11.2 Air Infiltration and Blower-Door IR
Key Takeaways
- Air leakage moves conditioned air through cracks and penetrations; IR reveals the resulting cold or warm streaks when a pressure difference drives flow across a temperature difference
- Combining a blower door with infrared multiplies contrast: depressurization draws cold outdoor air in during winter, making leak paths show as cool streaks on interior surfaces
- Heating-season interior air leaks often appear as irregular cold streaks at outlets, baseboards, windows, attic hatches, and rim joists—geometry differs from full-bay missing insulation
- Distinguishing air leak vs missing insulation uses pattern shape, response to pressure change, and whether the anomaly follows air paths versus cavity fill
- Document blower-door pressure (e.g., −50 Pa), indoor/outdoor temperatures, and wind; IR shows where leaks are, not a substitute for quantified airtightness metrics alone
Air leakage is often a larger energy and comfort problem than modest insulation gaps—and it is one of infrared’s best building applications when pressure and temperature differences cooperate. Level II thermographers must explain why a cold streak appears, how blower-door (or other induced-pressure) methods improve detection, and how to separate air movement from pure conduction through missing insulation.
Air Leakage Physics in One Page
Infiltration is outdoor air entering; exfiltration is indoor air leaving. Flow requires a pressure difference (stack effect, wind, HVAC imbalance, or a blower door) and a path (cracks, gaps, penetrations). Infrared sees the thermal consequence: cold outdoor air washing a warm interior surface (winter), or warm outdoor air heating a cool interior surface (summer), or interior air warming/cooling exterior surfaces at exfiltration sites.
| Ingredient | Role |
|---|---|
| ΔT (air and surfaces) | Temperature contrast of leaking air vs room/surface |
| ΔP (pressure) | Drives flow through leaks |
| Path geometry | Determines streak shape and location |
| Surface ε | Drywall/paint good; shiny metal trim needs care |
Without ΔT, leaking air may be nearly the same temperature as the room and invisible thermally even if the blower door measures huge CFM50. Without ΔP, many leaks barely flow and IR contrast collapses.
Heating-Season Interior Signatures (Natural and Induced)
Under winter interior conditions (indoors warmer):
| Leak type | Typical IR appearance |
|---|---|
| Electrical outlets / switches on exterior walls | Localized cool plumes or cool receptacle boxes |
| Baseboards and sole plates | Linear cool streaks along floor–wall joint |
| Window and door frames | Cool edges, corners, weatherstrip failures |
| Recessed lights / attic penetrations | Cool halos; sometimes wider plume under depressurization |
| Attic hatch / pull-down stairs | Strong cool perimeter if unsealed |
| Rim joist / band joist | Cool band; often mixed air + insulation defect |
| Plumbing and cable penetrations | Spot cool anomalies |
| Fireplace dampers / duct boots | Cool drafts at registers or chase |
Cold streaks follow air paths—often irregular, plume-like, or edge-concentrated—rather than filling an entire stud bay uniformly. Moving your hand or a smoke pencil (where safe) can correlate with the thermal plume; the camera documents the spatial map for the report.
Exfiltration on the exterior in winter can show warm streaks at eaves, ridge vents (context), and leaky joints where warm indoor air escapes—interpret with wind and stack-effect knowledge so you do not confuse exhaust vents with uncontrolled leakage.
Combining Blower Door with IR
A blower door is a calibrated fan sealing a doorway that pressurizes or depressurizes the building to a test pressure (commonly about 50 Pa relative to outdoors for airtightness metrics). For IR:
Depressurization (most common for interior winter IR)
- Building pressure set negative relative to outdoors (outdoor air pushed/pulled in through leaks).
- Cold outdoor air enters leak paths.
- Interior surfaces along those paths cool, increasing IR contrast.
- Thermographer walks interior (and sometimes attic) mapping streaks.
Pressurization
- Building pressure positive; indoor air driven out.
- Useful for exterior IR of warm exfiltration in winter, or for specific duct/chase studies.
- Interior winter leak contrast for cold outdoor air is generally weaker than with depressurization because outdoor air is not being sucked in as strongly through the same paths.
| Mode (winter) | IR emphasis |
|---|---|
| Depressurize + interior IR | Cold infiltration streaks—primary teaching case |
| Pressurize + exterior IR | Warm exfiltration locations |
| Natural ΔP only | Weaker, weather-dependent; still useful on windy/stack days |
Level II documentation should include approximate building pressure (or at least “blower door on, depressurized”), indoor and outdoor temperatures, wind, and whether HVAC was operating. Exact airtightness results (ACH50, CFM50) come from the blower-door instrumentation; IR locates leaks for sealing priority.
Practical sequence
- Prepare building per energy-audit protocol (as-found vs prepared state—follow the governing test standard/client scope).
