5 Comments

1. 
   tallpinescabin | Apr 12, 2022 09:33am | #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 | Apr 12, 2022 11:06am | #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.

2. 
   AbeVigoda | Feb 13, 2023 08:19pm | #3

   Without a basement or even a formal crawl space, how / should floors be insulated? Is creating a crawl space under an open cabin a necessary first step? Thanks.

1. Expert Member
   MALCOLM TAYLOR | Feb 13, 2023 11:53pm | #4

   AbeVigoda,

   You can add a skirt or insulate the floor. If you insulate the floor you need to create a heated service core to keep your water and drains from freezing.

   You may find these discussions useful: https://www.greenbuildingadvisor.com/article/insulate-raised-floor
   https://www.greenbuildingadvisor.com/article/crawl-spaces-vs-skirts

1. 
   andrewofsaintlouis | Jun 16, 2023 11:16am | #5

   If you don't *have* water and drains in your cabin, then you need not worry about any issues when insulating the floor? and you do not need to create a heated service core to keep your water and drains from freezing since you don't have any?