How to Insulate an Attic: Step by Step (Blown-In or Batts)

To insulate an attic, seal the air leaks in the attic floor first, put baffles at the eaves so the soffit vents stay open, then add blown-in insulation or unfaced batts across the attic floor until you reach your target R-value, which is R-38 to R-60 for most US homes. Finish by insulating and weatherstripping the attic hatch. On an open attic floor that’s easy to reach, most homeowners can do the whole job in a weekend.

Blown-in insulation is the easiest way to top up an existing attic, because it fills the gaps around joists, wires and pipes and goes on deep and fast with a rented blowing machine. This guide walks through each step, compares the main types of attic insulation and shows how deep each one needs to be. To count the bags and depth for your own attic, use our blown-in insulation calculator.

Before you start: check your attic

Look around the attic with a flashlight before you buy anything. Fix these problems first, or call a pro:

• Vermiculite insulation. It looks like light brown or silver-gray pebbles that pour like gravel. Much of it came from a mine that also held asbestos, so the EPA says to assume it contains asbestos and leave it undisturbed. Hire an asbestos professional before any work that would disturb it.
• Knob-and-tube wiring. Old wiring run on ceramic knobs shouldn’t be buried in insulation. Have an electrician check or replace it first.
• Moisture or mold. Wet insulation, dark stains on the roof sheathing or soft, rotten wood point to a roof leak or poor ventilation. Fix the cause before you insulate.
• Fans that vent into the attic. Bathroom, kitchen and dryer vents must run all the way outside, not stop in the attic.
• Recessed lights. Lights that aren’t rated IC (insulation contact) need clearance around them or an approved cover.
• Little or no ventilation. Without soffit vents and ridge or gable vents, moisture builds up in the attic.

ENERGY STAR’s do-it-yourself guide to sealing and insulating lists the same warning signs and recommends a contractor when you find them.

Tools and materials

• Insulation: bags of blown fiberglass or cellulose, or unfaced batts
• A blowing machine, rented or borrowed from a home center, often free when you buy enough bags
• Baffles (rafter vents) for each rafter space that has a soffit vent
• Canned spray foam, caulk and rigid foam board for sealing
• Sheet metal and fire-rated, high-temperature caulk for gaps around chimneys and flues
• Depth rulers, or strips of cardboard marked in inches
• A few boards or plywood strips to kneel and walk on
• A work light, a utility knife and a straightedge
• Safety gear: an N95 or better respirator, goggles, gloves, long sleeves and a hat

How to insulate an attic step by step

1. Measure the attic floor and pick a target. Multiply the length by the width, measured inside the outside walls. Our R-value chart by climate zone shows the R-value your area needs.
2. Make a safe path. Lay boards across the joists to kneel and walk on. Never step between the joists, because the ceiling drywall below won’t hold you. In summer, work early in the day, when the attic is coolest.
3. Seal the air leaks. Insulation slows heat but doesn’t stop air, so seal first. The table below shows what to seal and how.
4. Install baffles at the eaves. Staple a baffle between the rafters at each soffit vent, so outside air can still flow into the attic once the insulation is in.
5. Build dams. Put a box or dam around the attic hatch so insulation can’t spill out, and use sheet metal to keep insulation back from chimneys, flues and non-IC lights by the clearance the code and the maker require.
6. Mark the depth. Staple depth rulers to the joists or trusses, about one for every 300 sq ft, with the numbers facing the hatch, so you can check the depth as you go.
7. Add the insulation. Start at the eaves farthest from the hatch and work back toward it. See the tips for blown-in and batts below.
8. Insulate the hatch. Glue rigid foam to the back of the hatch, or cover pull-down stairs with an insulated box, and weatherstrip the edges.

How to air seal an attic

Tips for blowing in insulation

• Work in pairs: one person feeds the machine and the other holds the hose.
• Hold the hose level and let the insulation fall into place. Don’t push it into the baffles or pack it down.
• Fill evenly to the depth on your rulers, and cover the tops of the joists too.
• Check the coverage chart on the bag. It gives the minimum depth and the number of bags for each R-value, and it comes first.

