Appendix 3.A8: Thermal properties of typical constructions
4.7.2 Empirical data for air infiltration
Empirical data may be of use to provide basic guidance on possible infiltration rates that may be expected in buildings of typical construction in normal use in winter and under average annual conditions (perhaps to estimate allowances for cooling load). The data presented in this section are summarised from sample calculations using the calculation principles outlined in the previous sections.
4.7.2.1 Method 1: Empirical data based on air change rate at 50 Pa pressure test results
An estimate of the infiltration rate averaged over a year of weather data is sometimes inferred from the airtightness value given in air changes per hour at a reference pressure of 50 Pa. This approach was first suggested by Kronvall(29) who analysed data, principally of dwellings, to derive a
‘divide by 20’ rule. In other words, the estimated annual average ventilation correlates with the pressure test value at 50 Pa divided by 20. This rule was extended by Dubrul(30) to consider a wider range of building sizes, climate and exposure. This showed that for high rise buildings and buildings exposed to high winds, the ACH
value needed to be divided by a value as low as 10, whereas50
for sheltered and low rise buildings, the divisor could approach a value of 30.
Table 4.11 Example 4.3: calculation of combined wind and stack pressures
Opening Open country location Industrial (urban) location
Wind Stack Combined Wind Stack Combined
pressure pressure pressure pressure pressure pressure Facade 1:
— upper opening +3.8 –2.6 +1.2 +1.0 –2.6 –1.6
— lower opening +3.8 +2.6 +5.0 +1.0 +2.6 +3.6
Facade 2:
— upper opening –1.9 –2.6 –0.7 –1.3 –2.6 –3.9
— lower opening –1.9 +2.6 +4.5 –1.3 +2.6 +1.3
Figure 4.10 gives estimates of the divisor based on calcula- tions using a full year of CIBSE hourly average weather data for London (Heathrow) for a selection of building sizes and exposures. This must, however, be regarded as very approximate although, nevertheless, it is consistent with the previously cited observations.
4.7.2.2 Method 2: Tabular values
The data in Tables 4.13 to 4.21 provide a very approximate estimate of the contribution that air infiltration rate makes to the overall air change rate of a building. They are based on applying the calculation technique described in section 4.7.4.1. Air permeability values as defined by Part L of the Building Regulations(11,12) are used. The definition of air permeability is given in section 4.8.1. The values are:
— 20 m3/(m2ãh) at 50 Pa (this represents an existing
‘leaky’ building that does not comply with current regulations)
— 10 m3/(m2ãh) at 50 Pa (this represents a building that complies with 2002 regulations)
— 7 m3/(m2ãh) at 50 Pa (a moderately tight building that complies with 2005 regulations)
— 5 m3/(m2ãh) at 50 Pa (a tight building)
— 3 m3/(m2ãh) at 50 Pa (a very tight building).
For non-residential buildings, air leakage values (and hence calculations) are based on the single pressure testing of the entire closed space as described in TM23(5)(i.e. all internal doors are open). Air leakage values for residential buildings (and hence calculations) are based on individual testing of each dwelling.
These data should be used for guidance only.
The data presented in Tables 4.13 to 4.21 have the flowing format:
(a) ‘Peak’: infiltration rate for a winter design condition of approximately:
— outdoor temperature = –5 ºC
— indoor temperature =21 ºC
— windspeed = 4 mãs–1 Applications:
— estimation of peak infiltration heat loss for sizing applications.
(b) ‘Average’: annual average infiltration rate based on:
— CIBSE hourly 20-year average wind and temperature data.
