by Anthony Lymath
This is the second in an eight-part series of articles, Climate Change Adaptation in Buildings, examining the impact of climate change on the built environment, and the responses that can be made to those changes for both new-build and retro-fitting. The previous article provided an overview of the issues, and forthcoming articles will look at flooding, subsidence, drought and wind.
Until recently, the UK has only been considered to be a ‘cold’ country in design terms – but in reality its seasonal climate can also be hot, with temperatures peaking in the 30-40°C range in mid-summer. In fact, the highest daily maximum temperature record for England is 38.5°C, and even for Scotland is 32.9°C; and the highest daily minimum temperature records are 23.9°C and 20.5°C respectively1. It is interesting to note that all of these records have been set since 1990; and that when considered in the context of the lowest respective temperatures, the UK as a whole has a recorded temperature range of 65.7°C (64.6°C for England).
It is the author’s personal experience, however, that overheating in modern, highly-insulated houses was occurring as far back as the summer 2003 heat-wave...
Overheating has appeared in the news headlines during the summer of 2013, over concerns that home insulation and double-glazing improvements carried out through the government’s Green Deal scheme could increase the risk2. It is not a new concept, however: a news report from 2007 highlighted the problem with new school buildings3, and another example has recently moved to litigation4. The NHBC publication NF46 Overheating in new homes: a review of the evidence quotes anecdotal evidence that overheating began to become noticeable after the 2006 update of Building Regulations Approved Document L5. It is the author’s personal experience, however, that overheating in modern, highly-insulated houses was occurring as far back as the summer 2003 heat-wave, in a house built in 2001 (and probably therefore to the 1995 Building Regulations); and the BRE publication BR364 Solar shading of buildings itself dates back to 1999. Furthermore, it has been identified that some types of property are more prone to overheating than others – top floor flats and new-build detached houses, for example6 – and housing in built-up areas is more vulnerable than in rural locations. This is due to the Urban Heat Island effect7, whereby the temperature in towns and cities is generally higher than in the countryside. The contributing causes of this are:
- Hard and dark-coloured materials, such as paving, absorb heat during the daytime, and then re-emit it at night, causing night-time temperatures to remain high and prevent buildings being able to cool down before the next day
- Air conditioning/ chiller plant outlet vents discharge exhaust heat into the atmosphere.
As a result of this, health and comfort are more likely to be compromised (including, obviously, sleep disruption), and smaller, more compact dwelling spaces are more prone than larger ones8,9, for example flats and attic conversions.
Criterion 3 of the Building Regulations Approved Document L1A states that there is a mandatory requirement to make “reasonable provision … to limit solar gains”10, and goes on to say that this could be achieved “if the SAP assessment indicated that the dwelling will not have a high risk of high internal temperatures”11; the propensity for excessive solar gains being capable of being established by the use of the calculation in Appendix P of SAP 2009. This assessment procedure calculates a ‘threshold internal temperature’ to “estimate [the] likelihood of high internal temperature”12, and Table P2 is used to “estimate [the] tendency to high internal temperature in hot weather”13. According to this table, a ‘high’ likelihood of ‘excessive’ temperature is deemed possible if the calculated ‘threshold internal temperature’ is greater than or equal to 23.5°C. However, the opening paragraph to Appendix P states that it “does not provide an estimate of cooling needs”14 (by which one can infer mitigation measures against overheating), and that the procedure is “not integral to SAP and does not affect the calculated SAP rating”15. In any case, as one industry practitioner observes, limiting the effect of solar gains on internal temperatures “is often achieved by simply stating that windows in a dwelling can be fully opened during hot weather in order to cool the dwelling”16; or that internal blinds can be noted as provided (although once the heat has got inside the building, then the ‘damage’ has already been done).
Local Authorities in particular do not have the resources to be able to check submitted calculations thoroughly, and tend to have little option but to trust the design information submitted by consultants.
Therefore, the usefulness – and authority – of the SAP 2009 Appendix P assessment procedure is questionable, as noted in the DCLG Investigation into Overheating in Homes: Literature Review: “In a significant number of cases it is likely that Appendix P results are being presented to Building Control bodies alongside Building Regulations submissions for new dwellings, due to possibly incorrect interpretation of its legal status”17. In the companion publication, Investigation into Overheating in Homes: Analysis of Gaps & Recommendations, it is also noted that “SAP includes a means of determining overheating risks but as a monthly average model, which uses annual energy data, it is not considered to be an adequate tool to assess overheating risk at the design stage and nor is it intended to be”18. A straw poll of Building Control approved inspectors (both Local Authority and independent) suggests that the current mechanisms are not working as effectively in practice as might be hoped19. Issues identified include mitigating measures such as opening windows being specified for natural ventilation but not capable of being used to their full extent in practice due to noise, health and safety or security issues; or internal solar blinds are being removed by, for example, shop owners. Variations during construction, or poor communication in the design team, can also account for discrepancies between design and as-built calculation results, and the complication of the assessment software is commonly cited as a problem. Local Authorities in particular do not have the resources to be able to check submitted calculations thoroughly, and tend to have little option but to trust the design information submitted by consultants. Still other problems can occur post-approval, whereby end users are either not trained adequately in how to operate the building’s cooling apparatus effectively; or they are simply not aware of the risk of overheating and how to avoid it. Hence the current governing legislation can be seen to be failing to prevent overheating with sufficient reliability, due to a variety of factors.
