Concrete: Exploiting thermal mass

Arups report, 2004: Climate change and Thermal Mass

The report ‘UK Housing and Climate Change: Heavyweight vs. lightweight construction’ Ove Arup & Partners, 2004, opened the door to examine thermal mass in relation to future climate change. Naturally, concrete with its inherent superior thermal mass characteristics became a material to be talked-up as one that will play a part in sustainable construction.

‘With a significant proportion of the UK’s expected new homes to be in the Southeast where climate change will be most pronounced, it will be important to ensure these homes are future proofed for climate change. The alternative is homes that may become prematurely obsolete; requiring the addition of mechanical cooling, increasing energy costs and future increasing greenhouse gas emissions’ ¹

The ‘…research illustrates the relatively poor performance of thermally lightweight construction, as part of a response that includes solar, ventilation and internal heat gain control.‘¹

‘The characteristics that will help climate future proofed home include:

• Large areas of room exposed heavyweight thermal mass to absorb excess heat gains

• Generous volumes of cooler air ventilation particularly during night time with the increasing diurnal temperature swing.

• Good opaque shading to block direct solar gain, ideally outside the occupied rooms

• Limiting ventilation during the hot periods of the day.

• Positioning bedrooms north facing away from direct solar gain

• Domestic appliances with reduced heat output, particularly on 'standby'’ ¹

The report's summary concluded: ‘…mitigation of climate change effects can be achieved by designing homes with thermally massive passive features. This enables us to adapt the way we operate our homes to offset the expected temperature increases. On the other hand thermally lightweight homes would result in substantially higher room temperatures and levels of discomfort.’¹

Arups /British cement Association/The Concrete Centre report, 2007

A second Arups report, this time sponsored by the Concrete Centre in 2007, went a step further and connected EC02 (embodied carbon dioxide) with operational carbon expended in the heating and cooling of housing of different construction types. The report found that though a greater amount of embodied carbon was consumed in heavyweight construction, it was more than outweighed by savings made from the operational carbon load.

‘Four ‘weights’ of thermal mass were considered, ranging from lightweight timber frame to very heavy weight concrete construction. For each case, total EC02 quantities were calculated and predictions for operational CO2 emissions obtained from a 100 year dynamic thermal modelling simulation under a medium-high emissions climate change scenario for south-east England. At the start of the life-cycle, the dwellings were passively cooled in summer, but air-conditioning was installed when overheating reached a certain threshold. The inclusion of thermal mass delayed a year in the life cycle when this occurred, due to the better passive control of summertime overheating. Operational heating and cooling energy needs were also found to decrease with increasing thermal mass due to the beneficial effects of fabric energy storage. The calculated initial EC02 was higher in the heavyweight cases, by up to 15% of the lightweight case value, but these differences were offset early in the life-cycle due to the savings in operational CO2 emissions, with total savings of up to 17% in life-cycle CO2 found for the heaviest weight case.’ ³

However:

It's worth noting that the case study houses in the report where built to 2006 good practice energy efficient housing standards with the following elemental U values:

External walls: 0.27 W/m²K;

Windows and external doors: 1.30 W/m²K;

Ground floor: 0.20 W/m²K;

Roof: 0.15 W/m²K.

Background ventilation rate of 0.5 changes per hour

• The thermal performance of the report houses is less than the current Passivhaus standard where all elements of the external envelope of the building should combine to provide a U-value of not more than 0.15 W/m²K. Thermal modelling the same construction types now would produce different results.

Two other points worth making are:

• The projected climate change data used in the 2006 modelling was based on a medium-high scenario for this century. A different scenario would likely produce different results.

• Secondly, it's important to realise that the studies concentrated on climate change in the south-east of England where temperatures will be higher than at more northerly latitudes. Therefore in other areas of the British Isles, thermal mass becomes either less appropriate or inappropriate.

Concrete, thermal mass and housing

Before the current government closed down its zero carbon policy in 2016 - in addition to its cessation of the Code For Sustainable Homes in 2015, the Concrete Centre published useful guidance to designing cost-effective mainstream Code 5 and Zero Carbon homes² which obviously made much use of standard concrete products ranging from aggregate blocks to floor slabs to roof tiles. Technically obsolescent, both publications continue to be useful points of reference.

In using thermal mass (the) ‘… key function in the design is to reduce the risk of summertime overheating and provide a degree of adaptation to our warming climate. It also has the potential to improve the energy efficiency of the dwelling during the heating season through its ability to store and release solar gains and heat from internal sources.’ ²

Two ways of calculating Thermal Mass

The concept of thermal mass, the flowing of heat into and out of a material, is straightforward. The same can't be said of the ways to calculate thermal mass. Part L of the building regulations refers to SAP and the idea of the ‘k’ value, and this is the methodology most people use.

More sophisticated are the calculations involved in the ‘Admittance’ method set out in ISO 13786. Admittance calculations were initiated 40 years ago by BRE. As ever, the more data involved in the calculation, the more involved it becomes and Admittance calculations are no exception.

At the end of the day though, neither of the above match the current software available that models the thermodynamics of the building with some exactitude.

How SAP calculates Thermal Mass

• Thermal mass was added to SAP in 2009 to take into account its effect on heating and cooling.

• SAP uses the kappa (k) value to determine thermal mass. 'k' is the measure of the heat capacity per unit area in kJ/m2K of the 'thermally active' part of the construction element.

• The calculation uses the material properties:

- the density of the layer
- the specific heat capacity of the layer
- the thickness of the layer

• In addition to the thermal mass in external walls and ground floors, SAP

also takes into account internal partitions, separating walls and upper

floors.

• The 'k' value is a relatively crude way of determining thermal mass. It makes assumptions about the extent of the thermally active volumes of a material and ignores the effect of thermal conductivity in calculating the period over which heat is absorbed and emitted from the material.

Thermal Admittance: a more effective method

• Thermal admittance (Y) is a measure a material's ability to absorb heat from, and release it to, a space over time. This can be used as an indicator of the thermal storage capacity (thermal mass) of a material.
• Thermal admittance is expressed in W/(m²K), where the higher the admittance value, the higher the thermal storage capacity.
• Thermal admittance is calculated as the heat transfer (in watts W) / area (m²) x the temperature difference between the surface of the material and the air.
• The technique of calculating Thermal Admittance is set out in ISO 13786.
• Overall ‘Admittance’ calculations can sometimes underestimate peak cooling capacity where actual climate data isn’t used from say, five day meteorological cycles.

Publications

Freely downloadable from the Concrete Centre: www.concretecentre.com

• Thermal Mass Explained (2009 – 2015)

• Thermal Performance: Part L1A 2013 (2011 – 2014)

• Zero Carbon Performance – Cost-Effective Concrete and Masonry Homes (2014)

• Achieving Code Level Five with Concrete and Masonry (2009)

References

1 ‘UK Housing and Climate Change: Heavyweight vs. lightweight construction’ Ove Arup & Partners, 2004

2 ‘Meeting level 5 of the Code for Sustainable Homes’ MPA – The Concrete Centre, 2009

3 Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change. Energy and Buildings, issue 40, 2008

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