Crawl space vapor barrier: Figuring out the details

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Topic by Dan Lyke posted 02-10-2009 02:23 AM 5857 views 0 times favorited 9 replies Add to Favorites Watch
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Dan Lyke

331 posts in 3911 days

02-10-2009 02:23 AM

Topic tags/keywords: question basement

We’ve got a 1947 or thereabouts built on foundation and piers with a fairly low vented crawlspace. Looks like diagonal redwood or fir subfloor boards, and 5/8” oak floor.

And, in the winter (rainy season) here in Northern California, we have a sump-pump and the dirt down there is pretty damp. We also have some issues with mold on a wall, I’ve cracked the wall open and behind it is dry, but since our baseboards are yanked out right now I believe that could be extra moisture coming up from beside the floor.

A lot of web sites suggest that a good thing to do in this situation is a vapor barrier in the crawlspace. However, there seem to be three options:

1. A hard insulation. Foam or something similar cut and fitted up under the floor.

2. A vapor barrier attached to the bottom of the beams. This seems like a really bad idea, because I’m afraid that we’ll get condensation inside the vapor barrier, and water in places we don’t want it (ie: it’s akin to putting the vapor barrier on the outside in a wall).

3. 6 mil plastic on top of the dirt, taped 1’ or so overlaps, and as far up the edges of the foundations and side of the piers as I can go.

#3 seems like the best of the options, but I’ve seen so many different suggestions that I just don’t know, and I’m still concerned about what sort of mold issues I may be growing underneath that plastic in the dirt. Does anyone have experience with this or a reason that I should trust a particular source on what to do?

And, yes, if I can find a reason I should trust someone with an answer I’ll happily pay for it.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

View Scott Hildenbrand's profile

Scott Hildenbrand

33 posts in 3754 days

02-11-2009 04:42 AM

Option #3 is the one to go for. Also make sure that there is good ventilation to the crawl space area, which can be an issue all in it’s own. A power vent fan may be required.

Moreover, you will want to examine your property and see how the ground pitches. If it’s pitching to the house, any water on the uphill slide will end up under the home.

Down spouts from gutters need to be directed away from the home, NOT just emptying at the foundation. It’s amazing how many thousands of gallons can come off a roof. If possible dig a trench and install perforated drainage tube some 10-25’ away from the home; depending on the ground conditions. Have it end in a pop up emitter.

However, do NOT drain it beside or onto your neighbors property. You flood their place, they sue.. If you have a storm drain system by your road you could direct it there, depending on your property layout.

I noticed that you have an Irrigation system. Maybe that is one of the problems. If you have a crack in the system it could easily keep the ground around the fracture saturated and waste hundreds of gallons.

Either way, just having a barrier down is only a band aid at best if your ground is staying that damp. There are other moisture issues that need addressed first.

-- Planting Daylilies in Kentucky, zone 6b

View PaBull's profile


701 posts in 3858 days

02-11-2009 06:08 AM

I am with Scott, and maybe even put a french drain around the house, to keep any water from getting under the house.

Good luck and keep us posted.

View Dan Lyke's profile

Dan Lyke

331 posts in 3911 days

02-11-2009 07:05 PM

French-drain-wise, the yard is gently sloped, but in wintertime around here I can get standing water within a foot or so of the surface when it’s not raining. So there’s probably some room for putting in additional drains, but I believe there’ll always be water coming up into the sump. I think I could go to the storm drain system, I’ll check with the permitting office.

And the crawlspace has ventilation, but, yeah, I think I do need to look into a fan somewhere.

-- Dan Lyke, Petaluma California, http://www.flutterby.net/

View MRTRIM's profile


744 posts in 3912 days

02-12-2009 04:58 AM

dan im thinking it was bob#2 over at l j that recently did a crawl space vapor barrier , i could be wrong but you could check , hes a very helpful guy

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