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Old 12-23-12, 09:10 PM   #1
opiesche
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Default Crawl space vapor barrier and insulation

Hi all,

After installing my radiant floor heating system, I'm now looking at the need to treat my crawlspace. First, insulate betwen floor joists in my crawl space - I think I've got that part handled (fiberglass roll insulation with the facing pointing up, wire supports, done).
Secondly, I'll need to install a vapor barrier on the ground. There seems to be a lot of conflicting information out there, so I'd appreciate some advice

My house is in northern california (dry, hot summers, humid cool winters) on a vented 2' crawl space with a dirt floor. There's currently no vapor barrier. From everything I've read, I should use at least 6mil poly sheeting with as low a perm rating as I can find. Overlap seams by 8-12 inches, tape together - so far so good. Where I'm getting differing opinions is on how to treat the foundation walls with the vapor barrier. Some people say to leave it 6" away from the foundation walls. Some say to run it halfway up and tape or glue it to the wall. Some say to run it all the way up the wall to the sill plate (which other people argue against because, they say, it will make moisture from the ground wick up the foundation wall into the sill plate).

What have people here done about crawlspace vapor barriers, and what are your experiences?

Thanks!
Olaf

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Old 12-29-12, 03:34 PM   #2
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So, I bought some 6mil sheeting and a few rolls of Gorilla tape, and will hopefully get started on this next weekend, as the weather is supposed to be good. After reading some more, it looks like the best solution is to have the sheeting go up the foundation wall 8 inches or so, and to leave the rest of the wall uncovered.

Once the sheeting is in, I'll grab a bunch of fiberglass roll insulation and wire supports to install on another weekend. Once I'm under there and have crawled all the way over to the living room, I might find that I won't be able to install fiberglass there because the living room floor sits about 7" lower than the rest of the house - I'll have to see if there's enough room for me to work under there.

Does anybody here have any experience doing this? I'd appreciate some hints
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Old 12-31-12, 08:37 AM   #3
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We have a couple threads on crawlspace insulation:

http://ecorenovator.org/forum/renova...psulation.html

http://ecorenovator.org/forum/renova...-flooding.html

With your dry summers and humid winters I wouldn't worry about it horribly much. Doing just the floor and a bit up the wall would probably be enough. However, if it was me I'd probably just go all the way up the wall. Its not that much farther to go, and since you're down there anyways you might as well just do it so you never have to worry about it again. Depending on the age of the house, I'd think the sill plate should have a barrier of some sort under it so its protected from wicking water. Then again my house is over 100 years old, has a very moist basement and to my knowledge the sill plate is fine...
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Old 01-13-13, 05:43 PM   #4
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So, we've started on the vapor barrier and are about halfway done. We got some 6mil poly sheeting, cut 25ft long strips off of it (it's 10 feet wide) and that turned out to be the biggest we can handle reasonably well with the limited space under there (only about a foot and a half). Unrolling and then unfolding the pieces is a bit of a hassle, but works. We're unfolding each piece, then tack it to the foundation wall, straighten it and then run a solid line of tape along the wall. Gorilla tape seems to be working well for this, although only time will tell how it'll hold up with moisture and changing temperatures. Different pieces of sheeting are overlapped by about a foot, then taped together.







It's immediately apparent how much of a difference this makes. Even with only about half the floor area covered, it smells much less musty and feels much more comfortable under the crawl space.




This picture shows the foundation posts, which in our house are steel screw posts. Some of these are corroded and I'll replace them as we go along (there are two in the middle - the old one had rusted through in the bottom, and the thicker one next to it is the replacement).

Last edited by opiesche; 01-13-13 at 05:49 PM..
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Old 01-13-13, 07:41 PM   #5
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Opiesche, It looks like you are doing a great job. There is a dividing line on how you handle crawl spaces that involves either vented or unvented crawl spaces. In vented crawl spaces the poly up the wall of the foundation is not critical at all. In that case the 6 mil poly is just to keep ground water vapor from infiltrating the crawl space. Any vapor that gets in there by other means is assumed to be handled by air circulation from the vents.

