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Insulating a Crawl Space

Crawl Spaces

Theoretically, a crawl space can be insulated in either of two ways:

  • The walls of the crawl space can be insulated on either the inside or the outside, creating a heated area.
  • The house floor above can be insulated to keep heat from getting into the crawl space in the first place.

Insulating the walls is recommended for the following reasons:

  • Heating ducts and water pipes in the crawl space will not have to be insulated and will not freeze. Any heat loss from the ducts will not be a total waste.
  • The walls can be insulated on the outside to reduce the internal moisture problems that can develop in damp crawl spaces and keep the soil below the footings warm.
  • It is usually easier to do a thorough insulating job on the walls, especially when the crawl space is shallow or the joist spaces are uneven or oddly shaped.
  • Less material is usually required if the crawl space is of a typical height – less than 1.5 m (5 ft.).

How to Insulate and Heat a Crawl Space

Moisture Barriers

If there is no moisture barrier on the crawl space floor, add one. The barrier should be 0.10 mm or 0.15 mm (4 mil or 6 mil) polyethylene overlapped at the seams and held down with a few old boards or some other scrap material. If there is likely to be any traffic in the crawl space, you will need to protect the polyethylene with a 50 mm (2 in.) layer of sand.


From the Outside

  • Insulate the outside wall exactly as for an outside basement wall.
  • If outside obstructions (a porch, a paved driveway, etc.) make it impossible to completely encircle the crawl space from outside the house, then the inside of the wall may be insulated at those points.

Make sure that the inside and the outside portions overlap by at least 60 cm (2 ft.) and insulate the inside portion.

  • If your crawl space does not open into a full basement, it should have ventilation at a ratio of 1 to 500 (vent area to floor area). The best time to ventilate is in the spring because summer ventilation can increase condensation. Make sure these vents are closed and well sealed and insulated each winter!
  • If the foundation footings are above the frost line, insulate on the outside of the crawl space walls. By insulating on the outside, the walls will be warmer, avoiding any possibility of frost heave. Shallow footings can be kept warmer by placing a layer of horizontal insulation sloping away from the foundation.


From the Inside

  • If using polystyrene or semi-rigid glass-fibre insulation, insulate as for the inside of a basement.
  • Apply a polyethylene moisture barrier to the crawl-space floor and install adequate ventilation as described above.

Keep water away from the foundation walls. Slope the ground away from the house and install eavestroughing where necessary.

Partially Heated Crawl Space

Insulation on the walls and in the floor creates a partially heated crawl space

It is possible to insulate between the joists and create an unheated crawl space. However, this can lead to freezing pipes, frozen ground and possible rot at the joist ends. For these reasons, floor insulation is recommended only when combined with foundation-wall insulation to create a partially heated crawl space.

Note these points as well:

  • The air and vapour barrier must be applied on the warm (top) side of the insulation. If the floor above the crawl space is already covered with an impermeable material (such as linoleum or plywood), you already have a vapour barrier where you want it. The solid materials of the floor can serve as the air barrier, but be sure to locate and seal any air leaks. The airtightness at the perimeter joist spaces is critical. This area can be sealed with polyurethane foam.
  • Batt insulation may be held in place with heavy-duty permeable building paper stapled to the joists, or by chicken wire, sheets of polystyrene bead board or a commercially available insulation-support system.
  • Place the insulation firmly against the floor above. It should be installed so that it fills the space between the sub-floor and the support system (usually the depth of the joists).
  • Tape the seams in any heating ducts and insulate all ducts and water pipes in the crawl space. Remember, even insulated water pipes may freeze if the temperature of the crawl space is allowed to fall below freezing.
  • Make sure that the crawl space is adequately ventilated in the spring. Vents should be installed at a ratio of 1 to 500 (vent area to floor area). Do not ventilate in winter: the vents should be plugged and insulated.
  • There must be a moisture barrier on the crawl-space floor.
  • If your basement has both a full basement and a section of crawl space where the floor has been insulated, remember to insulate the wall separating the basement from the crawl space.
  • If the ground level inside the crawl space is lower than the ground level outside, there is a slight danger that frost heave can damage the walls by pushing them inward. Make every effort to keep water away from the foundation walls. Slope the ground away from the house and install eavestroughing where necessary.
  • There is an added safety precaution: if freezing becomes a problem, you may want to install a thermostat attached to a small heater in the crawl space. This unit can turn on and heat the walls when the temperature of the crawl space approaches freezing.