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Community and Q&A

How can I address condensation in my arched cabin walls?

waterfront165 | Posted in Mechanicals on

I built a 20×24 arched cabin(Gothic arch,2 inchsteel frame, 2×4 endcaps) in 2016. We were told that the provided insulation was r-49, but subsequent conversation with the manufacturer , solar gawd, said we are more like r 30, with two layers on the outside and two layers on the inside of the cabin ribs. We planned to make the end caps r 30 but the local contractor refused to install the rigid foam we had bought so we only have r 15 fiberglass in the wall. People here in our area of western North Carolina seem generally topay little attention to energy efficiency, so when a yankee, and a woman contractor, moves in, it is not easy to get willing help. The bottom line is that we have significant condensation . This is in the 2 inch rib bays. On the outside between the steel and solar yard, and on the inside between the solar yard and paneling. Since I have been building energy efficient homes since 1984, I find this quite distressing, as I was assured I had r49 . I believe part of problem with moisture. Is that we were supplied non perforated solar gard for the interior application.
At this point, and with limited budget I’m wondering if I can turn the rib bays into interior space by installing a vent at top and bottoms of each, and then use a lunos e2 HRV which is what the folks at rimland tales described using , along with a mini split which was in our plans.
I have also considered filling each rib bay with rigid spray foam, or perhaps mineral wool. We are very chemical sensitive , so somewhat reluctant to go with foam as we did that in a previous house and I believe it may have been done incorrectly so we had what I would call a “sick” house. I always felt better when I could drag myself out of the house. We can’t have that again.
At this point I am thinking of a Santa Fe dehumidifier, or a lunos 2e. And a small mini split that we can use primarily for cooling. We installed a vented Rinnai propane furnace that does a pretty good job of heating except in the bedroom alcove on the back side of the small cabin.
Any suggestions on the best way to correct this would be welcomed.

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  1. user-2310254 | | #1
  2. waterfront165 | | #2

    Well that is informative, and not great news. Any thoughts out there on draping this small cabin with a membrane and building a green roof over It? How would that help the thermal,inefficiency.? I was thinking it might be a decent solution . Also, if we are to put the insulation outside the ribs, what about outside the steel as,well? That is what led me to thin about a green roof. This would be properly assembled ,withmutiple layers of substrates over the membrane. It would probably red some sort of framework to provide for holding the dirt . Would 8 inches of dirt over the proper base create a nicely tempered interior? We would still have an exchanger and the mini splits.
    Or . Maybe just tear the whole thing down .

  3. Expert Member


    The first impediment to adding a green roof is probably the additional loading it adds to the structure.
    The second would be that it doesn't really add anything in terms of energy savings that couldn't be achieved a lot more efficiently by other means.

    A case can be made that having a lot of buildings with green roofs in a dense urban environment can lower the ambient temperature of the district, and they can be used to hold and release storm water runoff. But really when it comes down to it, the only reason to include one is because you like them. That in itself shouldn't preclude putting one on a building, as long as you are clear that's why you are doing it.

  4. waterfront165 | | #4

    I m always concerned and aware of the loading. We would habe,to do some calculations, but would be adding some weight bearing framework.
    Based on the earlier comment, I was hoping it would be a reasonable way to add some serious insulation, and fire resistance. I have heard mostly skepticism at this possibility.
    We are in the rural mountains; but I have been wanting to run the idea out there. Dies every one feel the same .
    Can anyone comment on the eos2 vs a dehumidifier like a want a fe.

  5. Expert Member
  6. waterfront165 | | #6

    It has been suggested that we can solve the condensation issue if we spray closed cell foam 2 or 3 inches, on the steel exterior of the arched cabin. This should add sufficient thermal block without having to gut the interior. We have been measuring moisture inside and when the outside temperature is below 50-60 , it has been in the 35- 45% range. We have a 110cfm bath fan and will be hooking up a 350 cfm downdraft vent in the kitchen . We expect to get a mini split that will also have a dehumidifier . The cost to do the foam, and paint it came in at $6900. I priced just the cost of mineral wool for half the cabin interior, not including labor at almost 4500. We would have trouble doing one side inside because it includes the bathroom, ceramic tile, laundry area with toung and groove pine, and kitchen wall with backsplash, so a real mess to tear back.

