In a region where January temperatures consistently hover below freezing, you really don’t want to be living in a house with no insulation.
But Christine Flynn and Liz Bagley, aunt-and-niece owners of the 1916 two-family house that covered in Season 28 of This Old House, were facing just that—until TOH general contractor Tom Silva came along. “People in houses built before WWII think there’s nothing they can do to protect against the cold,” says Tom. “Or they just don’t realize how much more comfortable their house can be.”
Interior Wall Insulation in an Old House
If a house’s attic (or roof) is already fully insulated, adding insulation to the walls may be the single best way to reduce heating and cooling costs. As is the case with most remodeling projects, many of this house’s walls were going to remain intact, so Tom had to consider how best to retrofit the energy-saving material without gutting the entire building—a job that would have blown the $250,000 renovation budget.
Where the walls were open—in the revamped kitchens and bathrooms, for example, and in the attic stud bays that had never been covered—he went with his top insulation choice: polyicynene, a cream-colored liquid polyurethane that foams up and stiffens after pros spray it in place.
However, finessing the fluffy stuff behind old walls is a lot tougher. In the living rooms and some of the bedrooms, Tom chose a slow-pour version of the foam, which goes in through holes drilled into the walls and takes longer to expand, minimizing the threat of cracking the existing plaster.
What is the Best Type of Insulation for Interior Walls?
A homeowner looking to similarly take advantage of a renovation to retrofit new or additional insulation has many choices—plastics, fiberglass, shredded paper, even denim scraps, and wool—in several forms. Of course, the cost, efficiency (expressed as an R-value that measures resistance to heat transfer), and skill level needed for installation of each type varies. So which type of wall insulation is best?
“That’s the e-mail I get every day, and there’s no simple answer,” says Andre Desjarlais, director of a program at Oak Ridge National Laboratory in Tennessee that focuses on how to build better buildings. But factoring in site conditions and budget, one kind of insulation will work better or may be easier to install. To understand which material would best suit your renovation, read on.
What it is
Made of either open-cell or closed-cell polyurethane (a plastic), or a special cement, this insulation goes on as soft foam or foaming liquid, filling all spaces and then stiffening in place. Applied only by professionals, it’s more costly than other options but it is the best at plugging air leaks.
Open-cell polyurethane, or polyicynene, is a low-density, spongy foam. It’s sprayed between exposed studs and expands to 100 times its volume in mere seconds. On finished walls, installers pour a tamer version through small holes; it expands over minutes to 60 times its volume. Closed-cell polyurethane foams to 30 times its volume and dries to a very hard shell.
Cementitious foam, which goes on like shaving cream but hardens over days into a meringue consistency, requires mesh across the studs to contain it.
Polyicynene gives about R-3.6 per inch of thickness; closed-cell polyurethane, between R-6 and R-7; and cementitious foam, R-3.9
When you can look beyond initial cost to long-term comfort.
Polyicynene may crack existing walls or leak out and stain a floor. Polyurethane is not stable when exposed to UV rays. Both closed-cell polyurethane and cementitious foam aren’t flexible, so as studs expand and contract, gaps may open.
Polyicynene and polyurethane are about $1.50 per square foot, including labor, if the wall is open, and $2.25 per square foot for existing walls. Cementitious foam costs about $1.40 to $2 per square foot. Installers can also spray a thin layer of foam to seal leaks, then fill in with less expensive insulation.
*Note: All prices are approximate for 1 square foot in a 2×4 wall
What they are
Fluffy blankets that come in long rolls or precut pads to fit between studs. These are most commonly made of fiberglass, but you can also find ones made from cotton (actually shredded denim scraps), mineral wool (made by melting slag from blast furnaces or rocks such as basalt), and real sheep’s wool.
Fiberglass batts can vary from R-3 to R-4.3 per inch of thickness; mineral wool delivers about R-3.6 per inch; cotton, R-3.4 per inch; and wool, R-3.5 per inch.
In walls gutted down to the studs on a do-it-yourself job.
The insulation must go in at full loft, not compressed. Stuffing insulation around pipes or leaving gaps in odd-shaped areas will make it less effective.
Also, sharp fibers in fiberglass and mineral wool can irritate skin—so you need long sleeves, gloves, a mask, and goggles when handling it—and some brands are made with a formaldehyde binder, which off-gasses over time.
Cotton and wool are natural products without these downsides, but they’re more expensive and harder to find-especially wool, which is only available via the Web from Canadian distributors.
