What Is a Crawlspace?

The crawl space is a traditional foundation construction in North America, Nordic countries, and Australia, and is becoming very popular in Central Europe too, especially for timber buildings.

It exists under the home like a basement, though it hasn’t the same height as a traditional basement. However, a crawl space is a middle ground between a basement and a simple foundation.

Crawl spaces tend to exist to provide access to ventilation and other systems beneath the home. Crawl spaces can be anywhere from one foot in height to more, though in order to qualify as a crawl space an adult should not be able to stand.


  • The investors usually appreciate lower investment cost and saving of time compared to the traditional slab-on-ground foundations.
  • Also, antipathy to artificial materials like plastic foils or concrete sometimes plays a role.
  • The crawl space can be a relatively safe way for using natural thermal insulation (e.g. straw bales, wooden or hemp fibres) in the base floor.
  • In some cases, it is the best or even the only suitable solution for building foundation (e.g. sloping terrain).


  • Many authors have reported major moisture problems in modern crawl spaces followed by mould growth and decay of building materials.

The base of a crawl space is the ground, which means that moisture-laden air is always present in the confined environment. Moisture condensation may occur and cause several housing problems.

For this reason, under-floor ventilation was originally designed to prevent excessive humidity, but under some conditions, it may have negative effects, such as an increase in relative humidity.

Can you dig out a crawl space?

The absolute answer is: yes, to make a crawl space for your house is an easy process, just by digging under your house floor. But is it really simple as I said? of course yes after asking an engineer and getting the permit to do that.

Real Statistics About Crawl Spaces

Houses with crawl spaces in the United States represent more than 21 million units from a total of 87 million single-unit structures (National Association of Home Builders, 2006).

Foundations of single-unit housing structures (excluding manufactured/ mobile homes) were classified in the 2007 American Housing Survey(U.S. Census Bureau, 2007) as: with a basement under all the building, with a basement under part of the building, with a crawl space, on a concrete slab, and others.

Insulating Crawl Spaces

Tools and Materials

  • Dust or respirator mask
  • work gloves
  • staple gun
  • batt or blanket insulation
  • heavy-duty staples
  • wire mesh


  1. Batts of fibreglass insulation should be installed in the bays between the floor joists.
  2. Staple wire meshes onto the joists to keep out animals and prevent the insulation from sagging.
  3. Staple fibreglass insulation batts onto knee walls above foundations. You can let the batts drape down over the masonry.

Crawl-Space Venting

Dirt floors require more ventilation than concrete floors. If the floor of a crawl space is concrete and the walls are insulated, you can ventilate with a series of small foundation vents.

The number of vents depends on the total square feet in a given space. A general rule is to have 1 square foot of vent area for every 150 square feet of floor space. Sliding metal vents are designed to replace the space of one 8 x 8 x 16-inch concrete block.

-A ventilated crawl space needs to be screened, with either pressure-treated-wood or PVC lattice and a welded wire netting.

-Plastic lattice provides venting in many shapes and colours—and when it gets dirty, you just wash it down with a hose.

-PT (pressure-treated) lattice has a built-in resistance to water damage and rot, even near ground level.

-Use galvanized wire mesh or plastic screening to keep insects and animals from entering.

Crawl space Ventilated to Exterior

  1. Provide at least one square foot of net free ventilation area for every 150 square feet of crawlspace floor in a ventilated crawlspace. You may reduce the net free ventilation area to at least one square foot for every 1,500 square feet of crawlspace floor if you cover the floor with a vapour retarder such as six-mil polyethene sheeting.
  2. Install covers such as screens or grates in the ventilation openings. Use screens, grates, grills, or plates with openings at least 1⁄8 inch and not more than ¼ inch.
  3. Subtract the space used by opening covers from the net free ventilation area of a ventilation opening. Example: a one square foot opening may be reduced to an effective 2⁄3 square foot opening when covered by a cast iron grill or grate. The cover manufacturer’s instructions should indicate the cover’s opening reduction amount.
  4. Locate a ventilation opening not more than three feet from every corner of the crawlspace wall. Unventilated crawlspaces are recommended by experts for most crawlspaces.
  5. There is considerable controversy about the effectiveness of crawlspace ventilation, particularly in warm, humid climates. Check with a qualified energy efficiency professional before adding insulation between floor joists in crawlspaces. Check the condition of existing floor joist insulation in crawlspaces at least annually.

Unventilated Crawl Space

  • You may eliminate crawlspace ventilation openings by insulating the crawlspace walls or floor system as required by general codes and by installing all the following moisture control and ventilation components: (a) cover all exposed dirt in the crawlspace floor with an approved vapour retarder, such as six-mil polyethene sheeting. (b) lap all vapour retarder seams by at least six inches and seal or tape the seams. (c) extend the vapour retarder at least six inches up the crawlspace wall and attach and seal the vapour retarder to the wall. (d) provide one of the following ventilation methods: continuous mechanical exhaust ventilation, or a conditioned air supply at a rate of 1 cubic foot per minute for every 50 square feet of crawl space floor area and provide a return air opening to the building interior.

Unventilated crawlspaces are recommended by experts for most crawlspaces.

  • Do not connect the return air opening for the building interior to a forced-air return duct. Use an opening in the floor or use an unpressurized duct between the crawl space and the building interior.
  • There is some controversy about providing conditioned air to a crawl space. Do not exceed the one cubic foot per minute conditioned air ventilation rate.

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