How Does Foam Seal?

Foam gaskets for HVAC systems and enclosure panels and doors are cut from elastomeric materials with a cellular structure that looks like sponge. Although compressible, sponges tend to absorb water, so we’re sometimes asked how foam can make a good sealing material.

Open or Closed?

Foam is made by trapping bubbles in the material as it solidifies. These form during the chemical reaction that creates the elastomeric material. Foam manufacturers control the size of these bubbles by managing the reaction process and using chemical additives.

An important property to consider in foam gasket material is whether the cellular structure is open or closed. When the structure is closed bubbles are closed off from one another and there’s no way for air or any other fluid, to move between them. In contrast, in an open structure most of the cell walls are missing, which lets fluid travel through readily.

Sponges are generally open because their purpose is to absorb water. Foam with a closed structure doesn’t have this ability, and that makes it fluid-resistant.

Open cell foam can be used as gasket material provided complete impermeability isn’t needed. Compressing it to less than 50% of its free thickness leaves very little space for liquid to pass through. This allows it to be used where light water exposure is possible.

Other Pluses and Minuses

A cellular structure makes elastomeric gasket materials less dense, which is beneficial when weight is a concern. They also tend to be soft and compress easily, although closed cell material is firmer because the air inside the cells has nowhere to go. While compressed, the air slowly permeates through the cell walls, and recovery is slower when the load is taken off.


Flexible cellular materials are covered by ASTM D1056. This standard uses a three character code to describe foam materials. The first character is either a “1”, meaning an open structure, or a “2”, meaning closed. When ordering a foam gasket material it’s important to be clear which is needed. If in doubt, speak with a Hennig Gasket material specialist.