Insulation: Sill/Rim Joist

Desired Outcome:

Properly align thermal boundary and air barrier between the conditioned and unconditioned space

SWS Detail: Subtopic 4.0401 Rim/Band Joist

Author: Brenna

Before

Uninsulated rim joist

After

Insulated rim joist



Steps

  1. Follow work order for air-sealing sill

  • Sill should be air-sealed at seams and corners with adhesive or foam prior to insulating

  • See the Air seal Basement or Crawl Walls when air sealing is called for.

2. Install insulation

  • Install one of the following insulation types

2a. Fiberglass batts

  • Select batt insulation that has a class II vapor retarder facing and that has a flame spread and smoke development index of 25/450 or less

  • Cut batt to fit sill with no gaps up against the framing.

  • Insulation must maintain full contact to ensure a continuous thermal boundary.

  • Do not compress batt insulation

2b. 2-part foam

  • Install 2-part foam up to 3 1/4" thick. may also be used for air-sealing and insulation.

  • See SDS material safety for restrictions of use.

  • The Sill is the one location on a home where foam plastics can be installed without an ignition barrier but shall meet the conditions of 4.0401.1f.

  • Foam is not thicker than 3 1/4" thick, listed as either ASTM E84 or UL 723, Flame spread of 25 or less and Smoke Developed Index of 450 or less.

2c. Pin and drape

  • Folding foundation insulation to insulate sill.

  • Fiberglass blanket can be notched and folded into sill, attach to joists with staples.

  • Edges of blanket must be sealed ensuring no insulation is exposed.

2d. Rigid foam

  • Use non-corroding materials to seal the seams around the insulation with adhesive.

3. Worst Case Draft Test

  • Conduct a worst case draft test to verify Category I, II, and III appliances are venting before leaving the job for the day.

  • Testing shall be recorded using the Utah WAP Worst Case Draft Test form.

4. Install Insulation Certificate

  • A dated insulation certificate signed by the installer will be posted on a wall in the space where the furnace is located, a utility room or an approved location inside the building.

  • The certificate must include:

    1. Installed insulation type

    2. Coverage area

    3. Installed thickness

    4. Installed R-value

    5. Manufacturer product name

    6. Manufacturer supplied material density

    7. Flame Spread rate

    8. Smoke Development index