1. Chapter 6A: INFILTRATION BACKGROUND
Agami Reddy (rev Dec 2017)
Definitions
Energy implications
Infiltration rates across building stock
Typical air leakage locations and background leakage
Scientific background for estimating infiltration- orifice flow
Time averaged pressure difference due to wind velocity
- roofs and walls
- corrections for different locations and heights
7. Pressure difference due to stack effect
8. Pressure difference due to combined effects
HCB-3: Chap 6A Infiltration Background

2. HCB-3: Chap 6A Infiltration Background
Definitions
Three mechanisms that contribute to the total air exchange:
 Infiltration is the uncontrolled air flow rate through all the unintentional openings such as little cracks and gaps between different components (such as ill-fitting windows or doors). It is balanced by an equal mass flow called exfiltration, since mass must be conserved
Natural ventilation is the air flow rate induced by deliberate opening of windows or doors. It is a variable quantity depending on prevailing outdoor conditions and one cannot control it properly.
Mechanical or forced ventilation is the air flow rate intentionally drawn-in by mechanical ventilation such as fans. It can be controlled and varied as necessary.  
The term passive ventilation is also used, and applies to both infiltration and natural ventilation  
Fig. 6.1
HCB-3: Chap 6A Infiltration Background

3. HCB-3: Chap 6A Infiltration Background)
HCB-3: Chap 6A Infiltration Background

4. HCB-3: Chap 6A Infiltration Background
Mechanisms
The cause for infiltration is basically a pressure difference due to wind and stack effects and imbalance in HVAC system ventilation:
In the U.S., residential buildings typically rely on infiltration to meet ventilation needs (though this is changing)
In commercial and institutional buildings, infiltration may not be desirable from the view point of energy conservation and comfort. Hence, efforts are made to reduce it. However, it may be significant especially in tall buildings
HCB-3: Chap 6A Infiltration Background

5. Infiltration Rates across Building Stock
Number of air changes per hour (ACH): 1 ACH is equal to a flow rate equal to one volume of the house per hour
New construction:
0.3 – 0.7 ACH
Low-income:
0.5 – 2.0 ACH
Figure 6.2 Variability of air exchange for three types of house plotted as relative frequency of occurrence versus air change rate.
Figure 6.3
Histogram of infiltration values
HCB-3: Chap 6A Infiltration Background

6. HCB-3: Chap 6A Infiltration Background
Tight buildings
Figure 6.4
Air leakage performance of a tight and a moderately tight building:
tight building (1.5 ACH at 50 Pa)
moderately tight building (10 ACH at 50 Pa).
In Swedish homes, it is
standard practice to limit
infiltration to about 0.2 ACH
Moderately tight buildings
HCB-3: Chap 6A Infiltration Background

7. Air leakage locations and background leakage
Three sources of air leakage:
a) Component perforations –
easy to identify
(vents, stacks, chimneys)
b) Openings – easy to identify
(windows, doors,…)
c) Background or fabric leakage
- depends on construction
impossible to identify all cracks,..
- generally assumed to be
uniformly distributed over surface area of the building
Represented in terms of “leakage/unit area of envelope”
HCB-3: Chap 6A Infiltration Background

8. Scientific Background
Flow thru opening in building envelope treated as ORIFICE flow
for sharp edge orifices
In reality, flow thru building cracks is a combination of laminar and turbulent
and corrections have been proposed based on experimental evaluations
HCB-3: Chap 6A Infiltration Background

9. Scientific Background: Wind Effect-
Very complex due to rapid changes in
magnitude and direction
Figure 6.6
Wind pressure acting on a building
HCB-3: Chap 6A Infiltration Background
From Liddament, 1986

10. HCB-3: Chap 6A Infiltration Background
For Time Averaged Wind Pressure
Figure 6.7
Wind pressure plotted as pwind/Cp = (ρ/2)v2 versus wind speed v following Eq. 6.8.
HCB-3: Chap 6A Infiltration Background

11. HCB-3: Chap 6A Infiltration Background
Low Rise Buildings
a) Walls
Figure 6.8
Typical values of wall- averaged wind pressure coefficient Cp of Equation 6.10 for a rectangular low-rise building as a function of wind direction.
6.9
b) For roofs of low rise buildings inclined at less than 20o,
HCB-3: Chap 6A Infiltration Background

12. HCB-3: Chap 6A Infiltration Background
Tall Buildings
Three different aspect ratios of length L and width W
Figure 6.10
Average roof wind pressure coefficients for a tall building.
Figure 6.9
Wall-averaged wind pressure coefficients for a tall building.
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For Time Averaged Wind Pressure
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Effective Wind Speed
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Table 6.1. Atmospheric Boundary Layer Parameters for Use in Eq. 6.11
HCB-3: Chap 6A Infiltration Background

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17. Scientific Background: Stack Effect
Caused by temperature differences (and hence air densities) on the inside and outside
The opposite occurs
in summer
Figure 6.11 Air leakage due to stack effect during heating season.
HCB-3: Chap 6A Infiltration Background

18. HCB-3: Chap 6A Infiltration Background
6.12
Or alternatively
6.14
h- height from neutral pressure line
T is in absolute units
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19. HCB-3: Chap 6A Infiltration Background
Figure 6.13 Variation of pressure difference due to stack effect with vertical distance from neutral pressure line following Equation 6.15 assuming indoor air at 24°C (75°F) and draft coefficient of unity.
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6.15
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Figure 6.12 Pressure profiles due to compartmentalization pressure difference.
Building with no internal partition,
building with airtight separation of each story, and
real building with open shaft
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Eq. 6.12
HCB-3: Chap 6A Infiltration Background

23. Combining wind, stack and mechanical ventilation effects
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Figure 6.14 Superposition of the stack and wind pressures along the height of a building. (a) Stack pressures. (b) Wind pressures. (c) Combined effect.
HCB-3: Chap 6A Infiltration Background

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Outcomes
Familiarity with the three causes of pressure difference across building envelopes that cause infiltration
Understanding the energy implications of air infiltration
Familiarity with different pathways/locations and types of air leakage: component perforations, openings, and background or fabric
Familiarity with the scientific background of the orifice flow equation used to determine volumetric air flow due to pressure difference
Ability to calculate time-averaged wind pressure over walls and roofs using graphs and equations
Ability to correct standard measured wind speed for terrain and height
Understanding stack effect and be able to calculate it for buildings
Ability to calculate pressure difference across buildings due to combined wind and stack effects
HCB-3: Chap 6A Infiltration Background