One of the new buzzwords people are using when they talk about energy-efficient homes is air sealing. Apparently, it’s essential if you want your home to be energy-efficient. But what is it?
Air sealing is simply finding and systematically sealing all the places throughout your home that leak air.
All homes leak. Like water coming through a hole in your roof, air leaks through gaps and holes throughout the structure of your house and all its components. But unlike the roof leak, you’re usually not aware of air leakage.
Maybe you can feel a draft in a room; this could be due to air leakage. Perhaps one room stays colder (in winter) or warmer (in summer) than others. Air leakage could cause or contribute to this.
But you’ll feel the main effect of air leakage in your wallet: Your typical home will lose 25%-40% of the energy you use for heating and cooling through air leakage. Another US government source says that you can save 15% on your heating and cooling bill by air-sealing and insulating.
An example used sometimes is comparing air gaps in a home to a leaky ship. You can bail out all the water constantly, but if it keep filling up through all the holes, it’ll never stay upright. This is just like your HVAC and other home systems trying to work together while air is leaking through all the time.
Read on to find out how you can stop your house—and your wallet—from leaking.
Table of Contents
- Let’s Start at the Beginning
- What is Air Leakage?
- What Causes Air Leakage
- Venturi Effect
- Benefits of Air Sealing
- When Should You Do Air Sealing
- Blower Door Test Audit
- Other Air Leakage Measurement Methods
- The Usual Suspects and How to Reduce Their Leaks
- Replacing Windows
- Electrical Outlets
- Recessed Light Fixtures
- Plumbing Pipes
- HVAC Registers
- Bathroom Vent Fans.
- Mail Slot
- Masonry Fireplaces
- Fireplace Surround at Masonry Chimney
- Gas Fireplaces
- Wood-Burning Stoves
- Laundry Area
- Wall and Window Air Conditioners
- Interior Walls and Ceilings
- Attic Floor
- Attic Wall With Attached Garage
- Attic Penetrations (Piping, Wiring, Ducts)
- Flue Pipes and Chases
- Chimney at Attic Penetration
- Soffit/Dropped Ceiling
- Attic Kneewalls
- Attic Access
- Whole-House Fan
- HVAC Ducting
- HVAC Plenum
- Exterior/Crawl Space
- What Sealant/Caulk Should I Use?
Let’s Start at the Beginning
Air leakage is complicated, and the more you understand it, the better you’ll be able to deal with it. This article will help you understand what it is, how it works, and how you can air seal your house.
What is Air Leakage?
Your house’s building envelope is like your skin. It completely encloses what is inside. You can think of it as the outside covering on your home. In newer homes, the actual envelope is somewhere between the exterior cladding and the inside walls.
Your skin has openings for your mouth, ears, etc., that serve specific purposes for what’s inside. Similarly, your house has special-purpose openings.
Doors allow you, your family, and your friends to go in and out of the house and bring in whatever you need. Windows let in light and fresh air when opened (and your teenagers can crawl out of them to escape).
These are the main ones, and they are individually the largest. You’ll keep them closed unless you are using them.
You have other openings. Your clothes dryer exhausts through a pipe to the outside. Your kitchen and bathroom fans vent through the walls or roof. Your furnace and water heater expel their products of combustion through flue pipes to the outside. Your fireplace is a big hole from your living room through your roof.
Those are functional openings. Generally, the pipes from your exhaust fans and your clothes dryer have dampers on them that are supposed to close when air is not flowing out through them, so they don’t allow outside air to come into your home.
But there are usually gaps around the openings in the walls or ceilings where they pass. The pipes don’t fit completely flush with the walls.
You’ll also find gaps in the wall around plumbing pipes, electrical receptacles, lights, electrical outlets, air conditioning system components—the list goes on. The various places that you can expect to find air leakage are listed below.
Air leakage in your home is simply the air movement in or out of all those places, whether you can see them or not.
What Causes Air Leakage
Several natural effects can cause air leakage. One or more of these may be happening at once at any given time. Mainly, air flows from areas of higher pressure to areas of lower pressure, and warm air rises.