- Establish baseline IR images before the fan if you want a before/after contrast lesson for the client.
- Bring building to target pressure; allow a short period for surfaces to respond (thin finishes respond faster than mass walls).
- Systematic room-by-room imaging; mark priorities (large cold plumes first).
- Optional: seal major leaks temporarily and re-image to prove reduction.
- Restore building; report thermal map + quantitative blower-door metrics from the tester.
Distinguishing Air Leak vs Missing Insulation
This separation is a core exam and field skill.
| Clue | Air leakage lean | Missing insulation lean |
|---|---|---|
| Geometry | Streaks, plumes, edges, penetrations | Area fill of stud/joist bays |
| Pressure change | Pattern intensifies or appears with blower door | Conductive pattern relatively stable with ΔP (same ΔT) |
| Outlets/windows | Strong local effects | Not required for a full-bay void |
| Between studs | May cool surfaces if air washes the drywall cavity face | Uniform cool bay if empty of insulation |
| Attic wind washing | Cool ceiling patterns that shift with wind/pressure | Settled insulation voids more static |
| Feel / smoke / theatrical fog | Draft correlates | Little draft if pure conduction |
Hybrid reality: rim joists, attic kneewalls, and drop ceilings often have both missing insulation and air leakage. Report both mechanisms when evidence supports them. Depressurization that dramatically expands a cool zone around a penetration points to air; a cool bay that barely changes with fan-on/fan-off but sits between warm insulated bays points to insulation fill.
Fan-on / fan-off test (teaching method)
When safe and time allows:
- Image suspect area at natural pressure.
- Depressurize; re-image same scene within minutes.
- New or intensified cool streaks → air-driven component.
- Unchanged cool bay with stable geometry → more likely insulation/conductive (still verify).
Massive thermal mass and solar-loaded walls can still confuse short tests—use judgment.
Cooling Season and Reverse Patterns
In summer with air conditioning, infiltration of hot outdoor air can create warm interior streaks at leaks under appropriate pressure. Conductive heat gain through missing insulation also warms interior surfaces. Pattern geometry and pressure response still separate mechanisms better than color alone. Always write “cooling season, interior, building depressurized to …” style context in notes.
HVAC, Ducts, and Pressure Imbalance
Forced-air systems create zone pressures. A house with leaky return ducts in a vented crawlspace can pull outdoor air through envelope leaks even without a blower door. IR may show cool baseboards whenever the air handler runs. Level II should note HVAC state and not assume every leak is “only envelope construction.” Duct leakage surveys (pressure pan, duct blaster) pair with IR for complete diagnostics.
Emissivity, Reflections, and False Leaks
| Artifact | Confusion |
|---|---|
| Cold window glass reflections | “Leak” on opposite wall |
| Metal blinds and shiny frames | Wrong absolute T; pattern still usable carefully |
| Recently opened exterior door | Transient cold plume—wait and recheck |
| Wet surfaces / evaporative cooling | Looks cool without air leak |
| Thermal bridging at corners | Cool without major airflow |
Change viewing angle, use matte tape if needed, and correlate with known penetrations.
Safety and Professional Boundaries
- Guard fan pressure on fragile chimneys, fireplaces, and some older buildings per blower-door best practice.
- Do not claim quantitative ACH50 from IR images alone.
- Coordinate with the energy auditor / testing technician when dual-certified roles are split.
- Respect occupant privacy and secure outdoor conditions (wind limits for meaningful airtightness tests).
Common Traps
| Trap | Correction |
|---|---|
| “All cool interior spots are missing insulation” | Many are air leaks or bridges |
| IR without ΔT or ΔP | Low sensitivity—state limitations |
| Ignoring blower-door pressure in the report | Document test pressure and mode |
| Exterior shiny metal as absolute leak temperature | Qualitative / high-ε reference |
| Sealing recommendations without locating paths | IR’s value is spatial prioritization |
Summary for Recall
Air-infiltration IR depends on temperature difference plus pressure-driven flow. In winter interiors, leaks appear as cold streaks and plumes at edges and penetrations. Blower-door depressurization pulls cold outdoor air inward and amplifies those patterns for mapping. Distinguish leaks from missing insulation by geometry and pressure response (fan-on/fan-off), and always document HVAC state, ΔT, and test pressure. IR locates; blower-door instrumentation quantifies whole-building airtightness.
Why does combining a blower door (building depressurization) with interior IR in winter improve detection of air leaks?
Which pattern is more characteristic of air leakage than of a full missing-insulation bay during a winter interior survey?
A Level II thermographer images a cool interior wall bay before the blower door runs and again at about −50 Pa. The cool bay geometry is unchanged, but a new cool plume appears at a cable penetration. Best interpretation?
What can IR alone not replace when a blower-door test is performed for airtightness contracting?