Air sealing: stop paying to heat the outdoors
Insulation slows heat. Air sealing stops the draught that carries it away. Done in the wrong order it wastes money, and done in the wrong house it can be genuinely unsafe - here is how to tell which house you have.
What is air sealing?
Air sealing means closing the unintended gaps that let conditioned air escape and outside air pour in - around wiring and plumbing penetrations, dropped soffits, attic hatches, rim joists and window and door trim. It is a different job from insulation: insulation slows heat moving through a surface, air sealing stops air moving through holes in it. ENERGY STAR groups the two together because doing either alone leaves most of the benefit on the table, and sealing is normally done first.
How air sealing actually works
Diagnosis, ideally with a blower door
A blower door depressurises the house so leaks become measurable and findable - you stop guessing which gaps matter and start ranking them. Many contractors pair it with an infrared camera, which makes moving air visible as a temperature streak.
Watch forA quote written from a walk-round with no measurement. Air sealing without diagnosis reliably ends up in the visible, comfortable places - window trim - while the expensive leaks stay open in the attic and the basement, where nobody wants to crawl.
Combustion safety and radon check, before anything is closed
ENERGY STAR is explicit that "sealing in some cases can cause naturally venting combustion appliances to back-draft gases back into your home, creating unsafe conditions", and separately advises considering a radon test and mitigation before sealing a home. Both checks belong at the start, not after.
Watch forAny crew that starts foaming before establishing how your water heater and furnace vent. If you have an atmospherically vented appliance and nobody tests draft after the work, you have bought an unquantified risk with your energy saving.
Attic-plane sealing - the big leaks first
ENERGY STAR names the attic locations that actually matter: behind and under kneewalls, the attic hatch, wiring holes, plumbing and pipe holes, dropped soffits open to the attic, recessed lights, and the furnace flue or duct chaseway. These are sealed with expanding foam, caulk, rigid board and, at flues, aluminium flashing with high-temperature sealant.
Watch forOrdinary expanding foam packed against a metal flue. Flue penetrations need metal flashing and a high-temperature sealant with the clearance the appliance requires - a foam-only detail there is a fire issue, not a performance one.
Basement, crawl space and rim joist
The bottom of the house is the other half of the stack. ENERGY STAR calls for sealing "the top and bottom of the inside of the rim joist cavity", and flags bay windows that hang off the foundation as especially important. Crawl spaces want "a continuous sealed groundcover of vapor diffusion resistant materials" before anything else.
Watch forSealing the top of the house only. Close the attic and leave the basement open and you can make the pressure profile worse, pulling more soil gas and basement air up into living space.
The living-space layer - trim, penetrations, ducts
Window and door trim, sill plates, dryer and bath fan penetrations, and any ductwork running through unconditioned space. Ducts leaking into an attic are functionally a hole in the envelope with a fan attached.
Watch forDuct sealing quoted as an afterthought. If your ducts run through the attic, sealing them often returns more than sealing the ceiling below them, and it should be priced explicitly rather than folded into an allowance.
Ventilation - deliberately, not accidentally
A house that leaks less needs its fresh air supplied on purpose: working bath and kitchen exhaust that terminates outdoors, and in tighter homes a mechanical ventilation strategy. ENERGY STAR flags kitchen, bath or dryer vents that exhaust into the attic as a problem needing professional correction.
Watch forA dryer or bath fan that dumps into the attic. It is common, it is a moisture source directly under your new insulation, and finding it is one of the more valuable things a sealing crew does.
Re-test and hand over the numbers
A second blower door reading after the work turns a claim into a result. You should receive before-and-after figures, a note of what was sealed, and a combustion-safety result for any vented appliance.
Watch forNo post-work test. Without one, "we sealed the house" is unverifiable, and it is also the document you will want if you later apply for a utility programme that requires measured performance.
What does air sealing cost?
Professional air sealing is usually a low-thousands job, and a small fraction of what insulation or window replacement costs - which is why it is normally the first money spent. Scope, house size and how much of it is crawl-space work drive the range. See the Cost Guide for current figures.
See the full air sealing cost guide - by material, size and region →
If the number is the problem, here are the ways people pay for it →
Insulation & sealing tools → Savings and R-value tools, including targets set by your climate zone rather than a national average.
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When air sealing is the wrong call
When saying yes is the wrong move:
- You have an atmospherically vented furnace, boiler or water heater and nobody will test draft. This is the one genuine safety veto. Tightening the shell changes house pressure, and ENERGY STAR warns plainly that sealing can back-draft combustion gases indoors. Either get combustion safety testing as part of the job, or address the venting first.