When the wind blows against your home, it can force air in through the different openings. The stronger the wind, the more it pushes air in through the gaps. You get whatever is in the outside air, which could be warmer or cooler than the inside air.
This wind forces air out through your leaks on the other side of your home, and you lose heated or cooled air.
The stack effect is a significant driver of air movement due to temperature differences. Warmer air is not as dense as cooler air so it rises; that’s why a hot-air balloon can fly. You heat up the air inside the balloon, and the air becomes lighter than the surrounding air, so the balloon rises. The same thing happens to warm air without the balloon.
Interior Stack Effects
In your home, air might be warmed by cooking, by your body, by lights, by your computer, TV or other equipment, etc.
The warm air rises to the ceilings, where it passes upward through leakage areas and out through higher leakage spots. That draws cooler air in through the leaks in the lower portion of your home, along with whatever else is in the air.
Some homes have a chase, which is a shaft extending from the basement through the structure to the attic that has pipes running through it. Any time you have a shaft or tube, air flows naturally out at the top and in at the bottom. A chase allows a lot of air movement all the time since the air at the top is usually warmer than the air at the bottom.
Exterior Stack Effects
In winter, the cooler air around the lower portion of your home seeps in through the various leaks. Then, it starts to warm and rise up and out through the upper leakage areas.
The reverse process occurs in summer or warm climates. Hot outside air enters the upper part of your home and cools off slightly (or more), creating a downward draft.
Your home might have some exterior features that could cause the wind to slow as it passes. This could suck some air out of your house if there’s a leak area near that point, creating a negative pressure that would suck air in through some other location.
Benefits of Air Sealing
There are many reasons you would want to air seal your home. This section details the benefits.
Lower Energy Bills
Because energy loss from air leakage accounts for 25% to 40% of your energy bills, you’ll get an immediate financial payback from air sealing through reduced bills that will start to pay back the costs. Leakage robs other energy efficiency measures like increased insulation and better windows of their benefits, too.
Improved Indoor Air Quality
When air leaks into your house, it brings with it whatever is in the outside air—dust, pollen, air pollution, odors, radon, unwanted humidity. The outdoor air could be hotter or colder than inside and make you less comfortable. The openings that allow the leakage can be pathways for insects to enter and cause problems.
You will be more comfortable with reduced air leakage when you get rid of those unwanted drafts, and the temperatures in different rooms aren’t very different. Less outside noise will find its way inside.
Less humid air will seep inside in warm, moist climates. In cold climates, your indoor air won’t be so dry that you have sinus problems and unwanted static electricity. Dry air also allows viruses to spread more easily.
Reduced Condensation Problems
Water is a building’s worst enemy and can find its way into your house with air leaks. Once moisture gets inside, it can condense and cause mold and mildew problems.
In hot, humid climates, moist air enters the home and condenses when it encounters the cooler parts of the structure, where it can cause wood to rot as well as provide drinking water for termites. Mold can form inside your walls.
In cold climates, moist air can enter the walls from inside and condense to cause mold and damage.
Reduced Possibility Of Ice Dam Formation
In colder climates, warm air loss through the attic melts snow on your roof. The liquid can freeze near the roof edges and create barriers to the runoff and cause roof leakage.
When Should You Do Air Sealing
The best time is when you are building your house from scratch. That way, each potential leak can be sealed up at the time of construction. In fact, the building codes in some places are now allowing houses only a certain amount of leakage; above that maximum, the building inspectors make the builder reduce the leakage.
Once drywall is applied to cover the building structure, leaks are much harder and costlier to find and seal.
OK, you say, I bought this house that was already built. What can I do?
The second best time is when you are remodeling. The more extensive the remodel, the more opportunities you have to expose areas of the structure where leaks typically occur and seal them before they are covered up.
But even if you’re not planning a remodel, there are obvious places that probably leak that you can seal. We’ll tell you where they are and what to do about them if you keep reading.
First, you have to locate the leaks so you can seal them.
Blower Door Test Audit
The most accurate way that you can locate the many air leaks in your house is through something called a blower door audit. This test quantifies the amount and identifies the locations of air leakage in your home.
