Sourcing Interior Air Leakage

Thought I would share this with those folks who perform thermography services with home inspections, especially our up-and-comers like Paul and Frank. I can never tell if the answer is going to be obvious to everyone, so I apologize if this turns out to be a no-brainer for you.

Here are two recent examples of air leakage at interior walls of new construction homes (blower door tested homes).

Both examples are interior 1st floor walls (no exterior exposure on either side) of 2 story homes. The houses are under slight negative pressure. Interior temperature has been stable for some time so we should be near thermal equilibrium. Pretty narrow temperature span on the images

In the first example outside is cool, interior is warm. Air leakage through switch boxes installed in the sheetrock.

In the second example outside is warm, interior is cool. This is a laundry room which backs up to staircase with the under-stair area fully sheetrock enclosed (no access). The first image shows the location of the studs as cool compared to the sheetrock (would you expect this on an interior wall?), the second image shows air leakage around a metal clean out cover for the washer drain.

Where might the air leakage be coming from? What would you look for? I have photos of the answers which I’ll share. Do we care about the air leakage when the temperature difference is so small?








No…I would call it unremarkable air exchange.

These are slab foundation homes with unconditioned air leaking into the center of the home. Where is it coming from and why?

Unsealed holes in the attic.


If the air leakage isn’t coming from the sides or below, it’s coming from the attic. If it’s leaking out at the first floor, it’s going to be coming through a chase.

In the first example, the chimney fire-stopping was not sealed in the attic so the air was entering through the chimney chase (you can see the fireplace in the first visible light image above).

In the second example there is simply an open chase adjacent to and above the staircase. You can see the plumbing vent from the clean-out image going right through the chase.

The builders acknowledged and corrected both issues. So while these thermal images would be unremarkable on exterior walls, on interior walls they usually indicate an issue in the attic that’s worth tracking down.

Maybe this one was too basic?



only basic for those who fully understand some of the past & current building practices & construction in general

after 2 replies i wouldn’t be so quick to use a roller where a cut-in brush is required

many may be in the wings unsure or afraid to reply due to ego

keep up the lessons & many will absorb sage wisdom…without any expected accolades ;~))

Agree keep them coming and never to basic. Sometimes its the basic ones that get ya. Thanks Chuck

If people truly knew what needed to be done to their homes to keep down the leaks. Most have no clue.

I just ordered a spray foam kit for my house. I plan on spending a day in the attic foaming knee walls, can lights and other holes next weekend.

I figured I would wait to insulate after my ductwork was replaced. Like I suspected, most of my existing insulation got trampled to death.

Mechanical chases and venting I suspect.

A home is under pressure. Add shear and up goes the points of entry.

1: The envelope under the most pressure.
2: The void in the two part system, the mid assembly is under middle pressure.
3: The conditioned space under the lowest pressure.

What type of venting / HVAC.

NOTE: Sealing homes too tight can have a devastating effect on air quality.

Thanks Chuck.

Yes, thank you.
Always a good reminder.