Building a Sealed Crawlspace, continued
Sealing the Floor
We install our sacrificial floor poly as soon as the house is
dried in. Once the house is enclosed, vapor that enters the
crawlspace will start to accumulate, so delaying this step can
spell big trouble.
For example, I was called earlier this year by a builder who
wanted me to seal up his crawlspace, but it soon became clear
that he had built his foundation almost a year earlier with no
vents, ground vapor barrier, or wall poly. "What's it like down
there now?" I asked him. "I don't know," he said. "I haven't
A lot of bad things can happen in ten months in a sealed
crawlspace with no vapor barrier and no conditioned air supply
— and in this case, they had. When I took a look, I found
the floor joists covered in a thick fur of white fungus
filaments. It wasn't mold, but the kind of rot you find under
dead trees in the woods. This type of decay can seriously
weaken wood before it's even visible. The necessary fix might
go well beyond a simple cleanup.
Mold will grow on surfaces at 70% relative humidity, but wood
rot means that the framing was saturated from long exposure to
90% or higher relative humidity. Vented crawlspaces don't
usually get that damp, but an unvented one with no ground cover
can. That's why it is so crucial to get your construction poly
in place at the earliest possible moment. Without it, your
whole project is at risk.
Installing the sacrificial
poly. We place temporary ground poly as soon as there is
tar paper on the roof. We lay the sheet out to completely cover
the floor area, but without sealing the seams. We use the same
poly for the temporary cover as for the permanent installation
— it's just the 6-mil all-purpose black plastic sheeting
that you can pick up at any building supply outlet in 50-foot
or 100-foot rolls, in widths of 15, 20, or 24 feet.
Drying the framing. The
temporary ground cover prevents soil moisture from evaporating
into the crawlspace. But by the time the poly goes down, the
floor framing may already have absorbed quite a bit of moisture
from the soil beneath and from rain falling on the still
unroofed structure. To promote rapid drying, we temporarily
install a small dehumidifier in the crawlspace at the same time
we install the poly. So we don't have to count on anyone
remembering to dump the reservoir, we rig the drain tube so it
discharges outside the crawlspace.
Dehumidifiers are rated on the basis of cubic footage, and
most crawlspaces are low enough in volume to make a large unit
unnecessary. We use a 25-pint model made by Whirlpool for most
of our houses. The dehumidifier stays in place until we remove
the sacrificial poly and replace it with the permanent, sealed
ground covering — anywhere from a few weeks to several
months, depending on the construction schedule.
Permanent floor covering. A
week or two before the house is ready for the owners to move
in, we take out the temporary poly and put down a nice, clean
By this time the construction poly has generally been damaged
and disturbed by the mechanical subs and other people who have
been working in the space. I suppose we could just cover it up
with the new poly, but it's better to take it out because it's
almost always sprinkled with chunks of wood, sawdust,
cardboard, and other scraps. We just bundle all that termite
bait into the poly and haul the whole mess out. It's a quick
Lapping and sealing. Then we
place the new permanent poly floor covering, sealing the seams
at the wall poly with duct tape and mastic, and also sealing
every joint in the field (Figure 6). Each joint is secured with
duct tape that is then coated with a brushed-on layer of
mastic. The stripe of mastic extends at least an inch beyond
the tape on each side.
Figure 6.After removing the temporary poly, the
author rolls out a new layer of 6-mil poly, carefully sealing
the seams with duct tape and mastic (top left). He also makes a
duct tape and mastic seal where the poly laps up onto piers
(top right) and around the grate that covers the
backwater-valve floor drain (bottom).
It matters how you lap the poly at joints: You need to create
a shingle effect to handle flowing water, but you're shingling
upside down. You're placing poly on a slope, and water will run
down the ground surface beneath the poly. If you lap the joint
one way, it will allow the water to flow past, but if you lap
it the other way, the lap will scoop water and hold it against
the seam, threatening your mastic seal (Figure 7). It's a small
thing that matters — mastic handles incidental water
beautifully, but like any material, it may not perform so well
if you let it get continuously soaked.
