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Insulating a Concrete Slab Floor
An Insulated Concrete Slab Floor will
Keep Your Home Warmer and Lower your Home Heating Costs
By Mark J. Donovan
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Uninsulated concrete floors can be
cold to walk on and make the living space uncomfortably cold. If you have plans
to finish a basement or your home sits on a cold concrete slab you may want to
consider insulating the concrete floor. Insulating a concrete slab floor can
make a living space feel much more comfortable and can save on your home’s
annual energy bills.
The key process in insulating a concrete slab floor involves building a plywood
subfloor over the concrete slab.
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Besides making the living space
warmer, a plywood subfloor is also the perfect surface for installing carpet or
wood flooring. The subfloor provides a moisture barrier and a nailing surface
for installing the carpet or wood floors.
Before installing a subfloor over the concrete slab it’s important to check for
adequate headroom height. Building codes typically require 7 feet 6 inches of
clearance between the finished floor and ceiling. The typical subfloor has a
height of 2-1/4” and a finished wood floor could add another ¾ inch. So from the
concrete slab to the finished ceiling you need a minimum of 7 feet 9 inches of
height to install a subfloor over the concrete slab.
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After confirming there is sufficient
height to build a subfloor and insulate the concrete slab floor, evaluate the
state of the concrete slab. If there are water problems they need to be
addressed first. Use hydraulic cement or some other type of concrete crack
repair product to seal cracks. If there are high spots on the concrete floor you
may need to grind them down a bit.
After evaluating and repairing the concrete slab, vacuum it thoroughly and apply
a concrete water sealer to its surface.
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Next, cover the entire concrete slab
with 6 mil thick polyethylene plastic. The plastic acts as a moisture barrier.
Make sure seams overlap by 6 inches and apply duct tape to the seams. Also, the
plastic should extend up the perimeter walls by 6 inches and should also be
sealed if possible with duct tape.
A grid of pressure treated 2x4 sleepers can then be installed over the concrete
slab. The 2x4s should be placed on their flat sides.
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Install sleepers by first installing
them around the perimeter of the concrete slab floor. Butt them end to end and
fasten them to the concrete floor with 2-1/4 inch long masonry nails. Nails
should be spaced every 3 to 4 feet.
Then fill in the perimeter sleeper framework with additional sleepers, making
sure they are installed on 16 inch centers. As you install the sleepers
constantly check that they are level and use shims when necessary.
The next step in insulating the concrete slab floor involves installing rigid
foam insulation (1-1/2 inch thick) in between the sleepers. Measure carefully to
ensure a snug, but not tight, fit. The goal is to leave no air gaps in between
the sleepers while not compressing the insulation in the process.
After installing the insulation, place ¾ inch tongue and groove plywood over the
network of sleepers. When installing the plywood start alternate rows with half
sheets of plywood. This helps to lock in the plywood pieces and create a
stronger subfloor surface. Fasten the plywood to the sleepers with 6d nails.
Apply nails every 12 inches under the plywood panel centers and every 6 inches
along the plywood panel seams.
With the subfloor installed and your concrete slab floor insulated you can then
install your preferred finished flooring.
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