Percent coverage of vapor barrier in crawl space
Arthur W. Kelley
In the winter our house has had some condensation on the interior of the
windows that causes mildew and some incipient rot. I decided to install a
vapor barrier over the soil in the crawl space under the house. I'm
getting conflicting information on whether the barrier should completely
cover the soil under the house (wall-to-wall) or whether the barrier
should leave some soil exposed (I've been told 20%). It seems to me that
the barrier should be wall-to-wall but some say that leaving 20% soil
exposed lets the house "breathe" (whatever that means).
Does anyone have an authoritative reference on this? I'm hoping to find a
citation to a reliable source rather than start a debate with both sides
expressing an opinion ad-infinitum. The reliable source would let me
answer those who say leave 20% exposed.
On a related topic: what to do with those foundation vents? Open all the
time? Closed all the time (seems pointless)? Open summer and closed winter
or vice versa?
I've enjoyed reading this group but this is my first posting (hope this
hasn't been answered previously while I was on vacation or something).
Thanks in advance for your help.
--
-Arthur W. Kelley
North Carolina State University
awke...@mail.ece.ncsu.edu
Unidexter
Arthur W. Kelley <awke...@mail.ece.ncsu.edu> wrote:
>................................................. It seems to me that
>the barrier should be wall-to-wall but some say that leaving 20% soil
>exposed lets the house "breathe" (whatever that means).
>
>Does anyone have an authoritative reference on this? I'm hoping to find a
>citation to a reliable source rather than start a debate with both sides
>expressing an opinion ad-infinitum. The reliable source would let me
>answer those who say leave 20% exposed.
Your .sig tells me you are located in the same general geographic area as
I am - in the Triangle. I'm sorry to say that I'm far from being an
"authoritative reference on this" but when we moved into our ne house I
had the same concerns when I noticed that the floor of our crawl space
was not completley covered by the polyvinyl sheets used as a vapor barrier.
I asked the builder's construction superintendent about this as well as
their warranty manager and both informed me that NO MORE than 90% of the
dirt floor should be covered - in my case the barier runs from the front
to back of the house between the cinder block piers that support the
house so that space between the piers equal to their width is exposed.
There is a perimeter drain around the crawl space in a trench that is
filled with gravel and connect to a pipe that drains into a surface water
easment on the lot behid mine - this trench is also not covered by the
vapor barrier.
Come to think of it I may have an "authoritative" soure that Ican refer to,
when the Terminex man came for the annual inspection, he commented on the
fact that teh system is working well in it and frequently he has to work
in crawl spaces with mini-lakes under the vapor barriers in them. I took
this opportunity to inquire about the open spaces and he too confirmed
what the builders people told me - not more than 90% covereage and in his
(the termite man's) estimation mine was just right between 80 and 90% of
the crawl space was covered.
As far as foundation vents go except for the fixed vents (I have 2 in the
front of the house) they should be closed in the winter and opened in the
summer. If you have trouble rembering or are just plain lazy about
opening and closing the things you can get tehm with temperature
controlled dampers that do this for you on a more precise basis -
reacting to changes in the temp, even opening them, in the AM and closing
them in the PM between the extremes of winter and summer, they are
available locally at HQ (I know definitely) and probably next door at
Home Depot, and across the street in Lowe's too here in Cary, for between
$12 and $15 each the last time I looked.
I don't have them so I can't tell you if they are any good or not - I'm
definitely lazy - but I'm cheap too so I didn't invest in them. I tend
to open or close them as I happen to pass them so usually they all get
closed before the end of winter and all are opened before the end of
summer when it is time to reverse the process.
Arthur W. Kelley
> Your .sig tells me you are located in the same general geographic area as
> I am - in the Triangle.
Yep - N. Raleigh. This question was actually generated for me by an Orkin
guy who came to do an inspection. In 1990 we moved into our house and had
about a 75% vapor barrier coverage. The soil under the vapor barrier
stayed wet because the barrier prevented evaporation. The adjacent
uncovered soil was dry because of evaporation of the moisture, I presume,
into the house. As I said, my windows sweated all winter with accompanying
mildew and rot. The house was built in 1975 and some windows have suffered
significant damage. I took out the old vapor barrier (OK - so I'm a neat
freak) and replaced it with one that is wall to wall. Since then I've had
no more sweating windows and the moisture is trapped as evidenced by a
film of water on the underside of the vapor barrier.
