Air Sealing Before Insulation: Why the Order Matters

ENERGY STAR sequences the two attic projects explicitly.

On its attic air sealing project page, EPA’s ENERGY STAR program recommends completing the attic insulation project after the air sealing project. Once insulation is piled on top, the leaks underneath are buried and far harder to reach. ENERGY STAR says sealing leaks and adding insulation together can provide up to a 10% saving on annual energy bills, and that combined they help prevent ice dams in winter.

The symptoms usually show up in the rooms below.

• Drafty rooms and uneven temperatures between rooms
• High heating or cooling bills
• Ice dams in winter
• Dry indoor air in winter
• Dust, especially in rooms directly below the attic

Most of the saving comes from a small number of large openings.

ENERGY STAR advises not to worry about every small gap and to plug the big holes first. Common locations are behind and under kneewalls, the attic hatch, wiring holes, plumbing penetrations, dropped soffits open to the attic, recessed lights, and the chases that hide ducts and flues.

Caulk suits gaps of a quarter inch or less, and expanding spray foam suits gaps from a quarter inch to three inches. Around furnace or water heater flues and chimneys, building codes usually require one inch of clearance from metal flues and two inches from masonry chimneys to anything combustible, including insulation. Those gaps are sealed with aluminum flashing and high-temperature caulk, not foam.

Some attic conditions have to be corrected before any sealing starts.

ENERGY STAR recommends a professional if you find difficult access, wet or damp insulation that suggests a roof leak, moldy or rotted rafters or joists, kitchen, bathroom, or dryer vents exhausting into the attic instead of outdoors, little or no attic ventilation, or knob-and-tube wiring from before about 1930.

Do not disturb vermiculite insulation — a light, pea-sized, flaky gray mineral — because it may contain asbestos. Have it tested and handled by a professional.

ENERGY STAR publishes recommended attic levels by climate zone.

R-value measures how well insulation resists heat flow; higher is better. The table gives the amount to add for existing wood-framed homes. ENERGY STAR also notes that many utilities offer rebates for professional assessments and for air sealing and insulation work.