How to Insulate a Crawl Space

A properly insulated crawl space helps regulate the temperature of a house. In colder climates, crawl space insulation keeps the floor warmer and helps to prevent frozen pipes.

How to Insulate a Crawl Space

Why Insulate a Crawl Space?

Unlike basements, crawl spaces tend to be out of sight out of mind parts of a house. Quite often months and even years pass without anyone paying a visit. Insulating crawl spaces provides year-round long-term benefits. Here are some of them.

  • Warmer in the winter. Cooler in the summer.
  • Protection against frozen pipes in colder locations.
  • Heating and cooling cost savings. Many HVAC systems have ducting in crawl spaces wasting energy heating and cooling an unused space.
  • Adds resale value. One more item a buyer does not have to consider.

Insulation vs Encapsulation

Crawl space insulation is not the same as crawl space encapsulation. Insulating a crawl space involves dealing with the underside of the floor. Encapsulation extends insulation to crawl space walls with a vapor barrier on the floor. Both methods are used to improve comfort in the house.

Crawl Space Insulation

Crawl spaces can be insulated with DIY spray foam kits (or contractor-applied spray foam), different types of batt material, and rigid foam board insulation. All are applied to the underside of the floor above.

Before installing or applying any type of insulation remove all existing insulation. Any holes and penetrations in the subfloor need to be sealed with spray foam in a can or acoustic caulking. Install rim joist insulation before any type of floor insulation.

Install ductwork insulation to any HVAC ducting exposed below the floor insulation. Subfloor insulation prevents house warmth from entering the crawl space.

Spray Foam Insulation

Spray foam insulation provides the best R-value of R-6.5 per inch. It completely air-seals the subfloor and is moisture-resistant. Mold and mildew will not grow on it and pests do not live in it. Foam also insulates any plumbing and HVAC ducting running through the floor joists.

Spray foam is the most expensive insulation option. It is next to impossible to completely remove it when dry and can be a messy installation–especially when installed overhead.

Batt or Roll Insulation

Fiberglass batts install easily and cost less than other products. R-value is about R-3.5 per inch. Batts require strapping to keep them from falling out. They absorb moisture and get heavier over time.

Install faced batts with the paper towards the subfloor. Stapling the paper to the floor joists helps prevent the batts from falling out.

Rigid Foam Board Insulation

Install rigid foam board insulation between the joists or attach it to the undersides of the joists. The most popular foam boards–in order of R-value and cost–are expanded polystyrene (R-3.5), extruded polystyrene (R-5), and polyisocyanurate (R-6.5).

When installing rigid foam between the joists ensure that gaps around the edges are sealed with spray foam in a can or acoustic caulking. Seams in boards attached under the joists also need to be sealed to create a vapor barrier. The foam must be at least 2” thick.

For extra insulation, install fiberglass batts between the floor joists, then apply rigid foam across the joist bottoms. The foam keeps the fiberglass in place and dry by providing a vapor barrier.

Reflective and Bubble Wrap Insulation

Reflective insulation and bubble wrap insulation can be stapled to the undersides of the joists. They reflect heat back into the living area but have no R-value. One of the big advantages of using these types of products is flexibility. They wrap around ducting and pipes that extend partially below the joists. Enclosing them inside the warmer area of the crawl space.

Insulating an Unvented Crawl Space

Crawl space encapsulation is the recommended treatment for unvented spaces. This method includes wall insulation and a vapor barrier on the dirt floor. The idea is to incorporate the crawl space inside the home’s insulation blanket.

Insulating a Vented Crawl Space

Vented crawl spaces have been a normal building practice for decades. Air movement is meant to keep the space dry. Good idea in theory but bad results in practice–in some areas. Venting works very well in dry locations.

Warm humid air mixing with cool crawl space air causes condensation. Wet crawl spaces attract pests, cause rot, and moisture can accumulate in fiberglass batts. Making it inefficient. It will even fall out of the floor joists because of the weight.

Insulation in vented crawl spaces should be moisture-proof or the space should be encapsulated.