Part O Overheating & TM59

Each new dwelling is now required to comply with the new Part O Overheating of the Building Regulations. Low carbon Box are accredited experts to undertake the Part O Overheating Simplified Method and TM59 dynamic insulation overheating assessments.

Part O Overheating & TM59

  • Low Carbon Box are experts on Part O overheating using the Simplified Method and TM59 dynamic simulation method.
  • All new build dwellings now need to comply with Part O Overheating of the Building Regulations.
  • The process is to demonstrate that the dwelling achieves the minimum requirements to comply with Part O Overheating of the Building Regulations.
  • This does not mean the dwelling will not overheat, but will seek to reduce the risk with an allowance of short periods during warmer weather.

Understanding Part O Overheating & TM59 Commonly Asked Questions

What is the Simplified Method for Overheating?

Part O overheating starts with the Simplified method approach. This measures the following 4 key criteria:

  • Glazing area in proportion to total floor area
  • Ventilation opening area in proportion to total floor area.
  • Glazing area in proportion to the most glazed room.
  • Ventilation opening area in proportion to the bedroom floor area.

Depending on location, orientation and whether the building is cross ventilated, the targets differ. So if the house is cross ventilated (windows on opposite sides of the dwelling) then the ventilation opening area should be a minimum of 9%. But if no cross ventilation, then this increases to 12%.

If you fail any of the criteria, then there should be an opportunity try and tweak the design to achieve a pass. But if the changes still do not get compliance, then you will need to undertake a TM59 Overheating Assessment using the dynamic simulation method.

What is a TM59 Overheating Assessment?

A TM59 Overheating Assessment is a dynamic simulation. There would be a 3D model built using an approved software that is accredited to undertake this assessment. The software could be DesignBuilder, IES VE or Tas along with other possible alternative systems.

The building geometry is input into the software including the building fabric specification. Windows and other glazed elements will have the whole window U values and glazing ‘g’ values (solar transmission) entered along with which windows open and by how much.

The building services are also entered into the simulation to confirm the heating system, the ventilation system, where any heat sources are (such as a hot water cylinder) and any other systems that may influence the results.

The weather file is selected from CIBSE Design Summer Year for either the most local weather station or the weather station that is the most appropriate based on the site location and topography.

The simulation is run for a whole year and results extracted to compare to the targets. For instance, a living room may only overheat for 3% of the year. And a bedroom may only overheat for up to 32 hours per year.

What are the key items to look for to pass?

If possible always try and ensure cross ventilation. This may not be possible on flats with a single aspect.

If the most glazed façade faces West (often the rear elevation) then you are restricted to 11% glazing ratio to floor area. This is difficult to pass. So, consider balancing the glazing of possible.

If you have large glazed bifold or sliding doors, then consider passive shading to assist with the overheating assessment.

On upper floor windows, the sills should be >1100mm from floor level (+/-100mm) so this means the build tolerance allows 1000mm from floor level. If lower than this, then window is restricted to 100mm unless it has internal guarding to a height of 1100mm. This rule conflicts with the 800mm cited in part K. The reason is because windows are left open longer for removing heat gains.

If smaller dwellings, then using continuous extract ventilation maintains a constant background air change which helps slowly remove heat gains at night.

West and south facing glass gets the most solar gains, so consider the area of glass and passive shading. Werst face windows to bedrooms for example can heat the bedroom up before night time, so is it possible to locate windows to an additional elevation?

Bedroom ventilation is very important. Ensure all casements can open where possible (no fixed side casements. And think about mid-terrace houses with no side windows. How will these achieve adequate areas for free ventilation.

Easily accessible windows are opening windows (to bedrooms) that are within 2000mm of external ground level (such as a bungalow or ground floor flat). Or the bedroom windows are within 2000mm of a roof that is less than 30 degrees and within 3500mm of external ground level. Under Part O Overeating, if the window is easily accessible, then this has to be modelled closed at night because it poses a security risk. Even in a secure back garden.

The result is often the need for mechanical purge ventilation. This is supply air to replace the ventilation lost by the closed bedroom window at night.

Ways to reduce the need for mechanical purge ventilation are considering higher window head heights and adding a high level fanlight. Consider a PAS24 secure night latch to allow the window to be securely shut at night but on the night latch to allow a small amount of free ventilation.

On a bungalow, a rooflight may also be effective, depending on the orientation.

How can Low Carbon Box help?

Low Carbon Box are accredited with Elmhurst Energy under the Part O Overheating Assessor Scheme.

We have been trained to model the building using the CIBSE TM59 Design methodology for the assessment of overheating risk in homes (2017). We are also accredited to use DesignBuilder software to undertake the assessment.

Our combined experience has delivered on assessment reports for multiple sites using the Simplified method and TM59 dynamic Simulation approach.

We have a number of options to help dwellings pass based on experience and try and pass recommendations on easily in the project stage (depending on when we are appointed) to reduce remedial measures required to pass.


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Client Thoughts

I would like to pass on our thanks and appreciation for your hard work, expertise and valued input to the scheme.

James Nellist
Lindum Construction