Hi All,
I’m hoping that someone can give me some advice on how to deal with the following situation. I am building a new cabin near the coast in Northern California. It will be a weekend place, about 600 square feet,, and will not be heated when we are not there. We don’t get snow, and temperatures are mostly reasonable, but there is a fair amount of rain, as well as a lot of fog. What I have noticed is that in the existing cabin (an uninsulated trailer), everything feels dampish when we get there – clammy blankets, books a little swollen, etc. I am assuming that this is caused by moisture in the air seeping into everything. The heating will be propane, and will only be on while we are there, and electricity will be from a generator. Are there any useful approaches that I can take in building the new place that will reduce or eliminate this problem? A clammy bed is not fun to climb into after a long drive!
Thanks for any help.
Loach
Replies
Electric blanket on the bed. Turn it on when you first walk in the door. That's what we do with our place.
It takes heat and ventilation to move moisture out. Turn off the heat and close the place up and guess what?
With your situation, I would look for a way to do some passive solar heat to keep it warmer inside so the moisture is less likely to condense
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I may in Alaska now, but I know of what you speak. I'm a fifth generation SFer, stay at my in-laws place on the NoCal coast several times a year, and have had numerous discussion with them about the current weekend use house, the cabin uncer construction, and the plans for a full-time retirement home.
Piffin is right, "It takes heat and ventilation to move moisture out. I would look for a way to do some passive solar."
You definitely need to add heat. And while leaks in the structure will always provide some ventilation, I would suggest you build as tight as you can. The structure will then require less heat input to get to reasonable % relative humidity.
If the house were always occupied (and therefore during the weekends when it is), you need some moderate ventilation to get rid of humidity creating by breathing, sweating, showering, cooking, cleaning, etc. When you leave, all the on-going sources of moisture do too. So I would NOT go hog-wild on the solar fans. I wouldn't have any going when unoccupied. Look for ways to add heat and I, too, like passive solar as a option. South-facing windows at a height or with a sufficient roof overhang to be mostly shaded in the summer.
If you are going to have propane, are you considering a hot water heater? I like direct-vent units because they allow you to build tighter structures and eliminate the possibility of back-drafting carbon monoxide. And I would inist on one in one-cabin (or put a traditional HWH in and OUTSIDE closet). But putting the HWH inside the cabin gives an additional heat source inside. Not large - just the standby losses, but every little bit helps during a stretch of cloudy weather. And, when you show up, you have 50 gallons of hot water for showers and/or to run a hydronic baseboard or fan-equipped kick-space hydronic heater. Nice on a winter visit!
Here's a psychrometric chart with a nice tutorial. Read it. Really. If you understand the concepts they present, you'll have made a lot of progress in developing a good design to deal with potential humidity problems in your cabin.
http://ianrpubs.unl.edu/generalag/g626.htm
Their second example (condensation on the walls) speaks to why and how insulation in the walls is helpful. Plus, for any given heat input, you'll retain that heat longer and have a lower %RH inside.David Thomas Overlooking Cook Inlet in Kenai, Alaska
That looks like worth keeping.
The attachment is something I have seen various forms of.
A box is built with insulated sides and back and glazing over it. The back side is painted flat black to absorbe solar btus. Vents are run through the wall from floor to puickup cool air and out under windowsill to discharge the heated air into the room. Louvers control airflow. It is something that will retro fit into existing places to add some extra heat in circumstances like this.
I worked building a fire station that had a similar semi-passive solar. The whole back south facing wall was a series of these with corrugated backs. A couple of fans ran on a thermocouple to circulate air through only when it reached a threshold temp. That air was ducted through a series of pipes in gravel under the slab.
A fire station doesn't need to be heated to a high degree. As long as the water doesn't freeze, the trucks don't complain. They don't need 72°F
But I'm told that the exterior temps were usually below zero before the normal gas heater came on to heat space due to this system..
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That passive solar block would help, for sure. I assume the louvers are closed in the summer to minimize heat gain. I like taking the coldest air off the floor, but would rather see the inlet and outlet at the same height, dropping into the solar box.
Then, during the night, cold air would stagnate low and, without any convection, heat losses would be minimized.
If his cabin has a downslope on the south side, a detached solar panel could be installed on the ground and connected with insulated ductwork. The thermosyphon would work to gather heat when it could and stop losses at night.David Thomas Overlooking Cook Inlet in Kenai, Alaska
I got to wondering if you all do much with solar heating in Alaska?
As a low cost solution for his situation, I liked your idea of the propane fired HWH and adding a loop to a radiator of it. If it could run a pump circulator off a solar panel for power....
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Or just set up a thermosyphon loop off the HWH. Take the hot off high and return it low to the drain valve port. Don't know if he has city water or spring or if he needs electricity to run a well pump.
