DIY Attic Insulation and Air Sealing Guide

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Posted June 1, 2017

While the air leaks around windows and doors are significant and most noticeable, you are probably losing a lot more air and energy in your attic and basement. In many homes, the combined air leaks in these two areas add up to a half-window being open all day long. Learn more in this attic insulation guide.


This is how it works in most houses: cold air comes in through windows, doors, and basement rim joists on the lower floors. Your heating system turns on, supplying warm conditioned air to your living spaces. As the warm air rises, it finds its way into the attic through leaks around recessed lighting, attic hatches, flue, and duct leaks. Lacking proper insulation and ventilation, your attic starts seeping the conditioned air to the outdoors.

Think of all the money you are spending to heat and cool your home. If someone left a window or door open, you would be pretty upset, right? Well, that is effectively what is happening in your attic and basement. There are many little holes around doors, windows, soffits, and where utilities enter the home. Combine this with inadequate insulation, and are wasting hundreds of dollars’ worth of energy every year and creating an uncomfortable home for you and your family.

Attic insulation improvements aren’t only for winter. Learn how to find and seal air leaks in the home for greater savings and comfort year-round.

Find Air Leaks

The most common air leaks in a home are found in and around the following areas:

  • Windows and doors
  • Plumbing, electrical, and HVAC holes, vents, and lines
  • Recessed lighting
  • Electrical fixtures, outlets, and boxes
  • Attic hatch
  • Behind kneewalls
  • Fireplaces and flues
  • Air ducts
  • Basement Joists
  • Attic hatches and floors

Now that you know the most common air leak locations, you can test air movement around these areas using a candle, incense stick, or thin piece of toilet paper.

Hold up your flame, smoke, or paper to the areas in your home where you suspect and air leak and if you begin to see erratic movements, you have air movement nearby. Hold up your hand to this area and you’ll probably feel the cold air.

Place a piece of colored tape or another clear marker in the place of the air leak so you can come back to it and seal it when you are ready.

How to Seal Air Leaks 

You can seal most air leaks with a combination of caulk, weatherstripping, expandable foam spray, and mastic sealant.

  • Weatherstripping – Use around doors and windows. Replace old weatherstripping. Follow manufacturer instructions.
  • Caulking – Use weatherproof silicone or acrylic latex caulk around doors, windows, and to fill any gap or crack that is less than ¼ inch. Use heat-resistant caulk for the areas around your flue and chimney.
  • Expandable Foam Spray – Use around wiring holes, plumbing vents, behind kneewalls, and for any gaps and cavities that are larger than ¼ inch (no larger than 3 inches).
  • Mastic Sealant or Aluminum Foil Tape – Only use mastic sealant and aluminum foil tape to seal leaks in your ductwork. Wherever there is a connection or seam in exposed ductwork, test for air leaks and then seal with mastic sealant of HVAC tape. Ironically, “duct tape” is not a good option to seal air ducts as they tend to peel and create an ineffective seal.

When You Should Hire a Professional

Call a professional as soon as possible if you notice any of the following signs while searching your home for air leaks:

  • Wet insulation, a sign of a leaking roof or HVAC system
  • Mold and mildew, a sign of a moisture and health problem
  • Condensation near HVAC system, a sign of a potential ventilation and/or carbon monoxide problem
  • Ice dams on the eaves of your roof, a sign of an attic ventilation and insulation problem
  • Knob and tube wiring, a potential fire hazard.
  • Damaged wiring, a potential fire hazard

Sealing around light fixtures and other electrical devices can create a fire hazard. Call a professional to help you add insulation around recessed lighting and attics. 

There are some potential electrical and fire hazards that could put you at severe risk of injury if you are not completely confident in your abilities. Extra care must be taken whenever insulating around HVAC, plumbing, and electrical equipment.

During your DIY attic insulation project, if you notice any of the following, stop and call a professional:

  • Wet or moist insulation
  • Mold and mildew
  • Ice dams
  • Knob and tube wiring
  • Damaged wiring
  • Critters
  • Uninsulated recessed lights and electrical boxes 

Attic Insulation

Increasing attic insulation is a fairly complicated DIY project (1-3 days or a full weekend), but is completely doable if you have the right materials and information.