To do this, you need some costly equipment and technical knowledge, so your best bet is to hire a professional. This will generally cost you around $450 for an average-sized house. But check with your utility company, most offer free energy audits, and some include a blower door test.
The professional will close all the exterior doors and windows and block all the obvious openings between the inside and outside, like the exhaust fans in your bathrooms and kitchen, the clothes dryer vent, the fireplace, and any other openings you might have.
Then she will set up a powerful variable-speed fan in a frame that blocks off and seals an entire exterior doorway. She’ll attach a device to measure the airflow as well as the pressure difference between the inside and outside of your house.
She will crank the fan up to create a standard pressure difference (50 pascals) and record how much air the fan is pushing out of the house through all the leaks at that pressure.
It’s like measuring the leakage from a bucket that leaks in a bunch of places. If you know the rate at which you have to add water to keep the bucket full, you know how much it’s leaking in total from all the places.
The measuring device will give a reading in terms of air changes per hour (ACH50—the standard reference) that shows how many times in an hour the amount of air from your leaks equals the volume of air in your house.
At that point, you can see where the leakage is with a smoke pencil—put it next to different places and watch the smoke blow into the house.
Even better, she will use a high-resolution infrared (IR) camera to spot the leaks. Her IR camera documents temperature differentials with the fan reversed, sucking air into the house at a lower pressure.
Hopefully, the air outside is significantly hotter or colder than inside the house. So wherever cold (or hot) air is leaking, and the fan is sucking it into your home, the IR camera will show a temperature differential at the leak. The IR pictures will give an idea of the magnitude of the leak, too.
You’ll get a full report, and then you can start sealing the leaks.
If you have hired a contractor to do all the air sealing, you should have him do another blower door test when he is finished to see how much he has reduced the leakage.
Can I Do a Blower Door Test Myself?
You can do a non-professional blower door test yourself to find some of your leaks. You’ll need something to generate smoke, maybe a cigarette, if you can’t find a smoke pencil.
You should close all the exterior doors and windows and other openings like the fireplace. Open all the interior doors.
You will also have to turn off the pilot lights if you have them in your water heater and furnace to prevent back-drafting that can cause carbon monoxide poisoning.
Then turn on all the exhaust fans in the house, including your clothes dryer. If you have a fan that sets in a window and you can close off the area around the fan, you’ll get good results.
Once the fans are running, use your smoke pencil at the different places around the house where there is the potential for air leakage. Keep reading to see a list of these places.
Other Air Leakage Measurement Methods
A very unusual method of measuring air leakage is the PFT (perfluorocarbon tracer) air infiltration measurement technique, which Brookhaven National Laboratory developed. This test can give detailed information on air infiltration patterns around the house.
You have to get the testing equipment from Brookhaven National Laboratory and send it back to them for analysis. This method is rarely used, so we’ll just mention it in passing.
The Usual Suspects and How to Reduce Their Leaks
There are obvious places in a house where you can expect leakage, especially in older buildings. Some of them are pretty easy to seal with products that at least reduce the air leakage if not eliminate it. Other places are difficult to access. And, of course, some are covered with structure or drywall, so you can only get to them during a remodel.
Let’s start the list with the places you access from the interior, which are easier. See the section below for information on what products to use for those that require caulking or foam.
Older windows leak a lot, especially as they age. Although they may not leak water from the outside, they will leak air to and from the outside in many cases. They let in air around the edges of the sashes (panes of glass) as well as between sashes if there’s more than one in a window, and they leak around the frames.
There are a couple of things you can do to older windows. The best thing you can do is caulk the junctions between the window frame and both the interior wall and exterior cladding. If the weatherstripping is worn, you can replace it to improve the seal. Or you can replace the entire window.
Newer windows are much better at preventing leakage through the windows themselves. If you chose to replace your old windows, you would get windows that are much more energy-efficient.
You can choose one of two ways to replace your windows. The easier, less expensive way is to put a new window into the old window frame. The new window frame has to be well-sealed to the old frame, and you still have to seal around the edges of the old frame.
