vapor barrier question

[powder11]

Had a flood in a basement level apt that is right above a crawl space. Going to reinsulate the floor joists. The previous vapor barrier was stapled directly to the bottom side of the floor joists, between the crawl space and the actual insulation. The genius at Home Depot where I bought the insulation told me to put vapor barrier on top of the sub floor, right under my flooring. The flooring will be a floating laminate, so no nails/glue, but I've never seen or heard of vapor barrior going under flooring. Is this person on crack or can I do it that way? Obviously it would be far easier than laying on my back stapling and the logic of having your vapor barrier on the warm side makes sense since thats how it goes on the wall, but I honestly have never seen it put down under flooring. What sayeth the people from know-it-all, interwebzland?

[tsproul]

Use of vapor barriers depends on the environment. Where is your house? What are the seasonal norms for temp and humidity?

To copy the same situation you had before, you would simply install faced roll insulation between the floor joists. The vapor barrier would go against the floor sheathing.

[L7]

Pretty typical to put vapour barrier under the laminate floor and essential over concrete. Most of the plastic wrapped styro balls for sound insulation are also vapour barriers for that reason. I never see faced batts here anymore and putting the laminate under the floor would be much easier as you've noted.

[Snow Dog]

The vapour barrier goes between the ground and the crawl space and not between the floating floor and subfloor. Normally the crawl space is heated because there's ducts or hot water pipes so putting a vapour barrier on the underside of the floor joists isn't normal but shouldn't hurt either.

In some places (like around Vancouver) a vapour barrier goes between the floating floor and concrete or between the subfloor and concrete. I don't think there's a hard rule; it depends on the builder and local inspector.

[jfost]

huh, here in RNO my VB is laid out on the dirt under the house basically wall to wall over the entire foundation. I just have a perimeter foundation with footings every now and then in the middle. There is no 2nd barrier stapled to the underside of the floor joists. It is also pretty f'ing dry here, so YMMV I guess?!?

[Foggy_Goggles]

It depends -- conditioned or un-conditioned crawler? Vented or Unvented? It's all about temperature gradients and dew points. I've never used faced batts in a crawl space. If it is a dirt floor crawler, I'd probably put 6mill on the dirt, unfaced batts in the joists and seal it up tight. Vapor barriers under the floating floor is usually only for concrete floors. PSA - home depot usually sucks for flooring.

[F.ing Mike]

Vapor barriers always go on the warm side of the insulation. Your best bet is to call your local building inspector and ask him what the code is. A barrier under your flooring won't hurt anything. If your replacing all your insulation, do yourself a favor and have it spray foamed. You'll have better insulation, won't have to worry about a vapor barrier and rodents don't like either. I am confused how you have a basement level apt and a crawl space though???

[Yanos]

Not that you were suggesting this, but never use two vapor barriers. If a vapor barrier is present on the ground in crawl space AND up the exterior walls - you're done. Otherwise, as others have already stated, you need to state the condition of the crawl space, heating type, ventilation, etc.

[jfost]

... never use two vapor barriers. ...

what if she's reeeeeeeeeally skanky though?

[powder11]

Vapor barriers always go on the warm side of the insulation. Your best bet is to call your local building inspector and ask him what the code is. A barrier under your flooring won't hurt anything. If your replacing all your insulation, do yourself a favor and have it spray foamed. You'll have better insulation, won't have to worry about a vapor barrier and rodents don't like either. I am confused how you have a basement level apt and a crawl space though???

Spray foam may be the way to go. The house is built on a steep slope, so the back 1/2 is sub grade foundation wall poured on to a rock slab and the front half is a crawl space with footings poured on the rock slab.

[tsproul]

having a contractor come in and do spray foam is going to be big $. materials alone will be around $10 per square foot for a typical floor joist - 2x10 (or so) 16" OC. Now add a few $ per square foot for labor......

Flash and batt will save considerable $ in the material department.

Spray foam is a good idea if you've got a lot of drafts in the floor - flash and batt or completely spray foam. If the floor is pretty tight, just use faced batts. Material cost is only $1-2 per square foot. And installation is an easy DIY. Really, vapor barrier for the floor is not nearly as important as it is for walls.

[F.ing Mike]

This is from my local building inspector (Western Massachusetts)

When the undersides of frame floors above crawl spaces are insulated with faced insulation, the
vapor retarder facing, generally kraft facing, should be placed on the top side, and in substantial
contact with the floor above . This prevents the kraft facing from being exposed and posing a fire
hazard . The opportunity for air to infiltrate between the floor and facing and bypass the
insulation is reduced . In many cases, it is standard practice to use unfaced insulation under
floors, with the assumption that the flooring materials provide adequate vapor resistance to inside
moisture.

When insulating perimeter walls, proceed the same as with a below grade masonry wall and use a
perforated flame-resistant blanket that is attached to the top plate, extended down the wall and
preferably extended two feet along the floor . Where the crawl space floor is bare earth, it is
highly recommended that the entire area be covered with 4 mil polyethylene sheet (ground cover)
to minimize the movement of underground moisture up into the structure .

The latest thinking is that it is best to have non-vented crawlspaces and insulated walls, treating
the crawlspace as conditioned space

[Foggy_Goggles]

11s...you live in BC right? What is the crawler like in the spring, damp or dry? What are your low temps for the winter? What are the humidity levels like?

Really, the right answer depends or the above. Living in Colorado, we have very low relative humidity levels. Water vapor is traveling from the inside of the home (warm and wet) to the exterior (cold and dry). On the EC and down south, the opposite can be true (people run de-humidifiers) where the outside air is hot and wet.

Vented crawl spaces are great if a. there is no plumbing in the floor joists or the crawler (where is the water main coming in) b. you have no real moisture problems. The idea with vents is to remove the water vapor via circulation. The problem with water vapor is that wood is a food source for mold (i.e. your floor joists). With the VB on the dirt floor the moisture can't get the the floor joists.

I built my house slab-on-grade as I hate everything about crawl spaces (other that the build cost). If I had a crawler I'd go with a "conditioned space" where the foundation walls are insulated, the VB goes on the floor, and you shoot some heat down there.

Really these crawl space problems in the mountains are actually site drainage problems. I've seen so many crawlers that look like swimming pools in the spring. Whatever you do, don't let your pipes freeze.

[KTHetzel]

It sounds like you have some great advice on the basement masonry side of the vapor barrier, question. I would suggest that I think the term "vapor barrier" is being used liberally. All flooring gets vapor barrier-Wood over plywood sub floor, Rosin paper-laminate over concrete, padding- I like the local building sub code official option, or perhaps check the local building codes online- There are significant regional differences when it comes to insulation.

Good luck
k

[Pow4Brains]

Foggy knows what he is talking about. Second on asking the building department in your area and spray foam is the way to go if you have the funds.

[hooli]

Vapor barriers always go on the warm side of the insulation. Your best bet is to call your local building inspector and ask him what the code is. A barrier under your flooring won't hurt anything. If your replacing all your insulation, do yourself a favor and have it spray foamed. You'll have better insulation, won't have to worry about a vapor barrier and rodents don't like either. I am confused how you have a basement level apt and a crawl space though???

This pretty sums up what I was going to say except don't trust guys at HD for correct info...