Overheating in homes

This post was written by BSRIA's Saryu Vatal

This post was written by Saryu Vatal, Senior Consultant of BSRIA’s Sustainable Construction Group

BSRIA’s Residential Network organised an event on the 22nd of July focussing on the issue of overheating in homes with an excellent line up of speakers. Nicola O’Connor started the day summarising an extensive research project by the Zero Carbon Hub that brought together input from government, industry and academic experts to understand the challenges around tackling the risk of overheating in homes (http://www.zerocarbonhub.org/current-projects/tackling-overheating-buildings). Chris Yates from Johnson and Starley made an appraisal of the assumptions and requirements within the Building Regulations and associated guidance as well as the implications for mechanical ventilation system manufacturers. Neil Witney from DECC explained the challenges around defining and regulating of overheating within homes, current policies and mechanisms that may be introduced in the future in response to the growing body of evidence highlighting the issue. Paul Ciniglio from First Wessex shared the organisation’s findings from several research projects and experience from their own developments, which resonated with issues highlighted by members of the audience. Bill Gething of Sustainability + Architecture and professor at the University of West England brought into perspective how changes in the way homes have been designed and built over the recent years has led to a shift in the performance of homes. James Ford, partner at Hoare Lea discussed some key considerations for designers to address the issue at early stages, to help minimise risk and dependence on active cooling solutions.

Extent of overheating

Evidence indicates that up to 20% of homes in England may already be overheating. Areas where additional risks have been highlighted include:

  • Common areas in apartment blocks, especially where community heating is installed – these areas are not assessed using SAP as they are outside the dwelling envelope. In reality, being unoccupied spaces these are often not modelled for their thermal performance (and energy use) at all. Community heating is being incorporated in an increasing number of projects and the supply network remains live even in the summer to meet the domestic hot water demand. Ensuring that the specification and installation of insulation for the distribution pipework is adequate is becoming increasingly important as buildings are made more airtight. Often stairwells and circulation areas have a high proportion of glazing and, with recent improvements in the general standard of construction and materials, tend to retain a large proportion of the heat gains. It is now important to incorporate a ventilation strategy for these spaces so that the accumulated heat can escape.
  • Urban areas – the average temperatures in city centres can be more than 4°C higher than rural areas. Flats are more common to city centres and these are often close to sources of noise and air pollution and have limited, if any, potential for cross ventilation. All these factors can combine to limit the effectiveness of natural ventilation in addressing the build-up of heat and not just in the summer. Building designs that incorporate large proportions of glazing in their facades, such as penthouses, if not carefully designed, can require air change rates that are unrealistic to achieve, using natural or mechanical ventilation systems.

Need for a definition

A number of sources and definitions are being referred to currently when evaluating for the risk of overheating in homes. These include CIBSE’s Environmental Design Guide A (2006) which sets standards for comfort, although it is not mandatory to use this to demonstrate compliance with the Building Regulations. Dynamic modelling through tools such as TAS and IES offer the opportunity of making a more comprehensive evaluation than SAP, but this option is skill, time and cost intensive. Building Regulations do not relate to limiting overheating for thermal comfort, just limiting the use of fuel and power for air-conditioning. The minimum evaluation for demonstrating compliance with Criterion 3 of Approved Document Part L of the Building Regulations needs to be carried out using SAP. While SAP is not intended to be a design tool, it is accepted that it is the default tool the industry uses widely.

Research projects have highlighted that dwellings can demonstrate a risk of overheating when evaluated against the CIBSE standard but not when modelled in SAP. Surveys from the Zero Carbon Hub study showed that nearly 60% of the housing providers surveyed had checks in place to assess the risk of overheating. However, only 30% of these housing providers explicitly included the requirement for considering the risk of overheating as part of their employees’ requirements to architects and designers. This suggests a missed opportunity for the issue to be addressed early on in the process, when cost and energy efficient measures may be effectively incorporated.

There are several challenges around the definition of conditions under which overheating can be said to occur as several factors contribute to this, including but not limited to air and radiant temperatures, humidity, air velocity, level of activity the adaptability of the individual. There are several checks that can be built into the design process which can help identify the risk at an early stage and allow for a method for mitigating these to be set up and followed through.

