Approved Document O: Overheating, frequently asked questions
Answers to frequently asked questions on Approved Document O, which sets standards for reducing overheating risk in new residential buildings.
Applies to England
These frequently asked questions (FAQs) refer to the Overheating: Approved Document O.
General
1. Does Part O apply to extensions and conservatories?
Part O only applies to new residential buildings. See paragraph 0.3 of Approved Document O where this is explained further. Conservatories on new residential buildings must meet the requirements of Part O.
Part O does not apply to extensions or conservatories added to residential buildings after they are built.
Part O does not apply to buildings undergoing a change of use.
Simplified Method
2. Why do the glazing limits for high risk locations sometimes allow for larger glazing areas than lower risk locations?
The simplified method for buildings in high risk locations includes shading standard, which is factored into the glazing area calculations. This means that glazing areas for west facing buildings in high risk areas can sometimes be larger than buildings in moderate risk areas. Paragraph 1.6 of the Approved Document states all the standards for limiting solar gains that should be followed.
For further information on shading in high risk areas see paragraph 1.9 of Approved Document O.
Residential buildings or parts of buildings in a moderate risk location can be designed to meet the glazing limits of the high risk location provided they also meet the shading requirements of the high risk location.
3. How are g-values calculated?
The g-values in Approved Document O are centre pane g-values and should not include the effect of frames.
4. Tables 1.1 and 1.2 give guidance for glazing areas. Which standards should be selected when the two most glazed façades are equally glazed?
When two facades of a building have the same area of glazing, the largest glazed façade orientation that results in the higher (more stringent) standard should be used.
5. Tables 1.1 and 1.2 give guidance based on the compass direction of the façade (north, east, south, west). Which standard should I use when the façade with the largest amount of glazing is midway between two compass points?
The closest direction should be selected. For example if a façade is facing east-south-east, the standard for east should be used.
If a façade is facing exactly between two compass points, the higher (more stringent) standard should be used. For example if a house points due south-east, the standard for south should be used.
Interpolation or averaging of the values in the tables is not suitable for the simplified method.
6. Do buildings in central Manchester have to follow the high or moderate risk values?
When following the simplified method for buildings in central Manchester, the minimum standard is to use the moderate risk values. However, central Manchester may have elevated night-time temperatures. Designers may want to consider following the guidance for higher risk locations for buildings in the postcodes listed in Appendix C of Approved Document O.
7. Paragraph 1.9 states that ‘Overhangs with 50 degrees altitude cut off on due south facings facades only’. What does this mean?
This is describing that a south facing window should be shaded completely when the sun is at a solar altitude angle of 50 degrees or higher. See figure below for overhang design.
8. Can you provide an example of how to use Appendix D to calculate the equivalent and free areas?
Both the total minimum free area and the bedroom minimum free area should be assessed when using the simplified method. Where the total minimum free area is being assessed, the equivalent areas of all of the openings in the building or part of building are added together. This example calculates the bedroom minimum free area for ease of explanation.
This example is for a house in a moderate risk area in a building with cross-ventilation. The bedroom has a floor area of 12m2. In the initial design the bedroom window is side hung, 2m high, 0.50m wide with a maximum opening angle of 30 degrees.
1. Select the bedroom minimum free area, given as a percentage of the floor area of the room, from Table 1.3 or 1.4.
This bedroom window is in a house with cross-ventilation, in a moderate risk location. The bedroom minimum free area is therefore selected from Table 1.3 and is 4% of the floor area of the room.
2. Convert the bedroom minimum free area for this bedroom from a percentage of the floor area of the room into an area.
The floor area of the room is 12m2 and the standard from Table 1.3 is 4%.
The bedroom minimum free area is therefore 0.48m2.
3. Using the tables in Appendix D, look up the equivalent area of the window in the initial design.
See Table D3 for a window that is designed to open to 30 degrees. The equivalent area of a window 2m high x 0.50m wide is 0.60m2.
NOTE: If your window has different dimensions to those provided in Appendix D, use the discharge coefficient calculator referenced in Appendix D of Approved Document O.
4. Following paragraph 1.12 of Approved Document O, check that the equivalent area of the window in the design is more than or equal to the minimum free area from Table 1.3 or 1.4.
The bedroom minimum free area is 0.48m2 (Step 2).
The equivalent area of a window is 0.60m2 (Step 3).
The equivalent area of the window in the design is larger than the minimum free area and therefore meets the standard.
In the scenario that the window in the design does not meet the bedroom minimum free area standard, consult the tables in Appendix D to resize the window.
Dynamic Thermal Method
9. When using the Dynamic Thermal Modelling Method do you need to submit a modelling report to support the assessment?
The building control body should be provided with a report that demonstrates that the residential building passes CIBSE’s TM59 assessment of overheating. This report should contain the details in CIBSE’s TM59, section 2.3. The compliance checklist in Appendix B of Approved Document O should be provided to the building control body. Additional information above that in the Appendix B checklist may be needed to satisfy section 2.3.
