How to Find Air Leaks While the House Is Depressurized

Why this matters

A blower door number tells you how leaky a house is. It does not tell you where. The only time the leaks announce themselves is while the fan is holding the house negative, and that window is usually the forty-five minutes the rater is on site. A crew that walks the house during that window with a plan comes away with a list a sealing crew can work from. A crew that wanders comes away with "the attic felt drafty," and the sealing crew spends a day foaming the obvious while the leak that mattered stays open.

The setup that makes the house ready for this belongs to How to Prepare a House for a Blower Door Test. This card is the search itself, and the record it produces.

What depressurization does for you

With the blower door pulling the house to about 50 pascals below outdoors, every hole in the air barrier becomes an inlet. Outdoor or attic air streams in through each one, at a speed you can feel on the back of a wet hand at the larger leaks. In heating or cooling season that incoming air is a different temperature from the room, so it also chills or warms the surfaces it washes over, which is what a thermal camera sees.

Some raters drop to a lower pressure for the search, often in the 20 to 30 pascal range, to keep the house quieter and to keep the doorway fan from overheating on a long walk. The leaks are still there, just slower. Ask the rater what pressure they will hold and for how long, and plan your route to fit it.

Three tools, used in this order of cost:

• The back of your hand, lightly dampened. Fastest, free, and good for any leak bigger than a pencil. Poor for small, diffuse leakage.
• A smoke pencil or puffer. Shows direction and speed at a specific spot. Hold it an inch or two from the suspect gap and watch: smoke pulled straight into the gap is a leak, smoke drifting lazily is not. Many smoke tubes release an acid irritant, so keep it away from your face and out of a closed closet, and do not use it near a smoke alarm you have not warned the occupants about.
• A thermal camera. Finds leaks you cannot reach and patterns you would never feel, such as a whole ceiling plane cooling from a missing top-plate seal. It needs a meaningful indoor-outdoor temperature difference to show anything; a difference of about 18 F (10 C) or more is the commonly cited working minimum. On a mild day it shows little. How to Use a Thermal Image as Evidence Rather Than a Picture covers taking images that hold up.

Steps, in the order that finds the most first

1. Before the fan starts, take a baseline image of each ceiling and exterior wall you plan to check. A thermal pattern seen only under depressurization proves air movement. A pattern that was already there at neutral pressure is something else, such as missing insulation or a framing member. Skip the baseline and you will log a wet stud or an insulation void as an air leak, and the sealing crew will foam a cavity that was never leaking.

2. Start at the attic hatch and the top floor ceiling. Warm air rises and the top of the house is where stack effect pushes hardest on a normal winter day, so ceiling leaks cost the most energy and carry the most indoor moisture into the attic. The hatch itself, its weatherstrip and its frame-to-drywall joint, is the single most common large leak. Then ceiling light boxes, the bath fan housing, and the line where the ceiling meets the exterior wall.

3. Go into the attic only if it is safe to, and look down at the top plates. Walk on joists or a laid board, never on drywall between them. Watch for roofing nails coming through the deck at head height. Wear a filtering facepiece against insulation fiber, eye protection, and long sleeves; if your shop requires respirator use, it runs under a written program per 29 CFR 1910.134. If the loose fill is vermiculite, granular and gray-brown or gold, do not walk in it or dig in it, because it may contain asbestos; note it and come back down. In summer, an attic can exceed 130 F by early afternoon, so go early or keep the visit short with water in hand. Look for dark, dirty patches on the insulation surface above top plates and around penetrations: that is filtered dust, a picture of years of air passing through.

4. Check every outlet and switch on an exterior wall, covers on. Do not take cover plates off live devices for this. Feel and smoke at the edges of the plate and at the device openings. A strong draft at an outlet on an interior wall usually means the wall cavity is open to the attic or the basement, which is a bigger finding than the outlet itself.

5. Go to the basement or crawlspace and walk the rim joist. The band where the floor framing sits on the foundation is often the leakiest line in a basement: the sill plate on concrete, the rim board seams, and every pipe, wire, dryer vent and hose bib through it. A thermal camera shows the rim joist bay by bay. Use a flashlight and look for daylight at the sill.

6. Check the plumbing and wiring penetrations into the floor and through the top plates. Under sinks and tubs, behind the water heater, where the main panel's cables go up into the wall. A plumbing wall's open drain chase can connect the basement straight to the attic.

7. Check recessed lights on the top floor, and any kneewall door. A recessed fixture that is not rated airtight is a ring of open space around a hot can, and a kneewall door is often an unsealed panel between a bedroom and a cold attic triangle. Do not touch the can trim of a light that has been on, and switch the fixtures off at the wall before you smoke them; they run hot.

