Air Sealing Before a Heat Pump: Where DMV Homes Leak and Why It Comes First

By Max Fisher · Published 2026-10-15 · Updated 2026-10-18

Short answer. Seal the air leaks before you add insulation and before anyone sizes a heat pump. Many of the leaks that matter are hidden in the attic, the basement and around chimneys and fixtures, not at the windows. Have gas and oil appliances checked first, because a tighter house can make a water heater backdraft. In BGE’s Home Performance program, heat pump rebates require attic air sealing.

This is step 2 of the Law of Sequence: measure, seal, insulate, then size and install.

Why sealing comes before insulation and equipment

The Department of Energy’s air sealing guide explains the difference with a simple picture. Insulation is a sweater: it keeps you warm when the air is still, but it does not block the wind. Air sealing is the windbreaker. Insulation slows heat moving through a surface, but it does not necessarily stop air moving through cracks and gaps.

Those air leaks make a house drafty, put extra strain on heating and cooling equipment, and can let in moisture that warps wood and leads to mold. Sealing first means the insulation that goes in next works as rated, and the heat pump installed last is sized for a house that holds its air.

ENERGY STAR estimates that homeowners can save an average of 15 percent on heating and cooling costs by air sealing and adding insulation.

Where houses leak

DOE’s guide shows 19 key areas where air sealing can improve a home’s energy efficiency, comfort and building durability. The ones worth checking first are rarely the windows:

The attic floor: gaps around recessed lights, plumbing and wiring holes, the attic hatch, and dropped soffits over kitchen cabinets.
Chimney and flue chases: the open space around a chimney or flue that runs from the basement up to the attic.
The basement rim: where the house framing sits on the foundation, plus sill plates and foundation penetrations.
Knee walls: where finished rooms on an upper floor meet unheated attic space.
Shared walls: the walls between attached rowhouses and townhouses.
Ducts: leaky supply and return ducts running through attics, crawlspaces and basements.

Many of these are invisible from inside the living space. That is why the step 1 assessment uses a blower door and an infrared camera to find them.

Safety first: combustion appliances and ventilation

ENERGY STAR is direct about the risk: sealing your home or ductwork can cause combustion appliances like water heaters to backdraft, pulling exhaust into the house. It recommends inspection and testing by a home energy professional to find combustion-related safety problems, and says those should be corrected before any air sealing begins. It also notes that sealing can trap pollutants such as radon and carbon monoxide, and that a tight home may need additional mechanical ventilation to keep the air safe.

If you have a gas furnace, gas water heater, oil boiler or fireplace, ask the contractor to test it before and after sealing, and make sure you have working carbon monoxide alarms on every level.

What to do yourself and what to hire out

DOE says a do-it-yourself approach can work for obvious leaks around door frames and exterior wall penetrations for pipes, vents, electrical fixtures and wires.

Reasonable do-it-yourself work:

Caulking gaps where pipes, vents and wires pass through exterior walls.
Weatherstripping doors and windows that open.
Sealing and insulating the attic hatch.
Foam gaskets behind outlet and switch covers on exterior walls.

Leave to a professional:

Sealing around flues, chimneys and furnace or water heater vents, which needs fire-rated materials and clearances.
Finding and sealing attic leaks buried under existing insulation.
Duct sealing and duct leakage testing.
Any sealing in a house whose combustion appliances have not been safety tested.

Don’t forget the ducts

ENERGY STAR says that about 20 to 30 percent of the air that moves through a typical duct system is lost to leaks, holes and poorly connected ducts. If a ducted heat pump will reuse your existing ducts, ask for them to be tested and sealed as part of the job.

Several companies in this directory say they seal ducts or offer Aeroseal, including Mannix Heating and Cooling in Gaithersburg, Rowan Heating and Air Conditioning in the Columbia area, Winters Heating & Cooling in Leonardtown and Sledge Total Protection in Capitol Heights. The ductwork listings on any city page show more.

The BGE rebate rule

On BGE’s Home Performance rebate page, heating and cooling equipment, ENERGY STAR windows and doors and heat pump water heaters carry a footnote: the project must include attic air sealing and attic insulation to R-49. The same page notes that air sealing must be completed before insulation is installed. In other words, sealing is not an optional extra in that program. It is the condition for the equipment rebates. Customers of other Maryland utilities should check their own program rules.

Working with your contractor

Bring the assessment report to the first meeting and ask the contractor to walk through which leaks they will seal, with what materials, and in what order. Ask for photos of the attic floor and basement rim before and after the work, since most of it will be hidden once insulation goes in. If the assessor offered one, ask for a second blower door test after sealing, so you can see how much tighter the house got. That number is also useful to the heat pump installer, because a tighter house can mean a smaller system. Finally, ask who will test the gas or oil appliances after the work is done, and get that result in writing.

Questions homeowners ask

Isn’t insulation enough on its own?

No. Insulation slows heat moving through a surface, but it does not stop air moving through gaps. DOE compares insulation to a sweater and air sealing to a windbreaker: you need both, and the windbreaker goes on first.

Can sealing my house make it unsafe?

It can if gas or oil appliances are not checked first. ENERGY STAR warns that sealing can cause combustion appliances to backdraft, and recommends a professional find and correct those problems before any sealing. A tighter house may also need mechanical ventilation.

Do leaky ducts count as air leaks?

Yes. About 20 to 30 percent of the air moving through a typical duct system is lost to leaks, holes and poor connections. If your heat pump will use the existing ducts, sealing them belongs in the plan.

What comes next

Once the leaks are sealed, insulation can do its job. Continue to step 3: attic insulation.