Friday, 28 July 2017

Bright future

With record June temperatures and the launch of CIBSE's new TM59, the built environment is starting to get tough on overheating. Sara Kassam, Head of Sustainability Development at CIBSE, takes a look at a paper presented at this year’s CIBSE Technical Symposium set out to evaluate both the overheating risk and suggest appropriate mitigation strategies

Retrofitting homes and building new homes with high levels of fabric insulation, is a key tenet of the UK government’s carbon emissions reduction strategy. But, while a well insulated home may require very little heating in winter, without minimizing solar gains and without the implementation of sufficient ventilation strategies, in the summer, high levels of insulation can increase the likelihood of a home overheating. 

Research by the University of Sheffield, based on computer modelling, has shown that with increased levels of insulation overheating is a problem, one that is set to get worse under the impact of climate change. However, the research also shows that with a few simple mitigation strategies a well-insulated home will enable the occupants to benefit from enhanced winter comfort without the risk of overheating in summer.

Computer modelling can predict overheating based on a variety of factors
The findings were presented in the paper Evaluating Overheating Risk and Mitigation Strategies in Low Energy Houses in the UK: Two Sheffield Case Studies, at the recent CIBSE Technical Symposium. For the study the researchers selected two detached homes in the Sheffield area. One was a new-build home built to Passivhaus standards. The other was an existing home, which had been retrofitted to Zero Carbon standards. 

Evaluation of the likelihood of overheating in both homes was undertaken using Passive House Planning Package steady-state energy balance software based on the East Pennines climate dataset with altitude adjusted to reflect the actual location. Future climate datasets used for the analysis were developed by the University of Exeter for both the 2050s and 2080s, under the high emissions scenario, based on the UKCP09 climate projections. 

Overheating metrics were based on the PHPP maximum default temperature of 25℃, which must not be exceeded for more than 5% of the time to ensure good comfort levels. If the 25℃ threshold is exceeded for 10% or more of the time then, under PHPP additional summer heat protection will be necessary.

Results of the modelling for the retrofit house:
Pre-retrofit, in the current climate, no overheating is predicted. 
With the building retrofitted to Zero Carbon standards overheating occurs for 2% of the time
By the 2050s, the Zero Carbon standard building will overheat 8% of the time
And by the 2080s, overheating will occur 13% of the time

Results of the modelling for the new build house:
In current climate, PHPP predicts overheating will occur for up to 32% of days annually.
By the 2050s, that figure will have risen to 52% 
And by the 2080s, 57% of days will be above the overheating threshold 

The researchers suggest some appropriate mitigation strategies for the retrofit house: 

Modelling predicted the new build house would overheat on the majority
of days by the 2080s

Reducing wall insulation
The results suggest that overheating could be reduced by 2%, in both the 2050s and 2080s climate scenarios by reducing wall insulation from Passivhaus to Building Regulations standard. Reducing roof insulation levels made no improvement on overheating frequency in the 2050s and only a 1% improvement by the 2080s.

Ventilation
Opening windows for night-time ventilation to provide 1.5 air changes per hour suggested overheating frequencies could be within the 10% figure in the 2050s. However, by the 2080s the same night cooling strategy would lead to a 12% overheating frequency.

Shading
Internal and external shading will reduce the amount of direct summer solar gain, reducing overheating risks. In the retro-fit house, internal shading reduced overheating frequency massively to keep it within 10% for both the 2050s and the 2080s scenarios. External permanent shading was also modelled with similar results.

For the new build, external vertical blinds with horizontal slats were most effective. External blinds eliminated overheating in the current climate and reduced overheating frequency to 10% and 14% in the 2050s and the 2080s future climate scenarios.

Strategies from solar shading to natural ventilation can be used to mitigate the
risk of overheating in the future
Reducing transmitted solar energy
In the retrofit building the glazing g-values (the value of solar energy transmitted through the glass) was reduced from 0.62 to 0.3, reducing solar energy permeability from 62% to 30% produced a significant reduction in the overheating frequency so that in the 2050s overheating would occur 2% of the time, rising to 5% in the 2080s. In the new-build, the g-value was reduced to 0.25, which decreased overheating frequency to 3% in the current climate context. However, reducing g-values was less effective in the future climate scenarios and led to an increase of space heating demand.

Combined strategies
For the retrofit house, it was found that night-time ventilation combined with temporary internal shading and permanent external shading eliminated overheating risk entirely in the 2050s, while in the 2080s overheating only occurred for 2% of the time. For the new build house a combination of reducing glazing g-values and adding external blinds eliminated overheating in the current climate and reduced it to 3% in the 2050s climate context. However, in the 2080s, there will still be 12% overheating frequency.

The findings show that while low energy housing standards such as Passivhaus can lead to reduced space heating demands the modelling also showed that over time the changing climate would lead to a higher frequency of overheating occurrences. However, the modelling also showed that with appropriate overheating mitigation strategies overheating risks could be predominantly eliminated in current and future climate scenarios. A robust solar heating mitigation strategy for houses in Sheffield would be: internal or external shading, night-time ventilation and reduced glazing g-values.

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