How to Build an Insulated Box for Recessed Fixtures

Lay a sheet of drywall material on a table top so you can mark it for the individual box panels. Use a tape measure and a pencil to draw five boxes on the drywall in the following sizes: Box A should be 29 inches long by 21 inches wide, box B and box C should be 29 inches long by 12 inches high and box D and E should be 21 inches wide by 12 inches high.

Cut out each box panel separately from the drywall using a utility knife to score through the pencil lines until the panels separate.

Don disposable latex gloves before proceeding to make cleanup easier and spread a plastic tarp over the work surface.

Lay panel A in the center of the tarp. Open a can of expandable polyurethane foam and shake it. Apply a line of foam down the top and bottom edges of the panel.

Insert panel B and C into the center of the foam on either side. It does not matter which panel goes on which side. Hold the panels in place until the foam hardens, which takes one to two minutes.

Apply two more strips of foam to the two remaining sides of panel A. Insert panels C and D into the foam and hold them in place until the foam hardens. You should now have a cube shape with an open bottom.

Reach inside the cube and apply more foam along all the edges where the drywall meets to completely seal it. Wait until the foam hardens.

Carry the insulated box to the attic and place it over the recessed light fixture. Trace around the bottom of the box with a piece of chalk and set the box aside.

Apply a continuous bead of spray foam along the entire perimeter of the chalk line and place the box back into position, pressing it down into the foam.