Skip to content

Crawl spaces

Crawl space insulation: floors, walls and rim joists

By Robert Stulpt Last reviewed

Short answer

Where crawl space insulation goes depends on how the crawl space is designed. If it’s vented, you insulate the floor above it, in the joist bays. If it’s a closed, conditioned crawl space, you insulate the foundation walls and the rim joist. Adding more insulation won’t solve a moisture problem. If your insulation is wet, has fallen or is contaminated, remove it and make sure the crawl space is dry before you insulate again.

On this page
  1. Where the insulation goes
  2. Insulation in a vented crawl space
  3. Walls and rim joists in a closed crawl space
  4. How much insulation you need
  5. Leave room for termite inspections
  6. Rim joist mistakes

Where the insulation goes

When you have a vented crawl space, the thermal boundary is typically the floor of the home. We can move it to the walls if we close and condition the crawl space. See vented vs. closed crawl spaces.

Vented crawl space Schematic section of a vented crawl space. Outdoor air enters through vents in the foundation walls (1) and leaves through vents on the other side. In humid weather, water condenses on cool ducts and pipes (2). Insulation sits between the floor joists (3). Moisture rises from soil that is bare or only partly covered (4). Schematic, not to scale. 1234
  1. Vents in the foundation walls
  2. Cool ducts and pipes, where condensation forms
  3. Insulation between the floor joists
  4. Bare or partly covered soil
Schematic, not to scale.
Closed crawl space Schematic section of a closed crawl space. The foundation walls have no vents (1) and are insulated (2). A liner covers the ground and runs up the walls, sealed at the seams (3). Moisture is removed by a dehumidifier (4), a small supply of conditioned air from the house (5), or both; some designs use a small exhaust fan instead. A narrow gap is left between the wall insulation and the sill plate so the wood can be inspected for termites. The floor above is not insulated. Schematic, not to scale. 12345
  1. No vents in the foundation walls
  2. Insulation on the walls instead of the floor
  3. Liner over the ground and up the walls, sealed at the seams
  4. Dehumidifier
  5. Conditioned air from the house
Schematic, not to scale.

Insulation in a vented crawl space

If I have a vented crawl space and the insulation is dry, in good condition, free of mold and rodents and held in place correctly, I leave it alone. I won’t remove good insulation just because. If it’s wet, fallen, compressed, contaminated or full of nests, or it’s been wet so long that I don’t trust it, I replace or remove it.

Floor insulation has to stay in contact with the underside of the subfloor, or fill the cavity, so it keeps its rated R-value (N1102.2.7 in the Virginia code). Batts that sag or fall away from the subfloor aren’t doing their job.

Walls and rim joists in a closed crawl space

On the walls of a closed crawl space, I typically use rigid insulation that’s suited to the application. I like it because it has good thermal properties, it won’t absorb water like a fiberglass batt and it leaves a clean finish. On the rim joist, I typically use closed-cell spray foam, because there are many joints and penetrations that need to be air sealed. You can also use rigid foam cut to size and sealed. It depends on the home.

Foam insulation and fire. Foam insulation normally has to be covered by a thermal barrier, such as 1/2-inch drywall. In a crawl space you only enter for repairs or maintenance, the code relaxes that: the foam can stay exposed if the product has been tested and approved for it, or it can be covered with a lighter ignition barrier, such as 1 1/2 inches of mineral fiber insulation or 1/4-inch wood structural panels (R316.5.4). Check the product’s documentation before you leave foam exposed.

How much insulation you need

It depends on your climate zone. Your residential code sets the minimum insulation for floors and crawl space walls, so make sure you know which edition your state uses and which zone you’re in. As an example, these are the minimums in the 2021 Virginia Residential Code (Table N1102.1.3), which follows the 2021 International Energy Conservation Code:

Climate zoneFloor over a vented crawl spaceWalls of a closed crawl space
3R-19R-5 continuous or R-13 cavity
4 (except Marine)R-19R-10 continuous or R-13 cavity
5 and Marine 4R-30R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous

“Continuous” means an unbroken layer, such as rigid foam boards over the wall; “cavity” means insulation installed between framing. The table has footnotes, so check the full code for your case.

When the walls are insulated, the code also says how: the insulation must be permanently fastened to the wall and run from the floor above down to the outside grade level, then at least another 24 inches down the wall or across the ground (N1102.2.10.1). In a closed crawl space, the 2021 Virginia Residential Code ties the drying options to insulated perimeter walls (R408.3). In climate zones 1A, 2A and 3A below the warm-humid line, insulation between the floor joists of a vented crawl space also needs a vapor retarder on its exposed face (R408.8).

Leave room for termite inspections

In Virginia, I like to leave the top of the wall exposed just below the sill plate, normally 1 to 2 inches. I don’t take my insulation or liner up that high. I want an inspector to be able to trace the entire perimeter. People forget about this. In an unvented crawl space, Virginia’s residential code (R408.3.1) requires the vertical face of the sill plate to stay clear, with an inspection gap of 1 to 2 inches below it along the top of any interior wall covering, except in areas not subject to termite damage or where another approved inspection method is used.

Rim joist mistakes

  • Stuffing in fiberglass without sealing the air leaks
  • Insulating wood that’s still wet
  • Spraying foam over damage that nobody inspected
  • Gaps left around pipes
  • Corners left unsealed
  • Covering areas that need to stay visible for inspection
  • Foam too close to hot equipment, without the required clearances
  • Using materials where the code doesn’t allow them
  • Not thinking about how the wood will be able to dry

First I want to make sure I have sound wood. Then I insulate it.

Sources