Recent research provides plenty of evidence that vents for
crawlspaces don't work, and that sealed and conditioned
crawlspaces are a better choice. But my business, the Healthy
Building Company, is the first I know of to specialize in
applying that science in the field. We've been sealing up
crawlspaces in houses with moisture and mold problems for five
years, and we're taking on an increasing volume of work
building dry crawlspaces for new homes.
We've developed a set of simple, effective, and repeatable
methods that produce predictable results. In this article, I'll
describe what they are, and how we deal with various obstacles
that may crop up along the way.
In general, our installations have four phases. The first is
the builder's job: Waterproof the foundation and install good
foundation drains. Next, we come in and apply poly and rigid
foam insulation to the foundation walls. As soon as there's a
roof on so that rain won't get into the crawlspace, we come
back a second time and put down a sacrificial layer of poly to
keep the ground vapor out for as long as it takes to finish the
house. Then, in the final two weeks of the job, we come back to
take out this temporary "construction poly" and install a
permanent version, carefully sealed at all seams and secured to
the ground. At the same time, we also install a quiet
ventilation fan that runs continuously to provide the newly
sealed space with a steady supply of conditioned air.
Managing Bulk Water
My vapor barriers are sealed with mastic, and they're both
watertight and airtight as well as vapor-tight. But I don't
intend them to cope with pressure from groundwater. Since I
work as a sub on the job, I hold the builder totally
responsible for managing bulk water. I tell him that if he lets
groundwater come up under my poly, presenting the risk of
flooding the crawlspace, then I won't guarantee the results.
Our mastic joints are very tough, but they can fail under
pressure. One good rip in the poly, and bulk water intrusion
could completely bypass our vapor barrier.
So I need to see waterproofing, or at least dampproofing, on
the foundation walls starting at the finish grade and extending
down to the footing. I want a perimeter drain at the level of
the footing, sloped to one corner of the building (see Figure
1). An exterior footing drain is the minimum; even better is to
have drains on both sides of the footing, connected across the
footing at the low corner.
Figure 1.Perimeter drains around the foundation
footing are necessary to prevent bulk water from threatening
the crawlspace's poly vapor barrier or its mastic-sealed
The crawlspace floor is also sloped to that low corner, so
that if a pipe breaks and floods the space, water will drain
that way on our poly. To provide the water with an escape
route, we install a backwater valve in that low corner (Figure
Figure 2.Managing bulk water is essential for
success in a sealed crawlspace. The author ties a backwater
valve into the foundation drain system at the low spot in the
crawlspace, so that any water from a plumbing leak will be able
to drain out through the perimeter drain.
The valve is connected to the perimeter drains; when we later
install the final poly, we cover the inlet with a grate that's
sealed to the poly with mastic. When the crawlspace is dry, the
valve stays closed and keeps the groundwater out. But if water
collects in the corner of the crawlspace, the valve flapper
opens and allows the water to drain out through the footing
drain. You can get these from your local plumbing supply or
from D.A. Fehr, a distributor of plumbing supplies
The crawlspace floor isn't usually graded perfectly smooth. In
the event of a flood caused by a plumbing leak, little dips and
hollows might collect puddles, but that much water can
evaporate out without wrecking our crawlspace. What we're
trying to avoid is 10 inches of standing water, and this setup
Complete dampproofing. The
North Carolina code calls for dampproofing only where the
exterior grade is higher than the interior crawlspace floor.
This means that if I have a 4-foot block wall with 2 feet of
earth inside it and 3 feet backfilled against the outside,
dampproofing is required only on the 1-foot portion where the
exterior soil is higher than the interior soil, not all the way
to the footing. This seems to assume that water won't come
through a block wall if there is soil on the other side. I
don't see anything to prevent it, so I say use the belt and the
suspenders: Coat the foundation all the way to the footing, and
install drains inside and out.
Wall Poly and Insulation
Once the builder has the foundation wall built and the
drainage installed, I install the poly and foam on the inside
face of the wall. We attach the 6-mil black plastic to the wall
with a water-based adhesive mastic (Figure 3), then fasten the
foam board over it with powder-driven masonry nails (Figure 4).
The wall poly extends onto the ground about a foot, leaving an
edge flap for us to seal the floor poly to later.
Figure 3.The author brushes mastic onto the block
wall (top), then applies 6-mil black poly (middle and bottom).
Mastic creates a strong adhesive seal on many different
materials, even if the surfaces are dirty.
Figure 4.The author attaches foam board to the
foundation wall using powder- actuated nails (five nails per
piece of foam). Although some building departments have
required foil-faced polyisocyanurate foam (top), he generally
uses extruded polystyrene (bottom). Either way, the R-10
foundation insulation meets the energy code without the need to
insulate floor cavities with fiberglass. But air sealing and
vapor sealing, not insulation, are the main reasons sealed
crawlspaces reduce the load on the hvac system.
No batts in the floor. Under
the energy code, insulating the foundation to R-10 means we
don't have to insulate the floor under the living space. We get
rid of the usual fiberglass batts in the joist bays, where
their performance is hindered anyway by air movement, poor fit,
and interruption by wires, pipes, and bridging; and we locate
the thermal boundary of the house with the air-pressure
boundary where it should be. People concerned about indoor air
quality are glad to see the fiberglass go, because it means no
irritating fibers will be floating around to get sucked into
Sidewall penetrations. Where
pipes or wires go through the side wall, we have to fit the
foam board to them carefully and seal the vapor barrier around
them (Figure 5). I'm working right now on a lot of houses with
package heat pump units, which add another large penetration
for us to fuss with.
Figure 5.The author carefully fits poly around a
pipe that runs through the foundation wall, then seals the poly
to the pipe with mastic. All penetrations through the
crawlspace wall must be sealed.
Detailing the duct penetration is just patchwork. Where the
builder runs flex duct from the unit through the wall, we have
to cut a half circle on one piece of foam board and a half
circle on the other, then piece them together around the duct
and seal the joint.
We use a water-based, nontoxic duct-sealing mastic called
PS-1, from RCD Corporation (800/854-7494,
www.rcdcorp.com), for all our sealing work
because it is so effective. It sticks to all kinds of
materials, rough or smooth, and even to dirty surfaces. It
dries in hours and is tough and strong; once a mastic joint in
poly sets up for real, two men can't pull it apart with all