You could replace the entire window assembly, which would involve removing the entire old window. You open up the exterior wall, weave the flashing paper that comes with the window into the building’s existing flashing, restore the exterior cladding around the window, and seal the window frame into the interior wall.
This procedure usually gives the most leak reduction and looks better, too.
Doors usually have gaps between the edges of the door and the frame, and the floor. There are also gaps between the door frame itself at the interior walls and the exterior cladding; the trim hides a gap.
Doors should be weather-stripped around their tops and sides. But weather seals can wear out or become loose. If your doors are already weatherstripping around their sides and top, check to make sure the seals are tight. You may want to replace the weatherstrips if they don’t look like they are tight.
The bottoms of your entry/exit doors should be sealed with a “sweep,” which is a flexible strip that sweeps along the floor and covers the gap under the door when it is closed. Make sure your door has a sweep that is flexible enough to seal securely against the floor when you close the door.
You should also seal around the interior and exterior edges of the door frame itself.
The baseboards all around the edges of your floors hide gaps between the floor and the walls, which leak. You can caulk around the tops and bottoms of the baseboards, although this may look sloppy.
The best way to seal your baseboards is to remove them and directly caulk the gap between the floor and the wall. However, this can be delicate work. It’s not easy to remove and reinstall baseboards. Here’s an article that tells you how.
You can reduce air leakage through electrical switches and receptacles by installing a gasket under the cover plate.
You can also replace receptacles with child-safe ones that cover the holes for the plug; you have to rotate the cover to allow holes to align holes for the plug.
Don’t forget about the boxes with cable and telephone connections; these can be gasketed too.
Recessed Light Fixtures
Older recessed lights are a problem to air seal because the light cans have openings to ventilate the heat from their incandescent bulbs. The ventilation prevents fires; please do not block these holes.
You can eliminate the leakage by replacing the light fixtures with ICAT-rated fixtures (insulation contact, air-tight). But this is expensive.
Alternatively, you can get thin LED lights with a bulb base that screws into your old can light. Push the can light up out of the way and install the LED fixture below it, caulked to the ceiling.
If you have access to the attic, you can put a hood over the recessed light and caulk it to the attic floor. It’s best if you switch to LED or CFL bulbs, so you aren’t generating as much heat.
The easiest and least effective option is simply to put a gasket under the trim.
Pipes come through the walls in bathrooms and kitchens to supply water to taps and drain it away. Often, you’ll find a metal or plastic escutcheon plate around the pipes to cover the gap between the pipe and the wall. Remove the escutcheon plates and seal the gaps, using caulk or foam for wider openings.
Don’t forget the gas pipes for the stove/oven and cooktop. These are not always easily accessible but are worth sealing.
Those grilles that provide the heating and air conditioning airflow are set into holes in the ceiling and walls, so there is a gap around them. These gaps are easy to caulk once you loosen the screws securing the register to the wall or ceiling. If you can access them from the attic, you may be able to seal them there more easily.
Bathroom Vent Fans.
Your bathroom exhaust fans have a trim plate that you can pull down from the ceiling so you can caulk the gap underneath.
If your house is old enough to have a mail slot in the door, you might want to consider sealing it up and getting a mailbox. If it is the open type, you can replace it with one that does a better job of sealing. Or, you can construct a mailbox that fits over the slot on the interior and has a well-sealed access door.
Most older masonry fireplace dampers don’t seal well. You can have a professional install a newer type of damper for the fireplace that sits on top of the chimney and does a better job of sealing.
Or, you can fill a plastic garbage bag with enough scraps of fiberglass insulation (or equivalent) to block the opening in the throat of the chimney. You should attach a string to the bag with a note on the end that hangs down into the fireplace to remind you to remove the bag before starting a fire.
Fireplace Surround at Masonry Chimney
You can caulk the gap between the brick chimney and the wall and between the hearth extension and the floor. Use a temperature-resistant caulk for this to be on the safe side
If there is a gap between the fireplace firebox itself and its surround, you should call a fireplace expert to seal the gap.
