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Insulating Basement & Crawl Space Walls

Basement walls and walls in unvented crawl spaces should be insulated to help prevent substantial heat loss. (A crawl space is an unfinished, accessible area below the first floor of a building. An unvented crawl space is part of the basement while a vented crawl space opens to the outside.) Remember, an uninsulated basement alone can account for 25% or more of a home's total heat loss.

Unframed, exterior basement walls should be insulated with blanket insulation, which can be installed horizontally or vertically. Typically, the blanket is held against the sill plate at the top of the basement wall with furring strips. In addition to the furring strips, you will need patch tape, and depending on the installation method you choose, possibly a Hilti-type gun and special fasteners for driving into concrete or cinder blocks.

It's a good idea to wear a work helmet when you're installing insulation in basements and crawl spaces. This can help you avoid injuring yourself on exposed framing members such as ceiling and floor joists, exposed nails and other hazards.

How to Install Basement Insulation on Unframed Walls

Horizontal Installation

  1. Seal around all penetrations in band joists. Any walls that leak water must be repaired before insulating.
  2. Pre-drill an ample supply of 1x2 furring strips.
  3. Measure the length of insulation you'll need for the full wall and cut it on the unfaced side.
  4. Grip the insulation by the flange, and with the faced side toward you, position it against the sill plate at the top of the basement wall. Then position a furring strip and nail through the furring and the flange to the sill plate to secure the insulation. Whenever possible, pull the insulation behind any ductwork, plumbing, or electrical that may lie along the walls. Otherwise, carefully cut and fit the blanket around obstructions.
  5. When you cover the full wall, you'll need to attach a second length of the blanket to the lower edge of the first one. If you have cut a piece to fit and there is no flange, create one by pulling back the insulation. Overlap the flanges of the top and bottom pieces so that the insulation butts together tightly. Then, staple through the flange to hold the pieces together. An alternate method is to attach furring strips to the wall at the mid-point and bottom of the wall. Then staple the blanket flange to the furring strips.
  6. To give the insulation a finished look, tape over all joints, seams and stapled edges with 3-inch wide, white vinyl patch tape.
  7. Cut small pieces of batt insulation to fit against the header joists and push them into place between each floor joist.

Vertical Installation

  1. Seal around all penetrations in band joists. Any walls that leak water must be repaired before insulating.
  2. Cut lengths of insulation a few inches longer than the height of the walls.
  3. Attach the blanket to the wall using furring strips or a Hilti-type gun to drive fasteners into the concrete or cinder blocks. Fit adjacent blankets tightly together.
  4. Trim the bottom of the insulation flush with the floor.
  5. To give the insulation a finished look, tape over all joints and with 3"-wide, white vinyl patch tape.

How to Install Insulation in a Finished Basement

Installation Tips

Finished basements are insulated like any other room in your house.

How to Insulate a Vented Crawl Space


If the sub-floor is already in place, the insulation is installed from below, much as it is installed in ceilings.

  1. Cover the ground with sheets of six-mil plastic film. The sheets should overlap each other by approximately twelve inches, and extend a few inches up the walls.
  2. Tape the film in place at the walls and hold the seams in place with tape, scrap lumber, or rocks. After the ground is covered, you're ready to install the insulation.
  3. Fit the insulation batts into the joists with the Kraft-paper vapor retarder facing against the sub-floor. Make sure the insulation fits snugly so that there is no air space between the flooring and the insulation.
  4. Staple the insulation flanges securely to the sides or bottom of the joists (called "inset stapling").

When installing Kraft-faced insulation, use wire lacing, screen or stiff wire fasteners to hold the insulation firmly in place. The fasteners are bowed upwards into the insulation, pressing it gently against the sub-floor without overly compressing it. Place the fasteners at least six inches from the end of each batt, and no more than 24 inches apart.

You may want to consider using JM's special ComfortTherm® for Underfloor, specifically designed for floors, with the stapling flanges on the bottom side, and the vapor retarder on the top side. It is stapled to floor joists in the same way as ordinary faced insulation is installed in ceilings and ensures insulation is up against the floor. The vapor retarder side is installed up against the subfloor, and the non-vapor retarder has flanges for stapling to the sides or bottom of the joists.
Where heating ducts between joists are exposed to cold air, insulation should be installed below the ducts to prevent heat loss.

In cold climates, if water pipes are running through the joists in the crawl space insulation should be installed below the pipes to protect them from freezing.

How to Insulate an Unvented Crawl Space


In an unvented crawl space, the general rule is to insulate the perimeter walls. This eliminates the need to separately insulate the water pipes and heating ducts.

  1. Cover the ground with sheets of six-mil plastic film. The sheets should overlap each other by approximately twelve inches, and extend a few inches up the walls.
  2. Tape the film in place at the walls and hold the seams in place with tape, scrap lumber, or rocks. After the ground is covered, you're ready to install the insulation.
  3. Locate the header joists, which run across the ends of the floor joists. Measure and cut pieces of unfaced insulation and place them against the header joists between each floor joist. Completely fill the spaces enclosed by the sub-floor, sill, and floor joists.
  4. Install lengths of standard batts or the wider basement blanket insulation to the sill using furring strips to nail the insulation to the edge of the sill plate. The insulation should be cut long enough to hang down the wall and extend two feet into the crawl space. It can also be installed horizontally in the same manner.
  5. Anchor the insulation as close as possible to the wall where it meets the ground using 2x4s.
  6. Locate the stringer joists, which run parallel to floor joists. Position an insulation blanket against the underside of the sub-floor and staple or nail it directly to the stringer joist. Or, attach the blanket to the top of the sill, and wedge smaller pieces between the sill plate and sub-floor. (This technique takes longer, but provides better thermal protection at the joist.) The insulation should be cut long enough to hang down the wall and extend two feet into the crawl space. It can also be installed horizontally in the same manner.
  7. Anchor the insulation as close as possible to the wall where it meets the ground using 2x4s.
  8. Make sure all pieces of insulation are tightly butted together. Be sure they fit snugly, without gaps between them. Taping is not usually necessary. An alternate way of fastening basement blankets to the walls in crawl spaces is using a Hilti-type gun to drive the fasteners into the concrete.

Related Products

ComfortTherm® Batts and Rolls


ComfortTherm® insulation is a lightweight, thermal and acoustical insulation made of long, resilient glass fibers bonded with an acrylic thermosetting Formaldehyde-free™ binder.

EasyFit® Batt Insulation


JM's EasyFit® Formaldehyde-free™ insulation is available unfaced or with kraft facing, and has vertical perforations for fitting non-standard wall cavities.

Unfaced Batts and Rolls


JM's Unfaced Insulation is a lightweight thermal and acoustical fiber glass insulation made of long, resilient glass fibers bonded with an acrylic thermosetting resin made without formaldehyde.

Basement wall insulation
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