Nope, not much solar in Alaska. When you need it most, you only have 5.5 hours of daylight. And at its winter highest, the sun is about 11 degrees above the horizon so it's pretty cool and often behind the trees.
In the summer, when you've 19.5 hours available (at 59.5 North), I don't need any heat. Summer days here are 60-75F and my heating season stops at 40F outside temps (really tight and well-insulated house).
It was odd to see a remote, automated Alaskan Railroad grade crossing with solar panels. Looked they were installed backwards because I drove by at midnight, a bit south of Fairbanks in the summer. So the sun was shining smack dab on the BACK of the PV panels. But they were pointing south.David Thomas Overlooking Cook Inlet in Kenai, Alaska
>> In the summer, when you've 19.5 hours available (at 59.5 North), I don't need any heat.
I did some calculations one time to see if it would be possible to store some of that surplus summer heat until winter. I started out with a 125,000 gallon tank, 1,000,000 pounds of water to make the BTU calculations easier. I decided it could probably be done, but it would take a really big tank and a whole bunch of insulation and a boatload of collector area. Payback time in generations rather than years or decades.
Yeah, the only way I see to do it is if the 125,000 gallon tank is groundwater with enough cover to be a buffer against seasonal temp changes (20 to 40 feet). Then use a slightly downgradient pump to recover it.
But then, the heat stored is pretty low grade, so you might need a heat pump to recover the heat at a useable temperature. Money is better spent on tight construction and lots of insulation.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Thanks Piffin. I've been wondering a lot about passive solar or something similar. My thoughts are to put in a couple of big picture windows that face south, something like 6 ft x 6 ft each. That should help warm the place in the daytime. Then maybe try a small solar panel and battery to run a fan to keep the place ventilated. If I used a light detector, I could set it to run during the day to clear out moisture, and then off at night to keep from drawing in more moisture. Either that, or try to find some sort of de-humidifier. If there are any solar experts out there, I'd be glad to hear from them. Also, would it help to cram in a ton of insulation in the walls, ceiling, and subfloor? Would all that work against me in the middle of summer, when it gets really warm outside?
Bruce
You said an uninsulated trailer . Im thinking metal walls exposed like a camper shell . That condensation . An insulated wood structure isnt near that bad because the temp dont change as radically . The attic will be vented with insulation below. Lots of difference , but still you need something to suck the moisture out .
You can buy a dehumidifier which will only run when needed automatically .
Tim Mooney
Didn't he say he would use a generator when cabin was competed? Probably does not have electricity now either.
Yup , he does say that .
Tim Mooney
I'm thinking some solar power ventalitionattic style fans as well. You could set them to run anytime they are charged and it would circulate the air with no utility cost.
Mike
If you're building new, make sure you damp-proof the crawl space under the cabin.
You can get a lot of moisture migrating up through the structure from moisture in the soil.
I don't remember being absent minded.
Yup, 6 mil plastic on the crawl space floor does wonders . I practice it on rentals too close to the ground . Its working .
Tim Mooney
Another idea for moldy books and musty linens is a trick from sailors. Little containers of a salt with a reservior underneath. Take humidity from the air and leaves salty water in the reservior.
You'd need a ton of them in even a small cabin (versus a boat which is really quite a small volume and perfectly water- / air-tight on 5 sides). But maybe a foot locker or big tupperware container or a dedicated and tightly sealed closet (with an exterior-type, weather-stripped door) with those salt-based dehumidifiers. Available at West Marine or most any chandlery.
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&productId=11648&catalogId=10001&classNum=380&subdeptNum=379&storeNum=12
and
http://www.westmarine.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=10001&langId=-1&catalogId=10001&productId=119932&catalogId=10001&classNum=380&subdeptNum=379&storeNum=12
Thanks everyone. Some real good suggestions there, and I'm open to all of them, as the time to deal with these issues is now, while we are still designing the cabin - I hope to start building in a month or two. The site is rather , er, rustic, with no power or water yet. We are going to drill a well, but the place will definitely be 'off-grid', as we are several miles from the nearest services. I'm going to handle the electricity by putting up solar panels and storing what I can in batteries, and use a solar pump and storage tank on the well. I like the solar fire-house ventilator, and can incorporate that into the south facing walls, as well as the 6-mil barrier under the place. My plan was to use a tankless water heater that would only run when we are there, as we won't be leaving the propane on when we are not there. (The propane guy won't deliver, so I get the joy of hauling a couple of big canisters in the back of the truck whenever we need them.) The bigest room has a south facing wall, and I want to put a cathedral ceiling in it, so I can get a lot of window space if I need it. I plan to use over-built trusses to boost the insulation I can put in the ceiling, and will run a ridge vent to let it breathe. The cathedral area will also be open to a second floor sleeping loft, so windows in the big room should be able to deal with the humidity issue for the whole cabin. I think that if I plan this right, it will work, so keep those suggestions coming!
Thanks, Loach