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Any reason for not venting the crawl cavity?


Robert Jones
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Custom home built in 2006. Located in Bremerton WA. There are no crawl space vents and the area is not conditioned. It has not led to any damage from moisture, but still not right in my opinion. Anyone think of any other reason why it was not vented?

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A guy had a slab porch poured in front and behind his house, vocered up all the vents on a 1875 house, the house had no moisture problems beneath it until 6 years after the slabs covered the vents, I had to vent the crawlspace after replacing all the wood work beneath the house on a job a while back.

Also, on mobile homes (experience from a mobile home park... sigh) unvented trailer houses will fair better on the wood because of the under webbing, but the 1/4 inch thick I beams will be about 1/8th inch in aobut 25 years (after you knock off the loose rust) from the older models we've had moved out. This is just my past experiences.

PS: Mold likes dark humid areas

But I also found this...

Matt

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I have a crawl space with 4 vents. I had a mouse problem beyond belief which is a result of having a few bird feeders. The mice were chewing through the screening on the vents and were a nuisance. Some of the mouse nests in the insulation were causing it to sag. About 5 years ago I sealed the four vents with plywood and caulk. Although the crawl has a sump pump and gets water periodically, I have not had any mildew or mold buildup. I get no musty odor. I have the mouse situation under control. (Norway rats under my greenhouse are another problem) I guess there are exceptions to every rule. Living in an urban environment, we have more wildlife than the wild kingdom tv show. Mice, rats, chipmunks, skunks, raccoons, possum, woodchucks, fox, coyote and deer. The two red tailed hawks that mate and nest nearby keep the squirrel population under control. A bird feeder or two will attract them all. But, back to the crawlspace. No problems with the vents sealed.

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I'm completely in the seal the floor, no vents, condition the crawl camp.

I like that heavy vinyl liner that the crawlspace contractors use; it's heavy enough to detail satisfactorily, you can use tapcons and cleats to seal it to the wall and columns, and you can crawl on it without tearing it.

It's amazing that we all did it so ass backwards for so many decades.

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A lot of factors involved and a little difficult if you can't be there to see it yourself.

Floor is insulated so i'm assuming the foundation walls are not.

Washington state, cold and humid climate.

Insulate the foundation walls down to the footing and use the crawlspace for an HVAC return air plenum.

Still a wild guess.

Marc

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Thanks for the replies everyone. Brandon, the home was built in 2006 and yes there were some drainage issues. The property ran downward from back to front. They have installed a rock retaining wall along the back side of the home. The crawl floor did have some patches of standing water and you can see the leaching going on from the stem wall/footing area. I advised my clients of the need for venting and that drainage repairs would be needed along the back side of the home to help redirect the ground water/runoff. For the ventilation, I simply quoted the 2003 IRC wich calls for 1/150.

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Washington state, cold and humid climate.

I'd call it a moderate, occasionally humid climate. During the summer, the humidity along the east and golf coasts makes this place seem like Arizona in comparison.

There's no point in insulating foundation walls here unless one plans to use the crawl as a plenum and wants that warm air under the floors to heat the floor so that the heat can radiate upward into the living space.

When the floors are properly insulated here and the crawls have the right amount of air moving through them, it's typically about 55° in the crawls here except for the coldest days of the year. Don't know why, maybe it's got something to do with the fact that this entire mountain range is nothing but a bunch of old volcanoes strung together and there's still magma under a lot of it.

It's a pretty rare day here in the summer when I actually feel all clammy due to the humidity like I used to experience in New England, New York where I grew up or in any of the eastern and southern states where I'd been stationed. I wouldn't call it "humid" at all.

A sealed crawl will work pretty well here if the vapor barrier has been put down well. Unless gobs of humidity is evaporating from the soil into the crawl because the barrier is a lousy install, I've found that most floor systems will allow the vapor to diffuse really well; even when under-ventilated. Still, the truss webs are OSB, and if the floor is OSB, I'd also recommend they get vents on every side within six feet of the corners and lots of air in there.

The guy that put down that barrier should be slapped upside his head for being too lazy to do a neat job.

John,

1/4-inch galvanized mesh not less than about 22 ga. is what you need. It'll keep out just about everything up to the size of a Wolverine if it's installed right.

Rob,

How close is that retaining wall to the back wall of the house?

ONE TEAM - ONE FIGHT!!!

Mike

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Assuming the soil vapor barrier is installed correctly and in fact is blocking the moisture from the soil, I would be very concerned about htis space not being ventilated. The interior humidity from the house will be driven downward from the living space into the cool space where it will have no way out except permiating through the concrete foundation.

It is not if the lack of ventilation will be a problem but when.

On another issue I notice two areas with apparent staining on the exterior plywood, above the confined rake on the left side of the ridge, and above the bathroom window. Any ideas on the cause, (or is it something in the picture?)?

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Ok, Thanks.

It's only a guess, but with that kind of elevation behind the home I'd say that the retaining wall contractor has probably put some pretty good drainage behind that wall - otherwise you would have been crawling over a water bed in that crawl. Except for behind those porches, it looks like adding vents won't be too difficult at all.

With all the knowledge of building science we have today, what kind of lame brain is still putting in gable end vents along with eave vents and a ridge vent. Tsk.

Dirt or stone under those porches? If dirt, you might try talking the client into getting some quarter stone over the dirt under there. I've never been under a porch with quarter stone yet where critters had been rooting around; but I bet if there's just dirt there those porches are going to have the rodent reel tours posting tiny toothpick sized signs around the home advertising the new condo to every critter around.

ONE TEAM - ONE FIGHT!!!

Mike

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I would be very concerned about htis space not being ventilated. The interior humidity from the house will be driven downward from the living space into the cool space where it will have no way out except permiating through the concrete foundation.

I doubt it. That never seems to be the case here. Besides, vapor diffusion moves from warmer to cooler and moist to dry - not the other way around. The soil around here is glaciel till and it holds moisture from rains for days in this climate. The foundation walls would diffuse to the interior, even when the outside is well treated, just by absorption through the footings.

There seems to be enough latent heat and ambient moisture in crawls to prevent that type of diffusion from happening here. I can honestly say that in nearly 14 years here I don't think I've ever seen a single instance where what you've described appeared to be the case.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Rocon

Besides, vapor diffusion moves from warmer to cooler and moist to dry - not the other way around.

There seems to be enough latent heat and ambient moisture in crawls to prevent that type of diffusion from happening here. I can honestly say that in nearly 14 years here I don't think I've ever seen a single instance where what you've described appeared to be the case.

Then we agree. "vapor diffusion moves from warmer to cooler and moist to dry"

You stated that the cool crawls are usually about 55 degrees (cool). The interior space would be conditioned and probably around 70 degrees (warmer). The humidity from people living in the house would drive from the living space into the unventilated crawl where it would be trapped.

If the soil vapor barrier were installed correctly it would not allow any moisture trapped in the crawl to percolate into the soil (assuming a low water table). You would have a bathtub under the house.

Just not seeing it happen does not mean that it does not exist. On another post someone just found their first chineese drywall. The condition existed before he saw it.

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  • 3 weeks later...

Robt,

The pic looks to me like those walls are poured. Around here in '06, with the speculative frenzy in full force, a block mason was often scarce, and I saw more than one foundation poured all the way up throught the walls. The guys who formed it up likely had no vents on site and were not thinking, just working and looking to getting paid.

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