Insulating an Off-Grid Cabin

[joel]

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.

 

[joel]

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....

 

[joel]

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.

 

[joel]

No sure if this is helpful, but here is the link.
https://www.greenbuildingadvisor.co..._content=gba_eletter&cid=63641&mid=1574568949

 

[Tootsie]

Only a year’s experience here. NY, cabin insulated with faced fiberglass, covered with panel, and thick sheets of foam insulation on floor beneath indoor/outdoor carpet. Limited use of propane heater more than enough for heat. I think a wood stove would be too much for a small cabin. Two vents on either end with a foam insulation plug for coldest times provides ventilation.
Humidity and mold are problems. A dehumidifier runs pretty much constantly, and bucket needs emptying every couple of days.
Last summer, with open windows, mold covered the walls, the floor, and most of the contents. There are fans in both windows.
I think we may have over insulated. Cabin is 12x36.

 

[Spikedriver]

I think, if you're going to insulate it tight, you need a really good roof vent. Gotta let that warmer moist air out. Whether that's in the form of a ridge vent or something else, I don't know. I'm not a builder. But I've seen older houses that got re-insulated both inside and out and then had moisture problems because they didn't get any added venting. @hashbrown would probably be the guy to ask on this topic, he builds residences in a very humid climate...

 

[hashbrown]

I think, if you're going to insulate it tight, you need a really good roof vent. Gotta let that warmer moist air out. Whether that's in the form of a ridge vent or something else, I don't know. I'm not a builder. But I've seen older houses that got re-insulated both inside and out and then had moisture problems because they didn't get any added venting. @hashbrown would probably be the guy to ask on this topic, he builds residences in a very humid climate...

In our climate I'd rather have a house that was more on the loose side. I've seen people try and build them to tight here and it leads to trouble.

 

[Weedygarden]

My home town, which has a population of less than 1000, has torn down more than 40 houses. Many of these homes were not well built, were on the smaller side and had had the last elderly family members who had lived there die. These homes sat vacant and were becoming a blight for the town. I don't know what they did to allow this to happen to so many homes, but it has given the town more of a park feeling. Why is she telling us this?

One house, was small, but was newer. Man was arrested and was in prison for a few years. An elderly cousin of mine and his wife lived next door to this home. They had contacted the owner and asked if he would sell them the home. They had several homes that they maintained and rented out. They would rather have kept the yard up and the place painted than to live next door to an almost abandoned home. Man said, "No." He wanted to keep the house. I don't exactly how long he was gone, but when he got out, his house was full of mold. Then he wanted to sell it.

Another home in the same small town had a former police officer who shot and killed his wife. That made national news. I knew their home well as a child. Husband is now in prison for life and that house was locked up for a few years, a crime scene, and husband kept thinking he was going to beat that rap and get out and live in the house again. Nope. By the time a few years had passed, and the house was able to be opened up and the extended family to go in, it was full of mold. Evidently a home needs air flow and movement and to be cleaned.

 

[Weedygarden]

Another story about buildings not being used. There is a very large garage in my area, holds about a dozen cars and is a private garage, that was closed up and not opened for a long time, years. A man bought it and it was full of mold. He got busy and started cleaning it out. He told me that he got really sick while cleaning out that moldy garage.

 

[backlash]

I have seen houses built so tight they had to cut holes for ventilation after it was built.

 

[joel]

I have seen houses built so tight they had to cut holes for ventilation after it was built.

Here in South Carolina, to be energy efficient, a house must pull a vacuum up to hg 0.05.
I think that what the builder told me.???

 

[Aklogcabin]

We have a remote cabin that we use in the winter. And have never seen any problem. The air during the winter is also very dry air. We will go out for a few days to a month. It's log but we have r42 in the lid with a 2" cold air attic design for ventilation.
And the wood stove will remove any moisture in the air. Not unusual to see a pot of water sitting on a woodstove because it will dry the air.
Insulated n use it . Remember what happens at the cabin stays at the cabin

 

[Cabin Fever]

We built our off-grid cabin in 1996. It has 12" of unfaced fiberglass insulation in the floor and ceiling and 4" of unfaced fiberglass in the walls. The floor, ceiling, and walls have 4mil plastic vapor barriers on the heated side of the framing. The attic area is ventilated with continuous soffit vents and a continuous ridge vent. The cabin is used several times a year, including winters. Never any condensation or moisture issues.