CIBSE TM59 Overheating Assessments for Homes
CIBSE TM59 is a recent publication, released in April 2017, which outlines the design methodology for the assessment of overheating risk in new domestic properties such as flats and houses.
Why CIBSE TM59 is Needed
With the ever increasing standard of new build domestic properties, such as improved U-values and air tightness, the heat lost from our homes have never been less. Popular architectural features such as large windows and skylights can also result in excessive amounts of solar gain. Along with poor ventilation, these factors can result in the significant overheating of our homes.
Overheating within the domestic sector is a serious issue, high temperatures can cause heat strokes during the day and affect our sleep at night, in August 2003 a heat wave alone caused an estimated 600 deaths. With the urban heat island effect and increasing impact of climate change, this problem is only predicted to get worse.
The CIBSE TM59 guide has been designed to assess the risk of new build homes overheating. Due to the range of different factors that can affect overheating such as occupancy rates, internal heat gains, and the dwellings overall design, a standard methodology such as the one proposed by CIBSE TM59 is required to assess the risk in any domestic property. The methodology proposed by the publication can also be used to assess the overheating risk to non-domestic residential like buildings such as care homes, student accommodation and hostels.
To take into account the variety of different internal gains and occupancy schedules for the different room types, Dynamic Simulation Modelling has to be used. Along with this, the correct weather file has to be used to accurately simulate summer conditions. The methodology states that the CIBSE DSY1 weather files most appropriate to the buildings location should be selected. This is for the 2020’s, high emissions, 50% percentile scenario.
Other weather files such as the DSY2 and DSY3 weather files that simulate different types of summer conditions, as well future weather files for example 2050’s or 2080’s can also be used to further explore the buildings risk of overheating, however these are not required under TM59.
The CIBSE TM59 Compliance Criteria
To be compliant with CIBSE TM59 both of the following criteria needs to be satisfied:
- For living rooms, kitchens and bedrooms. The number of hours during which DT is greater than or equal to one degree (K) during the period May to September shall not be more than 3% of occupied hours. (This is the same as the CIBSE TM52 overheating criterion 1)
- For Bedrooms Only. To guarantee comfort during sleeping hours the operative temperature in the bedroom from 10pm to 7am shall not exceed 26°C for more than 1% of annual hours.
These two criteria must be met to satisfy CIBSE TM59 for naturally ventilated homes only.
(Although the CIBSE TM52 overheating methodology mentioned previously has a further 2 criteria that can be used to assess overheating, these are for non-domestic buildings. CIBSE TM52 is widely used to assess overheating risk within offices and schools under the Education Funding Agency Priority School Building Programme.)
If a house has restricted window openings, due to noise or pollution issues, and is therefore mechanically ventilated, then the CIBSE fixed temperature test must be followed. The test is described in detail in CIBSE Guide A 2015 and states that all occupied rooms should not exceed an operative temperature of 26°C for more than 3% of the annual occupied hours.
How we can Help
At Ecodraw we have years of experience in producing accurate dynamic simulation thermal models for a variety of different building types, including both small and large scale domestic projects. We can input all of the occupancy and internal gain profiles in accordance with the CIBSE TM59 Methodology and investigate the overheating of your building using the required DSY1 weather files.
The results generated will be shared in an easy to understand report and if any properties fail the criteria, advice can be given on the most cost effective way they can be made to pass.