Figure 7.The components of a sealed crawlspace
must address the issues of bulk water, air sealing, vapor
sealing, and thermal insulation. Dampproofing and perimeter
drains are necessary details, because the poly ground cover is
not designed to handle constant water contact. The author
prefers drains both inside and outside the footing, joined at
the low point. He ties a floor grate into the interior footing
drain and protects the link with a backflow preventer valve.
Floor poly is lapped reverse-shingle fashion so as to manage
water flowing in the soil beneath the plastic. For maximum
resistance to air and vapor infiltration, seams in the poly and
at penetrations are sealed with mastic. R-10 insulation is
placed at the foundation wall, aligning the air, vapor, and
thermal boundaries of the space in the same plane. Finally, the
fan ventilates the space with conditioned house
Floor spikes. To make sure
the ground cover lasts and stays put, we nail the poly to the
underlying soil with 6-inch spikes and washers (Figure 8). We
use a lot of spikes — 5 to 10 per 100 square feet of poly
— but it's well worth the time and effort. Imagine a
worker in the crawlspace dragging a tool box along. If the
corner of the box snags on the poly and the poly is not
secured, he could pull out most of the ground cover at one
time. If it's nailed down, he might rip a 10-foot piece off,
but the rest of it would stay intact.
Figure 8.The author secures poly to the floor with
6-inch galvanized spikes through nailing tins (left). He then
applies duct tape and mastic over the tin for reinforcement and
a tight seal (right).
In case of accidents like that, we always leave behind a patch
kit consisting of a 250-square-foot piece of poly, a roll of
tape, a brush, and a gallon of mastic. The homeowner can fix
250 square feet with our kit by simply laying down some poly,
fastening it with tape, and brushing mastic over the
Preventing future damage. To
protect the permanent poly, we lay carpet runners in the space
as a path for service technicians, running from the access door
to the water heater, furnace, and any other appliance (Figure
9). One of the builders I'm working with right now doesn't put
any equipment in the crawlspace; for that company, we just set
a 10x10-foot square of carpet right inside the access door, in
case the homeowners want to store something.
Figure 9.The author installs carpet runners over
the poly as a pathway for tradesmen servicing basement
appliances (top). He posts a sign warning workers not to rip
the poly (bottom), but he also leaves a patch kit in case the
poly does get damaged.
Some builders like to make crawlspace storage a selling point,
but our crawlspace is virtually inside the house — it's
just like the family's living room, and they have to treat it
that way. That means the builder has to inform the homeowners
that they can't store lawnmowers, pesticides, fuel, and the
like in their sealed crawlspace.
Ventilation and System
At this point, the crawlspace is clean, dry, well insulated,
and both airtight and vapor-tight. All that's missing is a
steady supply of conditioned air. Builders always expect me to
install the fan, but, unfortunately, my hands are tied.
Although installing a ventilation fan is a simple job, my state
won't let me do it because I'm not a licensed mechanical
Ducted fan. We do recommend
a specific approach to ventilation that we've found to work
well. We first install a floor register in an open central room
that can't be closed off from the main body of the house (this
prevents our fan from having to compete with the home's hvac
system). We have the mechanical contractor place a
continuous-duty low-sone fan under the floor about 8 or 10 feet
away. Vent and fan are then connected with a run of flex duct
(Figure 10). Separating the register and the fan in this way
makes for quieter operation.
Figure 10.A quiet fan placed in the crawlspace runs
continuously, drawing air from the main living space through a
floor register and a short run of flex duct. The steady supply
of conditioned air maintains the crawlspace at a humidity close
to that of the main house.