Using batts

If the spaces between the joists are empty, lay the first layer between them with the paper facing down toward the living space. For extra layers, use unfaced batts laid across the joists at a right angle, which covers the wood and the seams below. Butt the edges tight, cut batts to fit around wires and pipes instead of stuffing them, and never squash them, because compressed insulation loses R-value. To count batts or rolls, use the insulation calculator.

Types of attic insulation

For most attic floors, blown-in is the best insulation, because it goes on deep and fills around every obstacle. Cellulose gives more R-value per inch and blocks air a little better, while fiberglass is lighter and settles less over time. Both work well. The values above are typical, so check the label on the product you buy.

Blown-in vs batts

Blown-in is faster and leaves fewer gaps, especially when you’re adding to old insulation or working around wires, pipes and odd joist spacing. Batts suit empty joist spaces, small attics and anyone who doesn’t want to run a blowing machine, but every gap and squashed spot lowers their real R-value, so they take more care to fit well.

Batts vs rolls

Batts and rolls are the same material. Batts come precut, often in 4 or 8 ft lengths, which are easier to handle in a cramped attic. Rolls come in long runs that you cut to length, which means fewer seams on long, open spans.

How deep should attic insulation be?

Blown-in depths are settled depths at about R-2.6 per inch for fiberglass and R-3.6 for cellulose, rounded to the nearest ½ inch. Products vary, so follow the coverage chart on your bag if it differs. When you add to old insulation, count the R-value of what’s already there and only add the difference.

How to insulate an attic door or hatch

An uninsulated hatch is often the biggest single leak in the attic floor. For a lay-in hatch, glue two or three layers of rigid foam board to the top of the panel, add weatherstripping where it sits on the frame, and build a dam around the opening so the attic insulation stays put when you open it. For pull-down stairs, weatherstrip the frame and cover the opening from above with an insulated box or a zip-up attic stair cover.

Should you remove old insulation first?

Usually not. If the old insulation is dry, clean and not crushed, leave it and add new insulation on top. Blown-in insulation can go right over old batts or loose fill. Remove it only when it’s wet, moldy, soiled by rodents or other pests, or damaged by smoke or fire, or when you need open access to seal a badly leaking attic floor. Removal is usually a pro job done with a large vacuum, and vermiculite should never be removed by a homeowner.

When you add to old insulation, use loose fill or unfaced batts. Faced batts or plastic sheeting laid on top can trap moisture between the layers.

Insulating the roof rafters or using spray foam

Most attics are insulated on the floor, which keeps the attic itself cold and ventilated. Insulating the underside of the roof instead makes sense when the attic is a finished room, or when it holds a furnace and ductwork you want inside the heated space. That usually means spray foam on the underside of the roof deck, which creates an unvented attic that must meet the code rules for that design, so it’s a job for an insulation contractor. If you put batts between the rafters of a vented roof, use baffles to keep an open air channel from the soffit to the ridge.

DIY or hire a pro?

Do it yourself when the attic floor is open, there’s enough headroom to move around, and air sealing is simple. Hire a pro for spray foam, insulation removal, vermiculite, knob-and-tube wiring, very low or cramped attics, attics with many recessed lights, and any moisture problem. To compare the two, enter your bag price and blower rental, or a contractor’s price per square foot, in the blown-in insulation calculator.

Worked example: blown fiberglass vs cellulose

Say your attic floor is 40 × 25 ft with 3 inches of old loose fiberglass, and you want to reach R-49. These are the default numbers in the blown-in insulation calculator:

• Attic area: 40 × 25 = 1,000 sq ft
• Existing insulation: 3 in × R-2.5 per inch = about R-7.5
• R-value to add: 49 − 7.5 = R-41.5

Cellulose needs less depth for the same R-value, but it weighs about twice as much, so it takes more bags. Bag sizes and coverage vary by brand, so enter your bag weight, or the coverage from the bag’s chart, in the calculator.

Heavy attic insulation adds weight to the ceiling below; our guide to drywall thickness explains when ceilings need 5/8-inch or sag-resistant board.

Frequently asked questions

Insulation rules vary by climate zone and local code, and products differ, so follow the label, the manufacturer’s instructions and your building department. Get a pro for vermiculite, old wiring, moisture problems or spray foam.

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