Decorative gas fireplaces don’t have dampers; they are just open holes through the roof. There’s not much you can do about that open flue.
Metal fireplaces that can burn either wood or gas have dampers that do a decent job of closing the flue, but they should be partially disabled for safety reasons: if the fireplace is burning only gas and the damper is closed, odorless and colorless products of combustion will spill into the living area and cause health problems.
Of course, if you’re burning wood, you’ll know immediately that the damper is closed as smoke fills the room.
You should seal around the outer edges of the firebox where it sets in the wall and around the flue pipes where they pass through the attic with a temperature-resistant caulk or a temperature-resistant spray foam.
Your wood-burning stove may have a damper if it’s an older model, but the damper is usually only partially closed. The flue pipe is just another big hole right through your home if the damper is not closed tight.
You’ll find gaps that you can seal associated with your clothes washer and dryer.
In newer homes, the washer discharges into a drain that is set into the wall. The entire recess containing this drain may have cracks and gaps that can leak. In older homes, you might have a pipe in the wall that you can seal around.
The clothes dryer discharges through a 4-inch pipe that may go through the wall, floor, or ceiling directly, or it could lead into a metal box set into the floor or wall. You should caulk the gaps around the pipe or box so there are no openings with a fire-resistant caulk, or use a temperature-resistant spray foam.
Wall and Window Air Conditioners
If you have an air conditioner in an opening in your wall or mounted in a window, it probably leaks around the edges. Seal the openings with caulk or spray foam.
Interior Walls and Ceilings
Although you may not want to caulk leaky corners for aesthetic reasons, you can put a fresh coat of paint on your interior surfaces to help seal them.
The attic in an older house can contribute as much as 50% of the air leakage in the whole house. So that is one of the areas to focus on to reduce your air leakage.
However, the attic floor is usually covered with insulation, which hides most or all of the places you need to seal. Some of the places are pretty hard to access. And if you’re not careful, you can fall through the ceiling.
As if that all weren’t enough, some attics are so cramped it’s hard to get around, and you may find all kinds of essential components that you can’t move blocking your way in many attics.
For these reasons, many folks prefer to call a professional to handle air-sealing the attic.
But if your attic insulation is old and settled, you might want to remove it and attempt the job yourself. After you finish air-sealing, you can install new fiberglass batt insulation yourself, or you can have blown-in cellulose installed.
The attic floor often has many cracks and some open gaps. You should seal the cracks with caulk, of course; use foam for the larger openings. You should cover the openings with a stiff material like drywall scraps and glue them to the surrounding floor with caulk. Make sure that you seal the entire perimeter of the cover.
Attic Wall With Attached Garage
If you have an attached garage, you may have a wall between the house attic and the garage. If you do, there are gaps and openings in this that you should seal.
Attic Penetrations (Piping, Wiring, Ducts)
You’ll find pipes and electrical boxes sticking through the floor of the attic, and you should seal the gaps around them with caulk or foam. If any electrical boxes are open, do not seal inside them—make sure they are correctly covered and seal only around their outsides.
You can caulk around ducts that penetrate the attic floor, too.
Flue Pipes and Chases
Flue pipes (which get hot) and some regular pipes often enter the attic in chases, which are channels that start (usually) in the basement.
It is best to cut sheet metal to fit around any older, single-wall flue pipes; click here for more information. You may have to cut a piece of drywall to help cover a large shaft.
Newer flue pipes have double walls, so they don’t get as hot; you can use a high-temperature foam to seal them. If the opening is too big, use pieces of drywall to cover part of the opening.
Chimney at Attic Penetration
Building codes mostly require a one or two-inch clearance from wood. You can cover the gap with sheet metal; click here for how-to instructions. Because the chimney can get quite hot with a wood fire, you should not use foam.
Soffits are features where the ceiling is lower in a portion of a room or along a wall. Sometimes they hide ducts or wiring; sometimes, they are just design features. They open into the attic, so if they are accessible from the attic, you should cover them with a sheet of drywall and caulk it around its edges to seal it to the attic floor.
