Customer Reports Drafts After New Window Install: Infiltration Test vs. Expectation Decision Tree

Why this matters

"Drafty after the new windows" is one of the most common callbacks in window replacement work, and most of the time the window is not the source. If you arrive and immediately check the window frame perimeter without considering the other two causes - radiant cooling effect and nearby wall penetrations - you will either find nothing and confuse the customer, or you will add unnecessary sealant to a window that is already performing correctly. This decision tree gives you a fast, low-tech sequence to reach the right diagnosis in a single visit.

Step 1: Clarify what "drafty" means to this customer

Before any physical inspection, ask two questions:

1. "Can you show me exactly where you feel the draft, and can you feel air moving if you hold your hand still near that spot?"
2. "Does it feel cooler near the window, or is there actual moving air?"

These two questions separate the three causes:

• Moving air that the customer can feel with a still hand = possible infiltration. Proceed to the infiltration test.
• General coolness near the window with no detectable air movement = likely radiant cooling. Proceed to Step 4.
• Moving air felt near a wall area that is not the window frame = nearby penetration. Proceed to Step 3.

Step 2: Conduct a simple field infiltration test

Do this on a cold, windy day if possible. Calm warm days cannot drive enough pressure differential to reveal small leaks.

Materials needed: one stick of incense or a single tissue.

Procedure:

1. Close all windows and doors in the room. Turn off the HVAC fan if accessible.
2. Hold the lit incense or the tissue edge within one inch of the window perimeter - the frame-to-drywall joint, the sash edges, and the corners of the sash.
3. Watch for smoke or tissue deflection. True infiltration will deflect the smoke or tissue consistently, not just once.
4. Move methodically around the full perimeter before concluding. A single bad corner can be the only infiltration point.

If the test shows no deflection at the window, the window is not the infiltration source. Move to Step 3. If deflection is present, identify the specific location and go to the Window Infiltration Repair section.

Step 3: Inspect for nearby wall penetrations

Window replacement work disturbs the wall around the rough opening. Existing gaps that were previously covered by old trim are exposed, and re-trimming can leave new gaps. Also, workers moving around the exterior sometimes displace existing sealant at unrelated penetrations (electrical conduit entries, pipe chases, cable lines) near the work area.

Check the following with the incense or tissue:

• The joint between the new interior trim (casing) and the drywall face. This joint is often left unsupported or with a gap behind the casing.
• Any electrical outlets or switch plates on the same wall as the window, or on the exterior wall within 24 inches of the rough opening.
• The baseboard-to-floor joint on the exterior wall below the window, where cold air can enter from the rim joist area and travel along the bottom plate.
• Any pipe, conduit, or cable penetrations on the same exterior wall.

If infiltration is found at one of these locations, the repair is at that penetration - not at the window. Seal with appropriate materials based on the gap size. This is also an opportunity to explain to the customer that window replacement sometimes reveals pre-existing envelope gaps that were not apparent before.

Step 4: Explain radiant cooling when no air movement exists

If no air movement is detected anywhere near the window, the customer is experiencing radiant cooling. This is not a defect and is not correctable with more insulation or better windows - it is physics.

What to say to the customer:

Glass surfaces are colder than the surrounding walls in winter. The human body radiates heat toward any surface colder than skin temperature. This creates a sensation of being cold near windows even when there is no air movement. It is the same reason sitting near a single-pane window in winter feels colder than standing in the center of the room. The new windows are performing correctly - better insulated glass reduces this effect compared to older windows, but does not eliminate it entirely at close range.

You can demonstrate this: use a non-contact infrared thermometer to show the customer the surface temperature of the glass compared to the wall. If the glass is 15 to 20 degrees colder than the wall surface, some radiant cooling sensation is expected and normal.

The customer's options are behavioral - not sitting directly adjacent to the window in cold weather, using insulating interior cellular shades, or adding a thermal curtain for nighttime use.

Window infiltration repair (when Step 2 confirms a leak)

If the incense test confirms infiltration at the window itself:

• Frame-to-rough-opening gap (between frame and drywall visible behind the trim): the gap should be filled with low-expansion foam or backer rod plus sealant. Over-expansion foam can distort the frame. If foam was already applied but is gapped or shrunken, remove loose material and re-apply.
• Sash edge infiltration (moving air along the sash perimeter when closed): the weatherstrip may be compressed or misaligned. Close the sash firmly and inspect the weatherstrip contact around the full perimeter. Adjust or replace weatherstrip at the failing location.
• Corner of the sash: a corner gap in the weatherstrip can leak even when the rest of the perimeter seals correctly. This is usually a weatherstrip join defect and the corner piece needs replacement.

Decision summary

References

• ASTM E283 standard test method for determining rate of air leakage through exterior windows, curtain walls, and doors
• Building Performance Institute (BPI) field diagnostic protocols for air infiltration in residential envelopes
• Standard guidance on radiant thermal comfort and glass surface temperature effects in cold-weather occupied spaces