INDUSTRY INSIGHT
Hot or not? T
he issue of homes overheating is a growing problem. New and refurbished homes are being built better insulated and more airtight than ever to satisfy increasingly demanding energy efficiency standards. With the focus on preventing heat losses in winter, the risk of homes overheating in summer is growing. In fact, there is evidence that some homes, in particular single aspect apartments with a high proportion of glazing, are already overheating. However, when increased summertime temperatures resulting from climate change and intense urbanisation of cities are factored into the mix there are worries that more and more homes will suffer uncomfortable or excessive temperatures for days or even weeks on end, impacting occupant wellbeing and potentially adversely affecting their health. The task for designers of both new-build
and retrofit homes is to provide dwellings that are energy efficient and warm in winter, but which stay consistently comfortable in the summer. Many factors influence the likelihood of a dwelling overheating, including its layout and orientation, occupancy, heat gains, shading and ventilation strategy. The complex challenge of assessing overheating risk in homes is currently not addressed adequately by the Building Regulations. Now CIBSE has developed a standardised methodology to enable designers to assess overheating risk in homes and to mitigate the causes of overheating. CIBSE’s Technical Memorandum 59: Design methodology for the assessment of overheating risk in homes will ensure the construction industry takes a common approach in assessing the complex issue of overheating risk. The institution’s aim with the document is to encourage good design that sets out to minimise incidences of overheating but without the methodology being so stringent that it over-promotes the use of mechanical cooling. TM59’s methodology draws on existing
guidance produced by CIBSE and others, including CIBSE’s industry-leading weather data products. These include the latest climate change projections to produce weather files for dates up to the 2080s, to allow designers to assess how a dwelling is likely to perform in the future. Overheating assessments are made based on 24-hour occupancy to reflect changing
40 September 2017
CIBSE has published new guidance to assess overheating risk in homes. Anastasia Mylona, CIBSE research manager, explains why the institute’s newly published TM59 is a must read for designers and developers
lifestyles including more people working from home during the day. The document is necessarily
prescriptive in its methodology to ensure it is consistently applied. The methodology implies the use of dynamic thermal modelling tools, a defined internal gain profile and specific weather files to provide a clear pass/fail result for a home. Based on tests undertaken in developing the methodology, CIBSE expects some single aspect apartments, particularly those with a lightweight construction and large amounts of glazing built in south east England will not pass the test. TM59 is intended to become an essential item in designers’ and developers’ toolboxes. Its methodology includes clear reporting requirements to enable all stakeholders to understand the basis of the assessment, to review the outcome, and to assess implications for a scheme’s design. And, by enabling overheating risk to be assessed early in design development, TM59 will enable mitigation strategies to be incorporated into a scheme early in design development, before the architectural drawings are finalised and before a scheme is submitted for planning approval. While the TM59 methodology is
prescriptive, it is worth bearing in mind that overheating is subjective. The point at which hot becomes too hot is dependent on a variety of factors and will vary from person to person. For this reason, the methodology cannot guarantee that people occupying compliant spaces will always be comfortable. Different strategies can be implemented
to reduce the frequency of overheating; passive strategies are preferable since these require no energy in use. Movable shading is one of the mitigation
strategies promoted by the methodology – fortunately CIBSE also publish a guide, TM37: Design for improved solar shading control, which gives guidance on how to implement this. Modelling shows that external shading is highly effective protection against solar gains. Internal blinds are another option, although less effective. Developers should note, however, that if blinds have been allowed for in the model then they must be incorporated into the final scheme because they will make a significant difference to a dwelling’s performance in a
Anastasia Mylona is CIBSE’s research manager
CIBSE’s newly published TM59 tackles the issue of overheating in homes
heatwave. Solar control glazing can also be used to reduce solar gains, although this has the disadvantage that it will also reduce heat gains in winter when they can contribute to meeting space heating demand. Increased ventilation is another mitigation option for overheating. Often the cheapest and most effective way to ventilate heat from a dwelling is to open the windows. However, this can be difficult in a noisy or polluted environment or where safety or security considerations prevent windows being opened wide. One solution could be to use secure acoustic-attenuated openings to help increase ventilation rates. Single-aspect flats are the most difficult properties to ventilate effectively because these render cross-ventilation impossible; such flats are common in urban areas because of pressures on developers to meet density and commercial targets. In addition to homes, the methodology can also be used to assess overheating risk for student accommodation, care homes and prisons. The methodology will allow different designs to be compared; it will support designs that improve comfort without cooling; and with all consultants using the same methodology to calculate overheating risk, CIBSE’s TM59 will provide consistency across the industry in assessing overheating risk. To encourage uptake of the methodology and to get a standard overheating measurement method adopted, CIBSE has made TM59 available as a download free of charge.
www.heatingandventilating.net
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