Fall and its mild temperatures present a great opportunity to accomplish home improvement projects around the home, such as finding and sealing attic air leaks.

Although it can get dusty and dirty, sealing air leaks (aka “drafts) in your attic can go a long way for keeping expensive heated air in and cold winter air out. It’s also a good time to check your attic insulation levels (R-Values) and see if they need an improvement.

Should You Hire a Professional Contractor?

While sealing attic air leaks is a DIY project you can accomplish on your own, there are some instances when calling a professional is necessary:

  • Difficult or dangerous to access certain areas
  • Wet and moldy conditions, which indicates a leaky roof or faulty exhaust vents
  • Inadequate or nonexistent attic ventilation
  • Old and dangerous wiring conditions, such as knob and tube wiring (pre-1930), which can be highly dangerous

WARNING: Some homes have vermiculite insulation, which contains asbestos—a serious health hazard. If your insulation looks like a flaky, pea-sized grey material, do not go forward with your attic air sealing or insulation project without calling your local health department first.

How to Find and Seal Attic Air Leaks

Most homes have insufficient attic insulation and multiple air leaks that draft expensive heated air into the attic and out of your home. Learn how to find and seal common air leaks in your attic this fall. Read the entire guide first before attempting to find and seal your attic air leaks. This project will take you a day or two to finish.

1. Get your supplies together and have a plan

  • Eye protection, gloves, and dust mask/OSHA-approved particulate respirator
  • Knee pads (not essential, but highly recommended)
  • Drop light (rough service lights) and flashlight
  • Boards to place on joists (never walk on ceiling drywall)
  • Hard hat to protect head from bumps and sharp objects
  • Batt or roll of fiberglass insulation and large garbage bags (13-14 gallon size)
  • Roll of 14-inch-wide aluminum flashing (to surround hot flue pipes)
  • Utility knife and sheet metal scissors
  • Tape measure
  • Staple gun (or hammer and nails)
  • Silicone or acrylic latex caulk and caulk gun for small cracks (less than 1/4 inch)
  • Expandable foam spray insulation for larger gaps (1/4-3 inches)
  • Special heat-resistant caulk for sealing around hot flues
  • Roll of reflective foil insulation to cover soffits, open walls and larger holes

Never walk on exposed ceiling or insulation as you could potentially fall right through! Instead, walk on the floor joists or truss chords. Use thick wooden boards to lay across the joists for more walkable space.

2. Know where common attic air leaks are located

Most air leaks can be found around light fixtures, plumbing pipes, ventilation, rafters, chimneys, and other areas. Use this map from Energy Star to learn where typical air leaks, holes, and gaps are located in the home:

Common attic air leaks include:

  • Behind kneewalls
  • Attic hatch
  • Wiring holes
  • Plumbing vents
  • Open soffit (box for recessed lights)
  • Recessed light
  • Furnace flue or duct chaseways
  • Windows and doors

Sometimes finding attic air leaks can be difficult since they are often hidden under insulation. If you have your home’s floor plan, it’s a good idea to use it to help you locate probable air leak locations. Make note of areas where there are dropped soffits over kitchens and bathrooms, slanted ceilings, where walls meet the ceiling, recessed lights, furnace flues, and any other dropped ceiling areas.

3. Start by looking around for common locations of attic air leaks

The first thing you want to do is look for the big holes and air leaks around the common attic air leak areas mentioned above.

Start with the biggest air leaks. The biggest improvements will come from sealing the biggest holes. These are normally found:

Where walls hit the attic floor
Dropped ceiling areas
Behind and under attic kneewalls—those short walls located under slanted ceilings

Look for signs of air leaks. Some of the most obvious signs of air leaks include:

Dirty insulation (evidence of air movement)
Drafts of air (if you can actually feel the air movement with your hand, that’s an obvious sign)

4. Seal the biggest holes first (dropped-ceiling areas/soffits)

Uncover stud cavities. You may have to remove or pull back insulation material in order to find your stud cavities near dropped-ceiling areas.

Fill a garbage bag with a 16-inch piece of unfaced fiberglass insulation. Once you have pushed back the insulation, you can plug these open stud cavities with your 13-14-gallon plastic garbage bags filled with fiberglass insulation. Fold it at the bottom of your garbage bag (the bag helps create a vapor barrier). Fold the bag, with the insulation inside, to fit into the stud opening. There should be enough insulation in the bag to form a tight fit.

Cover your open soffits with reflective foil insulation. With your sheet metal scissors, cut the foil insulation a few inches longer than the opening of the soffit. Use a bead of latex caulk around the opening and fit the foil into place. Finish by using your staple gun to staple it into place.

5. Seal air leaks behind kneewalls (usually for finished attics)

If you have a finished attic, you probably have kneewalls, those short walls underneath sloped ceilings. Kneewalls can help create a finished attic space, but they often create insulation and air leak vulnerabilities. Many builders fail to properly seal these areas, or if they do, the fiberglass batts fall out of place.

