Design elements that help prevent overheating

The current prolonged spell of hot weather will have highlighted to many of us the susceptibility of our buildings to overheating.

The issue of overheating in buildings is a serious health risk, figures published on mortality rates during heatwaves make grim reading but still, our construction methods and building regulations fail to deliver buildings resilient to overheating.

In this article, we look at why this is important, how we can design for cooler more comfortable buildings and what to look out for when planning building work.

What is a heatwave?

The World Meteorological Organisation definition of a heatwave is "when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5oC, the normal period being 1961-1990". They are most common in summer when high pressure develops across an area. High-pressure systems are slow moving and can persist over an area for a prolonged period of time such as days or weeks.

They can occur in the UK due to the location of the jet stream, which is usually to the north of the UK in the summer. This can allow high pressure to develop over the UK resulting in persistent dry and settled weather.

When was the hottest heatwave in UK history?

The scorching summer of 1976 was the hottest summer since records began. It led to a severe drought owing to the exceptionally dry conditions, although it is thought that 1995 was drier. In the summer of 1976, Heathrow had 16 consecutive days over 30oC from June 23rd to July 8th, and for 15th consecutive days from June 23rd to July 7th temperatures reached 32.2oC somewhere in England. But the single hottest temperature of 38.5oC was set on August 10th, 2003, the heatwave of 2003 led to the government creating a Heatwave Plan for England in response to the increase of deaths directly attributed to the heatwave.

You can read the Heatwave Plan here https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/711503/Heatwave_plan_for_England_2018.pdf

The current 2018 summer heatwave has seen the hottest day of the year so far recorded on Sunday, July 8th  a temperature of 32.4oC (90F) recorded in Gosport, Hampshire.

Outdoor Temperature Thresholds – the effects on health

Statistical analysis shows that maximum daytime outdoor temperatures are a predictor of heat-related mortality.

In London, mortality starts to rise when the maximum daily external air temperature goes above 24.7ºC and has been estimated to rise by approximately 3% for every further 1ºC increase in external temperature. In other regions, the thresholds at which mortality starts to rise are lower. For example, the threshold for the North East of England is 20.9ºC.

Heatwave Plan Regional Threshold Temperatures for Heat-Health Watch Alert Levels 2-4.

Design considerations to prevent overheating - decrement delay and thermal buffering.

Anyone familiar with spending a hot summer's day in a caravan and then another in a stone house with closed shutters will appreciate the meaning of ‘Decrement delay’. The inside of the caravan closely maps the rise and fall in external temperature to provide the familiar stifling effect on the occupants. In contrast, the internal air temperature of the stone house stays well below the midday heat, barely varies throughout the day and so provides relative comfort to those sheltering from the sun.

In the caravan, as soon as the outside cladding starts to heat up, output is detected within minutes on the inside face as most of the heat quickly transfers through the aluminium / lightweight insulation composite; whereas as the face of the stone wall heats up, the heat is absorbed by the stone and progresses slowly from the outside inwards. Hours later, some of that heat has arrived on the inside face of the wall whilst the remainder is released back into the cooler evening air.

The interesting, and often baffling, aspect of this phenomena is that the two materials can have very similar u-values - so that in steady-state conditions where heat applied at a constant rate over a period of time to the external face of both materials, there is an equally constant flow of (diminished) heat from the inside surfaces. Crucially though, for the purposes of thermal design, one material will start delivering heat to the inside before the other.

Heat transfer factors: Conductivity, Density and Specific Heat Capacity

But let’s start with understanding how different materials cause different heat flow rates.  What’s actually happening within the materials? The answer relates to the dynamics acting between three variable characteristics whose values are unique to each material. The rate of heat transfer is determined by:

And of course, a further factor is the quantity or thickness of the material the heat is transferred through.

Thermal Diffusivity

Thermal Diffusivity ties the above factors together into an equation that measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy.  In effect, it is a measure of thermal inertia or ‘buffering’.

The equation is:

Thermal diffusivity = thermal conductivity / specific heat capacity x density

Examples:

  • Rigid polyurethane insulation has a thermal diffusivity of approximately 4.46 x 10-7 m2/s
  • Timber fibre insulation has a thermal diffusivity of approximately 1.07 x 10-7 m2/s
  • Copper has a thermal diffusivity of around 1.11 × 10−4 m2/s

In a material with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.

