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Old 04-30-2009, 07:23 AM   #1
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Small cabin moisture problem


I need some help with this problem and am not sure where to post, so here it is.

I have recently built a small 8' x 12' cabin. From the inside out I used 6mil poly vapor barrier, Roxul R14 insulation, 2x4 wood frame construction, 1/2" plywood sheathing, Tyvek house wrap, vinyl siding.

The ceiling is also insulated with air allowance above to the shingles, but the floor is not yet insulated at all. The floor is built on 2x6 pressure treated wood, and 2 staggered layers of 3/4" tounge and groove plywood.

It is in a cold and moist climate (British Columbia, Canada).

There have been no cut-outs to allow for air exchange anywhere on the walls yet.

My problem is that I have noticed that there is a large amount of moisure build-up on the inside of the vapor barrier as well as the windows. Also, there can get a lot of moisture build up on the outside of the windows. This only happens if I do not run a small propane heater all day and night (but there is still moisure build up on the vapor barrier if there is something in front of it like a bag or box that keeps the heat from getting to it).

Now I only use the thing about 3-4 weeks a year, but I don't want to see it start rotting or have mold growing inside it. Plus there is no option to have a heater running 24/7/365.

Does anyone have any idea as to what the problem could be and how to fix/remedy it?

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Old 04-30-2009, 09:34 AM   #2
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Small cabin moisture problem


My guess would be that the interior vapor barrier has leaks at some penetrations, ie. wall outlets, window jambs, light fixtures, etc... What happens is the cold exterior plywood "sucks" the warm, moist air out of the heated space through any possible hole in the VB. A non-vented propane heater makes mass amounts of moisture also. Actually, just breathing in a cabin that size would probably create too much moisture! My family has a similar situation (cabin wise) at their vacation property. I think it's only like 12'x16'. It's used solely as a bunk house for hunting season, or when alot of people come up for the weekend & the main cabin is over flowing. With as many as 6 guys sleeping in the small cabin, it gets very warm, smelly, & musty! There solution is too run the small bathroom fan all night, thats mounted in the ceiling, for this same situation.

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Old 04-30-2009, 11:12 AM   #3
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Small cabin moisture problem


You will need to remove both the vapor barrier and the insulation to let the wood dry out.
Does the insulation have a paper covering or is it unfaced? I don't understand this, "There have been no cut-outs to allow for air exchange anywhere on the walls yet." Why would you do this?
Does the roof have ventilation? Has the vapor barrier been sealed top and bottom? Propane produces a lot of moisture when burned. What sort of heater are you using?
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Old 04-30-2009, 11:59 AM   #4
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Small cabin moisture problem


In addition, you have ply (.75 permeable) acting as another vapor retarder on the outside wall. The moisture is also coming from the ground as there is no ply (vapor retarder) to stop it. Is the floor insulated?

Vapor barriers: http://www.buildingscience.com/docum...gs?full_view=1

Material permeability: http://www.buildingscienceconsulting...gmaterials.htm Be safe, G
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Old 04-30-2009, 12:42 PM   #5
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Small cabin moisture problem


Quote:
Originally Posted by jomama45 View Post
There solution is too run the small bathroom fan all night, thats mounted in the ceiling, for this same situation.
I think that may work, having a fan run while inside. But not too sure as to what to do when I'm not there too.
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Originally Posted by Ron6519 View Post
You will need to remove both the vapor barrier and the insulation to let the wood dry out.
The wood isn't saturated yet as I have just recently finished the vapor barrier.
Quote:
Originally Posted by Ron6519 View Post
Does the insulation have a paper covering or is it unfaced?
It's unfaced, partially rigid insulation.

Quote:
Originally Posted by Ron6519 View Post
I don't understand this, "There have been no cut-outs to allow for air exchange anywhere on the walls yet." Why would you do this?
I'm just making a note to let all know that I haven't considered making a hole (with bug screen mesh) in the wall to allow air to flow in/out of the cabin as a solution, but if it's necessary I will.
Quote:
Originally Posted by Ron6519 View Post
Does the roof have ventilation?
Yes, the roof does have ventilation. It has foam ventilation barriers attached to the top at ever second rafter opening. Once those were in place, then the insulation and vapor barrier were installed.
Quote:
Originally Posted by Ron6519 View Post
Has the vapor barrier been sealed top and bottom?
Yes, the vapor barrier has been sealed from top to bottom. Any seams were covered with tuck tape. However, I have not yet used the acousic sealant at the bottom of the walls where they meet the floor. I have an extra 12"-14" of poly still sitting on the floor.
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Originally Posted by Ron6519 View Post
Propane produces a lot of moisture when burned. What sort of heater are you using?

