Moisture load and airflow

Air Infiltration

Air infiltration is the unintended flow of outdoor air into a building, room or enclosure through cracks, leakage paths and normal use of exterior openings.

Aliases: infiltration air, air leakage into a space

QUANTITY, SYMBOL AND UNIT

Quantity / objectAir infiltration flow

SymbolQ_inf, ṁ_inf

SIm³/scubic metre per second

CommonL/slitre per second

Commonkg_da/skilogram of dry air per second

Definition

Air infiltration is the unintended flow of outdoor air into a building, room or enclosure through cracks, leakage paths and normal use of exterior openings.

It is driven by wind, indoor–outdoor density differences and pressure effects from fans or other air-moving systems. When outdoor humidity ratio is higher than the controlled space, infiltration adds moisture load.

Relations and distinctions

Infiltration and ventilation

Ventilation is intentional outdoor-air exchange for a defined purpose. Infiltration is uncontrolled entry through leakage and use of openings.

Infiltration and exfiltration

Infiltration enters the boundary; exfiltration leaves it. Both depend on the pressure distribution and leakage paths.

Engineering meaning

Include door operation, penetrations, envelope leakage and pressure regime in the moisture-load review.
Convert infiltration to dry-air mass flow before applying an indoor–outdoor humidity-ratio difference.

Common misconceptions

MisconceptionMaintaining positive pressure eliminates infiltration everywhere.

Accurate explanationPressure varies across surfaces and with weather, fan operation and door use; local inward flow can still occur.

Application boundaries

Infiltration is time-varying and cannot be represented reliably by a guessed constant in every operating mode.
An airflow value alone does not give moisture load without indoor and outdoor moisture states.

Supporting sources

ASHRAE / SRC-ASHRAE-F17-CH16

ASHRAE Handbook - Fundamentals, Chapter 16: Ventilation and Infiltration ↗

2017 SI edition / Chapter 16, terminology and driving mechanisms for ventilation and infiltration
Supports: Infiltration is unintentional outdoor-air entry through cracks, openings and normal exterior-door use; Wind, stack and mechanical pressure differences drive infiltration; Infiltration differs from intentional ventilation

Maintenance record