My new-to-me 40-yr-old house in PacNW has a 36-48" high crawlspace that
didn't have nearly enough vents and in the warm weather the dank/musty
odor comes into the house. I just had existing vents enlarged, 10 more
installed, a fan put in, and it's been a huge improvement. Next week I'm
having the crawlspace cleaned out, mildewcide/fungicide sprayed
everywhere, and a new vapor barrier put down. The extremely clayey soil
is dampish looking, but not actually wet.
Would there be any additional benefit to putting in a 1 or 2" layer of
sand under the vapor barrier? The downside might be if I ever needed to
pump out water the sand might making pumping difficult, I suppose. Any
If the soil is clay and damp, the moisture is going to collect on the
underside of the vapor barrier. Sannd below the vapor barrier will
give it a place to collect.
I would suggest your moisture problem may begin outside the crawl
space. Is the ground in t(he area damp/ Does the surface tend to
direct run off toward the house? Does the roof shed water close to the
I hope the moisture is just from the crawlspace never having had enough
ventilation; my neighbors houses, just 20-30 ft away on either side and
the same age, both have very dry crawlspaces. My house has very wide
eaves and the soil almost never gets very wet next to the foundations.
All run-off is into drains, and the ground is sloped away. It's at the
base of a hill, but there are berms with drainage across the hill.
My thought was that sand would, as you suggest, give the moisture
somewhere to collect, plus it might be a "vapor pre-barrier" before the
plastic. If I have the sand put in and then plastic put down right after
the mildewcide etc. is sprayed on, my theory is that maybe there will be
less opportunity for musty mildew to grow under the plastic.
Until the crawlspace has had a few months to air out with the new vents,
I didn't want to go to the next step of looking at drains in there until
I know that the cleanout & new vapor barrier didn't do the trick.
Of course, I haven't the foggiest how I might get the sand in there or
spread it ... maybe get a couple of large, low, garden wagons and drag
them around with pulley & ropes. Is there some machine that could blow
sand instead of bark mulch?
Your description of your neighbor's dry crawl space and the layout of
your property suggests a dry crawl spce. So,you are probably correct.
The moisture is surface moisture and given some time will evaporate.
I'd suggest digging down - say - six inches to see how wet the soil
is. If it's dry, give the surface a few days to dry off before you put
down the vapor barrier.
In my neighborhood, there are a lot of highschool kids looking to do
yard work. They don't seem to mind crawling around in small spaces,
but I have to choose the more careful ones.
If it were my crawl space, I'd put down the vapor barrier on the clay
and weight it down with some sand. That's what we've been doing for
Habitat houses and it appears to work.
<sigh> nobody warned me that as a new homeowner I might suffer pangs of
"neighbor's crawlspace envy".
I now see why theirs is so dry. Their opening is a vertical 3ftx3ft
under the porch, on the prevailing wind side. They have a wire mesh
screen over it, that's all. By contrast mine is the same size, but it's
just below grade, with a 3ft-deep+wide access "dugout" lined in concrete
in front of it. So the cover lays horizontally (slight angle) on the rim
of the concrete "dugout", and because of this it has to be solid surface
to keep the rain out. Time for some creative crawlspace cover elevation
and redesign, I guess.
Tom Baker wrote:
Yes, the potential masking possibility is one I'd wondered about. My
goal in thinking about sand was just to be proactive in drying up the
earth as much as possible under the plastic. My theory is that if sand
or gravel would give a little room for air flow, and keep the earth out
of direct contact with the plastic, then that might keep any existing
moisture from causing new mold under the plastic.
New homeowner goes overkill, maybe <g>, but this musty smell is quite
literally "getting up my nose" as my cousins in Canada would say.
I placed pea gravel under the vapor barrier for a few reasons.
1. To provide some drainage for any moisture that does get in the
2. To level out the dips and bumps in the crawlspace.
3. To provide something easier on the knees to crawl around on than the
bare hard dirt.
4. To provide a little protection for the plastic (i.e. No sharp sticks,
rocks, etc. to poke holes in my vapor barrier).
Thanks, Anthony, that sounds like a more flexible idea than sand.
Unfortunately the crawlspace entrance is at the back of the house so any
mechanical delivery would have to go around about 50ft of corners and
What depth did you put in, 1 to 2" or ?
If the vapor barrier is not fastened tight at the edges - we typically
turn up the edges along the foundation in crawl spaces - the vapor
will tend to migrate through the gravel and out around the edges of
the vapor barrier into the crawl space.
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