Basic fiberglass batts cost about 40 cents per square foot uninstalled, while extra-dense ones run about $1. Mineral wool is also about 40 cents, cotton costs about 60 cents, and wool tops the chart at $2.75.
What it is
Dry bits of insulation that get blown into wall cavities through holes 1 to 2½ inches wide. It either goes in from the interior walls, which requires patching holes, or from the exterior, which requires prying up siding and drilling through the sheathing, adding to the cost of professional installation.
There are three main types: fiberglass, either treated with formaldehyde (made as a by-product of batt manufacturing) or untreated; cellulose, which is about 80 percent ground-up newsprint and 20 percent borate, a mineral added as a fire retardant; and mineral wool.
Fiberglass allowed to densely fill the cavity can give as much as R-4 per inch; cellulose, between R-3.6 and R-3.8 per inch; and mineral wool, R-2.7 per inch.
To beef up attic-floor insulation, or inside existing walls when the budget is tight.
Cellulose uses less energy to manufacture, so it costs less and is a better environmental choice in many cases. But fiberglass or mineral wool may be a better option in wet, windy areas, particularly in houses with wood siding, because, unlike cellulose, they don’t absorb moisture.
Cellulose and fiberglass are about $1.20 per square foot installed when blown in from the inside, $2 per square foot from the outside. Uninstalled, the prices are about one-third the cost; a homeowner doing a DIY installation can rent a blower for about $70 a day.
What it is
A variation on loose fill that’s suitable only for stud walls that haven’t yet been drywalled or plastered. A professional installer adds water and adhesive to the same basic insulation materials and uses a hose to spray the mixture between studs, which helps ensure full coverage. Plus, the adhesive makes fibers less likely to settle.
Sprayed-on cellulose and fiberglass work about as well as loose fill, but sprayed-on mineral wool performs better than loose fill, at R-4.1 per inch. Any spray-on insulation will seal air leaks better than loose fill.
On open walls when you’re on a tight budget but you want a contractor to handle the installation.
To avoid mildew, the insulation needs to dry for at least two days before it is covered with drywall.
About 50 percent more than loose fill.
What they are
Boards made from extruded polystyrene (XPS) or polyisocyanurate foam (“iso board”). These panels can go on the exterior of a house, over the studs (typically a weak spot in an insulation system) but under the siding, a technique that works well in hot, humid climates, as the boards become an effective vapor barrier.
In colder climates, the boards can go inside the walls, where you want the moisture barrier to keep warm air from escaping. Before switching to spray foam insulation, Tom used to use polyisocyanurate panels that way—he would build a 2×4 wall, insulate between the studs with batting (paper face removed), then cover the whole thing with foil-faced panels and seal them with foil tape before putting up wall board. The combined R-value of the batts and board on the 2×4 wall was more than batts alone on a 2×6 wall, and he would gain a little extra space in the room.
Extruded polystyrene provides about R-5 per inch. Foil-faced polyisocyanurate delivers from R-7.2 to R-8 per inch.
During siding replacement or an interior gut job to add insulation on a 2×4 wall.
To meet fire rules, you must cover interior boards with drywall at least ½ inch thick. Polystyrene will break down if exposed to sunlight, so it can’t be left uncovered too long.
One 4-by-8-foot sheet of 1-inch-thick expanded polystyrene foam costs about $10. A 2-inch-thick panel of foil-faced polyisocyanurate costs a little under $30.
Tom Silva’s Advice on Handling Half-Insulated Walls
“If your house has some insulation in the finished walls but not enough to keep out the cold, you’ll need to remove it before you can add more because the old stuff can get in the way and lessen the effectiveness of new insulation.
Figure out where you need to beef up by hiring an energy auditor with an infrared camera, or by peeking in around electrical outlets or behind trim pieces that you carefully remove. If you find some insulation, cut out a 12- to 16-inch horizontal band of drywall or plaster midway up the wall. Then just pull out the old insulation.
If you choose loose fill or expanding foam, it can go in through the same hole—though you’ll need to make more holes along the top of the wall to complete the job.”
Where To Find It
Valley Forge, PA
Bonded Logic Inc.
Good Shepherd Wool Insulation
Rocky Mountain House
Applegate Insulation Manufacturing Inc.
Johns Manville Corp.
Delfino Insulation Co.
Mississauga, ON, Canada
Lake Forest, IL
Federal Conservation Group
SDI Insulation Inc.
South San Francisco, CA
Insulation and Drywall Contractors
Southland Insulators Inc.
Building Envelopes Program, Oak Ridge National Laboratory
Oak Ridge, TN
Do you need protection for your home? Read our home warranty resources.