In the above three examples, we can see that heat races through copper while it moves more rapidly through rigid polyurethane than it does through timber fibre board.

Introducing the variable heat source - Periodic heat flow

‘Thermal diffusivity’ then accounts for the different rates of heat transfer through the variety of materials from a constant heat source.

However, when we look at the transference of the heat from the sun striking real-world roofs and walls, the heat source is not constant. The variability caused by such as the sun’s passage through the sky is known as ‘periodic heat flow’ and, because the sun behaves, in the same way, every day (‘diurnal’), its effects can be designed for.

Decrement delay

So, what is ‘Decrement delay’ and what can it do for us? In discussing its use we’re looking to take advantage of the fact that some materials are slower at transferring heat than others. In buildings, the benefits of decrement delay are only realised where the outside temperature fluctuates significantly higher and lower than the inside temperature. So, ideally, if the maximum heat delivery to the wall or roof is at around midday, and because we can achieve a delay in heat transfer, it should be possible for that heat to finally penetrate the wall or roof to the interior of the building some time later when the inside of the building is relatively cool and the outside temperature has fallen as the sun goes down. At this point, the heat ‘stored’ in the wall can be released from the wall in both directions without overheating the inside.

The time it takes the peak temperature in the middle of the day on the outside of a wall or roof, to make its way to a peak temperature on the inside face, is called 'time lag'’ phase shift or, more commonly, 'Decrement delay'.

By controlling decrement delay it is possible to control and prevent the overheating of a building.

A delay of between 8 and 12 hours might be considered optimum in normal conditions during extended periods of hot weather and cloudless skies up to 16 hours may be required.

It’s worth noting too at this point that in a construction element containing several layers, the sequence of the material layers that heat passes through is also a factor in determining decrement delay. For example, in masonry construction where insulation forms one layer, locating it on the exterior of the masonry can significantly enhance the decrement delay effect.

The image below shows a roof make up with a variation in decrement delay (phase shift) achieved only by changing the insulation type, the overall section U value remains the same but the difference in phase shift is 8.8hours between the maximum at 16 hours and the minimum at 7.2 hours.

Image courtesy of Steico


How Decrement delay / Thermal buffering works on a rendered stone wall



 

Amplitude damping and the ‘Decrement factor’

A stable indoor temperature is an aspect of thermal comfort. In conditions where the outside temperature fluctuates relatively widely, a constant indoor temperature is desirable. For example, an outside temperature might vary between 10ºC and 30ºC while internally it might vary just 1ºC above or below 20ºC. The fabric of the building envelope has effectively dampened the degree of oscillation.

The ability to attenuate the amplitude of the outside temperature to that of the inside is known as the 'decrement factor'.

It is calculated as f (the decrement factor) = Ti (the maximum swing from the ambient temperature on the inside) / Te (the swing in external temperature)

Example: From above, the peak to peak amplitude of the outside temperature is 20 degrees, and for the inside temperature it is 4 degrees.

f = Ti / Te = 1 / 10 = 0.1

Hence the closer the decrement factor approaches zero, the greater the effect the construction element has on attenuation. I.e. the smaller the decrement factor, the more effective the wall / roof at suppressing temperature swings.

The decrement factor is determined by the type and thickness of the materials that make up the wall or roof that the heat passes through.

(Confusingly, the same effect or 'amplitude suppression' is often expressed as a straight ratio. Taking the example above, the ratio would be 10:1 or 5:1 or more often just '10')

For a construction element containing several layers, the sequence of the material layers that heat passes through is a factor in determining decrement delay. For example, in masonry construction where insulation forms one layer, locating it on the exterior of the masonry can significantly enhance the decrement delay effect.

Calculating decrement delay

The response of construction elements to periodic cycles in temperature and heat gain can be quantified by using the thermal admittance framework as described in EN ISO 13786:2007. The framework also provides the basis for the CIBSE 'Simple Dynamic Model' for calculating cooling loads and summertime space temperatures (CIBSE (2005) Guide A: Environmental design).

