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Open Them, or Leave Crawlspace Vents Closed?


Tim Maxwell
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I'm in GA, West of Atlanta. I had a crawlspace for 13 years and only closed my crawlspace vents when there was a big freeze warning down into the 20's or lower. Then I would only close them till we got back to more normal temps.

I always liked to keep the air flowing through the crawl. On a windy day I could feel a breeze in mine.

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Joe may be correct in theory, but my dad's house, a 45 yr old brick sided ranch on a vented crawl in Atlanta, has never had a problem with excessive moisture/mold/condensation/etc.

Might be because half of it is a tall crawl (walkable - you have to lean over to get through the 60" door) --- I was just in there helping him get his mower started yesterday.

I just don't see the types of problems Joe describes in vented crawls around here (unless there's plumbing or waste leaks involved.)

But, i don't know nuthin' 'bout them houses up north....

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Hi Jimmy,

While I agree that there is a way to safely close and seal every crawlspace, I don' t think it's wise to simply refer someone to Dr. Joe's top 10 unless you also explain to them under what circumstances they can safely seal a crawlspace in a given climate. Seal a crawlspace incorrectly, and, I don't care where you are, you can damage the home.

OT - OF!!!

M.

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I would have to agree with Mike. The house needs to be designed for it to be closed. Simply closing up a crawl space in the wet humid South or the Northwest just does not make it on my logic chart. Does anyone know of any other experts that recommend the same as Dr. Joe? It would be nice to see varying opinions.

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I've never inspected a crawl space outside of New England, so I don't pretend to know anything about the world of crawl spaces beyond, but...

When was the last time you saw a properly sealed crawl space?

We didn't have a lot of crawl spaces where I did inspections, but the ones we did have were generally...unpleasant. I'm guessing that crawl space creators and sealers found them similarly unpleasant, and (sad, but true) did not spend a lot of time putting forth their very best efforts into properly sealing them off. Verily I say unto you, that I have never, ever seen anything in a crawl space that approached "old world craftsmanship". For me, discussing "properly sealed crawl spaces" is about as meaningful as discussing what the offspring of a unicorn and a sasquatch would be.

Maybe that's a regional thing, but I'd be interested in hearing from other corners of the world.

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Originally posted by Jim Morrison

I've never inspected a crawl space outside of New England, so I don't pretend to know anything about the world of crawl spaces beyond, but...

When was the last time you saw a properly sealed crawl space?

I've seen probably a dozen since 1996. Completely sealed off from outside but the air inside conditioned. That's one of the things that Dr. Joe advocates - turning them into conditioned space. The typical crawl is not conditioned space. If you seal it off from the house without introducing it into the conditioned envelope of the house, you can make those already horrible conditions worse.

We didn't have a lot of crawl spaces where I did inspections, but the ones we did have were generally...unpleasant. I'm guessing that crawl space creators and sealers found them similarly unpleasant, and (sad, but true) did not spend a lot of time putting forth their very best efforts into properly sealing them off. Verily I say unto you, that I have never, ever seen anything in a crawl space that approached "old world craftsmanship".
That's a reasonable statement that I can agree with. However, just saying to seal them, and giving folks a link to Dr. Joe's top 10, without going into what Dr. Joe advocates doing in those areas when you do seal them, could result in lots of folks telling clients, "Seal 'em," with only half the story.

For me, discussing "properly sealed crawl spaces" is about as meaningful as discussing what the offspring of a unicorn and a sasquatch would be.

Maybe that's a regional thing, but I'd be interested in hearing from other corners of the world.

Fair enough, but I spend a lot of time in crawlspaces. To me, and others in crawlspace country, a discussion of how to seal them, or not seal them, is meaningful.

I've seen vented crawlspaces under homes built in flood plain areas that were 100% flooded with several inches of water under them, but with well installed vapor barriers, where there were no issues. Then I've seen crawls where the houses were high and dry and owners thought they'd seal them in the winter, without understanding what can happen when that's done, and they created a situation that ended up with fungi growing all over the place, wood rot, and insects.