Contributing factors
The energy efficiency of homes in the UK has improved significantly in terms of reduction of space heating loads. This has come about in new homes through Approved Document Part L 1A of the Building Regulations and in existing homes through schemes such as the Green Deal. Homes are now less leaky and better insulated to keep warmth in but attention and emphasis is needed on measures to facilitate the expelling excess heat adequately when temperatures rise.

Homes are expected to provide comfortable conditions for occupants all year round and through a range of different occupancy patterns, which may in reality be considerably different to the standard assumptions made in modelling tools like SAP. It is possible that if modelling for thermal comfort is carried out assuming worst case assumptions for occupant density, external conditions and hours of occupancy, many homes would require mechanical cooling. There are, however a number of common sense measures that can be applied to ensure the impact of key contributing factors are minimised. These include controlling solar gains from south and west facing glazing and making provisions for adequate, secure ventilation especially when thermal mass has been incorporated in the structure.
The current extent of overheating in homes must be seen in the context of the anticipated changes in climate. With external temperatures expected to rise with an increased frequency of extreme weather conditions, homes built today must be fit for purpose for warmer summers.

Mechanical cooling?
There has been a rise reported in the installation of mechanical cooling systems in homes in the UK, more noticeably so in the south. While this may be an expected feature in high end homes, the cost of running these systems can be prohibitive, or at least perceived as so, for households where minimising expenditure on energy and fuel is a priority.
There is potential to develop low carbon mechanical cooling systems such as reversible heat pumps. The large scale uptake of these can however have some serious implications for energy supply and the capacity of the grid to accommodate a draw in peak summer months.

Way forward
In addition to affecting comfort, exposure to high temperatures over prolonged periods can have a significant impact on the health and well-being of residents. It is critical therefore to agree on a set of parameters that can help define overheating in homes and this should be carried out with input from bodies such as Public Health England.
Until a definition and modelling strategy is developed, designers and housing providers can refer to several good practice guides and research studies that help embed a common sense approach to design. There is significant potential to mitigate the risk of overheating in homes if early stage design decisions are taken with due consideration for the issue. The limitations of mechanical ventilation systems to help achieve comfort in homes must be acknowledged so that the final burden of an ill-considered design does not rest with the occupants.

References and further reading
http://www.zerocarbonhub.org/sites/default/files/resources/reports/ZCH-OverheatingInHomes-TheBigPicture-01.1.pdf
Design for Climate Change, Bill Gething and Katie Puckett, RIBA Publishing Feb 2013
http://www.arcc-network.org.uk/wordpress/wp-content/D4FC/01_Design-for-Future-Climate-Bill-Gething-report.pdf
http://www.zerocarbonhub.org/sites/default/files/resources/reports/Understanding_Overheating-Where_to_Start_NF44.pdf

To find out more about our Residential Network and to download the presentations from this meeting check out BSRIA’s Network pages.  To find out more about all of BSRIA’s networks contact tracey.tilbry@bsria.co.uk.

Proving the future – how to keep up with Building Regulations

"From a standing start in 2006 to today, the builders have grasped the importance of air tightness testing as a proxy for quality of construction and the contribution good airtightness makes to energy efficiency" Mike Smith, Engineering Director

“From a standing start in 2006 to today, the builders have grasped the importance of air tightness testing as a proxy for quality of construction and the contribution good airtightness makes to energy efficiency” Mike Smith, Engineering Director

The rapid adoption of airtightness testing and the ability of the industry to achieve the right result first time in 89% of tests is one of the success stories of the UK construction industry over the past decade. The BSRIA Compliance team tested over 10,000 dwellings and 720 non-dwellings in 2012 and found the average dwelling airtightness value was 4.89 m3/(hr.m2) envelope area at 50 Pa (against a maximum regulatory value of 10 m3/(hr.m2)).

From a standing start in 2006 to today, the builders have grasped the importance of airtightness testing as a proxy for quality of construction and the contribution good airtightness makes to energy efficiency. The testing itself is rigorous, robust and, arguably, now at a very low economic price. It has respectability provided by UKAS accreditation for non-dwellings testing, the training of testers and, in the case of dwelling testing, registered testers through the Airtightness Testing and Measurement Association (part of the British Institute for Non-Destructive Testing).