Applications:
— estimation of annual average infiltration loss for heat loss calculations
Table 4.12 Summary of estimation methods
Method Application Data requirements Advantages Disadvantages
Empirical data Air infiltration rate Building type, Easy to use Does not provide detailed
assessment when little height and exposure predictions
is known about the airflow characteristics of the building
Standard formulae Estimation of natural Design wind speed(s) and Easy to use Limited to narrow range of ventilation rates for simple temperatures, wind pressure building configurations buildings with openings coefficients, required
on opposite sides or on one ventilation rates or
side only opening areas
Theoretical calculation:
— single zone model Estimation of internal Design wind speed(s) and Relatively easy to use, Requires detailed knowledge airflows for buildings with temperatures,wind pressure predicts magnitude and of the building
simple internal layouts coefficients, location, size direction of airflow, and flow characteristics calculates internal pressure, of each opening to outside, changes may be easily airflow paths, building accommodated volume
— multi-zone model Estimation of internal As above plus internal As above plus internal Requires extensive input airflows for complex configuration and its pressure distribution data and considerable
buildings with known airflow characteristics computational effort
characteristics
Divisor
25 20 15 10 5 0
50 000 40 000
30 000 20 000
10 000 0
Volume of enclosed space / m3 Figure 4.10 Approximate ACH
50divisor to obtain average annual infiltration rate
— estimation of contribution of infiltration air to fulfiling ventilation needs
— estimation of infiltration contribution to summer cooling potential or infiltration heat load.
(c) ‘Air change at 50 Pa’: the equivalent air change (for the particular building dimensions used) given the 2002 Part L(11,12)leakage value of 10 m3/(m2ãh) at 50 Pa.
(d) ‘ACH
50 divisor’ (see also Figure 4.10): the value obtained when dividing the 50 Pa air change rate (determined from the 2002 Part L(11,12)value) by the calculated average annual infiltration rate.
Notes:
— On severely exposed sites, a 50% increase above the tabulated values should be allowed. On sheltered sites, the infiltration rate may be reduced by 33%.
— The air change rate in rooms in tall buildings may be significantly higher than the values given in Tables 4.13 to 4.21. The design of tall buildings should include barriers against vertical air movement through stairwells and shafts to minimise the stack effect. If this is not done, the balance of internal temperatures can be seriously disturbed.
— Allowance must be made in the sizing of heating or cooling plant to meet the needs of both the ventilation air and air infiltration. Where ventila- tion is achieved by mechanical supply only (or extract only) systems, infiltration (or ex-filtration) is partially inhibited and converted to ventilation airflow. In this case, the plant sizing may be based on the ventilation rate plus an air infiltration rate of 50% or less of the tabulated value, depending on the relative magnitude of the internal pressure generated by the mechanical system and the natural external pressures. Where ventilation is by a balanced mechanical ventilation system, infiltra- tion is not inhibited and the values given in the table should be applied without modification.
