TM59 Overheating Assessment – What is it?
Introduced in June 2022, the new approved document Part O has become a fundamental part of the Building Regulations for new domestic properties.
There are 2 ways to meet compliance:
A TM59 calculation is generally carried out when compliance cannot be met via the simplified method.
It is a much more detailed calculation that can allow for design flexibility.
For naturally ventilated dwellings, compliance is based on 2 main criteria:
1. For living rooms, kitchens and bedrooms: the number of hours during which ΔT is greater than or equal to one degree (K) during the period May-September inclusive shall not be more than 3% of occupied hours.
2. For bedrooms only: to guarantee comfort during the sleeping hours the operative temperature in the bedroom from 10pm to 7am shall not exceed 26oC for more than 1% annual hours.
The criteria for predominately mechanically ventilated dwellings are as follows:
1. For homes with restricted windows openings, the fixed temperature test must be followed – i.e., all occupied rooms should not exceed an operative temperature of 26oC for more than 3% of annual occupied annual hours.
Communal corridors should be included within the analysis, however, there is no mandatory target to meet. If an operative temperature in these areas of 28oC is exceeded for more than 3% of the total annual hours, this will be flagged as a significant risk within the report.
Key points to know about the calculation:
- Accurate weather data is used within the modelling to ensure accurate results
- Fabric construction must be modelled accurately – e.g., U-values
- Glazing must be modelled accurately, - e.g., u-values, g-values, % openable area and opening angle
- Mechanical ventilation must be included if present in dwelling
- Mechanical cooling cannot be included in the assessment
- External shading can be included – e.g., glazing overhangs
- Internal shading such as blinds cannot be included in the assessment
How can UKBC help?
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