The mantra should be “Build tight, ventilate right”. As fabric standards improve, driven on further by the 2013 Building Regulations, the role of passive and mechanical ventilation systems increases in importance. Unfortunately in the world of unintended consequences, we are seeing dwellings achieving better airtightness values than the designer intended which of course means less air leakage (and associated energy waste), but this is only useful if the designed-in ventilation systems can cope with these outcomes. In a nutshell the infrastructure supporting domestic ventilation engineering has not developed at the same pace as the improvement in building airtightness.

There is of course significant current activity to help remedy this problem but, as is so often the case, we are now on the back foot with increasing numbers of examples of poor installations and the inevitable questioning of the value of mechanical ventilation solutions.

The systems we are talking about are not complex but they are sensitive to errors. What is missing is not so much the technology or science but the widespread creation and adoption of proper codes of practice. Mechanical ventilation (MV) systems and the more complex MV heat recovery (MVHR) systems have to be site tested to ensure they are extracting and supplying appropriate amounts of ventilation. In the course of its compliance testing BSRIA is seeing two main kinds of problems.

The first is the performance of the specified equipment in a given situation, i.e. that the fan is correctly selected to match both the actual application and the inherent system losses that the system components will introduce. In simple terms this comes down to understanding the resistance characteristics of ductwork and its routing and the resistance of terminal units both inside and out. There is a widespread misunderstanding that ventilation fan outputs are usually quoted with outputs measured in “free air”. In reality they have to overcome backpressures from fittings. Even where kits are bought we see alternative terminal units used, usually to meet architects demands for aesthetics.

The second is the actual installation of the associated ductwork where there is a very poor understanding of the dramatic effect on performance that can arise from bad workmanship.

In a recent case BSRIA found approximately one metre of flexible ductwork that had been stuffed into the cavity wall for a straight through the wall installation that is approximately 300 mm thick. An additional 100 mm dogleg had been introduced on site to match the actual positioning of a porch structure. The result was a lot of fan noise with almost zero movement. The fan, when bench tested with zero back pressure, had a performance of 22 l/s, the designed performance including the ducting was 20 l/s however the actual performance was 5 l/s.

As part of the “catch up” in dealing with the rapid rise in the use of domestic ventilation we have identified that the act of measuring MVHR performance using published guidelines will give false results if the correct equipment or correction factors are not used. There is an easy remedy but not widely used at present. The automatic volume flow meter with pressure compensation – more commonly known as a “powered diff” will provide an instantaneous and accurate value. A more common hooded anemometer will impose a back pressure on the terminal, ducting and fan under test and the readings must be corrected (post use) specifically for both the anemometer model and the actual fan under test. More detail on this can be found in BSRIA’s “Domestic Ventilation Systems – a guide to measuring airflow rates – BG46/2013”.

And all of this is compounded by a lack of thinking regarding operational needs, limited controls, and poor instructions to the user, especially on what maintenance is required to keep performance at its peak.

So, airtightness demands have led to unforeseen consequences and something of a reaction against the use of mechanical ventilation. What then can be done to avoid making the same mistakes on other systems and concepts?

With fabric issues now largely dealt with in the Building Regulations it is likely that new focus will fall on the efficiency and operation of the MEP services in dwellings. If modelling and measuring the thermodynamics of a brick wall is difficult imagine how complex a multivalent heating system is going to be! And before being put into use, these complex integrated systems will need commissioning and possibly proving as well.

The Zero Carbon Hub has recognised that we will need to devise new test methods and regimes that, for example, will evaluate how the solar thermal collector performance meets expectations when linked with the ground source heat pump system that serves hot water generation, underfloor heating and thermal storage, in concert with a biomass boiler or room heater. Before regulation stimulates the market we need to have good practice guidance and proven on-site commissioning and test processes in place. This work is urgent and needs significant central support. With the next revision of Part L expected for 2016 – this time aimed at achieving zero (or nearly) carbon homes, time is not available to embark on a protracted negotiation with innumerable and varied industrial interests. Certainly industry’s support will be available but only for a properly directed and centrally funded programme.

If we fail to put into place a mechanism to improve the on-site verification of performance of new systems we will only have ourselves to blame for the next set of well publicised “failures to launch” and the consequent set back of achieving national aims.

BSRIA provides a range of Compliance Testing services for stress-free compliance to Building Regulations including airtightness (Part L), sound insulation (Part E) and ventilation testing (Part F).

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