Measuring and modelling window opening

The first published study of domestic ventilation behaviour is presented in Brundrett (1977), who observes a ‘strong association between temperature or external humidity and window opening’

in a study of 123 homes (Figure 15, see also Dick and Thomas (1951)). The author notes that British people have a tendency to open windows all year round, and identifies seven main

‘reasons for open windows in winter’ (Figure 16). This early piece of research acknowledges the importance of occupants in influencing ventilation rates, indoor air quality (IAQ) and energy use;

however, the study is presented as a technical investigation of building performance parameters, rather than as one based on social science research methods, despite using occupant interviews to ‘elicit the person’s own opinions and reasons for opening windows’

(p.290).

62 Term first coined by Blake (1999).

63 This term is defined and discussed further in the Methodology (section 4.2).

Figure 15: Window opening versus humidity and temperature⁶⁴

Figure 16: Reported reasons for opening windows during winter⁶⁵

A large scale study of how people ventilate was carried out during the 1980s under the International Energy Agency (IEA) Annex-8 project “Inhabitant behaviour with regard to ventilation”. The findings are documented in the Air Infiltration and Ventilation Centre (AIVC)

“Technical Note 23” (TN23), which presents the first conceptual framework for understanding ventilation behaviour (Dubrul, 1988) (Figure 17).

64 Images from Brundrett (1977), p.290 (left) and p.293 (right).

65 Table reproduced from Brundrett (1977), p.296.

Figure 17: Earliest conceptual model of ventilation behaviour⁶⁶

The findings can be grouped into the following four types, and are discussed briefly below:

 Correlations between window opening and other variables

 Impact of window opening on heating energy demand

 Variation in window opening in homes with different ventilation systems

 Attempts to reduce wintertime window opening through information campaigns

Correlations between window opening and other variables

These studies form the main findings of the (IEA) Annex-8 research. They try to identify why and when people open windows, generally by attempting to correlate certain ‘factors’ or

‘variables’ with either observed or self-reported window opening. A list of reported reasons for opening and closing windows was compiled in Dubrul (1988), while a wide variety of variables which relate to window opening are reported in Dubrul (1986). Van Dongen and Phaff (1989) divide these into ‘climatological’, ‘human’, ‘environmental’ and ‘architectural’ factors, while Dubrul (1988) groups them by ‘dwelling fabric’, ‘lifestyle’ and ‘weather’. The range of variables identified in these studies is summarised in Table 10. These studies effectively communicate the complexity of domestic ventilation, as it is clear from the wide range of identified variables that predicting people’s actions will be challenging.

Table 10: Factors influencing ventilation behaviour from IEA Annex 8

Variables identified Reference

Reasons for opening windows Provide fresh air to bedrooms Dubrul 1986 and1988 Remove cooking smells

66 Figure reproduce from Dubrul (1988), p.2.

Remove cigarette smoke or stale air Climatological Outdoor temperature

Orientation of windows with regard to sun

Environmental Presence of condensation or mould Moisture production in dwelling

Noise or odours annoyance from outside Architectural Level of infiltration

Room volume

Occupants' perception of mechanical ventilation

Dubrul (1988) reports a strong correlation between external temperatures and window opening (Figure 18). This is an example of the kind of complexity which is not captured by models such as the Standard Assessment Procedure (SAP), which assumes that in the heating period ventilation is independent of external temperature. The study also referred to an investigation of socio-demographic variables, which found that ventilation behaviour decreases with age of occupants. Figure 19 shows a plot of balcony door opening frequency against external temperature (Phaff, 1986). When the weather is mild and warm there is a strong overall correlation; however, the gradient is much shallower during colder periods, suggesting that during winter other factors may influence balcony door opening. Complementary findings are reported in Weihl and Gladhart (1990).

