Crawl Space Vent Fan?


Old 01-07-15, 10:18 AM
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Crawl Space Vent Fan?

Hi. My first post and probably a long one, so please bare with me.

My parents have a small, old cabin on a lake in Central PA. This spring I plan to do some work to address some of their neglect, starting with drying out the crawl space (last time I was there I noticed the washed pipe was disconnected and drained into the crawl space). Cold water pipes were also sweating, but otherwise the crawl space looked in decent shape for being 70 yrs old (no visible mold or wood rot).

A couple notes about the cabin:
1. It is not conditioned. No AC and they close it up in the winter (if they do go there's a wood-burning stove)
2. The cabin interior always feels damp or musty. Toilets sweat quite a bit.
3. They use it maybe 8-10 times a summer, so it doesn't justify too much expense.
4. The crawl space is tight and I can only probably reach about 50% of it.
5. Crawl space is passively vented (several around the foundation). There is some vapor barrier already down but could use some repair/addition.

Anyhow, my current plan is to:
1. Address gutters and downspouts to get roof water away from the foundation.
2. Remove any debris from the crawl space
3. Fix washer drain pipe and insulate all the cold-water water pipes that I can reach to address the condensation.
3. Temporarily seal the vents and dry out crawl space with a dehumidifier, space heater, fans while I'm repairing other stuff.
4. Repair/replace vapor barrier and seal where needed. Remove dehumidifier, heater, fans.

So my question then is what to do next. I've searched a lot on this forum and others and what I've determined is that I have 4 choices:

1. Seal crawl space
2. Seal crawl space and add dehumidifier
3. Add crawl space vent fan with humidistat (powered ventilation)
4. Do nothing further then vapor barrier repair (passive ventilation)

Reading online, #1 and more so #2 above sound like the preferred choices particularly for conditioned homes, but I don't think the cost (initial and monthly energy) of these options are feasible for my parents, particularly since I can't reach about 1/2 the crawl space (though it does have vapor barrier everywhere).

So, then it's down to #3 or #4 (vent fan or not). I've read conflicting info on vent fans, but most of them are in regards to air conditioned homes and humid climates. This cabin is not air conditioned and Central Pennsylvania, though it can get humid, isn't Florida. Where the cabin is (Somerset area) it stays pretty cool in the summer.

This is why I can't find an answer through my searches online - Can't find any recommendation for a unconditioned cabin (where the inside, outside, and crawl space would likely share the same temperature and possibly humidity).

Thanks for making it this far. I would be grateful for any response and thoughts on the vent fan in this unique situation. - Matt
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Old 01-07-15, 10:26 AM
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I would put down a vapor barrier throughout and increase the number of vents, if there are not enough.

Conditioning this space does not make sense since the cabin is rarely conditioned.
Old 01-07-15, 10:47 AM
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Current ventilation is into the crawlspace and up through the house, thus the "feels damp or musty. Toilets sweat quite a bit" comment. The source of that moisture is most likely the crawlspace floor. Other source would be relative humidity in that area. I assume those comments are associated with the summer visits.

In any case, as Mitch stated, the cabin is rarely conditioned so converting that crawl to sealed conditioned space will not work.

That leaves isolating the cabin from the crawlspace. Ideally you would insulate and seal the bottom of the cabin floor in addition to repairing or replacing the vapor barrier currently there. If addressing the cabin floor is not possible, at least seal all penetrations and increase the natural ventilation.

When it comes summer, you will need to see if added ventilation is introducing more moisture than it is removing. In the summer you might need to run that dehumidifier and close off the vents.

Old 01-07-15, 12:01 PM
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Wow. Thanks for the quick and helpful responses. I agree the dampness in the cabin is most likely due to the air from the crawl space and to a lesser extent the outside air (inside feels damp even on hot, dry days). My hope is that initialing drying out the crawl space, addressing the vapor barrier, cold-water pipes, and downspout issues would noticeably decrease the crawl space moisture and thus inside dampness. I don't know how to seal the cabin floor (do you mean from below or looking for gaps/spaces in the floor from above?)

My initial thought supporting the vent fan was that humidity in the crawl space would never be less than the outside air (because of the lack of conditioned interior and moisture in the ground coming through an old foundation and the cold water condensation). And that more ventilation can't hurt. But maybe this won't be true after my fixes, especially on a hot humid day?