So, either I trap the moisture in the crawl space with a wall to wall or
let it into the house and onto the windows with an 80% coverage. My
question is: what's wrong with trapping the moisture in the crawl space
where it won't damage the soil as opposed to letting it into the house
where it will damage the windows? The Orkin guy had no answer to this - it
seems he was quoting the 80% figure as a matter of faith without actually
being able to justify it. Maybe they just like to work in dry crawl
spaces!
Two other people have e-mailed me. One (from Colorado) says that their
county extension has a pamphlet that recommends 100% coverage. Another
(from parts unknown) says a discussion in Fine Homebuilding recommends
even going so far as to bond the edges of the vapor barrier to the
foundation for maximum effect. Can it be that the answer depends on
geograhical region?
Now I'm really confused. I'm still hoping to find something that I can
quote to the Orkin guy next time he comes out or something that will
convince me to cut holes in my vapor barrier. Anybody out there a member
of the American Society of Civil Engineers?
>
> As far as foundation vents go except for the fixed vents (I have 2 in the
> front of the house) they should be closed in the winter and opened in the
> summer. If you have trouble rembering or are just plain lazy about
> opening and closing the things you can get tehm with temperature
> controlled dampers that do this for you on a more precise basis -
> reacting to changes in the temp, even opening them, in the AM and closing
> them in the PM between the extremes of winter and summer, they are
> available locally at HQ (I know definitely) and probably next door at
> Home Depot, and across the street in Lowe's too here in Cary, for between
> $12 and $15 each the last time I looked.
>
I went and looked closely and I _do_have_ the thermostatic type. Now I
guess I have to pay attention to see whether they're opening and closing.
I guess I also have to remove the overgrown foundation plants that block
the foundation vents regardless.
Thanks for your help. I'll post if I can ever find out _the_ answer.
Bill Seward
< el snippo>
>As far as foundation vents go except for the fixed vents (I have 2 in the
>front of the house) they should be closed in the winter and opened in the
>summer. If you have trouble rembering or are just plain lazy about
>opening and closing the things you can get tehm with temperature
>controlled dampers that do this for you on a more precise basis -
>reacting to changes in the temp, even opening them, in the AM and closing
>them in the PM between the extremes of winter and summer, they are
>available locally at HQ (I know definitely) and probably next door at
>Home Depot, and across the street in Lowe's too here in Cary, for between
>$12 and $15 each the last time I looked.
IMHO, don't buy the automatic foundation vents. My house had them when
we moved in. It doesn't now. The things get corrosion on the bimetal
coil and in the hinge mechanisms, and stick in place--usually open it
seems.
I replaced them with a plastic vent that slides from side to side. Big
improvement.
-----------------------------------------------------
Bill Seward
w...@infi.net
I speak for myself. No one else will.
Randy Brown
|> From: unid...@nando.net (Unidexter)
|>
|> Arthur W. Kelley <awke...@mail.ece.ncsu.edu> wrote:
|>
|> >................................................. It seems to me that
|> >the barrier should be wall-to-wall but some say that leaving 20% soil
|> >exposed lets the house "breathe" (whatever that means).
|> >
|> >Does anyone have an authoritative reference on this? I'm hoping to find a
|> >citation to a reliable source rather than start a debate with both sides
|> >expressing an opinion ad-infinitum. The reliable source would let me
|> >answer those who say leave 20% exposed.
|>
|> their warranty manager and both informed me that NO MORE than 90% of the
|> dirt floor should be covered - in my case the barier runs from the front
|> to back of the house between the cinder block piers that support the
|> house so that space between the piers equal to their width is exposed.
And what should it be telling you if, underneath your vapor barrier,
are lots and lots of water droplets clinging to the underside?
I put down vapor barrier under about 1/3 of my house and that is what
it looks like.
Thanks.
Randy
--
Randy Brown SAS email: ra...@unx.sas.com Work #: (919) 677-8000, x6852
SAS Institute UNC email: br...@cs.unc.edu Home #: (919) 732-5458
Rob Henderson
>Two other people have e-mailed me. One (from Colorado) says that their
>county extension has a pamphlet that recommends 100% coverage. Another
>(from parts unknown) says a discussion in Fine Homebuilding recommends
>even going so far as to bond the edges of the vapor barrier to the
>foundation for maximum effect. Can it be that the answer depends on
>geograhical region?
Here in Indiana, we have subterranean termites to take into account.
Our termite guy recommended the 80% coverage and went on to explain
that what that really meant was that you wanted to keep the vapor
barrier a foot or two away from the foundation. The rationale being
that trapping the moisture next to the foundation would give the
termites a nice moist path by which they could reach your house.
In Colorado, where I believe termites are rare, this would not be
a concern.
--Rob