Photo: Ömer Sav

Should the underside of a roof be insulated?

The airflow from the soffits to the ridge vent keeps the roof cool and prevents ice dams, and the material will block that flow. For the same reason, insulation shouldn't touch the roof's underside.

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Insulating Your Attic Can Save You Money

There's no getting around it: If your house is in a cold climate, keeping it warm in winter is expensive. The Department of Energy predicts that prices for natural gas and heating oil will rise again this winter compared with last—and that's after a year in which heating-oil prices rose sharply in most parts of the country. Sure, you can dial down the thermostat and get used to wearing bulky sweaters indoors to cut costs. But if you've got an unfinished attic, giving it proper insulation is one of the simplest ways to keep a lid on your heating bill this season. "It's a DIY project you can tackle in a weekend, and the savings you'll get add up every year," says TOH general contractor Tom Silva. The Department of Energy estimates that a properly insulated attic can shave 10 to 50 percent off your heating bill. And it works the opposite way for warm climates; in summer, it helps stabilize your house's indoor temps to keep cooling needs in check. In a perfect world, you'd hire an energy auditor to tell you exactly how much protection you're getting from the few inches of attic insulation you may already have and to pinpoint things like air leaks that you can seal to make sure your insulation will do its job well. But if you can't afford to shell out a few hundred bucks for this service, never fear: We've rounded up all the most important know-how about attic insulation costs, products, prep work, and installation right here. Keep reading for the details.

Attic Insulation Costs

Type and material of your insulation

Square footage of your attic

Contractor or insulation installer’s fee

Note: If you have to insulate around electrical boxes or cables, then support from an electrician might be needed as part of the process too.

Start with The Attic Floor

Stop using your attic for storage. Why? Because the simplest and cheapest way to insulate an attic is to add material to the floor. But if the floor is covered in plywood, you can't stuff enough insulation beneath it to do the job sufficiently—not even in warm climates. Plan to pull up the flooring and layer new insulation on top of the old. With the floor gone, you'll have to find a spot elsewhere for stashing those off-season clothes and that holiday decor.

Choose Your Insulation Type and Material

For DIY attic insulation, you've got two choices: loose fill or batt (the common term for blanket insulation). Both can be added to uninsulated attics or layered over existing material. Once you've decided which type is best for you, examine the material options and prices to home in on the right product. Always check labels for specifics on whatever you buy.

1. Loose fill

Insulation fibers are packaged in bags and blown in place to the desired depth and density using special machinery you can rent from a home center. You can pour the fill in place and spread it manually, but the process is much more labor-intensive and the results won't be nearly as good.

It works best for:

Attics with irregular or nonstandard joist spacing

Attics with lots of obstructions and penetrations to work around

Attics where there is existing insulation to be topped, since it fills gaps and joints well

Low-clearance attics with limited headroom for maneuvering during installation

DIYers who want to get the job done quickly and are comfortable working with power equipment

Insulation material options:

Fiberglass

R-value per inch: 2.2–2.7

Made of: Recycled glass or sand that's melted and spun into fibers

Bottom line: Lighter in weight than cellulose or mineral wool, but it settles more than those materials, so you've got to put in a thicker layer to get the protection you need.

Cellulose

R-value per inch: 3.2–3.8

Made of: Fibers from recycled post-consumer paper that are treated for insect and fire resistance Bottom line: The most common blown-in material used, but it can rot and grow moldy if exposed to moisture.

Mineral wool

R-value per inch: 3.0–3.3

Made of: Fibers from rock or recycled slag from blast furnaces

Bottom line: Offers natural fire resistance, but costs more than other loose-fill materials.

2. Batts

This flexible insulation material is most often packaged in rolls that come in various thicknesses and standard widths, usually 16 inches and 24 inches, to fit between joists or studs in a house's framing. They come with or without a paper or foil facing that acts as a vapor barrier. You add one or more layers to achieve the desired level of insulation.

They work best for:

Attics with standard joist spacing, especially those with no insulation

Attics with few obstructions or penetrations to work around

Attics with sufficient headroom for maneuvering during installation

DIYers who don't mind cutting the material to fit around obstructions

Insulation material options:

Fiberglass

R-value per inch: 2.9–4.3

Made of: Recycled glass or sand that's melted and spun into fibers

Bottom line: Commonly used and inexpensive, but fibers can irritate lungs and skin (though less so than in years past, thanks to better manufacturing), and it's less effective at blocking airflow than other materials.

