Ji, Y., Fitton, R., Swan, W., & Webster, P. (2014). Assessing overheating of the UK existing dwellings – A case study of replica Victorian end terrace house. Building and Environment, 77, 1–11. http://doi.org/10.1016/j.buildenv.2014.03.012
Introduction
This paper deals with the dynamic modelling of the Energy House in predicted future climate scenarios. A validated model was created in IES VE (IES 2016). This was achieved using a comparison between accurate data gathered experimentally and a simulated version of this experiment, knowledge of the materials and properties of the EH were inputted to make the model as accurate as possible.
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Main Outcomes
A layer of External Wall Insulation (EWI) was added to the model to represent a basic retrofit in the form of a 45mm EPS slab. This may be argued against in terms of breathability and aesthetics, but ultimately EPS is homogenous and has a well characterised R-value and is not significantly affected by moisture compared to other materials, so this variable becomes of less concern. (Jerman & Černý 2012).
Morphed climatic models with data taken from UKCIP02 (Hulme et al. 2002) were used (Belcher et al. 2005) leading to the following analysis of overheating. Following the retrofit, the living room will take until 2050 to start overheating for significant periods, and the bedroom will start to overheat in 2020.
The bedroom is a sensitive area as people spend up to 33% of their time sleeping, are more prone to sleep disturbance at high temperatures (Wang et al. 2015). Lack of sleep has also been shown to have significant effects on health of humans in particular young adults. (Roberts et al. 2009).
The paper is focussed on excess summer temperatures but also considers that whilst overheating is a consequence of the average temperature increase, the model also identified that space heating demand would significantly reduce under the morphed weather data set with a reduction of 30% from 2005 figures to 2080 figures.
Limitations
Due to limitations of the article in terms of size, a number of issues that could have been investigated were not covered. These issues would have been:
• A comparison is not made to the original building, what would happen to energy performance and thermal comfort where the dwelling is kept in its unmodified state. This business as usual or retrofit question is one asked by many researchers, and policymakers. Also the original model was validated on the untreated EH not an insulated one so some very accurate future predictions could have been made.
• This study is also limited to one geographic area, other areas such as southern England have similar types of stock and are likely to have greater changes in average
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• No mitigations or limitation methods are suggested to overcome the overheating issues found.
Research Updates
Many publications have been published since this one on the topic of overheating, however most of them consider new builds rather than retrofit. Research that relates directly to this topic is found to agree with the finding of this paper: Psomas et al declare that most energy renovations for single-family dwellings in moderate climates (central and northern Europe) will overheat. This is particular pronounced when floor insulation and airtightness measures are increased. This can be mitigated by including window upgrades and decreasing the window g-value. (Psomas et al. 2016)
Summary
While this work does provide some detailed understanding of overheating within the Salford Energy House, the major consideration for the wider work is the relationship between the models and measured data. This modelling exercise is unusual, as the model has been calibrated using detailed in situ data. This is unusual as the process of calibrating a model to reflect an actual building is one that takes a significant data collection period, with many measurements required, such as u-values and air infiltration (Marini et al. 2016). This is made slightly easier in the Energy House as many variables can be taken away or added when needed. These calibrations allow for future environmental conditions to be predicted with a higher degree of accuracy.
Section Conclusion
In this section the performance gap issue is introduced showing areas where the performance gaps in terms of the modelling gap can be uncovered. Two types of energy models have been considered, steady state and dynamic. The steady state models are not complex enough to accurately predict actual energy usage in a dwelling, this is well referenced and justified; they are not designed for this purpose. Their use should be to compare buildings, and to assist in some basic design decisions, whilst also acting as a compliance-checking tool. These models are also susceptible to manipulation and error and can be extremely sensitive to errors in the
Page 53 of 85 input of variables such as materials or u-values. However, they are being used to calculate the modelled energy consumption in dwellings, and further used in studies to state whether there is a performance gap or not, this should not be the case. Next the implications of future overheating were analysed using predicated climate data for a building with EWI insulation added. This work was carried out using a calibrated model, thus making the work as accurate as is currently possible, given that the weather file uses predictions. If we consider these isolated pieces of research as a whole, then this leads to the conclusion that more accurate models should potentially be generated before we consider using these figures to calculate a performance gap.
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