— Office buildings are based on the definitions given in Energy Consumption Guide ECG019(31). These are:
— Type 1: naturally ventilated, 100–3000 m2
— Type 2: naturally ventilated, 500–4000 m2
— Type 3: air conditioned, 2000–8000m2
— Type 4: air conditioned HQ-type building, 4000–20000 m2
Table 4.13Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — office type 1:
naturally ventilated up to 6 storeys (100–3000 m2); partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
1 storey; 250 m2 2 storeys; 500 m2 4 storeys; 2000 m2 6 storeys; 3000 m2 (20 m ×12.5 m ×3 m)* (20 m ×12.5 m ×3 m)* (25 m ×20 m ×3 m)* (25 m ×20 m ×3 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 1.20 0.90 0.95 0.70 0.75 0.55 0.75 0.55
10.0 (Part L (2002)) 0.60 0.45 0.50 0.35 0.40 0.30 0.40 0.30
7.0 (Part L (2005)) 0.45 0.30 0.35 0.25 0.25 0.20 0.30 0.20
5.0 0.30 0.25 0.25 0.20 0.20 0.15 0.20 0.15
3.0 0.20 0.15 0.15 0.10 0.15 0.10 0.15 0.10
Air change rate 9.30 5.95 3.50 2.95
at 50 Pa (/ h–1)
ACR50divisor 22.0 16.9 13.5 11.5
* (Length × width ×height for each storey)
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.14Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — office type 2:
naturally ventilated up to 10 storeys (500–4000 m2); partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
2 storeys; 500 m2 2 storeys; 1000 m2 4 storeys; 2000 m2 8 storeys; 4000 m2 (20 m ×12.5 m ×3 m)* (25 m ×20 m ×3 m)* (25 m ×20 m ×3 m)* (25 m ×20 m ×3 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 0.95 0.70 0.80 0.60 0.75 0.55 0.80 0.55
10.0 (Part L (2002)) 0.50 0.35 0.40 0.30 0.40 0.30 0.40 0.30
7.0 (Part L (2005)) 0.35 0.25 0.30 0.25 0.25 0.20 0.30 0.20
5.0 0.25 0.20 0.20 0.15 0.20 0.15 0.20 0.15
3.0 0.15 0.10 0.15 0.10 0.15 0.10 0.15 0.10
Air change rate 5.95 5.15 3.50 2.65
at 50 Pa (/ h–1)
ACR50divisor 16.9 17.1 13.3 9.7
* (Length × width ×height) for each storey
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.15 Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — office type 3:
air conditioned up to 8 storeys (2000–8000 m2); partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
2 storeys; 2000 m2 4 storeys; 4000 m2 6 storeys; 6000 m2 8 storeys; 8000 m2 (20 m ×25 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 0.75 0.60 0.75 0.52 0.80 0.55 0.85 0.60
10.0 (Part L (2002)) 0.40 0.30 0.40 0.26 0.40 0.30 0.45 0.30
7.0 (Part L (2005)) 0.25 0.20 0.30 0.18 0.30 0.20 0.30 0.20
5.0 0.20 0.15 0.20 0.13 0.20 0.15 0.25 0.15
3.0 0.15 0.10 0.15 0.08 0.15 0.10 0.15 0.10
Air change rate 4.30 3.05 2.65 2.45
at 50 Pa (/ h–1)
ACR50divisor 15.4 11.7 9.7 8.7
* (Length × width ×height) for each storey
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.16 Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — office type 4:
air conditioned HQ-type building up to 20 storeys; sheltered (up to 4 storeys), partial exposure (up to 12 storeys), exposed (above 12 storeys) Air permeability / Infiltration rate (ACH) for given floor range / h–1
(m3/m2ãh at 50 Pa)
< 2 storeys* < 4 storeys* < 8 storeys* < 12 storeys* < 16 storeys* < 20 storeys*
Peak Ave Peak Ave Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 0.60 0.34 0.60 0.35 0.65 0.45 0.80 0.50 0.90 0.65 0.95 0.65
10.0 (Part L (2002)) 0.30 0.17 0.30 0.20 0.35 0.25 0.40 0.25 0.45 0.35 0.50 0.35
7.0 (Part L (2005)) 0.20 0.12 0.25 0.15 0.25 0.15 0.30 0.20 0.35 0.25 0.35 0.25
5.0 0.15 0.08 0.15 0.10 0.20 0.15 0.20 0.15 0.25 0.20 0.25 0.20
3.0 0.10 0.05 0.10 0.05 0.10 0.10 0.15 0.10 0.15 0.10 0.15 0.10
