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Crawl Space Insulation

A crawl space is an unfinished, accessible area below the first floor of a building. Remember, the main purpose of insulation is to enclose the living space in a thermal envelope. Therefore, if the space below a floor is unconditioned, either the floor or the walls below it must be insulated.

There are two types of crawl spaces: vented and unvented. Vented crawl spaces open to the outside, while unvented crawl spaces are part of the basement. How and what is insulated depends on whether the space is vented or unvented.

In a vented crawl space, the underfloor should be insulated, much as it is installed in ceilings. This may be done either before or after the sub-floor has been applied. JM ComfortTherm®, Kraft-faced and unfaced insulation are all possible products to use for this application. With Kraft-faced insulation, the facing is generally against the sub-floor.

In an unvented crawl space, you should insulate the perimeter walls.

How to Insulate a Vented Crawl Space

Installation Tips

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

Installation Tips

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.

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