Understanding home insulation, from fiberglass to foam

There are several types of foam insulation, according to Keith, but the two most common are called open cell and closed cell. Foamtech, one of many foam insulation manufacturers, describes the difference this way:

Alexandra Marks

Alexandra writes about the 'green' and budget-friendly renovation of a 100-year-old farmhouse in south-central Connecticut.

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Open-cell foam is soft – like a cushion or the packaging material molded inside a plastic bag to fit a fragile object being shipped. The cell walls, or surfaces of the bubbles, are broken and air fills all of the spaces in the material. This makes the foam soft or weak, as if it were made of broken balloons or soft toy rubber balls. The insulation value of this foam is related to the insulation value of the calm air inside the matrix of broken cells. The densities of open-cell foams are around 1/2 to 3/4 of a pound per cubic foot.

Closed-cell foam has varying degrees of hardness, depending its density. A normal, closed-cell insulation or flotation polyurethane is between 2 and 3 pounds per cubic foot. It is strong enough to walk on without major distortion. Most of the cells or bubbles in the foam are not broken; they resemble inflated balloons or soccer balls, piled together in a compact configuration. This makes it strong or rigid because the bubbles are strong enough to take a lot of pressure, like the inflated tires that hold up an automobile. The cells are full of a special gas, selected to make the insulation value of the foam as high as possible.

There are advantages and disadvantages to both types of spray foam, and I’ll get into that later. But both share one big advantage, which is that they pretty much seal the house up, almost as though it's wrapped tight in plastic.

That reduces almost completely any air infiltration – which is now as important, if not more important, than assessing the R-values or thermal resistance of a particular type of insulation. The less air that gets in and out, the better the overall insulation of the house.

“I’ve seen a lot more foam spray in older homes, 1800 and 1900 vintage similar to yours,” says Keith. “With all of the nooks and crannies that are difficult to fill with fiberglass, the foam lends itself better, it tightens the building up immensely.”

The problem is the cost. Spray foam insulation can cost two to three times as much as traditional fiberglass batting. That’s why Keith believes he’s seeing fewer builders using foam for new construction.

“It’s a tough sell to get away from the fiberglass because it’s inexpensive compared to the foams. But I think if you look to the long-term, the foam does provide a solid envelope and solid savings. It really is the way of the future.”

Next: Choosing the best insulation for Sheep Dog

Editor's note: Alexandra Marks will be blogging twice a week about her green and budget-friendly restoration of a 1902 farmhouse in Connecticut. See a photo gallery of the early days of the project by clicking here. You can read all she's written about the project so far by clicking here and then looking for Sheep Dog Hollow under Topics on the right side of the page.

You'll find numerous articles about the environment at the Monitor’s main environment page. Also, check out our Bright Green blog archive and our RSS feed.

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