Before you think about heading into your attic to seal air leaks and add insulation, here are the things you will need first:

  • Insulation – you will need batt, roll, or blown insulation.
  • Garbage Bags – fill the bag with insulation to insulate stud cavities under kneewalls and dropped soffits.
  • Silicone or Acrylic Latex Caulk – use the right kind of caulk to seal any cracks or gaps smaller than ¼ inch.
  • Heat-Resistant Caulk – use this type of caulk when sealing air leaks around fireplaces, chimney, and flues.
  • Caulk Gun – you need a caulk gun and know how to use it in order to fill air leaks with caulk.
  • Expandable Foam Spray – get several cans of expandable foam spray to seal larger gaps (1/4 inch to 3 inches).
  • Aluminum Flashing – this keeps the insulation from getting too close to the flue pipe, which can create a fire hazard.
  • Utility Knife and Heavy-Duty Scissors – you will need to cut the tips of off caulk, excess insulation and air sealing materials.
  • Measuring Tape and Staple Gun – use the tape to make accurate measurements and the staple gun to hold materials in place.
  • Work Gloves, Safety and Dust Masks – always use these items for your health and safety.
  • Head Lamp or Flashlight – there are a lot of dark corners and crevices that you will need to see.
  • Thick Wooden Boards – you may need boards to lay across your attic joists in order to walk safely across your attic.
  • Knee Pads and Appropriate Clothing – wear knee pads to avoid knee and back pain. Make sure you aren’t wearing any clothes that you care about. You WILL get dirty.

All successful projects are planned beforehand. Make sure you have all of the tools and materials you need before you begin. Ensure there is adequate lighting and safe conditions.

Safety First!

  • Adding attic insulation can be a dangerous job if you’re not careful. Make sure you examine your work area and materials before beginning, and that you have a proper plan in place.
  • Don’t work in the attic when you are home alone.
  • When you are in the attic, always wear an OSHA-approved dust mask to avoid breathing in hazardous airborne particulates.
  • Be on the lookout for exposed nails coming up for your roof deck.
  • Walk on rim joists and boards that you have laid on top of the joists. NEVER walk insulation or ceiling drywall.
  • Make sure the work area is well-lit. Have a powerful flashlight on hand.

Seal Attic Air Leaks First

Before you start adding any attic insulation, you want to make sure that all (or most) air leaks have been sealed first. Don’t worry too much about the small leaks. You want to be paying attention to the big ones.

The first spot to check is where the floors meet the wall. Investigate the perimeter of your attic floor for any air leaks around walls (inner and outer) and dropped soffits and dropped ceilings.

Next, look for large cavities behind attic kneewalls.

Dirty insulation is also a clear indication of some kind of air movement. If you see patches of dirty insulation in your attic, you probably have an air leak very close by. If you pull back the insulation, you should be able to see the cavity that was covered with insulation rather than sealed first.

You can seal large stud cavities in your attic, around dropped soffits, under kneewalls, and around recessed lights by stuffing insulation into a garbage bag and then plugging the cavity with it. Add or remove insulation from the bag as necessary to ensure it fits.

Once you have plugged all of the large stud cavities with insulation, you can now cover the soffits with your regular batt, foam, or blown insulation. Remember, dirty insulation indicates an air leak. Make sure you are plugging all cavities to stop leaking air.

Cover Dropped Soffits

After removing the old insulation from the dropped soffit, you can cut some reflective foil or other insulation blocking material (you can also use rigid foam board) and cover the opening with them. Use a line of caulk or other appropriate adhesive around the opening and seal the foil to the frame. You can also use a staple or nail gun if necessary. When you are done, cover with insulation.

Seal Behind Kneewalls

Insulate and seal the area behind kneewalls by filling a large garbage bag with a 24-inch piece of fiberglass insulation and stuffing it into the open joist space underneath the wall. You can also use a rigid foam board, combined with some spray foam to insulate the open joist cavities. After filling the kneewall cavities, again cover with insulation.

WARNING: Some attics have insulation that contain asbestos and other health hazards. Always use an OSHA-approved particulate respirator or dust mask to prevent inhaling airborne contaminates. If your insulation looks like small, flaky grey material, don’t touch it until you have had it tested. Contact your health department for approved lab references.

Sealing Attic Air Leaks

Furnace Flues

The openings around you HVAC system and water heater flue and chimney can leak a lot of air and cause safety issues as well. Since chimney, furnace, and water heater flues can get very hot, it’s important that you maintain at least 1 inch of clearance around metal flues (2 inches around masonry chimneys), including insulation (Seal and Insulate with ENERGY STAR).

Furnace, water heater, and chimney flues (metal/masonry) should be sealed with aluminum flashing and heat-resistant caulk. Plumbing pipes made of cast iron or PVC should be insulated with expanding foam or caulk depending on the size of the gap.

