There are several different kinds of foundations that can be
used for homes. In our area, the most common one uses a concrete
perimeter, with a crawl space underneath. This results in a dark,
often damp space under the house, which can be an ideal space
for mold to grow.
We decided to use a slab on grade foundation. This way, there
would be no air under the house to get contaminated. This type
of foundation consists of a large, single slab of concrete with
the same overall footprint as the entire house. We ended up using
about 110 cubic yards of concrete for the foundation.
The structural design for the house resulted in having all
of the interior walls be load bearing, in additional to the external
walls. This meant that a concrete footing was required under each
wall. The footings were 18 inches thick, compared to six inches
for the rest of the slab. The structural design required all of
the walls to be bolted to the foundation (which required a total
of 525 bolts).
Since we had decided on a slab on grade foundation, and because
we didn't want to have cold floors, and to go along with the design
guideline of no natural gas in the house (and therefore no gas-fired
forced-air heating), we decided to use a radiant heating system.
We chose the one made by InFloor (a division of GypCrete Corp.).
We also ended up with an electric heat pump in our ventilation
system. Although this could theoretically heat the house by itself,
electrical heating is much more expensive than radiant, and forced
air heat not only leaves the floor cold, it's also relatively
The radiant heat system required laying several thousand feet
of tubing in the foundation. Hot water is circulated through the
tubing to heat the foundation, which in turn heats the house.
InFloor makes a couple of different kinds of tubing. I selected
a cross-linked polyethylene type, with an oxygen barrier. This
is an extremely durable tubing, and the oxygen barrier will help
to protect the pump and boiler from future corrosion. Some vendors
suggest using copper tubing; however, I wouldn't recommend it.
The problem is that if the foundation settles slightly, copper
is much less forgiving than the polyethylene, and it is therefore
more likely to leak. In fact, radiant heat in our area has developed
a bad reputation because many buildings that were made with copper
radiant heat systems later developed leaks, requiring the foundations
to be extensively ripped up in order to fix the problems. The
photo below shows the red radiant heat tubing.
In order to keep moisture from getting into the concrete, which
could result in mold growth, staining, and other problems, the
foundation was built up in a layered fashion. First, the dirt
was compacted to 92 to 93% of optimal, to limit settling and subsquent
cracking of the concrete. Then six inches of gravel was put on
the dirt to serve as a capillary break. This limits the amount
of moisture that can seep up from the dirt into the foundation.
Because the heating is in the foundation, a two inch layer of
aluminum foil-faced extruded polystyrene insulation was put on
top of the gravel. Because soil also acts as an insulator, the
polystyrene only needed to cover the first eight feet from the
edge of the house. The insulation and the remaining bare gravel
was then covered with a 7 mil layer of plastic sheeting. The seams
were overlapped about two feet and taped together with duct tape
to prevent any "leakage" of water or mold from the dirt
into the concrete. A two inch layer of dry sand was used to help
level out the area, and to provide a better base for the concrete.
Finally, 6 inches of concrete was poured on top of the sand. Although
extruded polystyrene is a somewhat toxic material, the fumes are
drastically limited by the aluminum foil facing and the other
materials layered on top of it.
At this point in the construction, we had planned on having
a hardwood floor. In order to attach the floor to the slab, metal
strips would have to be nailed to the concrete. Although 1 1/4"
nails were recommended, our installer tested 1" nails and
found that they would work fine. Because we were concerned that
the nails might poke a hole in the radiant tubing, we decided
to add an extra inch of concrete on top of the tubing as a safety
buffer. It also made the foundation a little stronger, and therefore
hopefully more resistant to cracking. We used #4 rebar on 12"
centers (about 5 tons worth of steel) instead of the usual #3
rebar on 18" centers to help give us some additional crack
resistance. We wanted to reduce cracking as much as possible to
limit moisture and mold coming up from under the concrete. In
some areas, radon can also seep into homes through such cracks.
We also used 5,000 psi (7 sack) concrete, instead of the standard
3,000 psi (6 sack) concrete, for the same reason (and not for
structural reasons, so we didn't have to do any extra cost strength
testing). One option we looked into was including metal strips
in the concrete to help with crack resistance. We decided against
it because of the cost, which would have been about another $50
per cubic yard.
Conduits and plumbing stubs were put into the foundation before
the concrete was poured. The conduits brought the main electrical,
phone and cable TV in from the garage, and they provided a run
from the breaker box to the utility shed, which is about five
feet away from the house to the south, against a fence in the
back yard. The plumbing drain and vent lines and some of the supply
lines were also put into the foundation. The bulk of the electrical,
as well as most of the plumbing supply lines, were installed later
above the ceiling.
In order to keep our costs down a little, we decided early
on that we would have finished concrete floors in a couple of
parts of the house. The project room, laundry room, all three
bathrooms, the master bedroom closet and the copier room in the
office area all have concrete floors. These were poured at the
same time as the foundation, and because they were 1 1/4"
higher than the foundation (to allow for the thickness of the
then-planned hardwood floors), special forms were required. We
also planned for the shower pan to be sunk down, so that the shower
floor would be exactly level with the bathroom floor, to make
wheelchair access easy. This also required a special form.
We made sure to tell the company that did the foundation work
for us that we wanted fresh wood for the concrete forms, and that
we did not want them to use release oils of any kind. Some companies
use diesel oil or paint or wax on the forms so that the concrete
doesn't stick to the forms. However, if the forms are removed
within a week or so after pouring, sticking isn't a problem with
After the concrete was poured, I sprayed it with water several
times a day for three days to keep it damp and to therefore limit
cracking. Concrete completely hardens within 28 days. At that
time, there were no visible cracks. Seven months later we did
have a few small surface cracks, probably due to standard concrete
We initially wanted to leave the concrete bare, but we decided
that a sealer would be a good idea to protect against moisture
penetrating the concrete from above and causing mold problems.
So after 28 days, we had several coats of a non-toxic sealer called
sodium silicate applied. This greenish liquid has almost no smell,
goes on easily, and dries quickly.