The infiltration rate is the volumetric flow rate of outside air into a building, typically in cubic feet per minute (CFM) or liters per second (LPS). The air exchange rate, (I), is the number of interior volume air changes that occur per hour, and has units of 1/h.
General
- Below Grade or Interior Spaces——No infiltration losses or gains are taken for rooms located below grade or interior spaces.
- Buildings that are not humidified have no latent infiltration heating load.
- Winter sensible infiltration loads will generally be 1/2 to 3 times the conduction heat losses (average 1.0 to 2.0 times).
Heating Infiltration (15-mph wind)
Air Change Rate Method
- Range 0 to 10 AC/hr.
- Commercial buildings:
- 1.0 AC/hr. one exterior wall
- 1.5 AC/hr. two exterior walls
- 2.0 AC/hr. three or four exterior walls
- Vestibules 3.0 AC/hr.
CFM/sq.ft. of Wall Method
Range 0 to 1.0 CFM/sq.ft.
Tight buildings 0.1 CFM/sq.ft.
Average buildings 0.3 CFM/sq.ft.
Leaky building 0.6 CFM/sq.ft.
Crack Method
Range 0.12 to 2.8 CFM/ft. of crack
Average 1.0 CFM/ft. of crack
Cooling Infiltration (7.5-mph wind)
Cooling load infiltration is generally ignored unless close tolerances in temperature and humidity control are required.
Cooling infiltration values are generally taken as 1/2 of the values listed earlier for heating infiltration.
FREQUENTLY ASKED QUESTIONS
No infiltration losses or gains are taken into account for rooms located below grade or interior spaces, as these areas are not directly exposed to outside air. This assumption is based on the fact that air exchange between these spaces and the outdoors is negligible.
Winter sensible infiltration loads will generally be 1.2 to 3 times the conduction heat losses, with an average of 1.0 to 2.0 times. This means that infiltration loads can be a significant contributor to heating loads in buildings, especially in cold climates.
The 15 mph wind speed is a common reference point for estimating infiltration rates in buildings. However, actual wind speeds can vary significantly depending on location and building orientation. As wind speed increases, infiltration rates also increase, leading to greater heat losses and energy consumption.
The Air Change Rate Method estimates infiltration rates based on the number of air changes per hour (AC/hr). For commercial buildings, the range is 0 to 10 AC/hr, with higher rates corresponding to more exterior walls. For example, a building with one exterior wall might have an air change rate of 1.0 AC/hr, while a building with three or four exterior walls might have a rate of 2.0 AC/hr. Vestibules typically have a higher air change rate of 3.0 AC/hr.
The CFM/sq ft of Wall Method estimates infiltration rates based on the flow rate of outside air per unit area of exterior wall. This method provides a more detailed estimate of infiltration rates, taking into account the building’s envelope characteristics. The range for this method is 0 to 1 CFM/sq ft of wall.
Yes, infiltration rates can be used to estimate energy consumption in buildings. By calculating the infiltration rate and air exchange rate, building designers and engineers can estimate the heating and cooling loads required to maintain a comfortable indoor environment. This information can be used to optimize building design, HVAC system sizing, and energy efficiency strategies.