If Ken closes off the vents, he needs to make sure that any soil under the home is fully capped and sealed, and then he should condition that space, because, unvented, he's going to get a situation where vapor diffusion from the interior of the home will drive moisture down into that crawl where he'll have created a huge petri dish.

The probability of someone creating the large petri dish is a lot higher with an existing crawl than it is with one of Dr. Joe's ideal situations, which are aimed at new construction, because if the homeowners got a nasty crawl that's all dank, littered with rat droppings and everything else that comes with that, he's certainly not going to want to vent that into his interior, so he'll probably leave it as sealed unconditioned space and ultimately get himself in trouble.

Most crawls I've been in are well above freezing, even in the worst winter weather here. I'm sure that I couldn't say the same if I were in New England, but I remember that our unheated and vented basement (a root cellar, really) where I grew up in New York only got down to freezing during unusually cold winters, so closing the vents when the temps in there are above freezing just doesn't make sense to me; especially if the floors and the pipes are insulated.

A while back, I had a really nasty crawl and recommended that my client consider having it cleaned out and getting it sealed using Jeff Tooley's methods, as described by Advanced Energy Corporation's study and explained numerous times in JLC. The only thing I recommended that he not do was close the vents, because he didn't want to spend money conditioning that space during the winter. The contractor hired to do the job talked to me and then cleaned it out and installed the barrier. It turned out beautifully - more like the interior of an operating room than a crawlspace. Even with the vents open, it's going to be a pleasure for anyone to inspect that area from now on.

There are regional differences that mean a lot when it comes to crawls. The humidity in the southeast is so bad in the summertime that leaving a crawl open during the summer can cause the exact things that closing them off improperly in the winter in northern climates can cause. Anyone in the southeast who has a crawlspace beneath their home, and is considering closing it, should spend some time checking out Advanced Energy's website.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by Jesse

Ken

I wait until a little later in the season to close my crawl vents, probably when temps are consistently below freezing. By then, our area is bone dry, and moisture in the crawl isn't really an issue.

Hi Jesse,

What you don't realize is that when the winter weather is dry like that and you close your crawl the crawl becomes cooler and drier than the interior of your home. All that moisture that you create in your interior is going to be seeking 24/7 to find it's way into the crawl through a process known as "vapor diffusion." Under certain conditions, that can cause that vapor to become trapped between the underside of floors and insulation, condense on pipes in the crawl, and lead to fungal growth.

You know what the conditions in your crawl are, but conditions in other folks' crawls might be more prone than yours to cause those conditions if they seal them up.

This is a case-by-case basis thing. The only blanket statement that I think applies is that a completely sealed crawlspace will be fine, regardless of climate, only if it is conditioned space.

Others might have had different experiences than I.

ONE TEAM - ONE FIGHT!!!

Mike

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Mike, you make a good point - that I really shouldn't give advice on this gentleman's crawl without being in it. I understand the system of vapor diffusion. I see only a few crawlspaces, but I've never experienced one with notable moisture during the really cold months, regardless of ventilation. That certainly doesn't mean it doesn't exist. It's entirely possible the home in question is at the bottom of a hill or within 100' of a river. You are 100% right that the only way to guarantee a good sealed crawl is to condition it.

What I said was that I close my vents later. It works well for my house on my lot. Ken, as a lot of the brothers here are saying, it's a safer bet to leave them open. If you're going to leave it vented for winter, spend a little cash, and a little time, and insulate any plumbing down there.

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I inspected a crawlspace in a 1980's Street of Dreams neighborhood that was completely sealed. There was a layer of clear plastic laid on the dirt with pea gravel spread across the plastic. I believe the depth of the gravel was several inches. There were heat registers in each room of the home as typical, but no ductwork. There was an open plenum from the furnace that blew into the crawlspace and entered the home via the registers. This is the only sealed crawlspace (as designed) I have come across. The crawlspace was very nice with no issues whatsoever after 20 years or so.

Has anyone else ever run across this design/ style of sealed crawlspace in the NW?

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Yes,

See my post above from 4:39 pm. That's what I was referring to when I said that it was conditioned air. Sometimes they are used as plenums and they're under positive pressure; sometimes they simply have floor registers opening into them, but what they had in common was the fact that they were conditioned.