In the case of non-domestic buildings, the situation is no better. According to the BRE, “The overheating calculation implemented in SBEM version 3.0.b was originally developed to provide guidance for recommendations accompanying EPCs [Energy Performance Certificates]”20 and that an overheating compliance check needs to be carried out “separately outside SBEM”21. What’s more, Criterion 3 of Approved Document L2A places the onus on the design team to determine what an “acceptable indoor environment”22 is, and the following paragraph only refers to solar gains through windows – not through the remainder of the building fabric.
Whereas a number of technical sources offer guidance on overheating, including CIBSE Guide A: Environmental Design (2006), the NHS Heatwave Plan for England (2011), HTM 03-01 and BREEAM credits Hea 02 and Hea 03 (and even these can be claimed simply by demonstrating that the building occupants are able to open a window); none are mandatory, and there is no universally-accepted definition of overheating/ temperature thresholds. Many studies and reports refer to a temperature ‘peak’ threshold of 27°C (e.g. the Orme et al. study Control of overheating in well-insulated housing23 of 2003, and Energy Saving Trust CE129 Reducing overheating – a designer’s guide of 200524) or 28°C (CIBSE Guide A: Environmental Design25) but sleep interruption is thought to occur from 24°C26. Hence there is a direct health impact at a lower temperature than has until now been used to quantify overheating, and on this basis the current design temperatures are probably too high.
So the key issues would seem to be that:
- There is not an effective means of preventing (or dealing with) excess heat build-up in buildings
- There is not a universal standard definition for excess heat
- Increased air-tightness and insulation requirements (due to step-changes in Approved Document L) have not been matched by cooling and/ or ventilation requirements, to prevent or expel excess heat build-up.
One property of materials that can be utilised in construction has been largely ignored in the UK: the concept of thermal mass. This is probably due to the perception that the UK has a ‘cool’ climate, and also because the Building Regulations Approved Documents tend to steer designers to lightweight, highly-insulated solutions which are the antithesis. There are a number of different aspects to high thermal mass design:
- Thermal admittance
The ability of a material to absorb and release heat from an internal space, as that space’s temperature changes, is termed thermal admittance (or heat transfer coefficient), and is defined in EN ISO 13786:2007 Thermal performance of building components. This also provides the basis for the 'Simple Dynamic Model' in CIBSE Guide A: Environmental Design, which is used for calculating cooling loads and summertime space temperatures. The higher the thermal admittance is, the higher the thermal mass will be.
- Thermal mass
Thermal mass, on the other hand, is derived from the specific heat capacity (the ability of a material to store heat relative to its mass), density and thermal conductivity (how easily heat can travel through a material). This is used by SAP 2009 in the form of the ‘k’ (or kappa) value, in calculating the Thermal Mass Parameter (TMP). The ‘k’ value is the heat capacity per unit area of the ‘thermally active’ part of the construction element (only the first 50mm or so of thickness of the element has a real impact on thermal mass, as it reduces with increasing depth into the element; beyond 100mm the effect is negligible). It should be noted that the 'k' value is an approximation, as assumptions are made about the extent of the thermally active volumes of a material; in addition it ignores the effect of thermal conductivity in calculating the period over which heat is absorbed and emitted from the material. ISO 13786 provides a more effective method of determining thermal mass. Thermal mass should not be confused with insulation.
The final related property is called decrement, which describes the way in which the density, heat capacity and thermal conductivity of a material, can slow the passage of heat from one side to the other, and also reduce those gains as they pass through it. This therefore has an influence on the thermal performance of a building during warmer periods. These are termed decrement delay and decrement factor respectively.
By way of illustration of the significance of thermal mass, a wall comprising 200mm brick and 13mm ‘wet’ plaster has a poor U-value of 2W/m²K, but a high admittance of 4.26W/m²K and a high thermal mass of 169kJ/m²K. In contrast, a more modern construction comprising 100mm brick, 150mm mineral wool-filled cavity, 100mm aerated concrete block and 13mm plasterboard drylining on 10mm dabs, has a good U-value of 0.19 W/m²K, but a poor admittance of 1.86W/m²K and also a poor thermal mass of 9 kJ/m²K. By substituting 13mm ‘wet’ plaster for the drylining, the admittance can be increased to 2.74W/m²K and the thermal mass to 60 kJ/m²K. Decoupling the plasterboard can thus be seen to almost completely remove the effective thermal mass in a house built to modern standards.
By alternately storing and releasing heat, high thermal mass effectively diminishes the extremes in temperatures. In warm climates where there is significant temperature variation between day and night ('diurnal' variation), heat is absorbed during the day and then released in the evening when the excess can be either expelled through ventilation, or it can be used to heat the space as the outside temperature drops. Provided the thermal mass has cooled sufficiently by morning, the entire process can then be repeated the next day.
Thermal mass should be directly exposed (or 'coupled') to the internal space. Thermal mass needs to be protected from the outside in order to avoid the influence of high external air temperatures, i.e. where directly exposed to the sun. (However, as has been noted above, the ‘useful’ thermal mass is limited to a relatively small thickness on the interior, and the heat can be conveyed through convection and radiation between other surfaces.) This is achieved by positioning the mass within the insulated building envelope.
Thus it can be seen that the inclusion of thermal mass (in conjunction with effective night-time cooling) is exceedingly beneficial in reducing summertime heat build-up in lightweight construction.