How To Use Foam Board And Canned Spray Foam To Insulate A Basement Ceiling

Spray polyurethane foam is ideal for use in insulating basement ceilings, but large-scale application of foam can be difficult for do-it-yourselfers. Successful spray foam application demands the use of specialized sprayers and full-body protective gear, including respirators. In addition, applying spray foam to large areas is best performed by trained technicians who understand the process from start-to-finish. If you decide to perform a do-it-yourself insulation job on your basement ceiling, the use of rigid foam board coupled with canned spray foam is a satisfactory alternative in most instances. Below are the tools and materials needed as well as a guide to installation:

What you will need

  • 4-foot by 8-foot sheets of foil-lined polyisocyanurate rigid foam board

  • Long straight edge

  • Measuring tape

  • Thin-point black magic marker

  • Utility knife

  • Construction adhesive

  • Caulk gun

  • Canned spray foam

  • Stepladder

  • 10d nails

  • Claw hammer

Step-by-step installation procedure

1. Determine how many foam boards to purchase - Begin by measuring the gap between the ceiling joists to determine how wide each foam strip should be. Next, count the number of gaps between joists, then measure the length of the gaps in inches. Multiply the number of gaps by the width of the gaps, then multiply this total times the gap length to determine how many square inches of foam board will be needed.

As an example, if the gaps between joists measure 14-1/4 inches wide, which is the most common distance found in American homes, and there are 10 gaps across your basement ceiling with a length of 120 inches apiece, you will need 17,100 square inches of foam board. Considering that each 8-foot by 4-foot foam board contains 4,608 square inches, that means you will need 4 whole foam boards to provide coverage for the entire area.

2. Measure and cut the foam board strips to size - After determining how wide each foam board strip needs to be, layout the width of the strips and mark cutting lines on the boards using a thin-point magic marker and straight edge. Next, cut out the strips with a utility knife and the straight edge to help maintain a straight line as you cut.

3. Fit the foam boards to the ceiling - Once you have cut out your foam strips, test fit them by holding them up to the ceiling to ensure the strips fill the gaps and aren't obstructed by any obstacles, such as pipes or ducts. Trim away any excess pieces of foam board, so the board will fit around the obstacles.

Next, use a caulk gun to apply several thin beads of construction adhesive to one side of the foam strips, then press them into position on the ceiling. Hold the strips in place until the adhesive sets and the strips won't fall. If desired, hammer a few 10d nails along the edges of the strips into the joists to provide extra holding strength. Allow the adhesive to dry completely before moving to the final step.

4. Fill the gaps with canned spray foam - After the foam board strips have been installed and the adhesive has cured, you are ready to apply spray foam to seal gaps. However, before using the spray foam, be sure to read the directions for application and heed any safety warnings regarding its use. Open any windows and use portable fans to help ventilate the basement. When you are ready to begin, carefully spray a line of foam from into the spaces along the edge of the strips. Work slowly and be sure not to spray too much foam, as it is wasteful and could crack the foam board due to over-expansion. If you notice any remaining gaps after applying the foam, go back and fill these openings with additional foam. After finishing, restrict persons from entering the basement until the spray foam has cured completely.