To stop air leaks, find where conditioned air escapes and outside air gets in, then seal each gap with a material that suits its size. Start with the biggest openings, which are usually hidden in the attic, basement or crawlspace. Seal them before you add insulation. Caulk, weatherstripping and can foam handle small, visible gaps. Large or concealed leaks, especially under attic insulation, are better handled by a professional crew with spray foam.
TL;DR
- Air infiltration can account for 30% or more of a home’s heating and cooling costs, according to Cornell Cooperative Extension.
- The biggest leaks are typically in the attic, basement or crawlspace. Sealing those first has the largest effect on bills and comfort.
- Match the material to the gap: caulk for small cracks, foam for larger gaps, weatherstripping for doors and windows that move.
- Air seal the attic before adding insulation. Insulation alone does not stop air moving through a house.
- Spray foam is a strong air-sealing option, and it should be installed by trained crews wearing proper protective equipment.
Table of Contents
- What an air leak is and why it costs you
- Where air leaks hide
- How to find air leaks in your house
- Match the material to the gap
- Doors and windows: the leaks you can feel
- The attic comes first
- Where spray foam fits
- Our perspective: seal first, then insulate
- What air sealing and insulation cost
- How tight is tight enough?
- A practical plan for your home
- Get your leaks sealed in the Bay Area
- FAQ
- Sources
What an air leak is and why it costs you
An air leak is any gap in your home’s shell that lets air pass between inside and outside. Building codes call it infiltration: the unintentional introduction of outside air into a building, typically through cracks in the envelope and through doors being used.
The cause is mostly physics. Warm air is less dense, so it rises and escapes through the upper levels, typically the attic. That lowers pressure at the base of the house and pulls cold air in through doors, windows and other openings. This stack effect is strongest in winter, when the indoor-outdoor temperature difference is greatest. The result is infiltration low in the house and exfiltration high in it.
The consequences go beyond a draft:
- Higher heating and cooling use, since your system keeps working to replace lost air.
- Uneven rooms, with cold floors and warm upper levels.
- Moisture problems. The same Cornell source links air infiltration to moisture, noise, dust, and the entry of pollutants, insects and rodents.
- Wasted insulation. When heated indoor air or cold outdoor air touches your insulation directly, you lose significant heat, because insulation slows heat flow but is not an air barrier.
In the Bay Area the swings are milder than in Minnesota, but the physics are the same. Foggy summer evenings, cool nights in Santa Rosa and Petaluma, and heating-season drafts all show up as leakage in older homes with unsealed attic penetrations.
Where air leaks hide
ENERGY STAR lists the usual suspects: behind kneewalls, the attic hatch, wiring holes, plumbing vents, open soffits, recessed lights, furnace flues or duct chaseways, basement rim joists, and windows and doors.
Colorado State University Extension breaks down where air escapes in an average U.S. home. The shares below come from its air sealing table:
| Leak location | Share of total leakage |
|---|---|
| Floors, walls and ceilings/attics | 31% |
| Plumbing, electrical and HVAC penetrations | 17% |
| Ducts | 15% |
| Fireplaces | 14% |
| Doors | 11% |
| Windows | 10% |
| Electrical outlets | 2% |
Two lessons follow. First, doors and windows, the leaks people notice, together account for about a fifth of the total. Second, the large share sits in the structure itself: top plates, penetrations, ducts and the attic floor. ENERGY STAR says the biggest leaks are typically hidden in the attic, basement or crawlspace, and that sealing them has the biggest impact on energy bills and comfort.
How to find air leaks in your house
Leak hunting takes an afternoon. Work through the house in this order:
- Walk the obvious spots. Check door frames, window trim, and where pipes, vents, wires and fans pass through exterior walls. Look for gaps, cracked caulk and worn weatherstripping.
- Go on a cold, breezy day. Close windows and exterior doors, turn off the furnace or air conditioning, and move a hand or a stick of incense around suspect areas to see drafts.
- Check the attic. Look for dark, dirty insulation, which often marks where air has been filtering through, along with unsealed wire holes, plumbing stacks, the attic hatch, and the tops of interior walls.
- Inspect the basement or crawlspace. Rim joists, sill plates and duct boots are common trouble spots.
- Note what you cannot reach. The U.S. Department of Energy says a do-it-yourself approach works for obvious leaks around door frames, pipes, vents, fixtures, wires, ducts and fans, but not for small, hard-to-detect leaks, especially attic leaks covered by layers of insulation.
That last point matters. In many existing homes, the leaks that cost the most are buried under insulation. A crew that works in attics every day will find and seal them faster and more completely than a weekend of searching.
