Air Sealing andYour Heating Bill

Insulation slows heat loss. Air sealing stops it from leaving early.

Homeowners usually ask about insulation first, because R-value is the number printed on the package. Air leakage is harder to picture, which is probably why it is so often skipped, even though EPA and the U.S. Department of Energy have been publishing the same finding for years: a large share of a typical home's heating and cooling energy is lost to moving air, not conducted heat.

This page pulls together the published figures on how big that share is, what happens when it gets sealed, and how a Minnesota winter changes the math. As with the rest of this section, these are industry research figures, not measurements from a specific Trust Works job.

The 15 percent and 11 percent figures are ENERGY STAR's current modeled averages; the 20 percent figure is the earlier EPA ceiling estimate from 2009. Both describe air sealing and insulation done together, since ENERGY STAR's own methodology assumes a 25 percent reduction in air infiltration as part of the modeled improvement.

Sources: ENERGY STAR / U.S. EPA, Air Sealing: Building Envelope Improvements, ENERGY STAR, Methodology for Estimated Energy Savings, U.S. EPA, Seal and Insulate with ENERGY STAR (EPA 430-F-09-052, 2009)

The blower door: how researchers actually quantify a leak.

A blower door is a calibrated fan mounted in an exterior doorway that depressurizes a house and measures how much air is pulled in through every gap, crack, and penetration in the envelope. The result is reported as air changes per hour at 50 pascals of pressure, or ACH50. A lower number means a tighter house.

Oak Ridge National Laboratory used this method before, during, and after spray foam retrofits on a set of older East Tennessee homes as part of a deep energy retrofit study. The results are some of the more concrete before-and-after numbers publicly available on what air sealing actually changes in a real house, not a lab model.

Sources: ORNL/TM-2011/437, Spray Foam in Accessible Spaces (Christian & Gant, Oct. 2011)

A tighter house needs a plan for fresh air.

This is the honest part of the research that sales copy usually leaves out: once a house gets tight enough, mechanical ventilation matters more, not less. ORNL's report is explicit that a proper Building Performance Institute combustion-safety check should happen before and after sealing work, and that homes approaching the ASHRAE ventilation standard may need a fresh-air strategy rather than relying on incidental leakage.

It is also why we always recommend a blower-door test or an on-site combustion-safety check alongside any significant air-sealing project, rather than treating tightness as the only goal.

Sources: ORNL/TM-2011/437, Spray Foam in Accessible Spaces (Christian & Gant, Oct. 2011)

Why the stack effect makes this worse here.

Air leakage is driven by pressure differences, and one of the biggest is the stack effect: warm indoor air rises and escapes through upper-floor and attic leaks, pulling cold outside air in through the basement and rim joist to replace it. That effect gets stronger as the indoor-outdoor temperature gap widens, which is exactly what a Minnesota winter does for months at a time.

Savage sits in IECC Climate Zone 6A, a cold, heating-dominated zone, so the same leak that costs a mild-climate home a few dollars a month can cost a Minnesota home considerably more across a full heating season. That is the reasoning behind why we treat air sealing as a first step, not an upsell, on most winter comfort complaints.

Sources: U.S. DOE Building America, Climate Region Guide