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A review of building control documentation was carried out in order to determine the potential range in Window Wall Ratios (WWR ) of ND office buildings. The regulations/laws were also used to apply an expected mean WWR by building control period, to which a stock level probability distribution of WWR could be applied.

The type of glazing (i.e. single/double/triple) influences the impact of WWR on thermal gains and losses. Different glazing types were associated with ranges in WWR according to periods of building control.

4.7.1 Window Wall Ratio

For all considered control periods (1894 to present) minimum window area is set as 10% of the total floor area of each room in a building (unless the room is intended for use for building services). In identifying the range of potential WWR for the building stock, 10% of floor area was used to set the minimum ratio.

The minimum WWR is, therefore, dependent on the ratio of floor area to exposed wall area.

For the periods up to 1984 a maximum WWR of 50% was set for standard walling construction. Higher WWR were allowed provided pier-wall construction was used to increase structural stability. Pier walling results in thicker wall construction than otherwise required by regulations. To aid model development such measures were not considered, so setting an upper limit on WWR of 50%. For post 1984 control periods this upper limit was increased to 66%, in structural regulations (Part A of Building Regulations).

For buildings built up until 1950 a study by Ove Arup (1984), as documented in [113], showed commercial buildings to typically have glazing ratios between 40-50%. Those buildings between 1950 and 1973 were expected to have higher glazing ratios (as high as 75%) and between 1974 and 1984 the typical glazing area was reduced due to energy concerns.

Higher levels of glazing are feasible as part of ’curtain-walling’ systems, where it is acknowledged that 100% glazed walling exists in the considered building stock [136]. However, only metal panel insulated cladding systems have been accounted for as consideration of curtain walling and development of other curtain walling systems (including high level glazing) is seen as an area for further research and model development.

For ease of modelling windows were considered evenly distributed across all external walls of the building model.

4.7.2 Glazing Type

Prior to 1977 (introduction of 1976 Building Regulations) no thermal consideration of window-wall ratio was given in building control. Single glazing is the only considered glazing type pre 1977. Between 1977 and 1984 thermal regulations introduced limits on use of single glazing in a wall adhering to minimum thermal transmittance value. For periods post 1984 the thermal transmittance of the glazing unit is linked to WWR. The regulations stipulate (in absence of manufactures values) level of glazing (single,double,triple), glass surface properties relating to radiative control, air spacing in multiple glazed

In the 1976 Building Regulations [128] a standard single glazed unit was given a U-value of 5.7 W/m².K and 2.8 W/m².K for a double glazed unit. The combined wall and window U-value could not exceed 1.8 W/m².K. With a minimum wall component U-value of 1.0 W/m².K, single glazing is restricted to∼18%

WWR and double glazing to∼50%.

For Part L Building Regulations (1985 to Present) standard glazing units of given U-value have maximum allowable WWR (see table 4.13). A glazing unit of appropriate U-value was applied in the probability model according to the randomly applied WWR and period of construction. The building energy model does not include door openings or roof lights and so window U-values represent the averaged U-value requirement of all these components according to building regulation requirements.

For the building stock probability model three basic constructions were identified as single, double and triple glazed. The model was designed to apply a construction representative of the required window unit construction according to the relationship to WWR - identified by building regulations.

Table 4.13: Standard Glazing Unit, U-value and maximum WWR according to period of building control.

Control Period U-value 2.2W/m².K for a notional building maximum WWR of 40% for double

glazed unit.

5.7 50 Single glazed unit

5.7 18 Single glazed unit

2 - Double glazed with low emissivity coating

or

Triple glazing (applied in model)

5.0 - 4.7 25 - 27 A standard Single glazed unit is expected to have a U-value between 5.8 and 4.7, depending on type of framing.

(A standard single glazed unit is applied in the model).

4.2 - 2.4 30 - 58 Double glazed U-value ranges between 4.2 and 2.4, depending on material of frame, gas fill, glass spacing.

2.3 - 2.0 60 - 74 Either double glazing or triple glazing of different glass spacing, gass fill, emissivity coating and unit framing.

3.3 -2.3 25-38 Double glazing of varying spacing (6/12/16 mm), argon filled.

