What is the best type of insulation to put into our crawlspace to keep the floor warm in the winter?


What is the best type of insulation to put into our crawlspace to keep the floor warm in the winter?

Asked by J. Brown

We have a new house built on a hill and it has anywhere from 2 ft. to 4 ft. space underneath the floor (no basement)

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Michael Holcomb's picture

By properly air sealing and insulating your home's crawl space, you create a comfortable and healthier living environment for you and your family.

Save on energy costs by not having to condition the air from the crawl space that is pulled into your home and create storage space for your home by creating a clean and dry space beneath your home.

There are two schools of thought with respect to insulating crawl spaces:

  • insulate the underside of the floor, or
  • insulate the crawl space walls essentially making the crawl space a conditioned space.

In almost every case my preference is the latter, making it a conditioned space.

Making the crawlspace a conditioned space

Assuming you do not have a concrete floor in the crawl space you should first remove any debris and rake the dirt/soil level.

  • Next lay a drainage fabric directly over the soil to protect the liner and improve drainage.
  • Next install a sealed liner over the drainage fabric that extends 6-inches up the walls.
  • Any seams should be overlapped and sealed with appropriate tape.
  • There are a variety of products for this purpose but 20 mil liners should be the minimum consideration.
  • Liner warrantees vary so look for something beyond 20-years.

Insulate the walls using rigid foam board or closed-cell, spray-applied foam. The latter acts as a moisture barrier and allows the walls to dry to the outside. I would recommend 2-inches of material (R-10 to R-14). The foam should be installed from the rim/band joists to the floor liner to create a monolithic barrier.

With this type of installation the crawl space would be "conditioned" so it will not need to be vented to the outdoors. I do recommend installing a supply and return duct to keep air moving which helps reduce mold growth.

Properly sealed and insulated crawl space based on recommendations given above.

Alternatively, insulating the floor

If you insulate the underside of the floor you have to ventilate the crawl space. If the crawl space has mechanical systems; ducts, water lines, furnace, water heater or well you should avoid this method.

If the crawlspace floor is dirt or gravel you might want to consider a serious vapor barrier in either case to keep moisture from rotting wood structures.

For more information:

Read "We have an unheated crawlspace under our house that we would like to insulate." a Q&A answered by Florian Speier.