Cellulose

R-value per inch: 3.7–3.8

Made of: Fibers from recycled post-consumer paper that are treated for insect and fire resistance. Bottom line: Won't irritate lungs or skin, but the product is made by a limited number of manufacturers.

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Mineral wool

R-value per inch: 3.0–3.3

Made of: Fibers from rock or recycled slag from blast furnaces

Bottom line: It's naturally fire-resistant, but the product is more expensive than others.

Cotton

R-value per inch: 3.7–3.8

Made of: Fibers from recycled denim cloth

Bottom line: Blocks airflow and sound transmission, but the material is more expensive than others.

Size Up Your Existing Insulation (and Get Rid of The Bad Stuff)

Grab a tape measure and a flashlight to see what kind of insulation you already have in the attic and how deep it is. Then use the numbers in the chart at right to estimate its R-value. Pull up and dispose of any material that's compressed, water stained, or moldy—it's useless. Owners of houses built before 1990 take note: If what you see is lightweight, grainy, loose-looking insulation with shiny flecks, it could be vermiculite from a mine with asbestos deposits. Get it tested, and if necessary, call in a pro to remove and haul it away safely.

Determine Your Target R-value

The Department of Energy recommends these minimum R-values for unfinished, unconditioned attics. The numbers are based on your house's location and climate zone.

Check for Rebates and Credits in Your Region

Most federal tax credits for weatherizing your house and boosting its energy efficiency expired in 2011. But your state's energy office or local utility may offer product rebates, discounts, or other financial incentives for insulating. See a state-by-state directory of such programs here.

How Much Insulation to Buy

Measure your attic's square footage. For loose fill, read labels; each bag lists the required depths for a range of R-values and the number of bags needed to cover 1,000 square feet at those depths. For batts or rolls, calculate the number based on the width and length of the product you're using. In either case, get an extra bag or roll—you really don't want to run out when you're almost done putting it in.

The Number One Prep Step: Seal Air Leaks

Gaps in the attic or between the lower floors and the attic will let heated (or cooled) air escape to the great, unconditioned outdoors, making any insulation you add essentially useless. (Find a diagram of common air leaks here.) Here's a quick primer on fixing draft-prone spots. Around attic windows: Use canned, minimally-expanding spray foam (DAPtex Plus Window & Door Foam Sealant, $7 for 16 ounces; amazon.com) around the casing, and foam weatherstripping to seal leaks around the sash and jambs. Around pipes, wires, exhaust fans, and ducts: Gaps of ¼ inch or less can be sealed with fire-blocking caulk (3M Fire Block Sealant, $6.57 for 10.1 ounces; homedepot.com); seal larger ones up to ½ inch with fire-blocking spray foam (Great Stuff Fireblock Insulating Foam Sealant, $16 for 16 ounces; amazon.com). Around chimneys and flues: Use metal flashing sealed with high-temperature caulk (SUPERBOND Silicone Sealant, $7.90 for 13 ounces; grainger.com) or furnace cement (Oatey High Heat Furnace Cement, about $21 per half gallon; amazon.com.

Other Key Prep Steps Before You Insulate

Knock these to-dos off your list to ensure that your insulation will last for years and keep conditioned air where it belongs: inside your house. Fix roof leaks. Water is insulation's enemy. It creates a breeding ground for mold and mildew and ruins the air-trapping pockets that block heat flow. Look for water stains on the roof sheathing or damp or moldy spots on attic joists and existing insulation as a clue to where leaks might be. Box out light fixtures. Unless you're using mineral-wool insulation or your fixtures are rated safe for contact with any type of insulation, don't allow the material to touch or cover recessed cans or lights from the floor below—it's a fire hazard. Use hardware cloth, metal flashing, or scrap plywood to create a safety gap of at least 3 inches all around fixtures. Direct all exhaust fans and vents to the exterior. Though it's against building code to vent any kind of exhaust to an attic space, many home builders have gotten away with this shortcut. Correct that mistake so that humid exhaust air doesn't get trapped in your insulation and ruin it.

Protect Yourself as You Install Your Insulation

Here's how to get the job done without discomfort or injury.

Wear a dust mask, goggles, work gloves, a long-sleeve shirt, and long pants to protect your lungs, eyes, and skin from fibers. goggles, work gloves, a long-sleeve shirt, and long pants to protect your lungs, eyes, and skin from fibers. Don't stand on the joists. If you lose your balance, you could crash through the ceiling. Use a piece of plywood or a couple of sturdy, wide boards as a standing surface, and move them around as you work. They must span at least three joists for stability. If you lose your balance, you could crash through the ceiling. Use a piece of plywood or a couple of sturdy, wide boards as a standing surface, and move them around as you work. They must span at least three joists for stability. Illuminate dark corners of the attic using portable battery-operated lanterns or clip-on workshop lights.