- The attic already has water damage, mould or wet insulation. Sealing traps the moisture source under new material and hides the evidence. Find where the water is coming from - roof, bath fan, duct condensation - and fix that before anyone air-seals over it.
- Your attic has vermiculite you have not had tested. ENERGY STAR notes some attics have vermiculite insulation "which may contain asbestos" and that it should not be disturbed unless lab-tested. Air sealing an attic means crawling through and disturbing insulation. Test first.
- You have knob and tube wiring. ENERGY STAR identifies pre-1930 knob and tube as a fire hazard in contact with insulation and a professional-only situation. Air sealing around it is not a caulk-gun job - get the wiring assessed by an electrician first.
- You are selling within months. Air sealing is real, but it is invisible. It shows up in comfort and in bills over a heating season, not in an appraisal or a listing photo. Money spent shortly before a sale usually does not come back.
- The house has never had a radon test and sits in a region where radon is common. ENERGY STAR suggests considering a radon test and mitigation before sealing. Reducing air exchange without knowing the soil-gas picture is a decision made blind.
Types of air sealing
| Option | What it means for you |
|---|---|
| Attic-plane sealing | The top of the stack - hatch, wiring and plumbing holes, dropped soffits, flue chaseways. Usually the highest-return zone in the house. |
| Rim joist and basement sealing | The bottom of the stack. Frequently skipped and frequently the reason a top-only job underdelivers. |
| Crawl space sealing and encapsulation | Ground cover and perimeter treated as one system, not a set of individual gaps. |
| Materials and where each belongs | Caulk, canned foam, rigid board, mineral wool and flashing all have a defined place. Compare them in the Materials archive. |
| Start from the symptom instead | Draughty rooms, cold floors, ice dams and creeping bills each have their own diagnosis page in the Problems archive. |
Can I do this myself?
Reasonable to DIY
- Caulking window and door trim, baseboards and sill plates from inside the living space
- Weatherstripping doors and replacing a worn door sweep or threshold seal
- Adding foam gaskets behind outlet and switch plates on exterior walls
- Insulating and weatherstripping the attic hatch - one of the highest-value hours of DIY on any house
- Sealing an accessible rim joist cavity in a dry, open basement with rigid board and sealant
- Sealing obvious gaps where plumbing or cable enters through an exterior wall
Call a licensed pro
- Anything on the attic floor plane where the insulation has to be moved, disturbed or crawled over
- Recessed lights - sealing over a fixture not rated for insulation contact is a fire risk, and telling the two apart from below is unreliable
- Furnace flue and duct chaseway penetrations, which need metal flashing and high-temperature sealant, not foam
- Any home with naturally vented combustion appliances, where the work needs a combustion-safety test attached to it
- Blower door diagnosis and the re-test that proves the work did something
- Crawl spaces, where ground cover, drainage and sealing are one interacting system rather than a caulk job
What to check when hiring for air sealing
- Will you run a blower door test before and after, and give me both numbers? The before number tells you where to spend; the after number is the only proof the work performed. A contractor who does neither is selling effort, not results.
- How will you test my combustion appliances for back-draft after sealing? The single safety-critical question here. ENERGY STAR warns that sealing can back-draft naturally vented appliances. There should be a specific test named, not a reassurance.
- How will you detail the furnace flue and any duct chaseway? You want to hear metal flashing and high-temperature sealant with the required clearance. If you hear "we foam it", stop.
- Are my bath, kitchen and dryer vents terminating outdoors - will you check? Vents dumping into the attic are common and become a moisture problem the moment the space is tightened and insulated over.
- Has this attic been checked for vermiculite and for knob and tube wiring? Both are professional-only findings that change the job entirely, and both are cheaper to discover during the site visit than mid-crawl.
- What ventilation will the house have once it is tighter? The honest answer sometimes is "your existing exhaust fans are adequate", but it should be an answer, not a shrug.
- What is included, and what is a change order? Duct sealing, hatch rebuilds, recessed-light covers and rim joist work are the four items that most often appear as extras after the crew is on site.
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Often done at the same time
Sources
Sources last checked
- Insulation Fact SheetU.S. Department of EnergyRetrieved 2026-08-04Federal guidance on insulation types, R-value, and where it matters.
- Occupational Employment and Wages — Construction TradesU.S. Bureau of Labor StatisticsRetrieved 2026-08-04Labor cost context for home-services trades nationally.
Sources for figures and claims on this page.
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