30th April, 2021
Approved Document [X] – Overheating
Draft guidance on the assessment of overheating in new residential buildings was published in January, along with the consultation on the Future Buildings Standard, Part L and Part F of the Building Regulations.
As part of the open consultation on The Future Buildings Standard, MHCLG has proposed a new Approved Document (Approved Document [X] – Overheating) designed to reduce the risk of overheating within new residential dwellings.
In the draft Approved Document [X] residential dwellings are defined as houses, flats, care homes and student accommodation, essentially, buildings where people are likely to sleep.
Two Compliance Methods
There are two methods of compliance outlined in the consultation:
- simplified method, glazing area to floor area ratio or;
- full dynamic thermal analysis
Simplified Compliance method
The Simplified Method provides guidance based on the location of the new building, split into two areas:
- a) England, excluding Greater London (moderate risk of overheating)
- b) Greater London (significant risk of overheating)
Dwelling types are then split into two groups, proportional to the likely overheating risk:
Group A Buildings (most houses)
Group B Buildings (most flats)
Group A types will include the following:
- More than two fabric elements (ground floor, walls & roof)
- Openings on opposite facades, allowing for cross ventilation
Group B types will include the following:
- Two or fewer fabric elements
- Window opening on facades which are not opposite
Unwanted solar gains, within the simplified method, are managed via setting maximum glazing areas, based on location and group of buildings:
For schemes within Greater London, shading is defined as providing one of the following:
- External shutter with means of ventilation
- Glazing with a maximum g-value of 0.4 and transmittance of 0.7
- Overhangs with 50-degree cut-off on due south-facing facades only
To gain the removal of excess heat there are also minimum free areas set and openings are assumed to be open to a minimum of 60 degrees.
Dynamic thermal analysis method
This methodology is suitable for all residential buildings. It provides the design team with more flexibility over the simplified route. This option will allow dwellings to be overglazed, but with appropriate measures to maximise passive & mechanism ventilation.
Clients can demonstrate compliance by adhering to:
- CIBSE TM59 methodology for predicting overheating risk.
- Defined window opening criteria followed, referencing the internal temperature increases.
- Limits on window opening due to security concerns.
Internal blinds and curtains and external plant are not to be included within the assessment to limit overheating.
Consideration must be made to night-time noise levels when reviewing a CIBSE TM59 strategy that relies on opening windows to achieve purge ventilation.
Noise within bedrooms should not normally exceed the following limits.
When openings are used:-
- 40 dB LAeq, T, averaged over 8 hours.
- 55 dB LAFmax, maximum no more than 10 times a night.
When a mechanical system is used:-
- 30 dB LAeq, T, averaged over 8 hours.
The following may be accepted by Building Control as satisfying this requirement:
a.) Documentation to demonstrate that the local planning authority did not consider external noise to be an issue at the site at the planning stage
b.) Noise measurements of the indoor environment once the building is completed. Measurements should be taken in accordance with the Association of Noise Consultants’ Measurement of Sound Levels in Buildings with the overheating mitigation strategy in use.
c.) Modelling of the noise of the indoor environment, using appropriate external noise measurements, with the overheating mitigation strategy in use.
Sample testing of one of each dwelling design, facing each orientation, will be required if noise measurements are used to satisfy this requirement, with a testing regime to be produced and approved by Building Control.
If planning stage external noise issues are identified either of the following should be followed:-
- Assess external noise and reduce the building’s exposure.
- Design the building to reduce the passage of noise from the external to the internal environment.