Crawl Space Vapor Barrier

Hello,
I have heard different opinions on the use of gravel along with a poly 6mil vapor barrier. I am trying to conclude what the best solution is to control the moisture in my crawl.
Here is what im working with:
1950's Cape Cod, Brick, 5' crawl (approx 1200sq feet). The ground is mostly soft dirt, mostly dry but the furnace is down there on blocks approx 2-3 feet off the ground, so its been running alot this winter and we just bought the house. There is a sump pump that is located in the crawl.
I have purchased 6mil Poly sheets to cover the crawl but have not installed yet.
My question is will just the poly sheets be enough to keep moisture down and prevent mold, or do I need to put some kind of stone either under or over the poly?
If so what size stone, how much (thick) etc... OR will it even help much one or the other...
Any suggestions or expierence is appriciated!
Jason
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have you had water problems in this crawl space? mold, rotting joists, standing water, etc?
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The joists are not rotting, but I have had standing water for a time when the old pump quit working, also along one wall there is almost what looks like a trench about 1 foot deeper than the rest of the dirt floor and it has water in it, im not sure if this has eroded (sp?) over the years or its part of the under ground drainage that goes into the sump pump hole. Most of the ground is dry but certain parts are damp and mushy
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Jason wrote:

Generally I would suggest stone under the poly to allow any water to have a way to move UNDER it. I certainly would use the poly if it were my home.
However different local conditions and different construction situations may call for different solutions.
--
Joseph Meehan

Dia duit
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Look at the Building Science Corporation web site. That has some useful - researched - info on sealed crawl spaces. lo To be of any use, the poly must be lapped and sealed and sealed to the surrounding foundation. If, as marson suggests, there is moisture, look at ways of keeping it out of the crawl space or removing it. If there is extra info - say existing rot or moisture - let us know. TB
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Andy replies:
Here's what I did to a pier and beam house in Texas.
1) I put down black poly plastic over the dirt in the entire crawlspace. I did NOT seal or tape. This was to provide a vapor barrier, not a hermetic seal..
2) The house had NO foundation vents. I put in a bunch spaced about 15 feet apart around the perimeter. In this manner the moisture from the earth would condense on the underside of the plastic. What leaked up was easily ventilated out by the foundation vents....
As a result, when I sold the house 20 years later, the floor joists were in NEW condition, with no damage whatsoever.
The ventilation is the key. The purpose of the plastic is to limit the rate at which moisture in the ground can get into the air.
I hope this helps. It ain't hard to do.. And not expensive. It is , however a thankless, dirty job.... :>))))
Andy in Eureka, Texas
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some people would say to put rocks down first (for drainage) but I would say if anything, put sand down first, then the poly. who in their right mind would want to crawl around on poly covered rocks? people with rocks between their ears maybe.....
you say its already pretty dry down there, thanks to the furnace and leaky duct work no doubt. If you plan on working down there, lay out the poly. If not, why worry about it? You'd be better advised researching mold and what causes it, and what conditions it requires for growth. Then address those as you see fit.
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Just overlap the poly a few inches. You're trying to cut down the moisture, not remove it totally. Forget rocks unless you need a few on top to keep the poly from flapping. Better ventilation is good too.
Just lay out the poly and see what happens, If you need to do more, then do it after you see how the simplest solution works.
Jason wrote:

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