Our remote fan setup under the floor isn't the only way to
provide ventilation. You could also locate the fan in the
above-floor room and duct it to the crawlspace through a stud
bay, for example. The code doesn't specify this detail one way
or the other. Code officials make various judgment calls on the
forced-air supply to crawlspaces. Some North Carolina
municipalities, for example, now require an air- conditioning
supply register for the crawlspace; a dedicated fan is not
considered acceptable. Where I've been working, it's the other
way around — the fan's okay, but not a duct
In reality, it probably doesn't matter much either way. Fan or
no fan, there's going to be constant air movement between the
crawlspace and the rooms above. I've seen this countless times
in houses with moisture problems. Humidity readings in upstairs
rooms are always virtually the same as in the crawlspace.
Monitoring humidity. When
we've finished sealing a crawlspace, we install a humidity
sensor under the floor; the sensor is wired to a digital
readout and alarm located upstairs. Homeowners tell us the
readout tends to settle at a relative humidity of around 50% to
55%. Mold starts to grow at around 70% relative humidity, so we
set the alarm close to the top of the safe range, at about 65%.
This prevents nuisance alarms when the whole family gets
together at Thanksgiving and spends the day cooking,
temporarily causing the indoor humidity (including that of the
crawlspace) to spike.
We've yet to hear of an alarm going off at that 65% threshold,
which seems to confirm that sealed crawlspaces do control
moisture. The state of the floor framing is another good
indicator. While the normal moisture content of joists in a
vented crawlspace is around 16% to 18%, ours are running at 10%
When you change from an accepted method to an
alternative one, building inspectors don't
automatically climb on for the ride. Building
officials are often uncomfortable about sealed
crawlspaces because the code doesn't address them
in any detail (see "Sealed Crawlspaces and the
Code," below). If you don't explain the details to
the building department in advance, inspectors may
be uncertain how to evaluate your work when they
see it — and may even suspect that you're
trying to sneak something past them.
That can lead to expensive and unnecessary
problems. On one of our jobs, the inspectors made
me install horizontal perimeter insulation below
grade, as if I were building a frost-protected
shallow foundation. It never freezes where we work
— they were just mad at me. In another case,
a half-million-dollar project was held up for 45
days while the building department investigated,
delaying the closing for two weeks.
So I've learned to prepare the groundwork. Right
from the start, I make sure that the owner,
builder, and code officials know what my intentions
are and what I expect from them.
Step by step. We
do that by handing everyone involved some simple,
clear documents. The builder gets a detailed
process outline that explains exactly what's going
to happen and when. In effect, it says, "You're
going to call us within a week or two of putting up
the foundation wall, and we'll come put poly and
foam board on the wall. As soon as you have
tarpaper on the roof, you'll call us again, and
we'll come put down some construction poly." The
script spells out each step and takes the builder
right to the end of the job. We also stress the
importance of making the sealed crawlspace part of
the original permit application so the building
department knows what to expect.
guidelines. We also have a one-page detail
sheet that the building department can use to
inspect our work. This document provides plenty of
specific information, down to the five mechanical
fasteners per sheet of foam board. We try not to
leave them guessing about anything.
Even though some building departments decide to
apply some different requirements after considering
our detail sheet, it at least serves as a solid
starting point. The detail sheet itself is backed
by a letter from an engineer, which endorses all
the specifications contained in it.
The International Residential Code is
less than crystal clear about how to detail an
unvented crawlspace. The main text requires vents
of a calculated size in all crawlspace walls.
Ventless crawlspaces are allowed under two
different exceptions, both of which require some
exception. The first of these, Exception 4,
says vents can be eliminated if continuous
mechanical ventilation is provided at a rate of 1
cfm per 50 square feet of crawlspace floor area,
but it doesn't say where the air has to come from
or mention insulation. The second, Exception 5,
says that vents aren't required as long as the
space is supplied with conditioned air. How much
air, though, and how it should be delivered, is not
specified. Unlike Exception 4, Exception 5 does
call for insulation of the perimeter walls to meet
the energy code.
There's a lot of room in that language to make
mistakes. In our hot, humid climate, we definitely
don't want a continuous ventilation fan to pull in
outside air. And we don't want it to pull air from
the crawlspace and push it outdoors, either,
because that would depressurize the crawlspace and
draw moist outdoor air in through cracks and seams.
