“I love what you’ve done with the place, that open cell spray foam in the walls feels great,” said no one ever. Insulation isn’t the sexiest home upgrade, but it, along with air sealing, should be at the top of your list if you’re interested in using less energy for heating and cooling.
When we say top of the list, we mean it: The best heat pump, tankless boiler, or radiant heating system won’t overcome a house with a poorly insulated exterior—what contractors call the “building envelope”—or one that leaks a lot of air. “It’s not that one is more important than the other, but people put on a heavier coat in winter so they can be warmer,” says L. Carl Fiocchi, senior lecturer of the Building and Construction Technology Program at the University of Massachusetts.
Insulation and air sealing go hand-in-hand because insulation stymies heat transfer, but doesn’t stop airflow—and holding a consistent indoor temperature requires both. A lot of homes in the U.S. would benefit from added insulation and understanding the source of air leaks. “In a typical house that hasn’t been improved, 40% of the energy bill over the course of the winter is from air leakage, not conductivity, it’s got to do with air changes per hour,” says Fiocchi. Having a contractor run a home energy assessment or audit to test your home’s building envelope and HVAC equipment is a good idea. Part of their report will have recommendations on what kinds of insulation to add and where the air is leaking. Some of the upgrades can be DIY, but before you hit the home center, here’s what you should know.
Energy savings from insulation
The benefits of upgrading your insulation are clear. The Department of Energy (DOE) says that, on average, you can save up to 20% on heating and cooling costs by beefing up the insulation in attics, floors, crawl spaces, and basement rim joists, and by addressing air leaks. And odds are your home could benefit from some insulation considering the DOE says that 9 out of 10 in the U.S. are under-insulated.
What does insulation do?
Insulation’s job, be it in exterior walls, attics, and sometimes basement floors, is to act as a barrier that makes it harder for thermal energy—that’s “heat”—to enter or leave the house, depending on the season.
Insulation comes in a few forms and from a range of materials, but they all essentially do the same thing: trap air, which is a poor conductor of energy and therefore good at holding heat at bay. Fluffy fiberglass, for one, has thousands of pockets that trap air as it sits in the exterior wall cavity, as does polystyrene rigid foam board.
You can add insulation to the interior side of exterior walls, meaning the product rests within the cavities between 2×4 studs. Outside, you can add insulation between the sheathing and the siding. But, since replacing drywall is usually easier than re-siding a house, most homeowners insulate from the inside.
What does R-value mean in insulation?
When you’re considering insulation, the key metric you want to pay attention to is its R-value, which is an indicator of how hard it is for heat to transfer through it. The higher the R-value, which is measured per inch of thickness, the better the insulator. R-values are cumulative, so if you insulate the interior of a bedroom and then also add foam underneath the siding of the same wall, you combine those two R-values.
Local building codes set the R-value requirements of the building envelope. For example, in Key West, Florida, building codes might want an R-value starting at R30 in an attic but in northern Minnesota, you’d want to double that to R60. Generally the less expensive the insulation the lower its R-value. Fiberglass rolls can offer an R-value of 2.9-3.8 per inch of thickness, meaning to achieve an R-60 in Duluth, Minn., you’d need about a 20-inch thick blanket of fiberglass in the attic. R-values on spray-foam insulations are some of the highest, ranging from 5.6-8, which is why they cost more.
What are the different types of insulation?
While you can still find massive rolls of pink insulation at the home center, a range of other options have joined the bygone go-to. These are made from everything from natural materials, like ground-up paper pumped in by air, to stiff batts made from molten metal spun into cotton-candy-like structures. There’s also polyurethane spray foam, a buzzy type of insulation you might have interacted with in the form of a can of Great Stuff.
All insulation materials work well inside a house, but how they react to moisture, and the form they come in, can make some harder to use outside. Fluffy insulations like fiberglass batts pack down if they get wet (as in from a leak), collapsing the air pockets and compromising performance, so they’re not ideal to use outside. Rigid foam board panels, on the other hand, are impervious to water, and you can nail right through them without squishing out all the air.
Polyurethane spray foam
This is unique in that it provides an air seal and insulation in one product. As you spray the liquid foam it grows, filling nooks and crannies that rigid foam or fluff batts miss. Spray foam can come in either open or closed cell: Open cell cures to a spongy texture, whereas closed cell dries stiff. Open cell foam is about half as insulating as closed cell foam, which can be twice as expensive, so it’s typically reserved for projects where you need a lot of insulation in a minimum amount of space, like the underside of a roof where the rafters are not that deep. While some homeowner-friendly spray foam kits will handle a small project, like the exterior wall of a bathroom, this stuff usually requires hiring a professional.
Fiberglass
This insulation is the least expensive and most familiar to homeowners, because it can be used everywhere from basement walls to attic floors. Manufacturers spin molten glass, some of which is recycled, into fine fibers. Fiberglass insulation is sold as fluffy blankets and rolls you trim with a utility knife, or as semi-rigid boards you slice with a serrated blade. Fiberglass is easy to handle but is difficult to install so that it fills all the voids without compacting.
Loose-fill cellulose
This stuff is typically made from up to 85% recycled content like newspaper, which is then treated with mineral borate to deter insects and ensure fire resistance. You can install cellulose in a few ways. In the attic, you can pump the crumbly insulation in with a blower, which home centers will often rent to you, until the fluffy layer covers the ceiling of the floor below. Outside, contractors can drill holes into the sheathing and pump the insulation in without damaging the drywall inside. Some contractors might pack the cellulose on the inside of a wall cavity, where the drywall is removed, using a net to keep it in place. Cellulose is easy to work with but it can settle over time in a vertical wall, which reduces efficiency. Plus, if a pipe leaks the insulation will soak up the moisture.
Mineral wool
No, it’s not from sheep. This is essentially molten basalt or volcanic rock, or iron ore, depending on the manufacturer. It’s about 75% recycled material, which can be about double what fiberglass insulation offers. You can find it in semi-rigid batts, to fill joist bays, sheets to cover the walls underneath siding, and in loose fill formats. Mineral wool is easy to work with, doesn’t mind getting wet, isn’t attractive to bugs, won’t create a toxic gas in a fire, and provides good soundproofing, but is pricier than fiberglass.
What about air sealing? What’s that do?
Anyone who’s sat next to a drafty window knows what air leaks feel like. The bad news is in most homes the air leaks can be everywhere—from the hole for the dryer vent in the basement to the gaps around the second-floor bathroom vent. But, unless you walk around the house with a smoke stick pen, which emits a white stream that is pulled in the direction of airflow, it’s hard to see if air is moving through. An energy auditing contractor relies on a what’s called a door blower test—think of it as an MRI diagnostic that sees what the naked eye can’t—to figure out how much air is coming and going through the house.
A well-sealed, tighter home consumes less energy, meaning it’s less expensive to heat and cool the building. Think of it this way: When you pay for your air conditioner to yank humidity out of the air and return cooler, dryer air into your bedroom, it won’t work nearly as hard in a tight home as it would in a leaky one where it is constantly kicking on to replace conditioned air that is leaving the building. Here’s what you need to know about air-sealing your home.