Identifying Leaks In Building Envelopes & Why It Matters

November 5, 2025|Southface Staff
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Overview

Building Envelopes & Leakage

When we say “envelope,” we aren’t talking about the kind that letters go into. We’re referring to the physical components of a building that separate conditioned and unconditioned spaces — such as roofs, walls, windows, doors, and foundations.

Air, heat, and moisture can often leak in or out of this envelope in unintended ways, significantly impacting comfort, energy use, utility expenses, and even occupant safety. One of the first questions to ask when assessing a building envelope is: How much is the building leaking?

To answer this question, you’ll need to run a blower door test. A blower door test is a diagnostic tool that uses a specialized fan temporarily installed in an exterior door or window frame to measure how much air is entering or escaping through the building envelope (see image below for an example). The test provides a reading of the total air leakage in the space you’re assessing.

Blower door testing at the Octagon Museum in D.C. [Photo Credit: Southface Staff]

Let’s say your blower door test reveals significant leaks that you want to seal. [Southface Blower Door Test Video] The next step is to determine exactly where your building is leaking.

Testing for chimney leaks by injecting fog in fireplaces at D.C.’s Octagon Museum [Photo Credit: Southface Staff]

Identifying Leaks in the Envelope

There are a few common culprits when it comes to air leakage across building envelopes. These include:

  • Missing weather stripping on exterior doors
  • Open plumbing holes behind sinks or shower heads
  • Any other penetration in the building envelope that leads to an unconditioned space or an adjoining unit not part of the envelope being tested

If left unsealed, these openings can cause air infiltration — one of the biggest contributors to energy loss.

Photo Credit: Southface Staff

According to ENERGY STAR, the gaps, leaks, and holes in the average U.S. home are equivalent to leaving a window open 24/7.1 As shown above, even a single unsealed penetration can have a major impact – clearly visible in the color changes seen through thermal imaging at these points.

You wouldn’t want your heat escaping into the attic on a cold night or your cool air “walking” out the front door on a hot summer day just because of a little missing caulk or dry-rotted weather stripping — would you? Seal those penetrations, replace that weather stripping, and enjoy the peace of mind that your home will stay comfortable year-round. Your wallet will thank you too!

  1. https://www.energystar.gov/saveathome/seal_insulate/why-seal-and-insulate ↩︎