Reducing crawl space moisture

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Old 12-22-09, 06:25 AM
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Question Reducing crawl space moisture

My house is apprx 14 years old. My crawl space floor is clay and is covered with plastic apprx 80%. I also installed an exhaust fan on a timer in the crawl space door to remove the humid air and pull in air from the outside. However, the moisture content of the floor joists is still above recommended. The termite guy suggested setting up a de-humidifier with a drain line to the outside. He also suggested removing the moisture barrier so that the clay floor could dry out. Is this a good idea? Any other suggestions?
 
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Old 12-22-09, 07:06 AM
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Moisture

Cover the remaining dirt with a heavy vapor barrier such as 6 mil plastic. Install a high capacity dehumidifier designed for low temperature conditions. Connect the dehumidifier to a condensate pump to move the water to the outside. You need to dry out the under side of the house first.

If you decide to remove the vapor barrier later to attempt to dry the dirt, be sure to re-install the vapor barrier once the dirt is dry. The vapor barrier and dehumidifier is a necessity.

Pulling in outside air during high humidity weather will aggravate your situation.
 
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Old 12-22-09, 07:26 AM
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Hi Back40, There is a way to do both, seal off the moisture from the clay floor and dry out the clay at the same time. May be overkill for your location, but the principle/application is interesting. In the NE we have to deal with radon, so applying a VB can trap the gas and create problems. The solution is to install the VB as you will be considering, and add to it a layer of gravel and a venting system that will remove the moisture and where I live the radon. It makes a class A system and in your application removes any concerns about trapping the moisture below.

When you cover the floor with the VB, seal it to the walls and around any supports. Here is a link on crawl spaces: BSI-009: New Light In Crawlspaces —

Enjoy
Bud
 
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Old 12-22-09, 05:06 PM
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You must put down a VP. All seams must be sealed. VB needs to run up walls to at least ground level. Vents need to be closed to keep RH from entering. (only do this if you don't have a fossil fuel appliance that uses this air for combustion) Fans will only bring more moist air into the crawl space. A dehumidifier still could be needed but know way to tell until you get the VB down. Chandler Your up!!!!!!!!!!!!!
 
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Old 12-23-09, 08:42 AM
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Originally Posted by Bud9051 View Post
Hi Back40, There is a way to do both, seal off the moisture from the clay floor and dry out the clay at the same time. May be overkill for your location, but the principle/application is interesting. In the NE we have to deal with radon, so applying a VB can trap the gas and create problems. The solution is to install the VB as you will be considering, and add to it a layer of gravel and a venting system that will remove the moisture and where I live the radon. It makes a class A system and in your application removes any concerns about trapping the moisture below.

When you cover the floor with the VB, seal it to the walls and around any supports. Here is a link on crawl spaces: BSI-009: New Light In Crawlspaces —

Enjoy
Bud

Very enlightening, Thanks Bud!. Regarding VP installtion , I have read different instuctions. Some say run the VP up the wall and some say to stop apprx 6" short of the wall. Which is correct?
 

Last edited by Back40; 12-23-09 at 09:11 AM.
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Old 12-23-09, 09:44 AM
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As airman stated above, you can run the vb up to ground level. If you were adding rigid foam insulation it could stop somewhere behind the foam, 6".

Bud
 
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Old 12-26-09, 05:57 AM
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Keeping timber dry and warm - stops mould!

Over time timber adapts to the average humidity in the area ending higher than factory adjusted.
This is not to say that the timer is damp enough for mould to form.
Timber is protected to an extent by the heat emitted from the home above.
The heat from the room above moves down by conduction and radiation usually keeping the timber just that little bit warmer than it surroundings.
Water vapour is held in warm air, at 30C there is 30 grams of water vapour in each cubic metre of air. At 20C there is 18 grams and blow 5 there is virtually nothing.
As water vapour always moves from warm to cold, or from a high pressure area to a low pressure area, your timber is normally OK.
However, on average about 200 days a year the outside temperature suddenly rises and meeting the cold ground outside frost forms.
In the same conditions warm wet air moves into the crawl space and deposits the water vapour on any smooth surface as condensation, in the case of timber and concrete or other rough surfaces it is deposited within the item itself.
Water vapour is a very tiny molecule, so tiny that it can move inside the structure of most things.
The key thing is to keep the warm wet air out of the crawl space and keep the timber warmer than its surroundings then the water vapour is deposited elsewhere, the timber says as dry as the average humidity in the area and mould doesn't form.
Insulating the space between and below the joists helps to keep the wood warm and dry fix sheets of two inch polystyrene under the joists making sure there are no holes, this stops mould it also helps make the rooms above so much more comfortable.
 
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Old 12-26-09, 03:37 PM
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In reading this I would like to ask about my situation. I have a small log cabin, 18x18. It is in the piedmont area of NC. There is a crawl space below and it has a block wall all the way around with 4 vents. My floor joist are 10"x14" forest pine timbers that are over 100 years old. I got them from a salvage yard. How is the best way too make sure they do not get to much moisture. I do have a VB on the clay floor.
Thanks
Rowdy
 
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Old 12-27-09, 05:07 AM
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Insulating rarely used cabin

Rowdy, I imagine you use it for weekends and holidays and for the most part it is empty?
You have a wide range of weather up there with lots of rain and snow?
Therefore there is not much warmth to be gained from the rooms above?
And the weather is quite changeable.
The best approach is to stop the warm wet air getting under the cabin, by blocking the vents and sealing any holes. Look at the joins between the walls and the floor, then the floor itself, stop the air from being pulled up through the floor and out through holes in the cabin walls and roof, seal every hole you can find. This will also mean the cabin is much cheaper to heat and much warmer to stay in.
The logic is that water vapour will always move from warm to cold.
Any moisture in the ground will stay put and the only source of damp then is the short wall.
With the space under the cabin being enclosed and sealed from the outside air, it will remain at ground temperature about 50f. At this temperature air is able to hold less than 5 grams of water vapour per cubic metre, (very little) any rise in temperature will be very slow (as the space is sealed) and any water vapour entering the space will head straight to the coldest (probably a wall on the other side) surface and condense.
If at the same time you insulate either above the floor, one inch of polystyrene will raise its temperature by about 4 degrees, or under the floor, where polystyrene being water proof and almost water vapour proof will help to keep the cabin both warm and dry, you will enjoy your weekends even more.
Perry
 
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Old 12-27-09, 08:31 AM
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VB on ground and up walls to ground height. Seal all seams with spray glue. close vents.
 
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Old 12-30-09, 05:57 AM
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sealing a crawl space, air supply to fire

A couple of things I didn't mention.
The quoted humidities are at 99%, the usual humidity would be lower.
While the amount of water vapour in a cubic metre of air is almost insignificant, the problem is compounded by the millions of cubic metres of water vapour that will arrive in an area over the course of twenty four hours.
Sealing all the holes in your cabin will stop the passing wind from stripping the heat from the cabin and, it will also stop your fire from working.
To deal with this latter point, you need to install a four inch pipe under the floor, from the outside to either under the fire or a close too as you can get this will enable the fire to burn correctly and will stop any drafts, avoiding that being cooked on one side and being frozen on the other side situation.
When leaving the cabin you need to consider how to block the chimney and block the air supply pipe, a child's ball or similar will often do the trick.
 
 

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