The other type of crawl space is encapsulated crawl spaces. There is a huge amount of confusion about encapsulated crawl spaces, even by inspection professionals. So take advice about it with a grain of salt unless you know the person is thoroughly up to date. All encapsulated crawl spaces should have no venting to outside air. That means the poly should go all the way up the sides and to the rim joists. All outside air access should be tightly sealed.

So far not much confusion. Where there is controversy is about bringing the encapsulated crawl space into the conditioned air space. The most advanced thinking I've been reading about says that all encapsulated crawl spaces should be conditioned air crawl spaces. This means venting inside conditioned air to the crawl space. This would also imean insulating the perimeter foundation walls and also the rim joists. It's a big task but from everything I've read it is well worth doing. Once it is done you don't have to insulate the floors and no longer have to worry about leaks from air ducting in the crawl space because it is all conditioned air space.

Like I said, I don't think the building industry is up to speed on this so it may contradict what you have been told. Here's a link.

http://www.buildingscience.com/docum...ace_2010r2.pdf
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Old 01-13-13, 07:52 PM   #6
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Yeah, I noticed that there seem to be two or three different schools of thought and nobody seems to really agree on how these things are supposed to be done. I guess in the end it all depends on the specifics of each building, and the weather conditions it is in.

Since my crawl space was built vented, I don't think I want to go for full encapsulation. I'd have to somehow insulate the dirt floor from the space as well (that floor gets cold in the winter!), and I think it'll be easier to just insulate in between the floor joists.

I've also noticed a bit of wood rot in the rim joists next to a couple of the vents. I'll be fixing that this summer - I'm hoping I can do it from the crawlspace, so I won't need to remove the siding from the outside - that would be a huge project. Since it's only a few inches of the rim joist that are rotten next to the vents, I hope I can cut off the rotten part of the rim joist, remove it and set a new one from the inside, and then caulk and seal everything from the outside so this won't happen again. I'll have to take a closer look to see if that's possible though.
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Old 01-13-13, 08:50 PM   #7
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The PDF you linked to shows an interesting approach of rigid foam insulation over the fiberglass batts. I'm considering doing that (for the reasons mentioned in the article, the additional R3 or so, and because it would do a better job at holding the fiberglass in place than tension wires).
Really, the only thing I'm worried about is not being able to see into it to check for moisture. I'd have to pull off one of the panels everytime I wanted to take a peek. Well, that, and then there's the problem of not having a crawlspace access hatch in the foundation wall, so I have no idea how I'd get the rigid foam panels into the crawl space in the first place
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Old 01-13-13, 10:13 PM   #8
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Yeah, it all depends on how your floor structure works if using the foam board will work. On my house, for instance, the floor beams are perpendicular to the floor joists and would stop me from getting a good seal. So it wouldn't work for me.

Amazingly, they don't recommend insulating the ground with conditioned air crawlspaces. If the crawl space floor is at or above terrain level they just recommend insulating 2 ft into the dirt space (over the poly). The reason it is done that way is so cold earth temperatures outside the warm envelope can't migrate to the crawlspace earth. So that is the reason it makes sense to just insulate the perimeter of the crawl space and vent it to the house interior.

Apparently the warm air is enough to warm the surface ground temperature over time. I think it must mean that dirt is a pretty good temperature insulator and cold temperatures from farther below the ground surface don't pull the warmth away. Yeah, it is hard to believe. But that is apparently what is behind the whole idea making sense. I eventually plan to do it as one of my (many) long term projects.

BTW, I'm only about 70 miles from where you live. Lakeport, Ca.
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Old 01-13-13, 10:36 PM   #9
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I guess that makes sense. Even now, with relatively cold nights, the ground in the crawl space is warmer than outside.

Hey, that's not far at all. If you're ever in the Santa Rosa area, let me know and we'll get together for a beer
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Old 01-13-13, 11:45 PM   #10
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Quote:
Originally Posted by opiesche View Post
I guess that makes sense. Even now, with relatively cold nights, the ground in the crawl space is warmer than outside.
I think the problem of hidden assumptions is the biggest problem humanity faces. I feel like the older I get the more suspicious I get of my own unexamined assumptions. And I have less than most others I know!

Oh, wait.

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