Match the material to the gap
The right product depends mainly on gap width. The guidance varies a little between agencies, so treat these as working ranges:
| Gap or location | Typical material | Notes |
|---|---|---|
| Small cracks, up to about 1/4 inch | Silicone or acrylic latex caulk | ENERGY STAR attic guide recommends a caulk gun for holes 1/4 inch or smaller |
| Larger gaps, 1/4 inch to 3 inches | Expanding spray foam | Same guide; several cans may be needed |
| Around flues, vents and chimneys | High-temperature (heat-resistant) caulk | Keep foam away from flammable applications |
| Doors and operable windows | Weatherstripping | Tension seal, felt, reinforced foam, rubber tape, door sweeps, metal gaskets, bulb or fin seals, interlocking metal channels |
| Big holes | Foam board or plywood, then sealant | Backing keeps sealant in place |
The materials are laid out in the DOE Consumer Guide to Air Sealing the Home, which covers caulk, spray foam and weatherstripping. Choose caulk that matches the surface, and clean the area first so it bonds.
Doors and windows: the leaks you can feel
If cold air comes in around a door, the cause is usually worn weatherstripping, a missing or loose door sweep, or a door that no longer sits square in its frame. Check the compression of the seal all the way around: if daylight shows or a sheet of paper slides out freely when the door is closed, the seal is not doing its job. Replace the weatherstripping, add a sweep at the threshold, and caulk the gap between the trim and wall.
Windows follow the same logic. Weatherstripping stops leakage between the moving parts, and caulk seals where the frame meets the wall. For step-by-step window methods, this guide on sealing windows to stop drafts walks through the materials and technique.
If your windows are old and leak heavily even after sealing, that is a conversation about window replacement, which is separate from insulation. Keep in mind that windows and doors are only part of the picture, so fixing them without addressing the attic often leaves most of the leakage in place.
The attic comes first
If your goal is lower bills and better comfort, the attic is where the work pays off. The stack effect makes the top of the house the main exit for conditioned air, and every unsealed opening there acts like a chimney.
Building Science Corporation’s attic guide is blunt: attics should be air sealed before adding insulation, because adding insulation alone does not save much energy and can lead to health and durability problems. One of its authors uses an image of a hot air balloon: a hole at the bottom matters little, while a hole at the top matters a great deal.
The Insulation Institute describes the right order of work for vented attics. Air seal the space, provide proper ventilation, make sure ductwork is sealed and insulated, and then install the correct amount of insulation. It also notes that leaks between the attic and living space are a major cause of moisture problems, including condensation, ice damming, attic “rain” and deterioration of roofing materials. For more on why this matters for the roof itself, see this explanation of how attic insulation protects your roof.
Typical attic air-sealing targets:
- Top plates of interior and exterior walls
- Wire and plumbing penetrations through the ceiling
- Recessed light housings (use products rated for the purpose)
- Duct boots and duct chases
- The attic hatch or pull-down stair
- Kneewalls and soffit areas
- Chimney and flue surrounds, sealed with heat-resistant materials
If you are planning an attic upgrade, our guide to when, what and how much attic insulation covers the insulation side. California homeowners also run into energy code requirements on larger projects, which we cover in our practical guide to air sealing and insulation for Title 24.
Where spray foam fits
Spray foam is both an air barrier and an insulator. It expands into cracks, seams and irregular spaces that batts and loose fill pass over, which is why it is widely used for sealing and insulating in one step. Our crews install it as spray foam insulation and as closed-cell insulation, and we explain the trade-offs in our comparison of closed-cell and open-cell spray foam.
Safe use of spray foam
Spray foam chemistry deserves respect. The DOE’s two-part spray foam guide for attics and kneewalls calls for durable backing in large gaps, clean surfaces, ventilation per manufacturer specifications, and installers wearing respiratory, skin and eye protection.
A national laboratory retrofit report adds that the isocyanate component is a hazardous material, with exposure linked to skin and airway irritation, chest tightness and difficulty breathing. It states that applying large quantities of foam requires professional equipment, training and protective clothing, that unprotected people should stay out of the area during application, and that the building may need to be vacated while the foam cures.
In practice, that means:
- Keep family members and pets out of the work area until the crew and the product instructions say it is safe to return.
- Make sure the crew ventilates the space as the manufacturer specifies.
- Ask your installer for the product’s re-entry guidance for your specific job.
- Do not use foam near flues or other heat sources unless the product is rated for it. Use heat-resistant sealants there.
If anyone in your household has asthma or a respiratory condition, talk to your doctor before the job and plan where they will stay during and after installation.
Our perspective: seal first, then insulate
We see the same pattern in Bay Area homes. Owners add insulation to a cold room, and a year later the room is still drafty because air was moving through the framing the whole time. Insulation slows heat. It does not stop air.
That is why we treat air sealing as the first step of every attic and wall project, not an add-on. Our in-house crews work across the whole building envelope, so the person sealing your attic penetrations understands how the roof, walls and floor interact. We install to California energy and building code standards, and every job carries a workmanship warranty.
Spray foam is the tool we reach for when a house has many irregular gaps, because it seals and insulates in one pass. When a lower-cost approach such as blow-in or fiberglass will meet your goal, we say so and price it plainly. Our pricing is transparent, with no upselling: you pay for what your house needs.