Check if you have open cavities in the floor underneath your sidewalls or kneewalls. Again, be n the lookout for dirty insulation, which is a sign of an air leak.

Seal under and behind kneewalls to prevent air from leaking underneath the floor of the finished space. Cut a piece of your fiberglass insulation and fold it in the bottom of a 13-14-gallon garbage bag. Then, stuff the bag into the open cavity, folding it over again and under the wall. You may also choose to use a piece of rigid foam board sealed around the edges with spray foam.

6. Seal air leaks around furnace flues

This is a fairly complicated DIY task. Since furnace flues and chimneys can get very hot, you have to be extra careful around these items. It’s probably best to call a professional for sealing and insulating round furnace flues, chimneys, and vents.

Since they can get extremely hot, nothing should get too close to chimneys and furnace/water heater flues. That’s why it’s important to separate the insulation from the pipe with metal flashing. Building codes normally require a minimum 1-inch clearance around metal flues and a 2-inch clearance around masonry chimneys to prevent fires. Check your local building codes for the proper clearance measurements.

Cut aluminum flashing to fit around the flue. If you have a round flue, cut two pieces of aluminum flashing to fit between the joists. Then cut semi circles into each of the two pieces so there is around 3 inches of overlap when you put them together. Once the two pieces of flashing are cut properly, press them into place around the round flue and use a couple beads of latex caulk and staples to connect them to each other.

Seal the gap around the aluminum flashing and the flue with high-temperature silicon caulk. Do NOT use regular caulk or spray foam near your masonry chimney, furnace or water heater flue.

Create a metal dam to prevent insulation and other materials from contacting the flue pipe. Use the image below to create a metal barrier around the flue pipe. Cut a large enough piece of aluminum to fit around the flue pipe with at least 1-inch clearance around the entire circumference. This is usually enough to wrap around the flue plus 6 inches. Next, cut slots at the top slightly larger than 1-inch deep and a few inches apart. Cut slots at the bottom 2 inches deep and couple inches apart. Then, bend the tabs in at the top and bend them out at the bottom. Wrap the aluminum barrier around the flue pipe and secure the bottom tabs to the bottom floor flashing with your staple gun.

7. Seal smaller gaps and cracks

Now that you have found and sealed the largest holes and gaps in your attic, you can look for the smaller ones. Have you noticed any dark areas of insulation? This is a sure sign that air is escaping nearby. In cold weather, you may notice frosty areas in the insulation. Another clear sign is if you can feel the leaks with your hand.

While you don’t have to replace the insulation (dirty insulation will work just fine), it’s a good idea to seal the air leaks that are causing it. Don’t forget your gloves!

Upgrade the weatherstripping around your attic door/hatch and any windows you may have. You can test to see if you have attic door leaks with an incense stick or thin piece of toilet paper first. If the smoke or paper begins to move erratically you have an air leak. You can use this air leak detection test in other areas of your home as well. Make sure the weatherstrip compresses when you close the attic hatch door or windows. Use a hook-and-eye- latch to keep the door in place.

Use weatherproof caulk around any gaps or cracks in the attic that are smaller than ¼ inch. Make sure you use heat-resistant caulk around the flue and chimney areas.

Use expandable foam spray for any gaps in your attic larger than ¼ inch. These large gaps are normally around where utilities enter the attic, such as wiring holes and plumbing vents.

Use expandable foam spray or caulk to seal the openings around your plumbing vent pipes, electrical wires, and where walls meet the floor. Always read the instructions on the can before using.

Once the foam or caulk has dried, you can add the insulation back into place. A small rake can be useful for blown insulation.

8. Seal air duct leaks

If you have air ducts located in the attic, it’s a really good idea to seal these cracks up too. Keep in mind that there will always be sections of ductwork that you cannot access. For comprehensive duct sealing, cleaning, and insulation, contact a professional HVAC company.

According to the EPA, the average home loses around 30% of their conditioned air through leaks in the ductwork. Even if you don’t detect any air leaks around the duct connections, it’s a good idea to insulate them anyway.

While it’s best to have a professional completely seal and insulate your air ducts, you can also DIY seal your air ducts with mastic sealant or aluminum foil tape. Before you start sealing your duct connections, make sure the surface is clean and dry first. And ironically, don’t use duct tape!

Learn more about sealing attic air leaks:

Learn more attic insulation tips from ENERGY STAR.

After Finding and Sealing Attic Air Leaks

After you are finished with your attic sealing and/or insulating project, it’s important to have a professional HVAC technician inspect your home for proper ventilation and combustion backdrafting.

A tighter home, while more energy-efficient, can also lead to indoor air quality and health problems. There needs to be some ventilation to get the stale air out. One major concern is carbon monoxide buildup.

Check to make sure you have smoke alarms and CO detectors on every level of the home, and that they are replaced every 10 years. Don’t forget to test the batteries every 30 days. Learn more about smoke and CO detector safety.


Contact OnTime Service at (205) 872-1944 to schedule your next HVAC and indoor air quality service.

We will troubleshoot your home and give you straightforward pricing options before any work begins.

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