Figure 18: Outdoor temperature versus percentage of open windows⁶⁷

67 Reproduced from Dubrul (1988).

Figure 19: Balcony door opening versus external temperature (daily mean)⁶⁸ Impact of window opening on heating energy demand

These investigations attempt to quantify the impact of opening windows on air change rates and, subsequently, heating energy demand. Phaff and van Dongen (1985) predict that 5000MJ/y of heat energy is wasted by opening windows, based on a sample of 80 homes from a single apartment block in the Netherlands. Kvisgaard and Collet (1986) calculated that 63% of a dwelling’s total air change rate is attributable to occupant behaviour, based on study of 25 dwellings. Wouters et al. (1987) reviewed data from three studies to estimate the percentage of heat demand attributed to window use, based on three scenarios (Table 11). As insulation levels are increased (lower U-value) heat demand from window opening increases (in % terms);

as current, new-build, average U-values are well below even the lowest of those measured here, awareness of this potential is all the more critical today.

Wouters and De Baets (1986) surveyed 2400 dwellings in Belgian social housing (via face-to-face questionnaires) and estimated that windows were open for 8% of time during winter and 35% in summer. The authors then estimated that the average increase in ventilation rate due to occupants is 0.26 ACH during winter and 1.5 ACH during summer. The previously introduced TN23 report, which reviews and summarises data from all the studies, estimates that a dwelling’s energy use may vary by 10-15% depending on ‘ventilation behaviour’ (i.e. window opening) Dubrul (1988). However, it should be noted that the figures presented in this section must depend on the permeability of the dwellings and therefore may not apply to contemporary low energy housing.

Van Dongen and Phaff (1989) investigated 279 single family dwellings in the Netherlands and concluded that energy use was more sensitive to window opening in living rooms than in

68 Reproduced from Phaff (1986).

bedrooms, as this was where thermostats were often located. They also reported that use of windows varied from room to room (Figure 20), with similar findings reported in Trepte (1986).

Table 11: Percentage of total heating demand attributed to window use in houses and flats⁶⁹ Um (W/m²K) Ti (°C) Low window opening

(%)

Average window opening (%)

High window opening (%)

Houses

2.0 17 2 5 11

0.7 18 6 15 28

0.4 19 9 23 40

Flats

7.0 18 5 13 25

1.4 19 15 33 52

0.8 20 20 45 64

Figure 20: Heating season window opening by room type⁷⁰ Variation in window opening in homes with different ventilation systems

The UK study conducted as part of IEA Annex 8 investigated 18 newly built terraced homes in London, all of which incorporated MV (Lilly and Makkar, 1986). Average window opening was lower than that reported by Brundrett (1977). The authors argue that this is explained by the presence of MV. The German study of 24 mechanically ventilated test apartments also found reduced window ventilation compared to naturally ventilated homes, by a factor of four (Erhorn, 1988).

69 Adapted from tables 14 and 15 in Wouters et al. (1987), p.11.8.

70 Reproduced from Van Dongen and Phaff (1989).

A study of the influence that type of domestic ventilation system has on occupant ventilation behaviour is reported in Van Dongen (1990). Questionnaires were distributed during a mild winter (~5°C) in 414 Dutch dwellings, comprising a roughly equal number of homes with natural ventilation (NV), mechanical extract ventilation (MEV) and balanced mechanical ventilation (MV). Different levels of window use were observed according to ventilation type, with the least use in dwellings with balanced MV and the most in NV dwellings, a finding supported by Lemaire and Trotignon (2000), who also found that approximately 72% of occupants reported regularly opening windows during the heating season, using a questionnaire study of 10000 households. By ventilation type, this figure increases to 76% for NV homes. However, residents of 64% of homes with balanced MV also claimed to regularly open windows during winter.

Reducing wintertime window opening through information campaigns

Phaff (1986) presents the findings of a study which attempted to measure the impact of information campaigns, aimed at reducing wintertime window opening, on people’s use of windows in the Netherlands.⁷¹ Before the second winter of occupation, written information about

‘how to make proper use of the windows’ (p.57) was sent to 80 (out of 140) dwellings in the block, of which 20 dwellings were also visited in person. Window opening was measured using sensors. The results appear promising as, overall, windows were used less in the second winter monitoring period. In particular bedroom windows were open considerably less in the second year (70% less), while kitchen window opening actually increased by 13%. However, as it was on average 5°C cooler that year the effect of the intervention may be exaggerated. Also, the study does not distinguish between slightly and widely open windows; therefore, it is challenging to interpret the results of this kind of window opening monitoring data.

3.2.3. Interactions with ventilation systems