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NC State Extension

Expanding Foam – Defined

Spray /Expanding foams are available in a variety of different forms, from small cans to larger industrial gallon sizes. Special care needs to be taken when using these products, as some expand more than others and some can exert significant pressure on the surrounding structure during expansion.

Expanding foam is a product which is designed to expand and harden upon contact with the air. It usually comes in canisters with spray nozzles, making it easy to apply the foam in a wide variety of situations. (video) There are a number of uses and hardware suppliers stock several types for different applications. Latex, polyurethane, and a variety of other materials can serve as a base for this product, and waterproof, fire resistant, and other specialty versions are available.

Some spray foam brands have different levels of expansion: high expansion for larger gaps, low expansion for around windows and doors. In certain situations, the expansion of spray foam around windows and doors can, in theory, pop the frame out of a wall. So, go for lower-expansion foam there.

General Tips and Safety

  • Carefully read instructions on the can label before use.
  • Cover the floor with drop cloth or newspaper.
  • Wear safety glasses, or goggles, and gloves when spraying the foam. – spray foam takes two or three days to come off skin and up to two weeks for finger nails.
  • Wear old clothing – spray foam will not come off fabric once it has bonded.
  • Practice dispensing the foam onto a sheet of newspaper or into a garbage can.
  • Foam insulation is flammable during dispensing. Remove sources of ignition, including pilot lights, until spray foam hardens completely (approximately 8 hours).
  • Mist the area with water before and after foaming to speed up curing.
  • If you overfill an area, trim the excess with a serrated blade or utility knife once it has fully cured.
  • Fill the opening less than 50% full when applying to allow for expansion of the foam. Applied foam will continue to expand for a few minutes after it’s applied. So if you fill a gap to the point of overflowing, it will become even more overflowing a few minutes later. Overfilling causes wasted product and messy cleanup.
  • If the foam in the straw applicator hardens, trim near the area where the foam has hardened and continue application.
  • Spray foam comes in compressed cans (similar to spray paint) with a straw-like attachment. You can poke the straw back into gaps and spray the foam into deep gaps and cracks.
  • Note that when you release the spray valve, the foam may continue to flow for a moment from the can while the foam in the straw expands.
  • Applying expanding foam takes some practice so you may wish to start out in less conspicuous areas. Have plenty of paper towels or rags on hand.
  • More Frequently Asked Questions (FAQs) for the Do-It-Yourselfer – Health and Safety

Insulating expansion foam (“spray foam”) is measured by how much the liquid form of it expands once it’s dispensed out of its container (hence the terms “high expansion” and “low expansion” foam).

Types of Spray Polyurethane Foam Products – EPA