Building an insulated chase for pipes
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Surround the pipes on all four sides with 2-in. rigid insulation, then create an inlet and an outlet for warm air at each end of the chase. Keep air flowing with a small duct fan.
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Duct booster fan
A small duct booster fan will keep warm air flowing through the pipe chase.
For starters, forget about using fiberglass
or the foam pipe insulation sold at home centers. At best, it provides an
insulation value of R-3.8. That’s not
enough to prevent frozen pipes during
extended cold periods. Plus, it’s difficult
to install on existing pipes, especially
when the pipes run along the
length of a floor joist. So we put our
heads together and came up with a
solution that can be adapted to most crawlspaces. One of the editors also used
this fix to successfully insulate freeze-prone
pipes on an outside wall.
Here’s how it works: Build a duct
system around the pipes with 2-in.-
thick rigid extruded polystyrene foam
(R-10). It works by drawing heated
household air through the foam duct
and back into the house. Start by locating
a spot near the pipes on one end of
the crawl space near a wall and cut a 5
x 10-in. hole in the floor above. This
exhaust hole should be near an electrical
outlet. Cut another hole at the far
end of the pipes to fit a conventional
floor vent—the size is up to you.
Next, construct the duct by running
the foam down a few inches below the
pipes to create enough room for airflow. Glue the lengths together with
foam construction adhesive and
pin them with screws or nails until the
adhesive sets up. Crosscut individual
foam pieces to “cap off” any open joist
areas. Cutting is easy with a circular or
Use the same method to encase vertical
riser pipes and pipes that run along
the length of a joist. Once all pipes are
enclosed, glue on end caps.
Buy a duct booster fan (available in
the HVAC department at home centers) and place it over the exhaust
vent. Run the fan full time at low speed
during freezing weather (that’ll only cost a few dollars per month).
Foam insulation is flammable and produces
toxic smoke when burning. You must follow
these safety precautions:
- Keep crawl space lights at least 6 ft. away
from the foam duct.
- Never use electrical heating tape inside the
- Do not install a fan inside the duct.
- Do not tap into an existing HVAC duct to
supply heat to the foam duct