Q: What is the best way to upgrade insulation?

A: Adding insulation can keep a home more comfortable in both the winter and the summer. It also saves on energy bills and is one way to help combat climate change.

But before you worry about insulation, you need to plug air gaps, which can cause drafts and uneven temperatures from room to room. Some building-science experts say that stopping air gaps is even more important than boosting insulation, because drafts make a huge difference in whether you’re satisfied with recommendations to set heating at 68 degrees and air conditioning at 78. The federal Energy Star program has a great primer, titled “A Do-It-Yourself Guide to Sealing and Insulating with Energy Star.”

Air gaps and insulation matter on all exterior surfaces, but it’s a lot easier to plug gaps and beef up insulation in attics than in walls. Start by checking for gaps around the attic hatch, recessed light fixtures, duct registers, plumbing vent stacks and dropped soffits. (Dropped soffits are framed-in areas where the ceiling in the room below is lower than usual, such as above cabinets or over a sink where recessed light fixtures were added.)

Move insulation aside in these areas to see whether there are visible gaps. Insulation that’s dark with mildew is a clue, because this shows where moist, warm air from inside the house leaked through. You can also use light to pinpoint gaps by checking in the attic at night to see where light shines through.

It can be dicey to crawl around in an attic where insulation covers framing, however, because of the risk of stepping in the wrong place and poking a hole in the ceiling below. So another option is a test that the Energy Star program recommends. This test is also a good way to find leaks elsewhere, including in the basement, where air that seeps in creates a chimney effect, making rooms above feel drafty.

Turn off all combustion appliances, such as gas stoves and water heaters, and shut all windows, exterior doors and fireplace flues. Turn on all exhaust fans that blow air outside. If you don’t have an exhaust fan in the bathroom and your stove fan isn’t vented to the outdoors, open a window and rig up a fan that blows indoor air out; seal gaps around the fan with cardboard. Then, light an incense stick and watch how the smoke wafts when you move the stick around ceiling lights, electrical outlets, door and window frames, baseboards, the attic hatch, and wherever pipes or wiring pass through walls or ceilings.

When you find leaks, plug them with caulk, expanding foam or insulation wrapped in a plastic bag, depending on the location and size of the gap. (Filling gaps with fiberglass or cellulose insulation won’t stop drafts unless the insulation is encased in plastic.) Around doors and windows, weatherstripping might be the best option. In some cases, you might need to add flashing. The Energy Star DIY guide gives instructions and includes pictures for a variety of situations.

You can also take a shortcut: Hire a company that specializes in air sealing. Ask your utility company whether it offers rebates for the service.

Once the leaks are plugged, consider adding insulation. The attic is the easiest place to do this and offers the highest payback in terms of comfort and savings on heating and air-conditioning bills. If you hired a company to do the air sealing, it can offer advice on whether you need to add insulation and, if so, how much. Otherwise, one quick test is to look through the attic hatch. If you see an unbroken expanse of insulation, you probably have enough. If you can see the tops of the attic floor joists (which are also the framing that the ceiling underneath is fastened to), you should add more.

Insulation is measured in terms of its “R-value,” or the ability to resist conductive heat flow. (This can be founded printed on the label.) The recommended level of insulation for most attics ranges from R30 to R50, depending on your climate; you can search for “recommended R-values” at energystar.gov. It’s the total thickness that matters, so factor in the depth of the existing insulation.

There is often quite a bit of insulation toward the center of the attic and very little on the edges, where the sloping roof meets the walls. Don’t pile insulation around the edges, however, because that would block the soffit vents. These are openings on the outside that allow fresh air to flow in at the base of the roof so that the attic doesn’t become musty and that air above the insulation stays cold in winter, preventing ice dams. Install rafter vents to create channels where air can move between the area above the insulation and the outside, even once you add more insulation. (A 10-pack of rafter vents 22 inches wide by 4 feet long costs $19.21 at Home Depot.)

You can add fiberglass or blown-in cellulose; there is no need to match what is already in the attic. Which is best? From an energy-saving perspective, it doesn’t matter, as long as you install the depth recommended on the package to bring the total insulation to the R-value needed where you live. However, for each inch of depth, cellulose achieves better insulation, and it makes good use of recycled paper. Fiberglass is more energy-intensive to make. And if you ever need to remodel, having to handle fiberglass can be an itchy ordeal.

You might also decide by how easy the insulation is to install: You can unroll fiberglass batts, but you need a blowing machine to install loose-fill fiberglass or cellulose. Stores that sell insulation typically rent blowing machines, and they may even lend them without charge if you buy enough insulation.

It can also help to insulate a basement or crawl space, but doing that correctly is more complicated, because it depends on the climate, whether the basement is finished and whether you have an enclosed or vented crawl space. Find climate-specific advice by typing “insulate basements” into the search box at energystar.gov. Many homes have ventilated crawl spaces, which are now frowned upon by building-science experts, because they tend to become damp and moldy. The best solution is to seal off the soil and treat the space as part of the indoor part of the house, with insulation only on the perimeter, not under the floor.

Insulating walls can also help save energy, but that upgrade usually makes sense only when the siding is being replaced. If the walls have no insulation, installers can drill holes in the sheathing and blow insulation into cavities. Or they can install foam panels on the sheathing before the new siding goes on. If walls already have some insulation, foam panels are the only option.

Have a problem in your home? Send questions to localliving@washpost.com. Put “How To” in the subject line, tell us where you live and try to include a photo.