Questions about sealing a crawlspace.

John_W

I need to seal the crawlspace of my house (and maybe add a dehumidifier) to eliminate the water condensation problem down there.

What effect might sealing the crawl space have on the performance of a radon mitigation system?

Will there be less of a pressure difference between the conditioned space and the crawlspace after the seal is in place?

Should the air quality in the conditioned space improve after the seal is in place?


The house:

We have a 2,000 square foot, single story ranch style brick house on a crawl space.

All duct work is in the crawl space along with the indoor heat pump coil (with UV light), gas furnace, blower and two stage filter unit (fan fold element and activated charcoal element). The entire HVAC/duct system is 3.5 years old. The main supply trunk line is 16” metal, running along the north and east ends of the crawl space. All ducts to the floor registers are 6” flex. The return duct is 18” flex.

There are air vents in the crawl space walls on three sides of the house (north, west and south). There is fiberglass insulation (R17) between all the floor joists.


The radon mitigation system:

The floor of the crawlspace is covered with heavy plastic. The plastic goes to the walls and part way up them, where it is sealed to the cinder block with adhesive. A duct, under the plastic, runs the length of the crawl space (west to east) and terminates at a pipe that goes through the foundation wall and connects to a fan and exhaust stack. The radon mitigation system is 2.5 years old – and in good working condition.


The condensation problem:

I found small puddles of water across the north and east ends of the crawl space, mostly within a few feet of the main supply duct. The problem is worst where the crawl space is nearly filled, ground to joist, with the supply duct - and the vents to the outside are nearby. I suspect that the air in this tight corner of the crawlspace is being cooled more than elsewhere, and the closeness of the vents lets outside, humid air in, making things worse.

The condensation is also causing the insulation to smell like cat urine. Its not cat urine, a dead critter, moldy coil, etc. I looked everywhere, including with a good UV flashlight.

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john-hvac-new

The urine smell has dissipated considerably over the last week (owing, it seems, to lower than normal humidity in the last week). I'm hoping that this particular smell will not be a problem once we get a permanent dehumidification scheme in place.

I have ordered a 70 pint/day dehumidifier and am going to seal all the vents and other holes into the crawl space.

There is another smell, an earthy crawlspace smell, that has always been there. The house is 45 years old. I suspect that this smell is going to hang around, at least to some extent, even after we dry the space out.

I like the idea of completely sealing (as in hermetically) all the surfaces down there, including the underside of the floor. If there is an easy to apply, no VOC treatment, I will give that serious consideration.

However, there is still the radon issue to deal with. The poly of the mitigation system is top notch, but not of swimming pool liner thickness and not a one-piece sheet. So, there is still some radon in the crawlspace.

After I seal the vents and install the dehumidifier, what we then probably need is to pull a slight vacuum on the air above the poly, exhausting that to the outside. That could allow us to put a few small holes/grills in the floor as a way to supply the air needed to accommodate the vacuum being pulled on the crawlspace.

Maybe just pulling conditioned air down into the crawlspace would eliminate the need for a dehumidifier in the crawlspace - and exhaust any residual radon to the outside - at the same time.

banzaitoyota

read up on the building science website

beenthere

That would create a negative pressure in the occupied area. And pull in unconditioned air to that area. And raise humidity.

john-hvac-new

Good to know.

The radon mitigation contractor told me, a couple of days ago, if I seal the vents and add a humidifier, there would be no effect the radon system. It will work as before. So, I don't need to add any extra vacuum to the space above the poly.

I do wonder about air quality in a sealed crawl. Is there any air exchange? Will it get really stuffy or unhealthy for workers?

I have sealed a few vents and will finish the rest when the humidifier arrives.

I think I actually have two problems, after all. One is the excess humidity and the other really is a cat urine type smell. The more time I spend in the crawl, the more my nose tells me that the cat urine smell is a localized thing, in the corner near the crawl door.

Last weekend, I tried finding the urine crystals with a UV light - no luck.

Yesterday, I sprayed the whole area (about 15' x 15') with the enzyme/bacteria type urine smell remover. Haven't been back down yet to see if that helped.

beenthere

The humidifier will prevent it from feeling stuffy.
You will still have some infiltration from your first floor to the crawlspace. So no worry about any build up of VOC's in the crawlspace.