Should we insist that our contractor use expanding foam insulation around our new windows?


Should we insist that our contractor use expanding foam insulation around our new windows?

Asked by Sally Kieny, Colchester, VT

We are replacing some of our home's windows in an effort to reduce air leakage and improve energy efficiency. I asked our contractor (unfortunately from the old non-green school) if he was using expanding foam insulation to fill in the cracks and crevices around the new windows. I had read that this is the best way to reduce air leakage. He said that the expanding foams can interfere with the function of the window if they expand too much.

Answer this question


Carl Seville's picture

The key to sealing around windows is to stop all airflow between the interior and the exterior.

  • Most heat gain and heat loss is due to the uncontrolled movement of air through the building envelope.
  • In addition to heat movement, air leakage also provides a conduit for moisture to migrate into and out of the house, reducing comfort by allowing the relative humidity in the house to be higher or lower than desired.

Fiberglass insulation alone will not stop air movement - you must use a material that will not allow air to flow through.

  • While foam provides an excellent air seal, there are other ways to accomplish this.
  • If the gap is small, a flexible caulk will work.
  • You can fill larger gaps with foam strips, such as "backer rod", then seal them with caulk, making sure that there are no gaps allowing air to flow through.

Some expanding foams are designed to seal around windows; these are low-expansion products and will not affect window operation.

  • Look for products labeled "for window and door sealing".
  • Also look for products that use HFC or propane, rather than HCFC or CFC, as propellants. HCFC and CFC contain chlorine, which is known to damage the ozone layer.

Keep in mind that HFC and propane propellants are flammable during the installation process.

For more information:

Read Daniel Glickman's Q&A We need to replace the windows on our Florida home. What's the greenest choice?