Indoor air quality in highly energy efficient homes: a review (NF18)
There is a concern that building homes to a high standard of airtightness could have an adverse effect on indoor air quality.
This report reviews existing research from the UK, Europe and the rest of the world into homes built to a high standard of airtightness and the mechanical ventilation systems commonly installed into them. It highlights a number of issues and recommends further research. In response to this report we are expecting to commence a monitoring programme of some occupied homes shortly.
Summary of content
In line with its intention that all new homes in England will be zero carbon by 2016, the government is committed to progressive tightening of the energy efficiency aspects of the Building Regulations. Changes to the Building Regulations are complemented by the Code for Sustainable Homes which measures the sustainability of a new home against nine categories of sustainable design and construction.
Recent research by the NHBC Foundation (Davis and Harvey, 2008) has highlighted concerns of homeowners and builders about the possible adverse consequences on indoor air quality of the greater airtightness of the building envelope that is required to improve energy efficiency. Lack of air infiltration could lead to poor air quality since stale indoor air is not replaced at a sufficient rate by fresh outdoor air – with potential for any or all of pollutants build-up, high humidity and condensation (leading to mould growth), damage to structures and proliferation of house dust mites.
Because of its central role in determining the exposure of the population to air pollution, a case could be presented for more research into indoor air quality in homes. For the purposes of this review, however, the following main features of highly energy efficient homes are considered (together with their possible impact on air quality parameters relative to current Building Regulation requirement): increased airtightness, increased winter internal temperature, summer internal temperatures, mechanical ventilation, heat recovery aspect, materials for construction and ground contaminants.
In order to satisfy the energy use demands of the Code for Sustainable Homes in homes built to Code Level 4 and above, it is expected that mechanical ventilation with heat recovery will need to be applied in order to achieve an acceptable indoor climate, which represents something of a culture change in the UK. With widespread use of such mechanical ventilation with heat recovery systems comes the need for good understanding of their operation, performance and maintenance. Also, there is a possibility that householders would seek to counteract poor air quality or lack of the feel of ‘freshness’ by opening windows on a regular basis, thereby serving to offset the inherent benefits of a structure built to standards of high energy efficiency fitted with a continuously operating ventilation system.
The operation of mechanical ventilation systems in very well sealed dwellings during warm periods also raises questions about their potential effectiveness at removing heat and minimising overheating.
This review considers the current state of the art as regards airtight houses in the UK and elsewhere in the world with regard to the indoor environment they provide, and identifies requirements for further research in this area. The review starts with a summary of typical characteristics of Code Level 4 to 6 homes built under the Code for Sustainable Homes.
In sections 1 and 2, research in the UK, Europe and the rest of the world into indoor air quality and other relevant factors which may impact on occupant wellbeing, is reviewed. This is followed by a review of current research and state of the art concerning both ventilation performance in dwellings and of construction and ventilation provision in highly energy efficient homes – in all cases considering both the UK and elsewhere.
Using the literature and information gathered and summarised in the review, the authors present a summary of perceived research needs concerning the indoor environment in energy efficient homes and the ventilation systems required to maintain it within acceptable standards.
There is no published study of highly energy efficient homes in the UK that monitors the range of air quality and other factors that can affect occupant health and wellbeing. Of course, only a limited number of homes currently exist at Code Level 4 and above and the definition of Code Level 6 is currently the subject of government consultation. However, it is notable that comprehensive studies of the indoor environment of UK homes are few.
Possibly more informative to our understanding of highly energy efficient homes are studies in other countries with experience of building airtight homes, particularly for very cold climates, such as in Canada, central Europe, parts of the USA and Scandinavia. Experience from other countries provides a useful insight into the likely construction practices that may be increasingly adopted in the UK. However, there are differences in climate, construction practice and the social and economic circumstances of occupants, and therefore direct transfer of knowledge is problematic. With regard to indoor air quality in particular there is a dearth of information relevant to highly energy efficient structures.
In conclusion, there appears to be an urgent need for research into the performance of highly energy efficient homes with respect to the quality of the internal environment ventilation systems used, and the impact on the health and wellbeing of occupants. Two broad but inter-related topics require investigation:
1) Performance of products and designs:
- Noise generated
- Ability to clean fans and ductwork
- Achievement of the required air supply
- Air filter efficiency
- Use of demand control ventilation
- Impact of chemical emissions from materials on indoor air quality.
2) Performance of Code Level 4 to 6 homes:
- Performance of systems at installation, during one year of use, and beyond.
- Evaluation of indoor air quality and ventilation in a representative sample of homes (temperature, relative humidity, volatile organic compounds, formaldehyde, carbon monoxide, nitrogen dioxide, particles, mites, bacteria, fungi, radon, ozone, semi-volatile organic compounds in dust, carbon dioxide and air exchange rate).