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Should I skip vapor barrier in a seasonal cottage?


guy48065's Avatar
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10-26-12, 09:30 AM   #1  
Should I skip vapor barrier in a seasonal cottage?

In northern Michigan it appears most cottages that get shut down for the winter have walls & ceilings that are just paneling, T&G wood or drywall applied directly over the studs & unfaced insulation. If I do go up and spend the weekend in my cabin I arrive to a house that's maybe 20F inside. I turn on the wall furnaces & gas woodstove & it will be 70 in an hour. The walls are practically dripping wet at that point. This is what's normal up there and the places seem to hold up quite well. I think seasonal users prefer "OFF" to dialing down the thermostat for the entire winter if they only come up once a month for some snowmobiling or ice fishing.

Given that kind of abuse is it best to NOT have a vapor barrier behind the walls? Maybe drywall is also not recommended? In my renovation I'm planning T&G pine on most walls and ceilings (replacing cheap paneling), but may use drywall in a couple rooms if it will survive the temperature cycles.

 
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10-26-12, 01:30 PM   #2  
It probably isn't the vapor barrier that is causing the sweating. The vapor barrier is keeping the moisture from migrating to your insulation and beyond, so it is doing its job. With that said, I am sure you are heating with a fossil fuel, which puts out an inordinate amount of moisture as a byproduct. Changing temperatures that quickly leaves nowhere for the moisture to go but to the walls and ceiling. It may help to leave a little ventilation open while you slowly (at least more slowly than an hour) raise the temperature of the house.

 
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10-26-12, 02:04 PM   #3  
I'm guessing these are UNvented wall furnaces and fireplaces? If so, then that is the reason for the extremely high moisture inside. Unvented gas appliances discharge a huge amount of moisture in their exhaust.

 
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10-27-12, 06:39 AM   #4  
No these are through-the-wall vented heaters & the gas woodstove is sealed so no combustion by-products here, just a huge change in RH with cold walls below the dewpoint. My experience with fast heat rise & sweating is in the cabin I've owned for 5 years which is layers of solid wood--no insulation. Now that I'm thinking more about it I have 3 different wall coverings: Bare wood on the original part of the cabin, drywall in the bedrooms (stud walls inside the wood-wall perimeter & insulation in the studs I think), and paneling in the family room addition (insulation in the studs I think). They all get damp to touch but the drywall shines with water. Maybe it has nothing to do with the insulation but paint makes it worse since it's a vapor barrier. The drywall sections SHOULD be less affected because they SHOULD be warmer due to the insulation and double-thickness but the problem there is worse.

I'm asking the question because I have a new cottage & am faced with my own decisions to make--not just maintaining someone else's choices like in the old place.

I know log construction with no insulation is a little unconventional (and is why I originally posted in the "Walls" subsection, but the thread was moved). Some of the choices most of you are faced with don't apply to my cottage but physics is physics so how should I minimize any chance for damage to occur to the interior walls?

 
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10-27-12, 09:19 AM   #5  
Hi guy,
I didn't reply because I went with Furd and Chandler. However, if there are no unvented, then we must look elsewhere. From the explanation, the moisture is already there in the cabin and adding heat simply evaporates a lot of it and carries it to the cold walls. Essentially, everything was damp, but as you dry out the interior you make more moisture available for the cold exterior. That would go back to Chandler's comment about opening a window to get rid of that initial conversion of moisture to humidity.

What's under your cottage? Is it open to the great outdoors or is it sitting on a dirt crawl space, slab or basement? When you are not there air continues to flow in and out of the cabin and any moisture underneath may be transferred to inside.

Where is the source of that moisture?

Bud

 
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10-27-12, 09:52 AM   #6  
If it is log construction, it is more conventional nowadays than back in the 80's when I built mine. No one back then understood "thermal transfer". Logs heat up during the day and that heat doesn't make it into the living space until evening when it is more desirable. Likewise the logs cool off at night and the house keeps relatively cooler during the day, when the cycle starts all over again.

Like Bud said, it could be a little to do with the insulation, or lack of, beneath the flooring, coupled with the walls' capability to insulate.

 
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10-27-12, 09:57 AM   #7  
The moisture is already in the cold air--raising the inside temperature just raises the relative humidity. That's the "relative" part--the amount of moisture air can hold depends on the temperature. Cold air is more dense so it holds more vapor. Warm it up & the air becomes saturated with vapor & will condense on any colder surface. Opening a window just lets in more moisture-rich cold air. Slowing the temperature rise does help because it allows the inside surfaces to warm up also so the vapor doesn't condense--but going to bed when it's still 40 degrees inside isn't an option so I'm stuck with fast rise.

If it were possible to warm up the walls at the same rate as the air this wouldn't be an issue, but that's not possible. I just need to know if a vapor barrier behind the wall will trap some of this excess moisture in the colder wall and cause damage to the paint or drywall. I don't think I need to worry about thick pine paneling (and might be why it's so popular in cabins--it survives).

If this was a house heated all year like you're all familiar with a vapor barrier can prevent moisture from condensing inside the insulation or flowing through to blister off the paint on the siding. But I'm talking about a very different and severe situation. One that's pretty common in northern vacation cottages. I'm wondering if it isn't wiser to let the moisture flow out rather than try to contain it inside. I'm not really sure it ever actually "flows out". Maybe it just evaporates back into the air as the walls eventually warm up above the dewpoint.

I wish there was a "seasonal northern cabin" forum

 
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10-27-12, 10:57 AM   #8  
Ring the bell, it physics class .
Relative humidity is the ratio of the moisture vapor in the air as compared to the amount of moisture it can hold when saturated. When you heat air, without adding or removing moisture vapor, you lower its RH. Moving it closer to those cold exterior walls then raises the RH again. Humidity - Wikipedia, the free encyclopedia

As for the VB, an air barrier would be more important, especially with T&G. Ensuring a wall or ceiling can at least dry in one direction is important. The use of vapor diffusion retarders is becoming more popular as they will allow eventual drying when needed.

Bud


Last edited by Bud9051; 10-27-12 at 11:00 AM. Reason: addition
 
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