Insulation, Radiant and Vapor Barriers - Vapor Barrier in crawlspace / celler

I have a dirt floor crawlspace under my house, and I'm planning to install a 6 mil vapor barrier over the dirt floor. We have slightly high humidity levels in our house (50-65%) and think that the moisure is seeping from the celler through our hardwood floors and causing our humidity problem. The walls of the cellar/crawlspace are brick and not insulated. Also, there is no insulation under the floor or inbetween the joists.

We do not use the cellar for anything, so whatever I do to fix this problem doesn't have to look too nice. Also, all the water pipes and electrical wiring are attached to the floor joists so I am reluctant to insulate between the floor joists.

I've done some research on vapor barriers but I still have a few questions :

1. Will a 6 mil vapor barrier be sufficient, or should I use a thicker plastic? And how much should the plastic seams overlap?

2. Is it sufficient to just lay the plastic over the dirt, or should I also install bats of insulation up the basement walls and adhere them to the joist sill? (I have heard mixed opinions on attaching insulation bats to the sill because moisture can wick up to the wooden joist sill.

Any help / advice with this would be appreciated.

Thanks!
David

The 6 mil is fine but thicker is better, whichever is easier for you is best. The sheets of plastic should overlap by at least 4 inches and they should be taped. You also apply the plastic at least 6 inches up the wall.

As far as the insulation, it gets applied to the surface area that separates a conditioned space from an unconditioned space. Another way of saying this is insulation gets applied to parts of the house that you deliberately want to heat and cool from areas you do not want your heating or cooling to go to. So if you insulate the crawl space brick walls and not the floor above the crawl space, then you intentionally want to heat and cool the crawl space. On the other hand, if you insulate the floor above the crawl space, then you intentionally do not want your heating or cooling in the crawl space.

Thanks for your quick response & suggestions.

So, if the plastic goes up 6" up the sides of the brick, do I just fasten it to the brick with some sort of masonry anchors? Or is there another method that is easier/better (duct tape)?

Thanks for your comments re: insulation. Since I am NOT wanting to heat/cool the crawlspace, I should not put the insulation down the sides.

Based on your experience, what do you think would be more effective in solving my humidity problem: 1) put insulation w/ vapor barrier between the floor joists, even though it will be complicated due to pipes & wiring, 2) put plastic sheeting down on the dirt floor, as described, or 3) both.

I would prefer just to put the vapor barrier on the dirt floor as that will be easiest & cheapest.

Sorry for all the questions! Thanks again.

DS

The plastic (moisture barrier) should alleviate your problem with the high humidity in your home. There are several ways to attach the plastic to the brick. My personal favorite is liquid nails. I usually use a board like a 2x4 or 2x6, which ever I have around and some cinder blocks to hold the plastic up against the wall for the liquid nails to cure. In most cases I just leave the wood and the cinder blocks there. The insulation in the floor would help but would not specifically address the humidity problem.

I think take a look At http://aboutsavingheat.com/crawlspace.html

http://www.taunton.com/finehomebuildingpage/h00107htm

Might get you started the right way here. Yes you do want to insulate the walls. Also up there on the sill plate.In each joist all around the home or belt board as it is called some times
ED;)

This has been an immensely valuable thread. Thanks to all.

I did , however, notice that Taunton has likely modified the syntax of their URLs. The one mentioned above is actually:

http://www.taunton.com/finehomebuilding/pages/h00107.asp