Crawl Space Insulation

By some estimates up to 30% of the preventable energy loss in a home comes from an uninsulated crawl space.  Even in a fairly mild climate such as Seattle, a large amount of energy can escape through conductive and convective heat loss. Yet despite this, few homeowners give much consideration to the role their crawl space plays in their heating costs.  This is a mistake!

How are most crawl spaces in Seattle insulated?

Typically our homeas in the NW have fiberglass batt insulation installed in between the floor joists.  This provides a means to limit conductive heat loss through the home’s floor.  The batts are held in place by either metal rods or plastic twine. Other, less common insulation materials are polystyrene foam sheet,  poly-iso-cyanurate spray-on foam, rockwool, and cotton batts.

While crawl space insulation serves to prevent heat loss from the home, it will also keep the temperature of the subfloor above dew-point.  When crawl space insulation is missing it is common for condensation to occur on the subflooring.  This condensation may contribute to mold growth in the crawl space, or moisture being trapped beneath impermeable flooring  (such as vinyl or linoleum).

When should I replace my crawl space insulation?

  • Crawl space insulation has been saturated with water.
  • Crawl space insulation has fallen and become crushed.
  • Crawl space insulation has been contaminated by rodents or other pest infestation.
  • Visible mold growth on or beneath the crawlspace insulation.

How should my crawl space insulation be installed?

  • Crawl space insulation should be installed so that it touches the subfloor and does not allow airflow between crawlspace and subfloor.  Typically this is achieved with string stapled to the joists or with steel rods pressed up against the insulation.
  • Crawl space insulation should be continuous, leaving no voids or missing areas.
  • If insulation with a vapor barrier facing is used, the vapor barrier should be installed against the subfloor.  The vapor barrier must always be installed on the warm side of the cavity.
  • All HVAC ducting in the crawlspace should be air sealed (with mastic) and well insulated.
  • Insulation in a sealed crawl space should be installed in a continuous layer extending from the bottom of the subfloor to a minimum of 24″ below the exterior grade.

What types of crawl space insulation would you recommend for a Seattle home?

Two  options are available, both with unique benefits and challenges.  The first option is a traditional vented crawl space with insulation located between the floor joists.  This creates a radiant barrier between the cool crawl space air and the warm interior of the home.  The concept hasn’t changed much in the last 50 years and at least 80% of the homes in the Seattle area are built using this technique. The design has served its purpose well, though most home efficiency experts prefer newer methods.

Two primary problems arise with underfloor insulation: poor energy performance and rodent issues.  The majority of homes in Washington state are built with ducting in the crawl space.  With a standard ventilated crawl space, these ducts are running through essentially an outdoor environment.  If it’s a cold day in January, the ducts are running through very cold air as well.  This requires duct insulation and sealing to prevent heat loss.  A sealed crawl space avoids this issue by simply insulating the entire perimeter of the crawlspace.  Rather than running through cold, outside air, the ducts now run through a heated environment. Remember, if you use a traditional insulation technique you must air seal the penetrations between the crawl space and the first floor of the home.

The second problem is rodents.  If a rodent finds a pathway into your crawl they will interpret the underfloor insulation as an invitation to create a home.  Rodents prefer warm, confined spaces and the small gap between the insulation and the underside of the subfloor fits this need perfectly.

The second option is a sealed crawl space system.  Nearly all high efficiency homes built on a crawl space use this technique.  This system places the insulation on the perimeter foundation wall and leaves the underfloor uninsulated.  Duct insulation and floor insulation are both unnecessary because the ground acts as an insulative layer.  Because the space becomes a conditioned area it’s important to install a sealed vapor barrier.