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Q&A Spotlight

Insulating an Off-Grid Cabin

Are there special concerns when the building is only used once in a while?

Design and construction: Lou Ureneck. Image credit: Brian Vanden Brink.

Andy Peifer is building an off-grid cabin in New York State that will be used once in a while as a getaway—heated for several days at a time with a woodstove and then left unattended.

If the building is used only intermittently, Andy asks in this recent Q&A post, is it any more susceptible to moisture problems than a continuously occupied house would be?

Exterior walls of this building in climate zone 6 will consist of mineral wool or fiberglass insulation in the stud cavities followed by CertainTeed’s MemBrain (a smart vapor retarder) and 5/8-in. wood paneling. On the outside of the building, Andy plans on applying 1/2-in. sheathing followed by 1-1/2 in. of ESP rigid foam with taped seams, housewrap, a vented rainscreen, and fiber cement siding.

He is concerned that the layer of continuous insulation on exterior walls may not be thick enough. (The International Residential Code requires R-20 of cavity insulation, plus R-5 of continuous insulation or R-13, plus R-10 of continuous insulation in climate zone 6.) Although the cabin will get occasional use now, it could become a semi-permanent home in the future.

“Is being thin on exterior insulation a wise assembly, knowing the four-season structure is only occasionally heated with a woodstove,  and there’s no AC and no mechanical ventilation?” he asks. “What happens when we blast it with warm air for four days when the whole cabin is 15°F? Will MemBrain and cavity insulation prevent moisture accumulation on the interior of the sheathing? Does using MemBrain mitigate the thin exterior insulation concern because the wall can dry to the inside?”

Is Andy worried about nothing? Or does he really have a cause for concern? That’s where we begin this Q&A Spotlight.

Moisture risks are probably lower

Peter Engle…

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2 Comments

  1. Nate Reik | | #1

    Interesting discussion! I'm in the process of building a part-time use cabin as well... I'm monitoring interior and exterior Temps, RH and dewpoint, but starting to look more at when one should start to get concerned.....

    "If electricity is available onsite, the answer to this problem is to provide air change in winter—not much, maybe a bathroom fan—to keep this “ping-ponging” moisture under control. If you want to go a step further, think about a dehumidifier, or a humidity-controlled fan, or monitor the humidity levels to see if you’re running up against the edge of risk. A more expensive option is to keep some minimal heating on during unoccupied periods—same indoor and outdoor dew point, but interior RH will be lower."

    I assume this relies out the exterior dewpoint being lower than the interior dewpoint when you're ventilating, then, correct? And, for a cool building, what is the "edge of risk?" You CAN grow mold in your refrigerator....

    -Nate

    1. Expert Member
      Peter Engle | | #2

      Ideally, you would ventilate whenever the exterior dewpoint was lower than the interior dewpoint by some margin, maybe 5-10 degrees. That would require some sensors and a custom control system, but not too tough to DIY for a tinkerer. If there are solar panels on the off-grid cabin, there would be enough power to run a fan at least, even in cloudy weather.

      The "edge of risk" is somewhere around 80% interior RH. That "somewhere" might be +/- 10% or more. As in much building science, it depends. Joe L. and others have done work on "water activity" - that is, the availability of free water molecules at the surface of wood cells. I haven't followed much of the actual research but my understanding is that, when the steady-state RH of the room air exceeds 90%, the water activity at the surface is high enough for at least some wood molds to grow, given other conditions are ideal. But this is where surface temperatures start to become very important. If the wood surface is cooler than the air, that's when even moderate indoor RH can result in high surface RH and high water activity due to condensation or near-condensation.

      While molds do grow in the fridge, they do so slowly and most of those molds are not wood molds. They are growing on far higher-energy substrates (food) and that gives them a little extra oomph in the cold. The literature is pretty settled (I think) that wood destroying molds and fungi don't grow much, if at all, below about 50F.

      So in broad terms, the edge of risk in an unoccupied building is probably as high as 90% when the temperature is well below 50F continuously. But those conditions result in saturated wood that is just waiting to bloom with mold when the temperature rises. There are some dehumidifiers that will operate down to about 50F. Running one of these at about a 60%-70% setting whenever the temperature exceeds 50F in an unoccupied cabin would help quite a bit, though it would cost something to run. But even a small ventilation fan as Kohta mentions would help.

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