To carry out the sensitivity analysis a notional building is required. The building form (described in table 5.2) results from assessing typical building form for office buildings observed in the Non-Domestic Building Stock (NDBS) project (see section 5.6). It has not, however, been possible to determine typical values for certain form aspects (e.g. WWR, LWR, roof type and window types). The values chosen are taken as mid-points in the potential range for these building parameters as estimated by assessing building controls (see section 5.6).
Table 5.2: Notional Building Form for Air leakage Sensitivity Analysis.
Building Description Side lit (day lit). Detached brick plastered walls. Square building (LWR of floor = 1:1)
No. of Storeys 4
Storey Height (m) 3
Footprint Area (m2) 400
Windows WWR - 0.4 LWR - 5:1 Sliding - 50% Opening
Floor Type Solid Concrete
Roofing Flat - (no leakage)
Roof Pitch (û) 0
Running the air leakage model for the notional building (described in table 5.2) 5000 times, a mean air leakage of 14.9 m3.h−1 (to 1 decimal place) per surface unit area was given. A normal distribution of the expected leakage results (see figure 5.3) with a standard deviation (σ) of 2.3 m3.h−1.m−2(to 1 d.p).
This is expected by the central limit theorem.
QT (m3.h-1.m-2 at 50 Pa !P)
Frequency of Occurrence
Figure 5.3: Distribution in expected unit surface air leakage for the notional building.
Figure 5.4 shows the influence of a window’s LWR and the WWR at a LWR of 5:1. Varying LWR and WWR causes significant variation in the mean and σ of QT. As the mean value increases with increasing LWR or WWR σ also increases.
The notional building assumes windows to have an opening area half of the window total area (as set by early control laws 1894 - 1965). The window opening mechanism is described as sliding. Varying this to a hinged opening and a non-opening window shows a significant variation in QT (table 5.3). These results suggest the window component and its proportion of the building surface area are key factors in the air leakage of a building.
Table 5.3: Influence of window opening mechanism on QT of notional building.
Window Opening Mechanism QT mean (m3.h-1.m-2) QT ! (m3.h-1.m-2) Sliding
Hinged Non Opening
14.86 2.27
31.35 8.36
5.28 0.44
Window LWR QT (m3.h-1.m-2 at 50 Pa !P)
(a) Sensitivity for LWR.
QT (m3.h-1.m-2 at 50 Pa !P)
WWR (b) Sensitivity for WWR.
Figure 5.4: Sensitivity of mean and 1 σ of QT for LWR and WWR.
Figure 5.5 shows the sensitivity of wall area as a ratio of total envelope area, varied according to: (a) the LWR of the floor footprint and (b) the number of storeys in the building. The variation of LWR is from 1:1 to 8:1 increasing the wall area ratio (FW) by 0.1 (20% of the notional building FW).
Varying the number of storeys from 1 to 8 shows a greater variation to FW than by varying LWR. The effect on QT is therefore more noticeable, suggesting greater sensitivity to number of storeys than the LWR of the floor footprint. As the number of storeys increases the increment in FW reduces. The significance of further storeys becomes less and less and as building methods are likely to be different for taller buildings, [10], assessing the influence beyond 10 storeys in a building is considered unnecessary.
The roof of the notional building is considered to be airtight and flat. Table 5.4 shows, without increasing the roof surface area, how the other identified roofing methods can significantly increase QT
for the notional building. These other roofing methods require a slope which will also increase the roof area as a ratio of total surface area (FR).
For a tiled, gable-ended pitched roof figure 5.6 shows how increasing FRby increasing roof pitch influences QT. Increasing the roof pitch to 60○from flat increases QT by 30% (on the value stated in table 5.4 )
QT (m3.h-1.m-2 at 50 Pa !P)
Figure 5.5: Influence of FW on component leakage model due to a) variation in LWR and b) variation in number of storeys.
The sensitivity of the roof leakage assumes that the roof cavity is considered to be open to the rest of the building. In model validation this assumption was highlighted as problematic (see section 5.5).
Table 5.4: Influence of Roof Type on Unit Surface Air Leakage at ∆P = 50 Pa.
Roofing Type (Flat) QT mean (m3.h-1.m-2) QT ! (m3.h-1.m-2)
The wall is itself considered as a leakage component and will therefore influence QT. Table 5.5 shows marginal variation in the mean and σ of QT for the majority of tested wall constructions. For cur-tain walling systems (usually associated with multi-storey, modern office buildings) QT is significantly reduced. Curtain walling refers to a glazed walling system attached to a load bearing (usually steel) skeleton structure. Few openings are expected in such a structure and so all opening-related leakage components were discounted when calculating QT.
Table 5.5: Influence of wall construction method on notional building’s QT.
Wall Construction QT mean (m3.h-1.m-2) QT ! (m3.h-1.m-2)
*WWR = 0.0 as considered as a glazing system.
Increasing the footprint area for the notional building will reduce the wall area ratio FW which is
F
RQ
T(m
3.h
-1.m
-2a t 50 P a ! P )
Figure 5.6: Sensitivity of QT to FR - varied by roof pitch for a tiled roof.
providing the majority of the component leakage to this building. Table 5.6 shows little sensitivity to small variation in footprint area for the notional building. In the building stock, however, the variation in footprint can be quite large and as seen in table 5.6, QT will be sensitive to large variations.
Table 5.6: Influence of Building Footprint on notional building’s QT.
Footprint Area (m2) FF:FW:FR QT mean (m3.h-1.m-2) QT ! (m3.h-1.m-2)
A study on air leakage of 12 office buildings was carried out by BSRIA [147]. The normalised leakage rate at 25Pa and 50Pa is given for each building. A basic description of each building’s size, form and year of construction is also given in [147]. No information is given on the WWR or LWR of windows.
With an assumed WWR of 0.4 and a fixed window LWR of 5:1, the model was run with a description of the BSRIA studied buildings. Table 5.7 shows good agreement of the model to flat roof buildings, assuming windows are all opening and applying a sliding window opening mechanism. For an Elizabethan building, reported to have been refurbished in the Victorian era (1840-1900), the model severely over predicts the expected normalised leakage rate.
The most noticeable difference is the inclusion of a tiled roof component, rather than the flat roof (non-contributing) elements as in the other buildings. The leakage rate of a roof will only be fully noticed in a leakage test if the roof cavity is directly linked with the rest of the building. For closed loft spaces the total leakage rate of the roof does not directly effect the leakage rate of the inhabited building interior.
Removing the effect of the roof leakage from the model showed better agreement with the experimental data. Further still reducing the WWR to 0.1 as a more representative WWR for buildings of this period brings the model results inline with the leakage test results.
It should also be noted that the Elizabethan building underwent a refurbishment program in the Victorian era. The impact of refurbishment on leakage coefficients is not understood by the model. In this instance, however, as the renovation is over 100 years old, it is not considered a significant factor for explaining the measured leakage rates.
The other buildings in the leakage study by BSRIA are complicated by irregular design and use of components such as atria that have not been leakage tested as building components. For these reasons it was not considered possible to use them to validate the model. This highlights a current limitation of the component leakage model in estimating the probability of leakage rates in buildings.
Validation with the BSRIA data required assumptions to be made about certain aspects of the form of the tested buildings. These assumptions are significant to the model results. A more detailed description of the form of the building than could be obtained from [147] was required.