Kneewalls are vertical surfaces inside the attic that create extra living space below; they are covered with drywall. These can be significant air leakage points. You should caulk all edges of the kneewall; click here for more information.
There are many different attic access configurations, so you may have to use your imagination to air seal yours.
Often, you can install a gasket. If you have a piece of drywall that sits on top of the opening, put a foam gasket around the edge of the drywall so it can fit securely.
If you have a drop-down stairway, you may have to build a cover of some sort that can seal to the stairs or the attic with a gasket. Here’s an article with more information.
Whole-house fans are the biggest leak in your house if you have one. You can correct it by building a box that fits on top of the fan in the attic, with a lightweight hinged top that opens up when the fan turns on.
Here’s a video that shows you how.
Your ducting leaks. Older ducting leaks a lot. If the ducting is inside your walls, there’s not much you can do about it unless you have a company spray an aerosol into the ducting system that seals the leaks from the inside. This may be expensive.
If you have sheet metal ducting in your attic or basement with a thin or cardboard-like white material covering it, you probably have asbestos-containing insulation, and you should leave it alone.
If you have older ducts made out of fiberglass—these are stiff, straight, and usually tan, from the ‘70s—you may be able to tape the joints but avoid disturbing the ducts so you don’t release fiberglass particles into your system. Consider replacing them.
If your ducts are the common flexible type with fiberglass insulation under a plastic cover, used since the 70s, these may be damaged or loose at joints. You can tape them. Don’t use “duct tape;” it is not for actual ducts. Use foil-backed tape that has a lot of adhesive.
Here’s a great video from Crawl Space Ninja that shows you how to air seal your ductwork:
The furnace itself has sheet metal components that attach the body of the furnace to the ductwork, called the plenum. Usually, this is not well-sealed to the furnace, but that’s easy to correct. Here’s a video that shows how to air seal around the plenum on your furnace.
Sill Plates (Slab Foundation)
If your house has a slab foundation, you can’t get to the sill plate to air-seal it since it is below the exterior cladding. The best you can do is carefully air-seal the baseboards on exterior walls from the inside.
However, if the interior of an attached garage is unfinished, you can seal the sill plate at the base of the wall with the house to the concrete foundation with caulk.
Rim Joist (Raised Foundation) and Floor
If you have a crawl space under your house, you can seal around the rim joist, which is the structural member that supports the rim of the floor. Seal all around it, and fill any gaps or holes for pipes or wires.
If you have a floor that looks leaky, caulk the gaps between the floorboards.
The best way to air-seal the floor and rim joists is to have a specialized contractor spray-foam the entire area with open-cell foam.
Various other places within the structure should be air-sealed but are probably inaccessible. If a porch roof is attached to the building structure, it may leak air into the house. An interior stairway on an exterior wall may have leakage points that caulking the baseboards cannot seal. Cantilevered floors are inaccessible from below. Etc.
What Sealant/Caulk Should I Use?
The best latex caulk is Dap Alex Painter’s Acrylic Latex Caulk. Latex caulk is general-purpose and adheres well to drywall, wood, and masonry. It dries in two hours and can function for up to 15 years. You’ll need a caulking gun to squeeze out the caulk. You can also use a silicone or polyurethane caulk.
The best expandable foam is DAPtex Plus Foam. This polyurethane foam is one of the best and most affordable expandable spray foams. Use it to fill gaps wider than ¼” or so.
The best butyl rubber caulk is Flex Shot Rubber Adhesive Sealant Caulk. This is good for sealing metals, plastics, concrete, mortar, and rubber substrates and is suitable for outdoor use.
Don’t Forget to Provide Ventilation
When you tighten up your home by eliminating air leaks, you are removing sources of natural ventilation. If you tighten too much, you should add a source of ventilation to avoid various potential problems.
There’s a lot to air-sealing your house. It is helpful to know the theory and specifics of air leakage because different houses have different construction features. We have tried to cover the main issues for most older homes here, but you may need to do some additional research or talk to a pro about specific features of your home.
The benefits of air sealing are numerous. You’ll have a healthier, more comfortable environment that costs you less to maintain. You’ll find it’s worth the work!