    What are thoughts on the outside foam job.? It sure would be easier than trying to move out, gut the inside, etc. thanks.

  7. GBA Editor
    Martin Holladay | | #7

    My first hurdle to understanding your problem is visualizing the type of building you are talking about. I did some Google searching and found the photo below. Is that what you have -- an arched-roof building entirely framed with steel?


  8. waterfront165 | | #8

    Yes. Go to arched I think the one in the picture is a 24 wide. Ours is 20' x 24 long. The solar guard insulation installed as they told me to is a. Not even close to r 49. B. Causing a double vapor barrier. With more input and analysis, I have determined that we have usually low humiditywith our vent fan operating a few hours a day. It has been in the 38-42% range. The problem is at the steel, where we get condensation due to the dTemp. If we had decent insulation between the cold outside and warmer inside, it wouldn't happen. There is a good article in fine HOMEBUILDING last month on wooden houses in Canada with similar issues, due to not enough insulation and vapor barriers. Thanks! Any help and suggestions are welcomed. At the moment I'm pretty set on adding the exterior spray foam, coating it and painting. Least invasive, but still way expensive. Will add a mini split as well.

  9. GBA Editor
    Martin Holladay | | #9

    I don't think there is any alternative to closed-cell spray foam, either on the interior or the exterior.

    If you install exterior spray foam, you will need roofing foam. This is a specialty foam (very dense) used as roofing. After the roofing foam is installed, it can be protected by a special type of paint, which needs to be periodically renewed.

  10. waterfront165 | | #10

    The fella who will do the foam seems pretty knowledgeable, but based on My bad experiences, can you give me any further details on the actual materials. I want to be sure to share with him. He did speak of spray, then coating, which he described as getting chalky if not painted over. He said the coating comes in white or gray. I would paint over . The joke is to paint it over with " invisible" paint , like in the cartoons, to make my problem disappear. This has been an awful experience. I even tried to get in touch with the HGTV GUY, Holmes makes it right. To see if they would take us on. I wonder how many other arched cabins have the same problem.

  11. GBA Editor
    Martin Holladay | | #11

    Here is a link to a relevant article: Roofing With Foam.

  12. waterfront165 | | #12

    The link doesn't seem to be there. Can you please try it again?

  13. GBA Editor
    Martin Holladay | | #13

    I've tried with two computers, and the link works for me. Did you click on the words "Roofing With Foam"?

  14. waterfront165 | | #14

    Working fine now! Go figure. And , why is it We all have multiple computers these days..thank you. Will read it shortly.

  15. walta100 | | #15

    The way I see your problem the only way to prevent condensation is to keep all the steel above the dew point.

    Any insulation inside of the steel is self defeating and must be removed. With a steel frame all the insulation has to be on the exterior and protected from the weather. Almost all commercial buildings are constructed in this way so focus on contractors with commercial experience.

    If you do go the spray foam with a liquid applied covering any laps in maintenance that allows water to get behind the foam will require the foams removal for a proper repair. The one roof I have seen constructed in this way was a “flat roof” it was hidden from view from the ground but to my eye it was butt ugly with lumps and bumps every were and each and every repair clearly visible.

    If the curves on your roof are not to dramatic a commercial roofer may be able to cover it in several layers of sheet foam and a layer of EPDM rubber on top.


  16. Jon_R | | #16

    You can use exhaust only ventilation to stop exfiltrating air (the primary cause of wall condensation) and lower the interior humidity.

    > two layers on the outside and two layers on the inside

    Say it's 15+F outside and 30% RH inside. Sounds like ~1/2 of your insulation is outside the steel, so the steel should be no less than 42.5F. That will be condensation free (dew point = 37F), even if you don't stop exfiltration.

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