1. Chapter 6: INFILTRATION
Agami Reddy (July 2016)
Definitions
Energy implications
Infiltration rates across building stock
Typical air leakage locations and background leakage
5. Scientific background for estimating infiltration
- Flow thru large and small orifices
- Pressure difference due to wind effect and stack effect
6. Empirical methods for air leakage
- Air change method (for residences and small commercial)
7. Basic LBNL model for one-zone buildings
8. Engineering model (residential-type doors and windows, closed swinging doors, open doors, curtain wall)
9. Multi-zone network models
10. Measuring air infiltration
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2. HCB-3: Chap 6 Infiltration
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  
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4. 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
Newer
Older
Older typical US homes: 0.5 – 2 ACH (seasonal averages)
Newer homes: 0.3 – 0.7 ACH
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Moderately tight buildings
Tight buildings
In Swedish homes, it is
standard practice to limit
infiltration to about 0.2 ACH
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Mechanisms
The cause for infiltration is basically a pressure difference due to wind, stack effect and mechanical HVAC system imbalances:
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 6 Infiltration

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 6 Infiltration

8. Scientific Background
Given in textbook: 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 6 Infiltration

9. Scientific background- Wind Effect- Very complex
HCB-3: Chap 6 Infiltration
From Liddament, 1986

10. HCB-3: Chap 6 Infiltration
For Time Averaged Wind Pressure
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For Time Averaged Wind Pressure
Plots to determine Cp are shown in next 2 slides
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Wind
Pressure
Coefficient
a) For roofs of tall buildings for three different aspect ratios of length L and width W
b) For roofs of low rise buildings inclined at less than 20o,
c) For walls of tall buildings for three different aspect ratios of length L and width W
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d) Walls of low
rise buildings
6.9
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Effective Wind Speed
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17. Scientific Background- Stack Effect
Caused by temperature differences (and hence air densities) on the inside and outside
Winter stack effect in tall buildings
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6.12
Or alternatively
6.14
h- height from neutral pressure line
T is in absolute units
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Variation of pressure
difference due to stack
effect with vertical distance
from neutral pressure line
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6.15
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Eq. 6.12
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22. Combining wind, stack and mechanical ventilation effects
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Superimposition of stack and wind pressures along height of a building
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Empirical Models
Air change method: for residences and small commercial
assumes that a portion of the air in the building is replaced with outdoor air which must be heated/cooled.
Number of air changes per hour (ACH): 1 ACH is equal to a flow rate equal to one volume of the house per hour
Range of ACH:
tight loose
Air infiltration volume = (ACH) x (room volume) / 60 min/hr
Tables in next slide allow determination of
ACH for summer and winter conditions
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Method
used by some
professionals
because of its
simplicity
HCB-3: Chap 6 Infiltration

27. Basic LBNL Model for Air leakage
Applicable for 1-zone small buildings WITHOUT mechanical ventilation:
6.25
Effective leakage area (ELA) is the equivalent amount of free open area of an
orifice that allows the same volume of air by infiltration as the actual building
(Eqn can be used for specific days as well as seasonal averages depending
on how the temperatures and wind velocity values are selected)
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Table 6.2
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6.25
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Engineering Models
Leakage Thru identifiable Components: Based on FIELD tests on Actual Buildings:
Residential windows and doors
Commercial swinging doors when closed
Opening of commercial swinging doors
2) Background leakage
Curtain walls for commercial buildings
Use
Table 6.2
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Background Leakage
6.20
HCB-3: Chap 6 Infiltration

35. IDENTIFIABLE Components
Residential Windows and doors
Figure 6.15
residential doors and
windows : n=0.65
k=1 is tight gap width
k=2 is average
k= 6 is loose
HCB-3: Chap 6 Infiltration

36. Infiltration for commercial-type swinging doors when CLOSED
because gaps
are larger
Figure 6.16
HCB-3: Chap 6 Infiltration

37. Infiltration through opening of doors due to traffic
Figure 6.17
Similar plots are available for revolving doors and automatic doors
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Curtain wall
Figure 6.18
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Multizone Models
Set up and solve
a set of simultaneous
non-linear equations
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Multizone Models
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Opening of Windows and Doors
Figure 7.2 Measured ventilation rates, as a function of wind speed, in a two-story house with windows open on lower floor
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Natural Ventilation Air Flow through Large Openings
Due to wind:
Eq. 6.28
If the openings are not of the same size,
the correction curve should be used:
Figure Increase in flow caused by excess area of one opening over the other
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46. Lab Testing for Airtightness of a Component
Controlled tests are done in the lab without
the influence of climatic parameters.
A static pressure difference (about 200 Pa)
is created across the test specimen
from which the ELA can be deduced.
Often results are presented in terms of
- Flow per hour per area or
- Flow per hour per unit crack length
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Blower Door Tests
Blower door tests are done to estimate aggregate envelope leakage and to locate and fix leaks:
Device consists of a door-insert
with rubber edge.
Variable speed fan and measurements for flow and pressure difference
Tests conducted till fairly high pressures (about 50 Pa) in 10 Pa incremental steps
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Presentation and Analysis of Test Data of Blower Door Tests
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50. HCB-3 Chap 4: Solar Radiation
Outcomes
Familiarity with the three causes of pressure difference across building envelopes resulting in 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 leakage
Familiarity with the scientific background for analyzing wind and stack effects and engineering methods of estimating the associated pressure difference
Be able to analyze situations involving wind and stack effects on buildings
Understanding of the ELA concept and be able to use it along with the LBNL model to solve simple problems
Be able to apply engineering models for estimating leakage through various types of envelope components
Familiarity with multi-zone modeling methods
Familiarity with lab testing of components and with the blower door test
HCB-3 Chap 4: Solar Radiation