2.2-1.7 40-55 Double Glazed 12mm spacing, and low emissivity to double glazed, low emissivity argon filled 16mm spacing. Or triple

glazing of 12 or 16 mm spacing.

1.6-1.3 58-75 Triple glazed 16mm spacing (argon filled) to Triple glazed 16mm spacing (argon filled) with low emissivity coating

4.7.3 Distribution for probability modelling of ND Building Stock

As part of the NDBS Project, Gakovic reports on types of glazing and other openings for ND buildings in [137]. Categorising surveyed buildings into six forms, the ’traditional’ (load bearing), ’framed, curtain walling’, ’framed, deep plan’ and ’framed, other’ forms are of greatest relevance to office buildings.

Regression analysis on glazing area to floor area and glazing area to wall area was carried out in this study and showed a strong correlation in these variables for the relevant building forms.

From the building regulations a minimum glazing area to floor area ratio of 0.1 is expected. From [137]

a minimum ratio of 0.08 was observed for deep plan buildings. The 0.02 variation in observed and

theoretically considered minimum window to floor ratio is likely to result from service areas (e.g. lifts and plant rooms) that do not count for glazing requirements in building regulations. Higher average window-to-floor ratios for the other building forms are observed.

The regression analysis carried out in [137] provides typical glazing ratios for different built forms, but offers no account of probable distribution in the building stock. As building controls have been identified as a source of influence on building window-wall ratios, they were used to develop a priori probability distributions of WWR for each period of building control.

In each case a basic triangular distribution was applied (see chapter 3), where the limits were set to a minimum of 10% of the floor area and a maximum of 50% WWR pre-1985 and a maximum of 66% WWR post-1984. From [38] the ’traditional’ buildings surveyed result in an expected glazing area to floor area ratio of 0.13. Of the surveyed buildings 74.9% of the total floor area is attributed to building built prior to 1960. For the building control periods 1894-1938, 1939-1952, 1953-1965 the minimum value in the range (0.1 of floor area) was considered as the modal value.

For framed buildings within the study by Gakovic [137] 77.3% of the surveyed floor area was attributed to buildings built between 1961 and 1980. For framed curtain walling, framed deep plan and other framed structures the expected WWR were calculated to be 0.6, 0.15 and 0.25, respectively. Considering these methods as typical of the time (but with no distinction between these ’framed’ structural types) an averaged modal value of 0.33 was applied to the model for periods between 1966 to 1984.

For buildings built under Part L Regulations - Conservation of Fuel and Power (post 1984) the elemental approach to thermal regulation sets maximum WWR (as a percentage) for different glazing types. In each of the identified periods post-1984 the regulations use a notional building to which a maximum thermal transmittance of any building structure is set. For the WWR the maximum value associated with a typical level of glazing is used for the notional building. As a guide for good thermal design, these WWRs are considered as the most likely within the given control periods.

For the two periods between 1985 and 1995 a WWR of 35% is used. For post-1995 periods a WWR of 40% was given as the modal value in the applied distribution to the probability model.

Figure 4.5 represents the applied distributions in determining the WWR for the probability model. As a ratio of window area to floor area is used to determine the minimum WWR, the minimum WWR potentially could exceed the maximum WWR. In such circumstances the model logic set the WWR to the maximum limit considered.

0.1FA / WA! ! ! 0.50 ! ! ! Max WWR P(x)

1.0 Distribution applied for periods:

1894-1938, 1939-1952, 1953-1965

0.1FA / WA 0.33 ! 0.50

! Modal WWR Max WWR P(x)

1.0 Distribution applied for periods:

1966-1971, 1972-1976, 1977-1984

WA - Exposed Wall Area FA - Floor Area! !

0.1FA / WA 0.35 ! 0.66

! Modal WWR Max WWR P(x)

1.0 Distribution applied for periods:

1985-1990, 1991-1995

0.1FA / WA 0.40 ! 0.66

! Modal WWR Max WWR P(x)

1.0 Distribution applied for periods:

1996-2001, 2002-Present

Figure 4.5: Triangular distribution for determining glazing area in ND building stock probability model.