8. Log each leak as you find it, by location, before moving on. Not at the end, from memory. A list written at the truck afterward always loses the third and fourth outlets.

The log the sealing crew works from

The log is the deliverable. Here is one from a real-shape walk on a two-story house, with the rater holding 25 pascals for the search.

LEAK LOG - JOB 5120     HELD AT -25 Pa    OUTDOOR 31 F
#  LOCATION                    METHOD  SIZE   WORK
1  Attic hatch, hall ceiling   hand    LARGE  wstrip+frame
2  Top plate over 2nd fl bath  IR      LARGE  seal, attic
3  Plumbing chase, bath wall   smoke   LARGE  rigid cap
4  Bath fan housing, 2nd fl    smoke   MED    housing seal
5  Recessed can x4, bonus rm   IR      MED    can covers
6  Kneewall door, bedroom 3    hand    MED    wstrip+latch
7  Rim joist, north wall       IR      MED    sill+rim
8  Outlets x3, west wall LR    smoke   SMALL  gaskets
9  Dryer vent sleeve, rim      smoke   SMALL  seal sleeve
LARGE = felt by hand at 25 Pa  MED = smoke pulled fast
SMALL = smoke pulled, no draft felt     Baseline IR: yes

Why the size column uses observed cues, not numbers. You cannot measure an individual leak's airflow with a hand or a camera, and writing "40 cfm" next to a crack is invention. What you can record honestly is how it behaved at the pressure held. A leak you can feel at 25 pascals is a large one; a leak only the smoke finds is small. That ranking is enough for the crew to work from the top of the list down.

Why the work column is there. The finder knows what they saw; the sealer will not have seen it. "Plumbing chase, bath wall" means nothing to someone standing in the attic unless the log says the fix is a rigid cap from above. Writing the likely fix also forces the finder to say whether the leak is reached from inside, the attic or the basement, which is how the crew plans its day.

Reading this log. Three of the nine are large, and all three cluster around the second-floor bath: items 2 and 3 above the bath itself, item 1 in the hall ceiling beside it, all within about ten feet. That cluster is the story: the bath's plumbing wall was built open to the attic, and the hatch is next to it. If the crew seals only items 1 through 3 and the rater re-tests, the number moves the most for the least labor. Items 8 and 9 are real but small, so they wait until the crew is already in the room for something else.

What would change the priorities. In a cooling climate, where summer is the season that matters, the crawlspace or basement leaks would move up, because humid crawlspace or basement air drawn into the living space by duct leakage and exhaust fans is often the moisture load. On a home with a moisture complaint at a specific wall, the leak nearest the complaint goes first regardless of size.

How this goes wrong

Logging conduction as air leakage. A cold stripe at every stud on an exterior wall is the studs conducting heat, and it shows the same with the fan off. Only the baseline image separates the two. Patterns that grow or appear when the fan starts are air; patterns that do not change are not.

Chasing a leak into the wrong space. A draft at a second-floor outlet may be coming from the attic, the floor cavity below, or the garage. Before logging "outlet," check the attic above that wall and the band joist below it. Logging the symptom sends the crew to gasket an outlet while the cavity stays open.

Running out of fan time. The rater's test runs on their schedule, not yours. Walk the large-leak locations first (steps 2, 3 and 5). If the fan stops at step 6, you have the expensive findings.

Smoking the wrong direction. Under depressurization, smoke moves into the leak. If the house is being pressurized (a rater may do that when vermiculite is present), smoke moves away from it and out through the gap. Ask which way the fan is running before you read any smoke.

How to know the walk was complete

Every room with an exterior wall, ceiling or floor appears on the route at least once. The attic hatch, the top plates above every plumbing wall, the rim joist on all sides and every recessed light on the top floor each have a line in the log, even if that line reads "checked, no leak." A log with only findings cannot tell the sealing crew whether the north rim joist was clean or simply never checked. After the crew seals, the re-test should drop CFM50; if it barely moves, the large leaks were not the ones logged, and the next walk starts at the places the first one recorded as clean.

References

• See related: How to Use a Thermal Image as Evidence Rather Than a Picture
• See related: How to Prepare a House for a Blower Door Test; Blower Door Numbers Explained for Other Trades
• See related: How to Air Seal Penetrations Into the Attic; How to Air Seal a Basement Rim Joist
• See related: How to Use a Smoke Pencil to Read Air Direction
• OSHA 29 CFR 1910.134, respiratory protection, where your shop's work requires a respirator