The report is an Energy Follow Up Survey (EFUS), based on previous analysis.
The project sought to answer the following question ‘Do measures which reduce home energy demand in the winter increase the risk of overheating in the summer.’?
The research involved a review of the high-quality evidence relevant to UK homes published in peer-reviewed literature. The data set was also re-analysed to calculate the prevalence of overheating and the frequency and intensity of exceedance of temperature thresholds ranging from those signifying cold discomfort to those indicating overheating. The sensitivity of overheating prevalence to changes in outdoor temperatures was also explored using a new analysis method, temperature signatures.
The dataset included temperatures measured in 616 living rooms and 591 main bedrooms between May and September 2018. Meta data enabled comparisons to be made for houses and for flats which had: a high or low energy efficiency rating; cavity or solid walls; reasonable loft insulation; insulated walls; and full double glazing. The results of analyses were weighted to be representative of the whole stock of English homes.
The prevalence of overheating in the living rooms of flats was three or more times greater than in houses. Therefore, around 16% of flats would be classed as overheated by current definitions. The top floor flats exceeded overheating thresholds far more frequently, and were more sensitive to outdoor temperature increases, than other flats. Ground floor flats were cooler and less sensitive than other flats. Bedrooms of houses were hotter at night than living rooms were during the day, they were also more sensitive to outdoor temperature increases.
The principal finding was that the overall energy efficiency rating of English houses and flats did not significantly increase the risk of summertime overheating in either living rooms or main bedrooms, neither did the installation of individual energy efficiency measures installed in the sample. There is evidence that increased loft insulation and wall insulation may significantly reduce the risk of serious overheating in the bedrooms of houses. The evidence review supported these findings.
The work provides evidence that energy efficiency measures are effective at raising internal temperatures in the heating season. Consequently, the occurrence of temperatures associated with cold discomfort was significantly lower in the living rooms of houses with a better energy efficiency rating, full double-glazing (with cavity2, rather than solid walls). Double glazing kept flats warmer without increasing overheating risk. No other energy efficiency measure had any significant effect on the exceedance of any of the temperature thresholds.
The review produced evidence of an increased risk of overheating in houses retrofitted with internal wall insulation (not a prevalent measure in the EFUS sample) - but that this may be readily mitigated with pre-existing ventilation and shading provision.
In conclusion it was found that more work is needed to understand the risk for overheating, and the effectiveness of mitigation measures, in dwellings built to energy efficiency standards beyond those found in the EFUS2017 homes.