Plastic in crawlspace

[DIYchik]

House is on a barrier island with an extreme high water table. I have no plastic barrier in my crawlspace. I have noticed that my next door neighbor does and when we have heavy rains our crawlspaces fill with water from the ground up. His plastic is often floating in his crawl. I have more than the required foundation vents and 2 are auto vents to allow for year round ventilation. I also dont have any batt insulation in my floor joist.

Should I install a vapor barrier(6mil) along with batts in the floor joist. It is impossible to install a sump and no one has one.

Thanks

[Phelps]

Hmmmmm.

I'm not too sure on this one. The idea SOUNDS good, but in all honesty, the vapor barrier is supposed to be put to the heated room side. This applies to walls and ceilings. Vapor laden air is bound to rise...but in houses saturated with moisture, it could wind up at and below the floor, I suppose, also.

Do you know that a very large housing development company had a big lawsuit against them when a whole subdivision of houses had wall rot/mildew in the walls due to an improper vapor barrier technique that caused moisture to be trapped in the walls?

Anopther possible answer for you wold be for you to use tightly fitting pinkboard (or blueboard) between every joist. This styrofoam-like material is waterproof. And it has a higher r-value per inch than fiberlass. it comes in 3/4, 1, 1 1/2 and 2 inch thickness and perhaps thicker? In light of the fact you have power ventilation, i think you'd be okay with this and wouldn't be trapping moisture.

But you could call up a mobile home manufacturing company, like Wick Building Systems in Marshfield, Wisconsin, and maybe they would help give you advice, because as you know, mobile homes are set over crawl spaces.

[suemarkp]

As crazy as it sounds, I think it may be worthwhile to have a plastic moisture barrier on the dirt floor of your crawlspace, even if it ends up floating sometimes. What you don't want is extreme vapor in your crawl space. Yes, its vented, but depending on where you live venting may not help a whole bunch. Its better to keep the water out in the first place.

I know my pool stays warmer when I put a cover over it (less water evaporates). I would expect the same mechanism to apply with a flooded crawl space. Its a huge vapor source, but a plastic cover over it should be much better than no cover. I'd try to get large sheets that overlap a lot so when it floats and eventually lands back on the dirt again its still doing a good job of sealing.

What part of the country do you live in? Are you considering insulating the floor because its cold in winter or too hot in summer?

[DIYchik]

Live on a barrier Island off the Jersey Shore. The house is concrete block .

[suemarkp]

You can install insulation in your floor, but I don't know if its going to be cost effective. I'm not real familiar with NJ to know how cold it gets there on the island. I imagine its not terribly hot in the summer, and this isn't Florida, so you'd follow building practices of most of the country. This means, if you choose to install insulation, you would put the vapor barrier on the insulation towards the house floor (so paper side up, fiberglass side down towards the crawl space dirt).

I don't think you want a vapor barrier on the bottom of your floor joists (although that would make installation a lot easier). Even though there's a lot of water in there, it is a vented space and usually a colder space. You can use wire rods to hold the insulation in place. I've seen these at Lowes and Home Depot and you buy them for whatever joist spacing you have (16 or 24 inch).

[DIYchik]

Here at the Jersey Shore we get cold wet winters and hot humid summers.

[Not a Pro]

...The idea SOUNDS good, but in all honesty, the vapor barrier is supposed to be put to the heated room side...Not necessarily true. In Wisconsin, yes -- but not in locations where cooling, rather than heating, predominates. In actuality, in most areas, other than Canada and Florida for instance, theoretically the best place for the vapor barrior is somewhere in the middle of the insulation pack where it will never be below the dew-point -- regardless of whether the potentially condensing moisture is coming from the outside (A/C conditions, i.e. NJ summer/Florida) or inside (heating conditions, i.e NJ winter/Wisconsin).

As far as crawl space vents, "it depends". ;) Some places they're needed to prevent moisture problems; other places they cause moisture problems. Best advice I've heard is -- if you've got 'em and you have problems, seal them off. If you don't have 'em and you've got problems, put 'em in. The ideal is to have vents, and open and close them (but must be "sealed" closed) depending on dew point and ambient temperature, but who's going to do that?