The problem in the absence of data and guidance – how should designers address the issue of opening windows, and incorporate considerations of ventilation and for mitigating overheating?
The Government’s planning practice guidance, summarised from Paragraph: 005 Reference ID: 30-005-20140306, How to recognise when noise could be a concern?, is reproduced in the table below:
|Perception||Examples of outcomes||Increasing Effect Level||Action|
|Noticeable and intrusive||… where there is no alternative ventilation, having to close windows for some of the time because of the noise.||Observed Adverse Effect||Mitigate and reduce to a minimum|
|Noticeable and disruptive||… where there is no alternative ventilation, having to keep windows closed most of the time because of the noise.||Significant Observed Adverse Effect||Avoid|
The key terms to consider are “where there is no alternative ventilation, having to close windows..”. Although the planning guidance also contains a reference to Part F of the Building Regulations, opening windows may only form part of the normal ventilation provisions for some of the template methods of compliance. This means that under normal living conditions (i.e. excepting the provision of purge ventilation) there will always be alternative means of ventilation other than opening windows. Ventilation systems that rely on air flow through trickle vents driven by natural forces – ie Systems 1 & 2 as described in Approved Doc F (see below), rather than Systems 3 & 4 – also rely on opening windows outside the winter period.
The meaning of “ventilation” is also greater than Part F of the Building Regulations, and there are other benefits of ventilation as discussed below. There is an implication in the guidance above that windows are considered to be normally open, and the adverse effect is “having to close” them.
Purposes of opening windows
Opening windows can serve many purposes, some of which are quantifiable and some may only be evaluated qualitatively. These are summarised with their intended purpose in the table below:
|1||Quantifiable||Purge ventilation||for indoor air quality following spillages or painting & decorating – Approved Doc F|
|2||High ventilation rate||to assist thermal comfort- reduce overheating in summer, or to provide whole dwelling ventilation outside the winter period for Systems 1 & 2.|
|3||Qualitative||User control of environment||intuitive and easy with immediate feedback|
|4||Feeling connected||with the natural world outside|
|5||Perception of fresh air||air movement also assists cooling effect|
Opening windows may be one part of a “natural ventilation” strategy. This summary aims to identify the uses and purpose of opening windows, so that the importance can be properly considered in the context of sustainable design.
1. Ventilation & IAQ – Part F of the Building Reg.s
Part F of the Building Regulations defines and sets out requirements for ventilation in order to provide adequate indoor air quality, based on performance requirements for water vapour and pollutants to avoid adverse environmental conditions for the habitants and building fabric. This purpose is described in the extract from Approved Document F (AD-F) below:
AD-F sets out four template “Systems” that can be adopted to demonstrate compliance. There is more discussion of the noise impact and considerations associated with each ventilation System described in our previous publication which are summarised here. The template Systems demonstrate adequate ventilation requirements as summarised in the table below:
|Ventilation component||Provision with AD-F System / purpose|
|Whole dwelling ventilation||Extract ventilation||Purge ventilation|
|Purpose||To breathe and control pollutants from building fabric Continuous when occupied.||Control of water vapour from cooking and bathrooms.May be continuous or intermittent||Intermittent|
|System 1 – “Natural ventilation”||Trickle vents in winter, opening windows at other times||Intermittent extract fans||Typically provided by opening windows|
|System 2 – Passive stack (“natural”)||Trickle vents and passive stack ventilation in winter, opening windows at other times||Continuous via passive stack|
|System 3: MEV||Continuous mechanical extract – normal level||Continuous mechanical extract – boost|
|System 4: MVHR||Continuous mechanical supply and extract – normal level||Continuous mechanical supply and extract – boost|
Associating ventilation conditions with noise level limits
The air quality requirements are essential – poor air quality is well correlated with a range of adverse health effects, such that it is nether desirable nor tolerable to suffer. There should be no doubt that the whole dwelling ventilation should be provided while maintaining preferred acoustic conditions at all times. Approved Doc F, para.s 4.34 to 4.36 suggests that the noise levels associated with this ventilation condition should not exceed 30 dB(A) in bedrooms and living rooms, and 35 dB(A) in kitchens and bathrooms.
There is almost no research data on the acoustic conditions that may be appropriate to achieve with the provision of extract ventilation. There is plenty of anecdotal evidence of people turning off intermittent extract fans because they are too noisy. Extract ventilation is at different rates if it is provided intermittently rather than continuously, and therefore different noise limits may be appropriate. It should be noted that for continuous extract ventilation with a centralised system when operating in boost mode, this will affect other rooms also, not just the room triggering the boost mode. Noise from mechanical ventilation systems is discussed in further detail here.
In the purpose of ventilation, the provision of purge ventilation is when there has been a spillage of water, burnt toast or painting and decorating – highly intermittent activities during which the level of noise is not likely to be very significant, and again for which there is no noise data to suggest that noise should be limited from either external or mech vent systems.
In terms of indoor air quality provision, it is suggested that if opening windows are used to provide purge ventilation, no noise criterion is necessary to associate with this condition.
In contrast, the main use of opening windows is not for indoor air quality purposes, but for the other purposes noted in the table above. These are considered below.
2. Thermal comfort and overheating
There is no universally accepted definition of overheating, but two competing methods of assessment. These are the Fanger model, described in ISO 7730, and the adaptive thermal comfort model described in CIBSE Guide A 2016. [As at May 2017, CIBSE TM59 describes a method for consistently assessing overheating in dwellings.]
