Part O Overheating – Common Questions

Part O Overheating – Common Questions

Part O Overheating of the Building Regulations has applied to new homes since June 2022. Despite that, the guidance still catches out architects and designers on a regular basis — particularly around window accessibility, sill heights, and when TM59 becomes unavoidable. These are the questions we get asked most often.

There 2 main methods of compliance:

Simplified Method

• The glazing is assessed based on the overall glazing proportion to the floor area (excludes window/door frame it is glass only). The most glazed facade orientation dictates the selection of how much glazing is allowed.
• The overall ventilation is based on achieving ventilation free area (equivalent area) equivalent to 9% of the floor area. if the dwelling does NOT have cross ventilation, then this area is increased to 12%.
• There is also the most glazed room which is similar to the overlall glazing allowance.
• And bedrooms need 4% ventilation free area in proportion to the bedroom floor area.
• You CANNOT include shading, solar glass, air conditioning, mechanical ventilation. It is solely, focussed on glazed areas and ventilation openings.
• The location of the dwelling may alter the allowance for glazed areas and ventilation openings.

TM59 Dynamic Simulation

• This dwelling is modelled in 3D in approved software (such as DesignBuilder, IES VE or Tas for example).
• Each (habitable) room is assessed under the criteria set out in TM59. This affects living spaces, kitchens and bedrooms. Other spaces will influence the results.
• There are maximum targets to achieve such as 3% overheating per heat and for bedrooms, a maximum of 32 hours per year over 26 degrees.
• This process CAN include shading, solar glass, mechanical ventilation, adjacent buildings (Not trees), stack ventilation and other factors.

When carrying out an assessment, you generally start with the Simplified Method.

Q1: The dwelling is a bungalow or ground floor flat with ground floor bedroom windows, can we use the Simplified Method?

A1: Not generally. ground floor Bedroom windows that are within 2m of external ground level are considered ‘easily accessible’ and should be modelled closed between the hours of 11pm,-7am for security reasons. In this case, a TM59 assessment is required to model this.

Q2: For easily accessible windows, is there a way to allow these to open at night?

A2: Yes, there are 4 main options to consider:

• Add a secure grilled, railing or lockable shutter (to be agreed with Building Control).
• Have a window opening at least 2m from ground level such as a high level fanlight.
• Have a separate insulated Louvre (this looks like a Louvre externally and has an insulated door internally that can open to allow secure fresh air). This is more suited to urban environments.
• Consider the 20mm secure night latch if this is part of the window’s secure locking system and Building Control accept it. (Simulating this using TM59 may require a script).

Q3: The bedroom window is above a ground floor roof. Can this open?

A3: There are 2 main options:

• If the roof is over 3.5m from ground level and/or the roof is in excess of 30 degrees then the window can open normally and be counted.
• If the roof is less than 3.5m from ground level AND less than 30 degrees AND the window is within 2m of the roof, then the window is easily accessible. To resolve this, you can consider one of the 4 options in Q2.

Q4: The bedroom window has to be closed at night because it is easily accessible. What are the options?

A4: This would need a TM59 Dynamic Simulation. The topics we would look at to help the bedroom comply would include:

• Solar glass to reduce solar gains.
• Mechanical purge ventilation (Supply air via a fan) at a noise level less than 40dB to achieve the minimum flow rate to remove heat gains.
• Mechanical ventilation with Heat Recovery (MVHR) with a summer bypass may help, but check the maximum flow rates allowed.
• Consider a rooflight if this is feasibly to help remove heat gains securely if not easily accessible.
• Some MVHR systems now have a low load comfort cooling add-on to deliver small levels of comfort cooling to temper the air to help comply without a full fledged comfort cooling system. Vent-Axia have a unit called the Econiq Cool-Flow and Zehnder have a ComfoClime add-on to their Q450 or Q600 MVHR. Feedback on these systems varies, but these are not traditional comfort cooling.
• If in warmer areas of the country, then there may not be any other option than comfort cooling of all other elements have been considered and flow rates for purge ventilation are too high.

Q5: My upper floor window sills are 800mm from floor level in line with Part K of the Building Regulations, does this comply for windows used in the Part O Overheating Calculation?

A5: No they do not comply. If the window is being counted in the ventilation area for removing heat gains (99% of the time it is), then the ‘Protection from falling’ criteria supersedes that set out in Part K. Under Section 3, items 3.8-3.11 of Part O, this confirms the height should be no less than 1100mm from floor level when the external level is more than 600mm lower than the internal floor level.

Q6: But my bedroom window is also a means of escape. This cannot be any higher than 1100mm from floor level?

A6: There is a build tolerance of +0/-100mm confirmed under the Approved Document O: Overheating, frequently asked questions on the Government website, item 16. This means as long as the guarding or sill height it more than 1000mm and less than 1100mm for windows being used as means of escape, this is acceptable (Please check with building Control).

Q7: My sill level is still below the minimum height, what can we do?

A7: There are 2 main options:

• Add a 100mm restrictor. However, this significantly restricts airflow and highly unlikely this will pass the Simplified method and will likely result in mechanical purge ventilation if modelled using the TM59 overheating assessment.
• Add internal guarding to the required height . Please check guarding rules for reducing risk of climbing. This allows the window to open normally. But bear in mind, if the window is top hung, then you may need to reach over the guarding to use the window latch.

Q8: I have glass balustrading to my french doors to my bedroom, how does this affect the assessment?

A8: The glass will reduce solar gains to the lower portion of the glass. However, it will also reduce airflow as the glass is acting like a fixed window. Open guarding provides more air flow.

Q9: There are large glazed screens or bifold doors that are exposed to the sun. What should we consider?

A9: Some options to consider:

• An external shading device to reduce solar gains in the summer months is beneficial such as an overhead shading device, awning, pergola, Brise Soliel. But ensure this will allow lower altitude sun to get into the space during Spring/Autumn seasons.
• Consider recessing the glazed doors/screens into the facade so you get shading above and to the side of that screen.
• Reduce the area of glass either by reducing the overall area of the opening, or consider 3/4 glazed screens/doors to reduce solar gains.
• If you use solar glass, this will lower the solar gains (while the glazed doors are closed), but this negatively affects the SAP energy assessment. So you may need to consider triple glazing to balance this out in some instances.

If your scheme is heading towards a TM59 assessment or you’re not sure which method applies, we can help. Please check out our services page for Part O Overheating and TM59.

Or contact us on our Contacts page.