Conditioning crawlspaces - Pro Construction Guide
Conditioning crawlspaces

Conditioning crawlspaces

Poorly designed or constructed crawlspaces create cold floors

Poorly designed or constructed crawlspaces create cold floors that chill both rooms and people, and waste energy and money.

They also result in problems with structural durability, mold, radon and other indoor air quality issues. Ventilated crawlspaces are the main culprit for these undesirable conditions, but they do present an opportunity for the industry to improve and retrofit them to more desirable conditioned crawlspaces.

A crawlspace is basically a short basement, but it should be treated like a full basement: air sealed, insulated, conditioned and controlled. Follow these tips for conditioning crawlspaces.

Ventilated crawlspace

A ventilated crawlspace has operable air vents, typically more than one, generally located across from one another at the upper part of the foundation wall. The intent of these vents is to allow airflow of outside air, but in reality they allow in uncontrolled and unconditionef air, which is often moisture-laden, hot or cold.

This air has many opportunities to condense on cold surfaces. And the increased humidity of the air can cause the moisture content of all wood materials in the area to reach dangerous levels – think mold growth.

Another source of moisture in a ventilated crawlspace comes from the evaporation of soil moisture into the space. It is imperative to create a moisture vapor-tight seal between the ground and the space to curb additional moisture from the earth.

Without this, the air in the crawlspace becomes more humid, raising additional moisture concerns. Insulating the floor system in a ventilated crawlspace provides a challenge. Insulation is typically placed between the floor joists and is difficult to place between irregular spaces, around wires, plumbing, mechanical penetrations and at the perimeter or rim joist and box sill area.

This type of crawlspace insulation strategy is very common in many older homes and even new ones. A ventilated crawlspace may well contain individually insulated pipes and ductwork, but unfortunately, they’re still exposed to a variety of potential problems, including temperature swings, critters and moisture.

The good news is that solutions are relatively easy to implement and proper treatment of a ventilated crawlspace will result in a range of benefits for the homeowner.

Conditioned crawlspace

A conditioned crawlspace has insulated walls and a well-sealed vapor barrier on the floor. It does not have vents in the walls to the exterior and there is typically no insulation required between the floor joists above.

It moves the thermal barrier from the first floor to the exterior foundation wall so insulation can be placed on either the inside or the outside of the foundation wall. The home is now placed above a conditioned area with the insulation moved to the walls – think conditioned basement.

Conditioned ventilation air is provided to the crawlspace via a supply duct or combination of supply duct and return ducts from the HVAC system or a small ventilation fan. In the latter, a small fan is placed in the crawlspace and exhausts air to the outdoors.

A transfer grille or air passageway from the main level of the home is connected to the crawlspace. The conditioned air is provided to the space at approximately 50 cfm per 1,000 square feet of crawlspace.

A conditioned crawlspace resembles an unfinished basement in that all of the piping and ductwork is protected from the elements within a semi-conditioned space that is climate-controlled, well sealed from outside air and moisture sources with the ability to dry. In recent years, conditioned crawlspaces have become increasingly common.

These enclosed spaces provide greater benefits in terms of comfort, energy savings, protection from frozen pipes, and the management of moisture and mold. That being said, ventilated crawlspaces may remain necessary in certain limited conditions, including areas where moisture cannot be controlled such as high water tables or floodplain construction.

A conditioned crawlspace should be constructed reasonably airtight and include an air barrier strategy that remains in place over time – this includes the sealing of unintentional leaks to both external and internal spaces. For example, old foundation vents, rim and band joists, holes around drainage pipes, and other openings in the building envelope created for electrical wiring, ductwork, or plumbing should all be air sealed.

A 10-mil polyethylene barrier should be placed over the entire crawlspace floor – the sheets should also extend 12 to 18 inches up the foundation wall. The polyethylene sheets must be glued or taped at the overlap seams and to the foundation wall, including any footings or other penetrations, preventing air and moisture infiltration from the ground at the edges and between sheets.

Without ground cover, the damp surface soil can add gallons of water per day to the crawlspace because the ground beneath the house is a very good source of moisture. This is a good moisture control strategy for all crawlspaces, even if you aren’t converting a ventilated crawlspace to a conditioned one.

Next, insulation boards (extruded polystyrene foil faced polyisocyanurate or fiberglass blankets rated for this application) R-10 to R-15+ should be glued or nailed to the foundation walls, covering the entire wall.

Building America research on conditioned crawlspaces has demonstrated 15 percent to 18 percent less energy consumption for heating and cooling, while reducing humidity more than 20 percent. These results have substantially influenced changes in the 2009 and 2012 versions of the International Residential Code (R408.3) allowing unvented conditioned crawlspaces.

Thousands of homes have now been constructed with this important innovation ( See Construction Instruction’s video on unventilated crawlspaces at building-resources/animated-details/conditionedcrawlspace/.

Building or retrofitting crawlspaces

If you are building or retrofitting crawlspace foundations:

  • Be sure HVAC equipment has sealed combustion and there are no water leaks from piping.
  • Keep it closed off from the outside.
  • Access should be from the interior of the home through the subfloor or a sealed, airtight and insulated door from the outside.
  • A continuous polyethylene vapor retarder should be applied over the soil. All seams should be taped, and the perimeter caulked and secured to the foundation wall.
  • If possible, pour a thin concrete slab on the polyethylene to provide easier use and to prevent damage to the ground cover.
  • The exterior foundation wall should be waterproofed and foundation perimeter drainage pipe installed, as site rainfall and below-grade water conditions require.
  • The crawlspace wall interior or exterior should be insulated with rigid foam insulation. Consider proper insect management when rigid foam is used.

conditioning crawlspacesSafety concerns

Of course, anytime a building is sealed tightly, there are potential health and safety concerns that should be addressed.

For example, in the case of conditioned crawlspaces, it is a best practice that combustion furnaces and water heaters are all power direct vent or sealed/ closed combustion units.

Additionally, in areas where radon is a concern, radon mitigation measures should be adopted. For example, the crawlspace floor should be sealed with a sheet of polyethylene without gaps or leaks and a ventilation system should be installed beneath the polyethylene barrier.

Other unique local conditions may exist that require additional measures. When crawlspaces aren’t treated as a conditioned space, serious building performance and health issues can occur.

Remember, whatever gets into the crawlspace moves into the house: soil gas, moisture, rodents, and even insecticide from local exterminators. For this reason, it’s also critical that all ductwork in the space be sealed with high-performance tapes and mastic.

Regardless of where a home is located, proper treatment of crawlspaces has multiple benefits, including increased comfort, improved indoor air quality, protection from moisture, and energy and cost savings. In short, there’s no need to have cold feet when it comes to committing to the proper treatment of crawlspaces in houses and plenty of reasons to take steps to improve this important, but all too often ignored, part of a house.

Before you select the insulation for a conditioned crawlspace, check your local code requirements. You may be required to use a fire-rated foam insulation board or to cover your foam insulation board with a fire-rated material.

In addition, areas with termites may need a termite inspection gap in the insulation, which is a 2- to 3-inch gap located near the top of the wall.

This special feature is provided by Construction Instruction. Among the most-respected building scientists in North America, Construction Instruction ( teaches best practice building methods. Use Construction Instruction’s mobile app to get construction videos, building science articles, technical data and best practices anytime.


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