WARNING: Flues get very hot. Use extreme caution when working around furnace, chimney, or water heater flues.

It’s a good idea to create a metal dam around your flue to prevent any insulation from getting too close to the pipe. Cut a piece of aluminum flashing to create a metal barrier. If your flue is round, cut a half-circle into two pieces of aluminum flashing (have them overlap by about 3 inches) and nail or staple them into place.

Once you have the base down, seal the gap around the two pieces of aluminum flashing and the metal flue with heat-resistance caulk (do NOT use expandable spray foam).

To prevent any insulation from getting too close to the flue, cut another piece of aluminum and cut slots around 1-2 inches at the top and bottom. Bend out the tabs at the top and bottom and wrap the aluminum around the flue. Secure it by nailing or stapling the bottom tabs to the wood or drywall. Now, you can put insulation around the metal dam without fear.

If you do seal air leaks around flues, make sure that you use heat-resistant caulk.

Foam or Caulk Small Gaps in Your Attic

While most of the air leaks in your attic are probably covered up by insulation, you can detect these air leaks by looking for patches of dirty or dark insulation. The dirt comes from dusty air leaking into your attic.

You might also notice some patches of ice or frost. This is a result of moist air leaking into your attic and then freezing when it hits your cold attic.

Once you seal the air leak with caulk or expandable foam, it’s not necessary to replace the insulation. Just put it back into place.

Seal small gaps with caulk (up to ¼ inch) and expandable foam spray (up to 3 inches). Wear gloves and clothes you don’t care about. Once released, the caulk or foam is very difficult to remove.

Caulk around plumbing and wiring holes. If gaps around plumbing pipes and vents is larger than 3 inches, you can use some fiberglass insulation to supplement the effort.

Fill small holes with caulk or expandable foam spray. If there are any gaps that are larger than 3 inches, you can stuff some fiberglass insulation into the gap and then fill up the remaining space with expandable foam.

Attic Hatch Insulation

Once you have finished sealing all of the leaks in your attic, the next step is to weatherstrip your attic hatch or door.

If you don’t already have wooden boards surrounding the perimeter of the opening, you can cut some 1×3 boards and nail them into place with 6d nails. Apply your weatherstripping to the top of the new wood stops.

After weatherstipping the hatch or door, cut a piece of rigid foam board insulation the size of the door and fit it into place on the back. You can also purchase pre-made insulated attic stair covers from your local home improvement store.

Use hook-and-eye fasteners to close the attic door. Make sure the weatherstripping is compressed when the door is closed and hooks are latched.

Air Duct Sealing

While you are in the attic sealing air leaks, look for any ductwork that may be up there. If you have ducts that aren’t sealed, you are probably leaking a lot of conditioned air into your attic space. Sealing duct leaks can improve the efficiency of your HVAC system by around 20%.

When the furnace or air conditioner is on, check for air leaks around the duct connections with your hand, a lit incense stick, or a thing piece of toilet paper. Use mastic sealant or aluminum foil tape (not duct tape) to seal all of the duct connections. Mastic sealant paste is a slightly more effective and durable than HVAC foil tape.

Learn how to seal your air ducts here

How to tell if you have enough attic insulation:

Attic insulation levels are rated according to their R-value. This measures the insulations ability to prevent heat flow. The higher the R-value, the more effective the insulation. According to the EPA, “the recommended level for most attics is to insulate to R-38 or about 10 to 14 inches, depending on insulation type.”

Watch this video from the Department of Energy to learn how to estimate your attic’s R-value with a ruler:



Adding the Right Insulation

You have two choices when it comes to DIY attic insulation: loose fill or batts/rolls.

Don’t worry about mixing different insulation types. You can mix loose fill with fiberglass batts and vice-versa. If you lack the proper insulation levels and wish to add loose fill insulation, hire a professional since this will require a large blowing machine. You can rent it, but it’s heavy and will require a truck and multiple people.

We recommend hiring a professional if you have recessed light fixtures or “can” lights. And don’t forget to create a barrier around your flues.

Click here for more Attic Insulation Tips from ENERGY STAR.

WARNING: After any significant improvements to your home’s insulation, make sure you call your local HVAC professional for a thorough inspection of your insulation and ventilation levels. In tighter homes, there is an increased risk of CO and other harmful gas buildup. Click here for more insulation, ventilation, and indoor air quality tips.

If you have any questions about how to properly insulate your home, don’t hesitate to give Hiller a call. We’d be happy to answer all of your questions and pay a visit to discuss your ventilation and insulation options.

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