10. If mechanical cooling is used to pass Part O, does this affect Part L compliance?
Yes. The SAP calculations must reflect that mechanical cooling is installed. Note that the SAP calculation will not calculate the cooling loads that were calculated through the dynamic thermal modelling method. If mechanical cooling is installed, changes may be needed to ensure that the building complies with the energy efficiency requirements of the Building Regulations.
11. If windows or doors are made secure, can they be modelled as open when rooms are unoccupied at night?
Following paragraph 2.6(c) of Approved Document O, when a ground floor or easily accessible room is unoccupied at night, windows, patio doors and balcony doors should be modelled as closed. This does not apply to ventilation louvres or other secure openings.
12. If mechanical ventilation is used at night, can homes still be considered predominantly naturally ventilated?
Yes, a home can still be considered predominantly naturally ventilated where mechanical ventilation is provided for cooling at night.
13. Approved Document O states that requirement O1 should be met ‘using passive means as far as reasonably practicable’. What does this mean?
If mechanical cooling is installed, then it should be demonstrated to the building control body that all practicable passive means of limiting unwanted solar gains and removing excess heat have also been incorporated into the building.
One way of demonstrating that all passive means have been incorporated is to consider the limiting standards of the simplified method. Another way of doing this is to design the building to pass using no mechanical cooling in use and assuming no usability issues. This will mean solar gains have been limited as far as practicable.
‘Passive means’ is defined in Approved Document O.
Usability
14. What can be used to provide noise attenuation?
For guidance on reducing the passage of external noise into buildings see the National Model Design Code: Part 2 – Guidance Notes (MHCLG, 2021) and the Association of Noise Consultants’ Acoustics, Ventilation and Overheating: Residential Design Guide (2020).
There are many different solutions to attenuate noise, for example, opening windows a smaller amount at night. The associated reduction in equivalent area should be accounted for in the overheating assessment.
15. How do I apply the 650mm window handle standard to protect people from falling?
The 650mm standard is the dimension that should be used to calculate the safe opening angle of a window. This opening angle should be used in the equivalent area calculations.
It is not necessary to use a physical restrictor to meet this standard.
16. How can both the standards for escape windows and guarding for ADO be met in practice?
Some build tolerance is acceptable when building a window that is a means of escape, with an opening at a height of 1100mm above the floor. While it is expected that the 1100mm guarding height in Approved Document O is achieved, a reasonable build tolerance is +0 / - 100mm.
17. Will guarding in front of windows, such as juliet balconies, effect the equivalent area calculations?
Guarding which significantly restricts the passage of air may reduce the equivalent area of the window they cover. This should be considered on a case-by-case basis and, where necessary, calculations of equivalent area should take obstructions into account.
18. Where the windows are not part of the Overheating strategy what are the guarding standards?
The guidance in paragraph 3.8 states that openings which are intended to be open for long periods to reduce overheating risk might pose a higher risk of falls from height. Only the proportion of openings which can be opened with a very low risk of occupants falling from height should be considered to form part of the overheating mitigation strategy. This only applies to windows used in the overheating strategy.
Where a window is not used as part of the overheating strategy the minimum guarding heights of Approved Document K should be followed. The homeowner should be informed, as part of providing information about the overheating strategy, of any windows that are not intended to be open as part of the strategy and therefore are not built to the guarding standards of Approved Document O.
19. Are horizontal bars as guarding acceptable if they do not allow children to easily climb them?
Section 3.9 of Approved Document O advises that guarding should not allow children to easily climb it. Therefore, horizontal bars should generally be avoided.
Horizontal bars can be acceptable where children cannot easily climb them. In the Building Control Alliance Guidance (Technical Guidance Note 16), it suggests that horizontal bars can be used as guarding where they start a minimum of 600mm above the inside floor level. It is important that the design of horizontal bars is carefully considered in the context of the room. If the only place for furniture such as a bed is under the window children may be able to climb horizontal bars.
20. Is extra guarding needed if windows have restrictors on?
Openings that can be opened wider than 100mm and are part of the overheating mitigation strategy should follow the standards for protection from falling in Paragraph 3.9 of Approved Document O.
A window, that is both part of the overheating strategy and uses a fixed restrictor to limit window opening to 100mm does not need to meet the standards of Paragraph 3.9 of Approved Document O. If the window has a fixed restrictor to limit window opening to 100mm, the equivalent area of the window should be calculated with the restrictor in place when using the simplified method.
All practicable passive means of limiting unwanted solar gains and removing excess heat should be adopted first before installing mechanical cooling. One practical passive means of removing excess heat is a window that opens beyond 100mm and has the minimum guarding standards in Approved Document O.