OT - OF!!!

M.

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Here in MS I've only seen 1 sealed crawlspace. I inspected the house before purchase and my client wanted a sealed space. She asked me to look at the project once it was finished. A heavy barrier was installed. The sections of the barrier were well taped and for the most part the barrier was well bonded to the foundation and piers. Expanding foam was used to seal the top plate and even to seal around the B vent from the furnace..................................I kid you not! She decided to install a new heat pump. No conditioned air was injected into the space but a dehumidifier and a sump pump were installed. I examined the workmanship but not the science of the project. It has started me on a new field of study.

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My vote would be to have the water lines insulated and close the vents in late fall/early winter when the daytime highs are averaging in the upper 30s to low 40s. There will be enough radiant heat fro the home to prevent freezing, especially with insulated water lines. Unless of course the crawl has huge air leaks or vents.

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Originally posted by Brandon Whitmore

I inspected a crawlspace in a 1980's Street of Dreams neighborhood that was completely sealed. There was a layer of clear plastic laid on the dirt with pea gravel spread across the plastic. I believe the depth of the gravel was several inches. There were heat registers in each room of the home as typical, but no ductwork. There was an open plenum from the furnace that blew into the crawlspace and entered the home via the registers. This is the only sealed crawlspace (as designed) I have come across. The crawlspace was very nice with no issues whatsoever after 20 years or so.

Has anyone else ever run across this design/ style of sealed crawlspace in the NW?

Yes. I see a couple each year. Plenum crawlspaces were a fad in the late '70s and early '80s. They work fine as long as the crawlspace actually stays dry. If they tend to take on water -- even a little bit -- then the house will have problems with humidity and nasty smells.

I don't test for radon, but I'd be interested to know whether or not these houses have higher radon levels than other crawlspace homes.

- Jim Katen, Oregon

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Yes. I see a couple each year. Plenum crawlspaces were a fad in the late '70s and early '80s. They work fine as long as the crawlspace actually stays dry. If they tend to take on water -- even a little bit -- then the house will have problems with humidity and nasty smells.

Jim, it's strange that we work in the same area and I have only seen one sealed/ plenum crawlspace. You can keep them.

Just wondering, would this style of construction be more energy efficient overall? Seeing how there are no vents or cool air rushing through the crawlspace sucking out some of the homes heat... I would think so-- could be wrong.

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Jim, it's strange that we work in the same area and I have only seen one sealed/ plenum crawlspace. You can keep them.

Just wondering, would this style of construction be more energy efficient overall? Seeing how there are no vents or cool air rushing through the crawlspace sucking out some of the homes heat... I would think so-- could be wrong.

Yeah, that was the whole idea -- that they were energy efficient. Of the ones I've seen, there was quite a wide range of quality. I like the pea-gravel ones the least. The really nice ones had concrete floors, though I can only remember seeing three of these they were all very clean and dry. In one of them, the seller even had a mechanic's crawler down there. It worked beautifully.

- Jim Katen, Oregon

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The really nice ones had concrete floors, though I can only remember seeing three of these they were all very clean and dry. In one of them, the seller even had a mechanic's crawler down there. It worked beautifully

I have only seen one crawlspace that was covered in concrete. It was a 50's or 60's ranch (er) style crawlspace with foundation vents installed as well. It was very nice.

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My parents home in Alexandra, VA had a sealed crawlspace, this was back in 1965. It had a concrete floor with the area acting like a return Plenum for the main floor of the home. The only major problem I can recall was when my older brother decided to try his hand at wine making. Have you ever seen glass bottles explode! I bet that crawlspace is still purple after 40+ years!

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Originally posted by chicago

Vents should remain open unless the space has been conditioned properly.

Other wise you are begging for mold.

If I leave my vents open in the winter here in North Idaho, then it takes half the summer for the snow in the crawl space to melt. That's mostly to do with where my house sits and the constant north wind and drifting we experience during the winter. Usually I just close the vents on the North and west sides of my house, leaving a little cross ventilation between the south and east vents.

I recommend the same to my clients in areas that look like they may get drifting snow and where there may be plumbing close enough to a vent(s) that could be effected by freezing winds.

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