Foam insulation: not exactly historically accurate

Scott Gibson

Spray-in urethane foams do a great job of insulating walls and ceilings, but are they really the right products for older period homes? After all, there's nothing historically correct about this kind of insulation.

Strictly by-the-book furniture restorers would stay away from repairs that aren't reversible. They would not, for example, substitute a polyurethane finish for the original shellac or yellow carpenter's glue for hide glue. For all practical purposes, spray-in foams are permanent, so you could have the same hesitation about using them in an old house.

Or not.

Using completely authentic materials makes sense in a historically important structure. I doubt spray-foam insulation found its way into James Madison's 19th century Virginia home when it underwent a meticulous restoration a few years ago. But the objectives of that kind of project are entirely different than making an old house comfortable and economical to heat and cool.

There are too many advantages to foam insulation to rule it out only because it wasn't available at the time the house was built.

Urethane foams are two-part compounds mixed in the application gun and sprayed into wall and ceiling cavities. There are two basic types--open- and closed-cell, each with different properties, insulating values, and costs.

Open-cell foam has an R-value of about 3.6 per inch, not that much different than fiberglass batts or dense-pack cellulose. A key difference is how effectively it seals air leaks. The foam expands instantly as it is applied, finding its way into cracks and fissures and hard-to-reach spots. Once cured, the foam makes an effective air seal.

Closed-cell, or high-density, urethane foam is a much better insulator, roughly R-6.5 per inch. It also lends structural strength to a wall or ceiling. Although it is more expensive, closed-cell foam also becomes a vapor barrier. That's an important advantage because it can prevent water vapor from condensing inside walls and ceilings and causing mold or decay.

Spray-in foams are usually applied when walls and ceilings are open, in new construction before the drywall or plaster goes up and in a retrofit when interior walls have been removed down to the studs. Closed-cell foam can also be pumped through holes drilled into existing walls.

In a major renovation of an old house, new interior walls and ceilings hide the foam. No one but the homeowner knows it's there, and the house ends up infinitely more comfortable than it was and heating and cooling bills are lower.

If the renovation is to include new exterior walls, also consider adding an inch of rigid foam board to the outside of the house before new siding goes on. That reduces the loss of heat through the wall framing.

The problem here is the extra layer of insulation may interfere with window and door casings and other exterior trim. If those original details could be preserved, I'd probably decide against rigid foam beneath the siding. Accepting slightly lower performance in order to preserve the look of the house would be worth it to me.

Otherwise, foam insulation is an excellent option. No, it's not historically correct. Then again, neither is your dishwasher, refrigerator, or gas-fired boiler.

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