How do I insulate the underside of a bay window?

A bay window can add square footage to an ordinarily squared-off area. Since bay windows are an extension off the main perimeter wall of your home, insulation should be installed on the underside. Insulation will help minimise the transfer of air and keep your bay window area at a moderate temperature compared to the rest of the adjoining space.

Remove the exterior sheathing material that covers the interior floor joists of the bay window area. Wear goggles and safety mask to protect from flying debris. Remove loose dirt and debris and discard.

Measure the space between floor joists and the width of each individual floor joists.

Cut the foam panels using the utility knife according to the measurements taken. Insert the foam panels between the floor joists on the underside of floor boards, to ensure panels have been cut to the correct widths.

Seal spaces around the foam panels with the expanding foam. Continue applying foam boards until the underside of the floor boards is covered. Foam boards that are placed vertically and adjacent to the floor joists should join at the expanding foam seams. Use a damp cloth to clean excess foam and drips.

Replace sheathing that was removed and seal up air spaces on the exterior to prevent air leaks.


For climates that have harsh winters or summers, two layers of foam can be applied for greater insulation protection.

Things You'll Need

  • Safety mask
  • Goggles
  • Utility knife
  • Level
  • 2-inch rigid extruded foam panels (polystyrene or polyisocyanurate)
  • Exterior adhesive
  • Expanding polyurethane foam
  • Measuring tape
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About the Author

Ronique Gibson has 14 years in the architectural field, as well as over 1000+ articles written to date in various home niche topics, from decor, DIY, organization, home staging to lifestyle. Gibson has an established expertise in home and garden topics online and in written publications.