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What air sealing and insulation cost
Air-sealing work is usually priced as part of the insulation project. These are the prices published on our service pages, one-off per project:
| Service | Published price |
|---|---|
| Spray foam insulation | $1.00 to $2.50 per board foot |
| Closed-cell insulation | $1.50 to $2.50 per board foot |
| Fiberglass insulation | $0.50 to $2.50 per sq ft |
| Blow-in insulation | $1.20 to $3.50 per sq ft |
| Insulation removal | $1 to $2 per sq ft |
A board foot is one square foot at one inch thick, so thickness drives the spray foam total. Final cost depends on access, the depth you need, the condition of existing insulation, and whether old material must come out first. Worn, matted or contaminated attic insulation is often removed before sealing so the crew can reach the leaks underneath.
What you gain is lower energy waste. With infiltration potentially 30% or more of heating and cooling costs, sealing the envelope is one of the most direct ways to cut that share.
How tight is tight enough?
Air tightness is measured in air changes per hour (ACH). The OSU Green Home Technology Center reports that a poorly sealed home can have a natural ACH above 1.5, an energy-efficient home typically sits at 0.35 to 0.5 under normal weather, and a home below 0.35 may need added ventilation for air quality.
That last point is worth planning for. A tighter house needs deliberate, controlled ventilation rather than random cracks. Bathroom and kitchen exhaust fans, and where appropriate a whole-house ventilation system, supply that. Combustion appliances such as furnaces and water heaters also need proper venting and combustion air, so tell your contractor what you have.
A practical plan for your home
- Do a quick walkthrough. Fix the obvious leaks: door sweeps, worn weatherstripping, cracked caulk.
- Look in the attic. Dirty insulation, open penetrations and missing hatch seals point to major leakage.
- Decide what is DIY and what is not. Small, visible gaps suit caulk and weatherstripping. Hidden attic leaks, large gaps, and anything involving spray foam are for trained crews.
- Get a written scope. It should list the areas to be sealed, the material, the thickness of insulation, and the warranty.
- Seal, then insulate. Confirm the order of work before the crew starts.
- Plan ventilation. Ask how the house will breathe once it is tighter.
For commercial buildings, the principles are the same, with larger envelopes and more penetrations. Our commercial insulation service and our spray foam roofing system address roofs and walls on warehouses, offices and retail spaces.
Get your leaks sealed in the Bay Area
USA Spray Me seals and insulates homes and commercial buildings across the San Francisco Bay Area, from attics and walls to roofs. Our certified, California-licensed crews handle the whole envelope in-house, build to California building code standards, and back the work with a workmanship warranty. We give honest recommendations, including when a less expensive option will do.
To discuss your attic or building, see our spray foam insulation services in San Francisco and the Bay Area, or browse all of our services.
FAQ
What does air infiltration mean?
Air infiltration is the unintentional entry of outside air into a building through cracks and openings in its shell. Its counterpart, exfiltration, is conditioned air escaping out. In winter, warm air rises and leaves through the upper levels, which draws cold air in low in the house.
Is it normal for cold air to come through a front door?
A little air movement around an old door is common, but it is not something you have to live with. Most door drafts come from worn weatherstripping, a missing sweep, or a gap between the frame and the wall. Replacing the seals and caulking the trim usually fixes it. If the door no longer closes squarely, the frame may need adjustment.
Should I air seal before adding attic insulation?
Yes. Building Science Corporation advises sealing the attic first, because insulation alone does not stop air movement and can lead to moisture and durability problems. Sealing the penetrations and top plates first lets the insulation perform as designed.
Can I use spray foam from a can to stop air leaks?
Can foam is commonly used for gaps from about 1/4 inch to 3 inches in the attic, according to ENERGY STAR. Keep it away from flues and other heat sources, and use heat-resistant caulk around them. For large areas or hidden leaks under insulation, professional spray foam installation gives more complete coverage and is done with the protective equipment the work requires.
How do I know if my house has too many air leaks?
Signs include drafts, cold floors, rooms that never reach the set temperature, dusty interiors, high energy bills and dirty streaks in attic insulation. A walkthrough finds the obvious leaks, while an inspection of the attic usually reveals the bigger ones.
Sources
- DOE BECP Resource Guide: Air Leakage
- Reduced Air Infiltration, OSU Green Home Technology Center
- Air-sealing, Cornell Cooperative Extension
- Locating Air Leaks, ENERGY STAR
- Air Sealing Colorado Homes, CSU Extension
- Seal and Insulate with ENERGY STAR
- Consumer Guide to Air Sealing the Home, U.S. Department of Energy
- Attic Air Sealing Project, ENERGY STAR
- Attic Air Sealing Guide, Building Science Corporation
- Retrofitting Vented Attics: Solutions That Work, Insulation Institute
- Insulate an Attic or Knee Wall With Two-Part Spray Foam, U.S. Department of Energy
- Best Practices and Case Studies for Retrofit in Mixed-Humid Climates, OSTI
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