Open Cell Spray Foam Insulation

Open cell foam is the lighter of the two spray foams. It goes on as a thin liquid and expands about 100 times in a few seconds, filling the cavity and sealing around wires, pipes and framing. Once it cures it is soft and a little spongy, and its cells are open to each other, which is where the name comes from.

Where open cell works best

• Under the roof deck of a house. This is most of what we spray in Dallas–Fort Worth homes. At 5.5″ to 7″ thick it seals the attic and brings the ductwork inside the house. See attic spray foam.
• Exterior walls in new construction. It fills 2×4 and 2×6 cavities completely and seals the gaps around outlets and plumbing that batts leave. See new construction.
• Interior walls and floors for sound. Open cell is better than closed cell at deadening noise between rooms, around a media room or under a bonus room.

Where we don’t use it

Open cell lets water vapor pass through. That is an advantage under a roof deck, where it lets a leak show up instead of hiding it. It is a problem anywhere moisture is constant:

• On bare steel. Humid air can move through open cell and condense on the cold metal behind it. Metal buildings, barndos and shops get closed cell.
• In crawl spaces. The damp air under a pier-and-beam house calls for closed cell.
• Anywhere it can stay wet. Open cell can absorb liquid water. It dries out when the source is fixed, but it is not the foam for a wet location.

How thick, and what R-value

Thickness	About	Typical use
3.5″	R-13	2×4 walls
5.5″	R-20	2×6 walls, most roof decks
7″	R-25	Roof decks where there is room for more

Why it costs less

Open cell uses much less material per inch than closed cell, so the price per inch is lower. Because it takes more inches to reach the same R-value, compare by R-value, not by inch. For a roof deck, where rafters leave plenty of depth, open cell is usually the better value. See current rates on the spray foam cost page.

Open cell questions

Is open cell foam an air barrier?

At the thicknesses we install, yes. It fills the cavity and seals the cracks and gaps that let air move through walls and roofs. That air sealing is a big part of why a foamed attic feels so different from one with blown-in insulation on the floor.

Will open cell foam hide a roof leak?

No, and that is one of the reasons we use it under roof decks. Water passes through open cell and shows up as a stain or drip inside, so you can find the leak and fix it. Closed cell is water resistant and can let a leak travel along the deck before it shows.

Does open cell foam shrink or pull away over time?

Foam that is mixed and applied correctly stays bonded to the framing and doesn’t shrink. Shrinking and pulling away are signs of a bad mix or the wrong conditions on spray day, which is why we check temperatures and the machine settings as we spray.

Is open cell better than blown-in insulation?

For sealing an attic that has ductwork in it, yes, because blown-in doesn’t stop air. For a vented attic with no ducts, a deep layer of blown-in on the attic floor can be the better buy. We will tell you which one fits your house.