Any fan you install should take air from the house
and add it to the crawlspace.
In theory, that will slightly pressurize the
crawlspace and cause air to move away from the
crawlspace in all directions. Whether or not the
crawlspace becomes pressurized in practice depends
on a variety of factors, including wind pressure
and the tightness of the subfloor and
In practice, subfloors in real homes are so full
of holes, and ductwork located in crawlspaces is
typically so leaky, that the uncontrolled air
exchange between the crawlspace and the house far
exceeds the amount moved by our fan. The fan is not
really necessary, as I know from all the existing
crawlspaces I've fixed without one. But bringing
conditioned air below the floor is the right idea,
and it can't hurt. I like the belt-and-suspenders
approach, as I said, so I don't mind the fan.
bedroom? Our application actually satisfies
the provisions of both exceptions: We provide the 1
cfm of continuous ventilation required under
Exception 4, and we do it with conditioned air, as
required under Exception 5.
But that brings up another potential problem. Once
you've conditioned the crawlspace, the code people
aren't sure how to treat it. Some of them want to
call it a plenum, while others want to consider it
habitable space, like an additional bedroom. Either
way, exposed wiring and plumbing can be an issue,
even though there's no sensible way to cover those
elements up inside a crawlspace.
Fortunately, most code people end up willing to
think of our crawlspace as a very short basement.
They've seen regular basements with all the same
details, and they haven't treated them as habitable
Paddling into the
mainstream. We still run into problems
occasionally. For example, I've been using extruded
polystyrene rigid insulation on crawlspace walls
for years, but on one current project the building
department decided that since the space is
conditioned, we couldn't leave that foam exposed.
We had to switch to foil-faced polyisocyanurate,
which isn't used much in our area and had to be
But we find that when we make the effort to
educate and inform code officials in advance, most
of them can relate to what we're doing. Our
engineer's letter reassures them that they won't be
blamed if something goes wrong.
Finally, we have one big thing going for us: What
we're doing works. When building inspectors compare
our dry, clean crawlspaces to the dank, moldy pits
under most houses, it's hard for them to complain.
Over time, they're going to start taking our
methods for granted.
Costs and Benefits
My company is currently charging production builders about
$1.75 per square foot to seal up a crawlspace, including
materials. Custom builders pay more like $1.95. But there are
offsetting reductions in other construction costs.
Up-front offsets. First of
all, my price includes insulating the foundation walls, so you
can subtract the cost of the floor insulation you don't have to
put in. That's 40¢ per square foot in our market. So from
$1.75, you're now closer to $1.25. You can subtract the cost of
buying and installing foundation vents, as well.
Also, by sealing the crawlspace, we cut the heating and
cooling load on a house substantially. This has an especially
pronounced effect on the air-conditioning load, because dehum-
idification accounts for much of the work an air conditioner
has to do in our area. By eliminating all the soil moisture and
a big chunk of the infiltrating humid air, we drastically
reduce that latent cooling load. On an average house, our work
lets you downsize the equipment by 1/2 to 1 ton of cooling and
reduce the ductwork accordingly. By the time you add up those
direct offsets, our crawlspace work is almost free.
Callbacks and customer
satisfaction. That reduction in load translates to lower
energy bills for the homeowner, year in and year out. But the
builder also gets a long-run benefit: Houses with sealed
crawlspaces are much more stable from season to season, which
means that some of the callback money you now spend caulking
trim joints or rehanging doors will stay in your pocket.
Finally, there's liability. As anyone who's spent some time in
a crawlspace knows, conventional vented crawlspaces are often
musty and moldy. A well-sealed crawlspace, on the other hand,
typically contains less mold than the outside air. In today's
legal climate, that's no guarantee that you won't get sued
anyway, but it can't hurt.Contractor and consultantJeff Tooleyis owner of the Healthy Building Company,
based in Bear Creek, N.C.