I live a 7-hour drive from the cabin so I really only have one shot to do this in the spring and won't be able to monitor the crawl space this summer. I'm still on the fence regarding vent fan but may leave it for another time.

Thanks again! - Matt
Old 01-08-15, 07:21 AM
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Install a VB and seal. Close all vents. use a remote RH meter $50 or so at radio shack. If RH is over 55% then look into other options. Everything is sweating because the RH is so high.
Old 01-08-15, 11:53 AM
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I have a similar situation, but I'm dealing with it in a primary residence. I only have vents at the back of the house, and it looks like they were added after the house was built. I called a contractor about installing vents in the front and he didn't want to do it. He was in favor of crawlspace encapsulation. More I read on it, it seems that's the way to go and adding more vents could make the issue worse. The research I've done basically says that if the air outside is a certain temp it can hold a certain amount of moisture. Hot air holds more than cold air. Once air is cooled and the capacity to hold water diminished, the excess the air cannot hold any longer turns into condensation. Crawlspaces are almost always cooler than the exterior ambient temperature, so if you ventilate more, you're just bringing more moisture into the crawlspace. Now stagnant air condenses more than circulating air, so it's possible to somewhat control the amount of moisture by forcing air exchanges, but crawlspaces are so cluttered with trunk lines, spaces between joists, wiring, plumbing, etc that it creates a lot of dead air space. So the only practical way to eliminate moisture ingress is to seal the crawlspace and condition it with either tapping into the HVAC system, which you said doesn't run often enough to provide benifit, or by using a dehumidifier.

not sure this is telling you anything you don't already know, but wanted to confirm your thinking that adding ventilation isn't the answer unless you add enough of it to turn the underside of the house into a wind tunnel and can be about ambient temp. That would create issues if the structure is in a area that has sub-freezing temps in the winter and pipe freezing is a concern.
Old 09-03-15, 08:20 AM
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Very late reply (8 months) to the last post, but i didn't get a chance to get to my parent's cabin until just recently. Recall the cabin is not air-conditioned (no cold ducts), nor heated (closed and winterized in the fall) and used only sparingly so encapsulation with a dehumidifier or HVAC supply is not feasible and hopefully not needed. As far as I can tell, the main sources of wetness in the crawl would be the cold water pipe condensation and ground moisture getting around the VB. Here's what I did in the crawlspace (it was still wet with condensation on the cold water pipes on this latest trip):

1. Cleared gutters and added long flexible downspouts to hopefully better divert rain water down and away from the cabin.
2. Removed any accessible loose trash/wood in the crawl space holding moisture.
3. Re-arranged the vapor barrier down there to cover as much of the floor as possible. Bought some more VB and duct tape and will put this down later.
4. Foam-insulated and duct taped all accessible cold water pipes.
5. Temporarily sealed vents and put a dehumidifier down there for a couple days just to dry things out a little.
5. Removed dehumidifier, re-opened vents and added small 110 cfm exhaust fan to one of the vents closest to the well-water entry and largest area of dampness.
6. I then asked my dad to inspect the crawl space and general mustiness of the cabin when he goes up this weekend and turn off the fan if there is no improvement (or it's worse).

My thought process is since I've insulated the cold water pipes and the cabin is unconditioned:

1. crawl space temp and outside air should be similar (i.e., similar moisture content in the air). If anything, the ground moisture in the crawl would increase the humidity/dew point relative to the outside air justifying the exhaust fan.
2. At the very least, the negative pressure created by the fan in the crawl should help reduce (maybe only slightly) the "stack effect" or amount of crawl air pouring into the cabin.
3. Knowing my dad was headed up back up within a week or two, if there was no improvement or crawl moisture got worse, the fan could just be turned off, and we could re-assess next steps.

I'll let you know how the fan works out from my dad's crawl space assessment next week.

Thoughts? Thanks. - Matt
Old 09-09-15, 10:14 AM
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Just realized this should probably be in the basements and crawl spaces forum but not sure how to move it.

Anyhow, update from my dad on his last trip to the cabin:

He said the cabin smelled considerably less musty when they entered and the crawl space was dryer than he previously remembered. It was a dry couple weeks since I installed the fan, but being that it will only get dryer in the fall, I asked him to keep the fan on and just turn it off (unplug it) when they go up to winterize the cabin. Will keep you updated, but so far, the crawl exhaust fan seems to be helping with both the wet crawl and the cabin's musty smell.
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