Crawlspace Venting for House on Posts.


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Old 10-10-11, 01:09 PM
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Crawlspace Venting for House on Posts.

I've got a cottage that is build on the side of a hill. its built on posts so no foundation, but the perimeter is skirted with a hardboard that prevents much air movement, especially to the back of the cottage where the clearance from the ground to the floor joists is minimal. (8" or less).

I'm having humidity problems in the house and I believe much of the humidity is coming from underneath. I've covered the ground down there with 6mm poly, but the critters have chewed many holes through it, so not sure what if any good it is doing.

I was considering adding some mechanical ventilation under there to keep the humidity from building up.

My thought was to install a whole house vent in the crawlspace with ducting going to the hard to get at areas and keep the air down there changing over.

From researching crawlespace venting, i'm confused and not sure if this is a good or bad idea.
 
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Old 10-10-11, 04:42 PM
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So u want to bring in 60 70 80 percent rh from outside into a cool crawl. I'd say a bad idea. 6 mil VB sealed and run up the walls. Close vents
 
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Old 10-10-11, 06:33 PM
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Thanks Airman. I think that the critters/mice will just chew through the 6m VB again if I replace it. Without a proper foundation wall it will be next to impossible to keep them out. So I think a functioning VB is going to be tough.
Any other suggestion? Maybe a dehumidfier?
 
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Old 10-10-11, 07:40 PM
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With out a VB a dehumidifier will never stop running. You need the VB to stop the moisture. Get 6 mil that is fire rated. Animals don't like the taste
 
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Old 10-10-11, 08:40 PM
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How is the bottom of the house sealed off from that crawl space? Plumbing, electrical, and other penetrations will be adding to the humid air that is flowing up into your home.

Do those critters have access to your insulation down there? If so, they can make a real mess out of it and create some real health issues.

Sounds like you need to upgrade the structure around the perimeter so you can keep the critters out and then apply the new VB as airman is suggesting.

Bud
 
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Old 10-10-11, 10:13 PM
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There is only a small area that is insulated with Roxul insulation or Rock Wool which is not affected by moisture. that is in the joist bays of one section that I re-did from the top and it sealed off from under side with 1/4 galvanized wire mesh and is doing quite well, but the 6mm on the ground is all chewed up. The rest has NO insulation. I use it occasionally in winter and wear slippers!
Sealing off under there from critters is going to be a major project. Getting the VB tucked up into the hard to reach places is next to impossible and having it go up the skirt wall will be impossible without digging out more access space.
I just finished installing a wood floor on the entire first floor and used a craft paper product under it that is the equivalent of 30 lb felt paper. I'm surprised the humidity is getting up through that. there are almost no penetrations up through it.

I was really hoping that mechanical ventilation would be a good solution, I thought that if i changed the air under there X times per hour, that it would be the same environment as outside the house and no condensation would occur. I guess I was wrong.

thanks.
 
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Old 10-11-11, 04:54 AM
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The key to the issue of air exchange is RH (relative humidity). Outside air at 70 and 80 % RH will increase in RH as the temperature drops. So, pull that air into your crawl space where the ground is 50 and the air becomes 100% humid. I'm guessing on the numbers.

Exposed ground can generate gallons of water per day. If the space is simply closed off the air will rise in humidity level until it is in balance with the moisture in the soil and since the soil can draw moisture from far below it gets very damp down there.

The last issue is stack effect, warm air leaks out of the upper half of your home as replacement air pushes in the lower half, often from that very crawl. You applied a new layer to the floor, but did you seal all of the holes for electrical, plumbing, chimney, seams under walls, ans so on? Even a good effort will never get them all unless it is done during construction.

If you could replace the plastic down there and then cover it with something to protect it, might work. Or come up with something the critters won't attack. I recently helped someone increase their 6" of clearance to 2' for similar reasons. As long as it can't become a pond down there adding more clearance is always useful.

Bud
 
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Old 10-11-11, 06:31 AM
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Thanks Bud,
How did you dig to increase the clearance? shovel? I'm in NH (tough digging), but I did increase the clearance on one side to about 12-14". That's the area that is insulated and wire meshed, the same area that the floor joists rotted out and needed replacing. The rest of the place looks quite good under there for a 50 year old building. There are very few penetrations into the house from below. one drain and two water pipes, and they come from within a joist bay that is insulated and closed off with plywood to keep the pipes from freezing. I didn't mention that there is a small 'basement' that has a concrete floor and block walls. its approx 4' x 4' x 10' long. that is where most of the penetrations into the house are.
A friend suggested replacing the 6mm on the ground and pouring a concrete slab over it. that would require a pumper truck since there is no access from the shallow area and would have to come down the hill and push it back up the hill to cover the poly in the tighter areas.
is there any other type of cover for over the poly that might keep the critters off of it?

If I can't get the entire area 100% sealed off, will the moisture escaping the ground from those spots simply continue until the air and ground are in equilibrium? If that's the case then, isnt an imperfect job of VB on the ground a waste of time?

Thanks again for your help.
Mike.
 
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Old 10-11-11, 06:57 AM
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Even the concrete floor in that small basement area is providing moisture. I'll add a couple of links for reading.
We were digging in a sand/gravel material so not as bad as it can get. My helper dug and I pulled the sled out and dumped it, very slow. That's what high school kids are for.
Addressing any drainage issues from the outside that can direct surface water away is always good.
As for critters, they are there for a reason, shelter, nesting, food, warmth, whatever. Making that space unattractive to them and adding an abundance of traps can virtually eliminate them. I found one opening where they were getting into my basement and instead of sealing it, I placed 6 traps around it. 20 or so mice later the problem stopped. Then I sealed it.
IMO, if you plan to stay there long term, you need access to under that floor. Insulation needs to be installed and protected and once the floor is sealed off there should be no moisture entering the house.

In the short term, more plastic and perhaps as airman suggested, fire rated if it taste bad.

Bud

BSI-009: New Light In Crawlspaces — Building Science Information
BSI-003: Concrete Floor Problems — Building Science Information
 
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Old 10-11-11, 07:26 AM
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thanks Bud, and thanks Airman. I read your response on my crackberry and missed the part where critters don't like the taste of fire rated Poly.
I will set out a bunch of mouse traps as well to help keep them out.
I guess I can try to seal off the skirt with 1/4" wire mesh. not sure if they will try to tunnel underneath. but worth a shot.
I considered having the bottom of the floor spray foamed from underneath, but that needs to be covered up and I'm afraid of enclosing those joist bays and having moisture rot out the wood. The other method of insulating that seems to have worked well is the Roxul rock wool and wire mesh. so that may be the way to go for insulating.
The majority of the floor joists etc under there look very good for a 50 year old building, so I hesitate to make any changes that could possibly be detrimental.

Thanks again for all the great suggestions.
 
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Old 10-11-11, 07:50 AM
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I don't think spray foam would be a problem if you got the moisture issue under control. That said, foam is spendy, the wool might be considerably cheaper and seems to be working well for you.
 
 

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