Preserve The Attic's Airflow

Covering up the soffit vents with loose fill or batts, which can happen if you stuff insulation along the eaves, is a huge no-no. The airflow from the soffits to the ridge vent keeps the roof cool and prevents ice dams, and the material will block that flow. For the same reason, insulation shouldn't touch the roof's underside. Staple plastic or foam baffles to the roof sheathing, near the eaves, to keep the material away.

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If You're Insulating from Scratch, Put The Right Vapor Barrier in The Right Place Though some batts come with a paper or foil facing that can act as a vapor barrier, Tom Silva prefers to use 6-mil polyethylene sheeting, cut to fit between the joists and with seams sealed by foil tape, to prevent moisture from seeping into insulation. Whether using loose fill or batts, put the vapor barrier closest to the warm side of your installation, where hot, moist air would get in—facing the house's interior in cold climates (beneath floor insulation) and the attic's interior in hot climates (on top of floor insulation). Some regions don't require a vapor barrier.

More Insulation Installation Tricks and Tips

Work from the perimeter of the attic toward the door or hatch so that you don't trample all over the insulation you just put in. Always cover the tops of the ceiling joists to make sure the insulation is deep enough to reach your target R-value and to prevent thermal bridging, the heat loss that occurs through the wood framing. Shower thoroughly after you're done for the day to remove fibers from your skin, and launder your work clothes after one wearing.

For loose fill

Fasten blocking around the hatch or door to allow the material to be installed around this area without escaping. Make sure the fill's depth is uniform across the attic. To make it easy to eyeball how level the material is as you blow it in, screw depth guides to joists throughout the space. To get the target R-value from the product you chose, use the number of bags your calculations showed that you needed to insulate your attic—never fewer. If you've reached your target depth but still have a few bags left over, keep adding the material at an even depth throughout the space until all the bags your calculations called for are empty. To help the material achieve the right density as you install it, hold the blower hose parallel to the floor and the floor joists. Blow in the fill between and over the joists rather than across them.

For batts

Always use unfaced batts, both when laying product for the first time and to prevent moisture from becoming trapped between new and old layers of insulation. You can buy them unfaced or simply remove the paper or foil backing. Place a new layer of unfaced batts perpendicular to the old layer, to cover any gaps in the lower layer. Adjoining batts should be butted snugly together, but not tight enough to compress them. Never lay heavier batts, such as cotton, over lighter ones, like fiberglass. You'll compress the lower layer and reduce its effectiveness. Always cut batts to fit around obstructions and penetrations. Stuffing or cramming them around ducts, piping, and the like will compress the air-trapping pockets in the material, reducing its insulating properties. Don't leave any gaps between batts and joists, obstructions, or abutting batts. Even narrow ones will let air escape. Instead, cut a thin strip of the insulation you're using to size.

Protect Access Spots

Attic hatch or door: Affix rigid foam insulation to the attic side of the hatch or door. Add weatherstripping around the perimeter and a sweep to the door of a walk-up attic. Affix rigid foam insulation to the attic side of the hatch or door. Add weatherstripping around the perimeter and a sweep to the door of a walk-up attic. Pull-down stair or ladder: Use a zippered, insulated tent to keep the enclosure draft-free (Attic Tent AT-7 Attic Cover/Insulator, $120; amazon.com). Tip: Use a Chef’s Knife to Cut Batts. Some pros prefer it to utility knives because the large blade easily slices through the thick material. Use a piece of plywood as a cutting surface, and stand on a scrap 24 as a straightedge to guide your cuts.

Thinking of Finishing The Attic Someday?

If you're considering turning your attic into conditioned storage or living space, it's worth changing tactics to insulate between the rafters and wall joists instead of the floor. You can use rigid foam panels or do as Tom Silva does with nearly all houses these days: Hire a pro to install spray foam insulation. Foam blocks airflow needs no vapor barrier and has a higher R-value per inch than loose fill or batts, so you'll get more protection with less depth. With spray foam, unlike with loose fill or batts, you must cover the soffit vents (they aren't needed to keep the roof cool). Then cover the foam with drywall as required for fire safety. Looking for more help with repairs around your home? A home warranty may help. Check out the This Old Houses Reviews Team’s in-depth reviews on:

thisoldhouse.com - Read This Before You Insulate Your Attic - This Old House