need be.

    Rigid polyiso is also a lot greener than most closed cell polyurethane too, since it uses much less damaging blowing agents. Foil facers also limit the fire retardent exposure risks too, since it can only leak out at the cut edges.

    The ties don't need to dry toward the interior- they really don't need to dry at all. Creosote protects the wood from mold/rot issues well enough to use wood in fully exposed and even partially buried applications such as railroad ties, or even fully immersed applications such as dock pilings. With any amount of roof overhang the drying rates after rain-wetting will be pretty high anyway.

  9. davebeller | | #9

    Hmmm...interesting. A few mentions of the foil faced foam board stuff. I know nothing about that. Dana, if I understand you correctly, the foam board would be less permeable that spray foam? Also, if i used foam board, I wouldn't really need to fill space between log rows, I guess.

  10. davebeller | | #10

    Also, with the differing log widths, I'm not sure I want to attempt at removing fur strips (and random shims) and then attempt to put them back on straight. My talents may not be good for this! However, I am intrigued at the option of using the foam board instead of spray foam.

  11. user-2310254 | | #11

    Dave. Dana is referring to this type of rigid insulation:

  12. davebeller | | #12

    Sorry. I'm not a prime member and can't view that link. Thanks for sending, though!!!

  13. user-2310254 | | #13

    Dave. The answer is clear: you can accomplish your goals without spray foam. If budget is a concern, you might be able to use recycled foam. See sites such as Insulation Depot.

    Also... I believe GBA offers a 30-day trial on membership.

  14. davebeller | | #14

    Thanks, Steve! So, can I simply add this foam board right over my fur strips and lay sheetrock right up against the foam board? This may be easier than I thought.

  15. Dana1 | | #15

    Air seal the gaps in the logs as best you can first, but yes, you can put the 1.5" polyiso directly on the furring, tape the seams with a high quality foil tape (eg Nashua 324a, sold in many box stores, but there are many others- don't use cheap stuff- the adhesives might give up over time.) Use expanding can foam to seal the gaps at the top & bottom to ensure a good air seal, and prevent air from convecting through the channels between the furring & logs back into the open space.

    You'd have to long-screw/long nail the wallboard to the furring through the foam. Overlap the seams of the wallboard with those of the foam, which makes for better long term air tightness and a flatter wall, with less movement at the taped seams.

    Each foil facer tests at less than 0.05 perms (and you have two of them), whereas 2" of closed cell polyurethane runs ~0.5-0.7 perms, and order of magnitude more permeable to water vapor than even one foil facer. As long as you make it air-tight, there are no gases that can diffuse through the facers at a rate high enough to matter. (Hydrogen and helium might be able to get through, but neither are likely to be present in high enough quantities to matter, if they're even detecible at all.)

  16. davebeller | | #16

    Can you please explain what you mean by 'air seal the gaps'? Are you referring to there large gaps between log rows? If so, how would I do that? Thanks!!

  17. Jon_R | | #17

    Odor from the walls on summer days - In addition to the above advice, I'd very slightly pressurize the house. Even the best air sealing leaks a little bit - so make sure it flows from inside to outside.

  18. user-2310254 | | #18

    Dave. Martin noted in comment 7 that you can use scraps of fiberglass insulation to "chink" the railroad ties and then seal any remain gap with canned spray foam (Great Stuff for gaps and cracks, for example).

  19. GBA Editor
    Martin Holladay | | #19

    If you want to air seal the log wall -- and I think that it's a good idea to try -- you'll need more than a few cans of Great Stuff. After you stuff some fiberglass between the logs, you'll still need a thin layer of closed-cell spray foam over the entire wall. That means that you'll need a two-component spray foam kit (or you'll need to hire a spray foam contractor).

  20. davebeller | | #20

    Martin - if I went the route of using the foam boards, I wouldn't need the thin layer of spray foam, would I?