Ron
I've been using a non-vented 1500 BTU radiant propane heater for the time being. Until I can find an inexpensive direct-vented propane heater.

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Originally Posted by GBAR in WA View Post
The moisture is also coming from the ground as there is no ply (vapor retarder) to stop it. Is the floor insulated?
No, the floor is not insulated or vapor barriered yet. I have to jack up the cabin a couple more feet higher to be able to get underneath it to do both. That will happen next summer though.

Thanks for the links too. I'll read them right after I post this for more help.
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Old 04-30-2009, 06:42 PM   #6
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Small cabin moisture problem


This reminds me of ice fishing in Minnesota. After about an hour the walls are always dripping wet. I didn't see any above posts that really pointed out your problem or solution for that matter. What you have is excess moisture condensating on a cold surface. Now that moisture comes from at least two sources that I can determine. #1 YOU. You release moisture into the air from breathing, using the sink, showering, etc. #2 The propane is also giving off a bit of moisture.
In a small cabin, you are going to notice this very quickly. You really have a couple option. You can either get rid of the barrier and allow the moisture to escape naturally or you can install an air exchanger. The second is more preferable, for energy reasons and the fact that option #1 still might not be effective enough. Good luck!
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Old 05-01-2009, 05:17 PM   #7
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Small cabin moisture problem


It's sure hard to compare S. Ga. to British Columbia. IF you can run a bath exhaust fan and it helps to remove the excess moisture, could you determine an amount of time each day you may need to run it and put a timer on the exhaust fan so that it runs for a predetermined amount of time each day? Just a thought, David
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Old 05-02-2009, 02:12 AM   #8
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Small cabin moisture problem


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Originally Posted by jaros bros. View Post
Now that moisture comes from at least two sources that I can determine. #1 YOU. You release moisture into the air from breathing, using the sink, showering, etc. #2 The propane is also giving off a bit of moisture.

In a small cabin, you are going to notice this very quickly. You really have a couple option. You can either get rid of the barrier and allow the moisture to escape naturally or you can install an air exchanger. The second is more preferable, for energy reasons and the fact that option #1 still might not be effective enough. Good luck!
Yes, I did notice a big difference when the vapor barrier was not completed and when it was fully sealed. The moisture was trapped way easier when it was completely sealed.

I don't really want to get rid of the vapor barrier, so what kind of air exchanger would you suggest for such a small area? I would also like to make sure that it stays warm enough throughout the night. It will also have to be something that doesn't rely on electricity as there is no power out there.

Quote:
Originally Posted by Thurman View Post
IF you can run a bath exhaust fan and it helps to remove the excess moisture, could you determine an amount of time each day you may need to run it and put a timer on the exhaust fan so that it runs for a predetermined amount of time each day? Just a thought, David
I had thought that might work, but I have no electricity out there at all yet. Eventually I will get some solar pannels and battery storage.....but that's still quite a ways in the future.

I had tried leaving a window open through the night about an inch, but that just seemed to aggrevate the problem and make it more moist.
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Old 11-30-2010, 08:12 PM   #9
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Small cabin moisture problem


Have you looked into a very small HRV (Heat Recovery Ventilation) unit? this will help tomove air as well as retain about 80% of the heat while ballancing out indoor and outdoor humidity levels.
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Old 12-01-2010, 06:59 AM   #10
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Small cabin moisture problem


sounds like you have a real tight cabin.
does it have a block foundation with dirt floor? if so , moisture will radiate up through the flloor . you need to lay down a minimum of 6MM plastic vapor barrier.
you also need a dehumidifier. if you use LP as a heat source ,this can add to the moisture.
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Old 12-01-2010, 07:00 AM   #11
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Small cabin moisture problem


sounds like you have a real tight cabin.
does it have a block foundation with dirt floor? if so , moisture will radiate up through the flloor . you need to lay down a minimum of 6MM plastic vapor barrier over the dirt .
you also need a dehumidifier. if you use LP as a heat source ,this can add to the moisture.

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