The Fanger model is based on the physics of human heat generation during different levels of activity(metabolic rate), insulation provided by clothing and seating, the effect of air movement (draughts) and the sensation of temperature in a room (operative temperature). There are fixed “universal” limits for comfort, based on average responses from climate chamber surveys.
The adaptive thermal comfort model is based on the experience of occupants in naturally ventilated buildings, and correlates overheating with the difference between the internal operative temperature and the running mean external temperature. It is therefore a description of the effect of the building on the difference between internal and external temperatures, on the basis that people are able to adapt their clothing accordingly. A key element of this approach is the control of the internal environment by the occupants.
The Zero Carbon Hub has recently produced a range of reports on overheating, here.
To determine if overheating occurs, and the extent of the problem, it is necessary to adopt a definition of overheating, so that adverse and significant adverse conditions can be determined in that context. A tick box assessment of overheating risk is included in the SAP assessment, but this is over simplistic and can be satisfied by “game playing” the assessment. A significant use of opening windows is to control thermal comfort. The studies on thermal comfort often cite noise as a reason that windows cannot be opened, and this becomes a contributing factor to the problem of overheating.
Thermal comfort may be controlled by many means of design and operation, such as:
- façade aspect and fenestration
- solar shading
- internal thermal mass
- reducing internal heat loads
- higher ventilation rates through opening windows or passive through-wall vents
- higher ventilation rates from mechanical ventilation
- mechanical cooling
There should be no presumption that passive ventilation through opening windows is the most appropriate method to limit overheating. Alternatively, a very prudent design approach is to design to meet both preferred acoustic and thermal criteria simultaneously for air conditioned buildings . The US currently expends more electricity cooling buildings than is used by the whole of Africa; it would be an undesirable outcome for sustainability if mechanical cooling is adopted in more residential buildings simply because we cannot assess the potential satisfaction with indoor environmental conditions.
3. Interdependence of thermal and acoustic comfort
It is not known at what combined levels of adverse thermal and acoustic environment occupants feel dissatisfaction with the indoor environment. It is not known what exchange rates people may choose between thermal discomfort and acoustic discomfort, where they have control over both factors by the use of opening windows. This is further complicated by the notion that acoustic discomfort may be a function of the character of the noise as well as the absolute level. It is not known if the exchange rates may be more applicable to threshold levels of thermal and acoustic discomfort, or to a dose relationship with either or both. Research is needed to inform these questions. This has started at Salford University.
Qualitative benefits of openable windows
4. Occupant control of the environment
The benefits for the occupants in being able to control their indoor environmental conditions should not be underestimated. The experience of control also changes the physical conditions that occupants report as acceptable. The adaptive thermal comfort model demonstrates that people report comfort levels while tolerating significantly higher temperatures where they have control of the environmental conditions through opening windows, compared with fixed windows, mechanical ventilation and air conditioning.
It would therefore seem likely that people may also tolerate higher noise levels where they have control over the environmental conditions. We are assembling a library of references that consider combined environmental effects, and the impact of occupant control on the acceptability of internal conditions.
5. Feeling connected with the outside world
Opening windows offer a feeling of connection with the outside environment that is valuable. There is a notion of biophilia, that humans have a natural affinity for other natural living systems, and having a sense of connection to the outside environment can be important. We know that rooms with no natural light create a sense of unease after some time; the ability to have a direct physical connection to the outside may be considered on this spectrum.
It is noted that some environmental health officers believe so strongly in this that they state that it is a “human right” to be able to open windows and enjoy reasonable internal noise levels. Dani Fiumicelli (Temple Group) notes that as yet this is not a right that is recognised by the courts. In regard to the ECHR Article 8 – the right to respect for private and family life, case law has established that “There is no explicit right in the convention to a clean and quiet environment, but where an individual is directly and seriously affected by noise or other pollution, an issue may arise under Article 8….” Hatton vs. UK (2003); 37 E.H.R.R. 28 – Paragraph 96”.
If acceptable indoor environmental conditions can be achieved without opening windows, what value may be assigned to the feeling of connection with the outside world – and what noise level may be tolerable for this benefit? This benefit may rest on the acoustic connection with the outside world, and as such higher noise levels may well be tolerable or even desirable for this condition.
6. Perception of fresh air
There is a perception that outside air is fresh, and overcomes a stagnant internal environment. While our human sense of air quality has not yet evolved to effectively assess many modern air pollutants, the perception of air quality is a complicated field. The feeling of air movement also assists in cooling, however, and the perception of this may again be more important than the physics. If acceptable indoor environmental conditions can be achieved without opening windows, what value may be assigned to the feeling of fresh air – and what noise level may be tolerable for this benefit?
The government’s Planning Practice Guidance makes vague references to ventilation and noise that are difficult to interpret. There are quantifiable requirements for ventilation at different rates in dwellings in accordance with Building Regulations that do not require opening windows. There would seem to be very little benefit in consideration of noise whilst using opening windows for the Building Regulations purpose of purge ventilation.
Limiting the extent of overheating firstly requires adoption of a definition of overheating; it appears that there is a lack of scientific literature to assess the combined tolerance to thermal and acoustic discomfort where windows are under the occupants control. As such if the thermal and acoustic criteria are applied without consideration of the combined tolerance, for the limited periods of time for which they may occur, buildings may be unnecessarily over-designed.
There are significant benefits of opening windows for occupant control, feeling connected with the outside world, and a perception of fresh air. If acceptable indoor environmental conditions can be achieved without opening windows, what value may be assigned to these qualitative benefits – and what noise level may be tolerable for these?
We all need to know these things so that we can design sustainable buildings.
Jack Harvie-Clark, 16/11/2015 (updated June 2017)