  21. GBA Editor
    Martin Holladay | | #21

    In theory, a layer of foil-faced polyiso can be installed in an airtight manner -- as long as the perimeter of the foam board (along the floor, at the vertical corners, and at the ceiling intersection) are perfectly sealed with spray foam, and as long as the seams between the polyiso sheets are perfectly sealed with high-quality tape, and as long as the foam never shrinks, and as long as the tape adhesive never fails.

    Many builders might feel better with a belt-and-suspenders approach that includes a redundant air barrier. But it's up to you to determine how to detail this wall -- your approach will depend on your goals and your budget.

  22. user-2310254 | | #22

    Martin. I think Dave was starting to feel comfortable with the steps Dana outlined in comment #16. With that approach, it seemed okay to me to chink the logs with fiberglass and then use can foam to seal the gap (with foil-faced polyiso, tape, and more foam or caulk to follow.) But I take your point that foaming the entire wall and then adding the other components would be a more bulletproof installation. Perhaps I incorrectly assumed there was a cost-to-benefit issue in play.

  23. davebeller | | #23

    Cost is definitely an issue, here. I had an architect friend come over this evening for some advice. I explained to him my very recent learning of foam boards as an option. He recommends spraying logs with a polyurethane to seal, place 3" fiberglass with foil facing towards the exterior along channels in between vertical log rows and then installing 2" foam boards onto fur strips (and sealing those, of course). Then, Sheetrock can go right over that. Many of you (whom I'm so grateful for your time and advice!) mention a foil faced foam board. Does the foil face towards exterior or interior?

  24. davebeller | | #24

    I did Google the afore-mentioned foil faced rigid polyisocyanurate foam and read up a little. Seems like an interesting option! I'm wracking my brain trying to see if I can pull this off without expensive spray foam.

  25. GBA Editor
    Martin Holladay | | #25

    If the polyiso has foil on only one side, it should face the interior, so that the seams are easier to tape. But most brands of foil-faced polyiso have foil on both sides.

  26. Jon_R | | #26

    I agree with #22 - no odor using just sheets will be hard. Maybe add a liquid-applied vapor barrier. For example:

  27. davebeller | | #27

    Both of these say not for interior applications. Is that a concern? Would these be better than a polyurethane or even a shellac (which is what they use in fire restoration work to seal things)?

  28. GBA Editor
    Martin Holladay | | #28

    I don't advocate the use of a liquid-applied water-resistive barrier for a purpose for which it is not intended. Use either closed-cell spray foam, polyiso, or both. (Codes require spray foam or rigid foam to be protected with a layer of drywall on the interior side of the foam.)

  29. davebeller | | #29

    Sorry...I'm starting to get confused 'liquid-applied water-resistive barrier', are you referring to the recommendation in #22, or the recommendations I received earlier in polyurethane and shellac? Also, by 'interior side', you're referring to inside the room? For example, in my case, up against the logs would be something (still undetermined) applied with a low volume paint sprayer, about 2" of foam board, and then drywall? Sorry! At the first reading, I took 'interior' as the interior of the wall and I sorta panicked. :-)

  30. GBA Editor
    Martin Holladay | | #30

    The suggestion to use a liquid-applied water-resistive barrier was made by Jon R in Comment #26; that's the comment I was reacting to.

    No type of foam -- whether spray foam or rigid foam -- can be exposed inside a house (except in some limited cases in crawl spaces, basements, or attics). So you need to have a layer of 1/2 inch drywall on the interior side of any foam insulation, for fire protection.

  31. davebeller | | #31

    Ah...yes, the walls in question are bedroom walls (2 walls in each) and one kitchen wall. I do plan to have sheetrock on these walls. Is this foam (spray or boards) super flammable?

  32. GBA Editor
    Martin Holladay | | #32

    Yes, foam is flammable -- hence the code requirements for thermal barriers and ignition barriers.

    For more on this topic, see Thermal Barriers and Ignition Barriers for Spray Foam.

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