Manually calculating thermal response simulations is not for the faint-hearted, but a number of programs are available to take the load - notably the freely available, Excel spreadsheet based 'Dynamic Thermal Properties Calculator' developed by Arup and distributed by the Concrete Centre.  https://www.concretecentre.com/Publications-Software/Publications/Dynamic-Thermal-Properties-Calculator.aspx

The importance of decrement delay

Decrement delay will nearly always be more important in hotter climes than in the UK. An exception though is in timber/steel frame construction. One of the more common criticisms directed at lightweight construction is the lack of thermal mass - which can lead to the familiar 'caravan effect' above. Whereas masonry construction has the obvious benefit of 'heavy' materials such as brick and block, framed structures are typified by combinations of a cavity and lightweight insulation - leading to low thermal diffusivity and so little in the way of decrement delay.


Steico Special Dry Wood fibre insulation boards used on a pitched roof. Picture Courtesy Kithurst Builders Ltd

Until recently, insulation products were chosen mostly on the basis of a combination of their u-value and their thickness. Since the most commonly used insulation materials such as polystyrene, polyurethane and mineral wool had broadly similar densities and heat capacities, their decrement capabilities were relatively insignificant. With the increasing availability of wood fibreboard insulation materials in the UK, boasting comparatively high levels of diffusivity, designers can look forward to realising thermal performances more closely mapped to traditional masonry construction.

Some examples of materials

Don’t building regulations already set a standard?

Approved Document Part L1A is designed to drive the conservation of fuel and power, rather than set thermal comfort standards. It requires housebuilders to make “reasonable provision to limit heat gains” in dwellings in order to reduce the need for mechanical cooling. Specific criteria or thresholds are not specified. The overheating ‘check’ in SAP Appendix P provides a means of demonstrating that reasonable provision has been made, but the calculation is not integral to the SAP rating and it is unclear what happens if a development fails the test.

The dwelling should have appropriate passive control measures to limit the effect of heat gains on indoor temperatures in summer, irrespective of whether the dwelling has mechanical cooling. The guidance given in paragraphs 2.38 to 2.42 of this approved document provides a way of demonstrating reasonable provision.” Criterion 3, Approved Document Part L1a

The Standard Assessment Procedure (SAP) is the Government’s procedure for rating the energy performance of homes.

Designers and developers in the UK need to show compliance with SAP for each of the domestic units they are designing. It is not a design tool, but rather a compliance tool intended to produce an energy rating.

SAP Appendix P is a simplified check of whether the home could have an overheating problem. It uses regional average external air temperatures for the months of June, July and August, heat gains and fabric characteristics of the building in order to calculate monthly mean summer internal air temperatures. These are then compared to a table of threshold temperatures. For monthly mean internal temperatures below 20.5°C, overheating risk is predicted to be ‘not significant’, whereas for temperatures of 23.5°C and above, the risk is ‘high’.

Issues with SAP

The temperature inside a home depends on many variables and changes throughout the day. Overheating risk during severe hot weather events cannot be calculated using monthly average temperatures. Neither can the impact of the Urban Heat Island effect or future changes in climate.

Housing Providers and experts consulted by the Zero Carbon Hub raised many concerns with SAP Appendix P. For example, it allows unrealistic assumptions to be included, such as that windows are constantly open, which make it too easy to pass.

SAP 2012 (Appendix P): Levels of threshold temperature corresponding to a likelihood of high internal temperature during hot weather.

To sum up

We have to acknowledge that overheating is a problem to be avoided and that we don’t need a heatwave to prod us into action, buildings can overheat for a multitude of reasons but the use of materials that help buffer heat preventing rapid transfer through the building can be used to substantially mitigate the risk. Typical areas that will benefit from such design are timber frame and lightweight structures, on a brick or masonry house this will usually be the roof. This is an important area to factor in heat buffering as it combines the warmest part of the house (hot air rises) with the largest solar collector on the building – the roof itself. Wherever possible design with overheating in mind, the Building Regulations do not require minimum standards for decrement delay and SAP is currently under review regarding overheating, so the choice to design and build to prevent overheating is one that rests solely with you.

 Further reading

 BRE Overheating in Dwellings

Tackling Overheating in Buildings

Overheating - a growing threat that mustn't be ignored

 

 Our thanks to Sandy Patience Dip Arch RIBA editor of www.greenspec.co.uk for material used in this article

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