Air change rate 3.80 2.55 1.95 1.75 1.65 1.55
at 50 Pa (/ h–1)
ACR50divisor 22.3 15.0 8.8 7.2 5.2 4.7
* (Length × width ×height) = 40 m ×25 m ×4 m for each storey (all cases) Note: tabulated values should be adjusted for local conditions of exposure
Table 4.17 Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — factories, warehouses, halls; partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
500 m2 1500 m2 5000 m2 10 000 m2
(25 m ×20 m ×5 m) (50 m ×30 m ×10 m) (100 m ×50 m ×20 m) (100 m ×100 m ×25 m)
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 1.00 0.65 0.75 0.45 0.55 0.35 0.45 0.3
10.0 (Part L (2002)) 0.50 0.35 0.40 0.25 0.30 0.20 0.25 0.15
7.0 (Part L (2005)) 0.30 0.25 0.25 0.15 0.20 0.15 0.15 0.10
5.0 0.20 0.20 0.20 0.15 0.15 0.10 0.15 0.10
3.0 0.15 0.10 0.15 0.10 0.10 0.05 0.10 0.05
Air change rate 5.80 3.05 1.60 1.20
at 50 Pa (/ h–1)
ACR50divisor 18.7 13.6 10 9.2
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.18Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — schools; partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
1 storey; 500 m2 1 storey; 1000 m2 2 storeys; 1000 m2/floor 3 storeys; 1000 m2/floor (25 m ×20 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 1.05 0.70 0.90 0.65 0.65 0.5 0.65 0.45
10.0 (Part L (2002)) 0.55 0.35 0.45 0.35 0.35 0.25 0.35 0.25
7.0 (Part L (2005)) 0.40 0.25 0.35 0.25 0.25 0.20 0.25 0.20
5.0 0.30 0.20 0.25 0.20 0.20 0.15 0.20 0.15
3.0 0.15 0.10 0.15 0.10 0.10 0.10 0.10 0.10
Air change rate 6.80 6.30 3.80 3.00
at 50 Pa (/ h–1)
ACR50divisor 20.0 19.7 15.2 13.5
* (Length × width ×height) for each storey; each storey is nominally isolated by structural design and fire doors etc.
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.19Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — hospitals and health care buildings; partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
< 2 storeys; 500 m2/fl. < 4 storeys; 1000 m2/fl. < 8 storeys; 1000 m2/fl. < 12 storeys; 1000 m2/fl.
(25 m ×20 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)* (40 m ×25 m ×4 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 0.75 0.60 0.65 0.45 0.65 0.45 0.85 0.60
10.0 (Part L (2002)) 0.40 0.30 0.35 0.25 0.35 0.25 0.45 0.30
7.0 (Part L (2005)) 0.25 0.20 0.25 0.15 0.25 0.15 0.30 0.25
5.0 0.20 0.15 0.15 0.15 0.20 0.15 0.20 0.15
3.0 0.15 0.10 0.10 0.10 0.10 0.10 0.15 0.10
Air change rate 4.3 2.55 1.95 1.75
at 50 Pa (/ h–1)
ACR50divisor 15.3 11.6 8.8 7.7
* (Length × width ×height) for each storey
Note: tabulated values should be adjusted for local conditions of exposure
Table 4.20Empirical values for air infiltration rate due to air infiltration for rooms in buildings on normally-exposed sites in winter — hotels; partial exposure
Air permeability / Infiltration rate (ACH) for given building size / h–1 (m3/m2ãh at 50 Pa)
1 storey 2 storeys 5 storeys 10 storeys
(50 m ×12 m ×3 m)* (50 m ×12 m ×3 m)* (50 m ×12 m ×3 m)* (50 m ×12 m ×3 m)*
Peak Ave Peak Ave Peak Ave Peak Ave
20.0 (leaky) 1.15 0.85 0.85 0.65 0.80 0.60 0.90 0.65
10.0 (Part L (2002)) 0.60 0.45 0.45 0.35 0.40 0.30 0.45 0.30
7.0 (Part L (2005)) 0.40 0.30 0.30 0.25 0.30 0.20 0.35 0.25
5.0 0.30 0.20 0.25 0.20 0.20 0.15 0.25 0.15
3.0 0.20 0.15 0.15 0.10 0.15 0.10 0.15 0.10
Air change rate 8.75 5.40 3.40 2.73
at 50 Pa (/ h–1)
ACR50divisor 21.3 16.9 12.1 9.1
* (Length × width ×height) for each storey
Note: tabulated values should be adjusted for local conditions of exposure