Part O Simple Method; watch points for designers

[edit] Building Regulations Approved Document O

The overheating mitigation regulations 40B took effect on 15 June 2022 in England. This regulation takes its lead from CIBSE TM52 and has arisen through lessons learned from previous guidance, such as BB101 and CIBSE Guide A.

It provides a set of criteria to determine whether a dwelling is at risk of overheating, to be evidenced through a process called the simplified method or more complex dynamic modelling method. It is likely that dynamic modelling would be carried out by specialist consultants, but it is possible for the simplified method to be implemented by a design team, at least during early design stages. In either case it is useful for design teams to understand the outline of requirements and areas to watch to avoid costly changes at later stages. This also helps better interrogation of any work done by an external consultant, either via the simple or the dynamic model approach.

The interface between intended, drawn and specified packages with assumed, modelled, and assessed packages is crucial, even before this is reconciled with what is built on site.

Approved document O requires residential buildings to be categorised by location and risk, to minimise solar gains (depending on cross-ventilation) and for excess heat to be removed via free areas (depending on cross-ventilation). It also asks, tests and requires evidence for these to be practicable in occupation

[edit] Dynamic thermal modelling approach.

This is only covered briefly here as it is likely to be led by an external consultant. This approach is loosely based on the CIBSE’s TM59 methodology, though there are some differences.

• Criterion 1: For Living Rooms, Kitchens and Bedrooms the number of hours during which ∆T is greater than or equal to one degree (K) between May - September inclusive shall not be more than 3% of the occupied hours.
• Criterion 2: For Bedrooms Only – The operative temperature from 10pm to 7am shall not exceed 26°C for more than 1% of annual hours (32.85 hours). The document highlights the limits of modelling as well as acceptable strategies.

Note that with this approach it is only necessary to assess a representative sample of units, not each individual unit (which is required in the simple method). The two criteria use an adaptive thermal comfort approach and test all main rooms, kitchens, living rooms, bedrooms but exclude bathrooms, hallways and internal rooms. There is the addition of a night-time hours of exceedance test for bedrooms, based on a fixed temperature threshold of 26 degrees. In general, there is greater flexibility with the dynamic route in terms of assessment and specific location requirements.

[edit] Simplified Method

Although theoretically it is simpler and quicker, in reality it is not so simple and is significantly more prescriptive and (a particular downside for larger schemes) every unit needs to be assessed individually. key elements of the approach are:

[edit] Categorising residential buildings

• ‘Moderate risk’ location – England, not cities.
• ‘High risk’ location – urban and some suburban parts of cities. It is suggested at least initially that both are treated as high risk to give some flexibility and avoid problems at later stages.

[edit] Limiting solar gains

1. Assess for cross-ventilation or no cross-ventilation (cross-ventilation is only considered if directly opposite).
2. Assess maximum area of glazing as a % of the floor area (glazing area not window area).
3. Assess maximum area of glazing in the most glazed room as a % of floor area.

(note: Max G value 0.4 and min light transmittance of 0.7 plus southern 50 degree overhangs)

[edit] Notes on shading

Glazing in high-risk locations must include shading for any window orientated between northeast to northwest via S - so most windows just not north. There are less options in the simplified method than dynamic modelling. Low G specification, which has to be lower than 0.4 G but centre pane value (not whole window G value). External shutters, or overhangs to South facades, 50 degree angle overhang, how deep that needs to be. No benefits to shading in moderate risk locations under the simplified method but different in the dynamic modelling route you are allowed to include.

[edit] Removing excess heat

1. Total minimum free area greater than 6% or 9% of floor area or 70% or 55% of glazed area depending on risk location from 1.
2. Bedrooms to have minimum of 13% or 4% of room floor area depending on location risks in item 1. Note again that cross ventilation is only on opposite sides.

[edit] Three methods to calculate ventilation

1. Use discharge coefficient calculator spreadsheet (assumes top hung windows) know width and angle of opening (depending on mechanism). Use equivalent area (not free area also given in the table -as that's the whole area of the window.)
2. Look up tables in appendix D, give results for increments of 10 degree opening angles and increments of 250mm size, height and width (assumes side hung windows) Top or side hung can make a slight difference.
3. Manufacturer’s data if it is available. ( this may be unlikely, at least initially)

[edit] Ensuring the overheating mitigation strategy is usable

[edit] Noise – key points

• Noise limits set for bedrooms at night specifically.
• Planning constraints of noise in the daytime may lead to complexity.
• Around 30% of all existing UK homes would exceed noise requirements, so more than just noisy sites. Roads, rail lines, high winds or bird song treated as noise.
• Noise requirements not met then, windows can’t be used for night ventilation.
• Acoustic vents ay be a possibility but warmer, noisier areas might mean mechanical ventilation or cooling.
• If planning constraints state daytime noise venting through openings is problematic this triggers TM59 criteria ie from predominantly naturally ventilated to predominantly mechanically ventilated, thresholds change and harder to pass.

[edit] Pollution – key points

• High air pollution sites; EHO and/or BCB may require filtered mechanical ventilation for background ventilation year round, but accept opening windows for mitigation purposes.
• In many cases, air pollution likely road traffic, and so noise requirements may drive this.

[edit] Security – key points

• Night-time ventilation windows must be secure, so no ground floor patio windows.
• Strictly lockable louvered shutters or grills.

[edit] Protection from falling - key points

• Protection from falling supersedes part K which means if windows that are part of overheating strategy and open more than 10mm, handles should have a maximum reach outwards of 650.
• Non-ground floor windows or difference of inside to outside height of more than 600 mil in terms of the inside floor level on the outside floor level. Heights need to be at least 1100mm, issue if window is means of escape, where the maximum is 1100mm (tolerance plus zero and minus 100mm to check)
• Fixed lower area and openable from 1100 mm or external / internal guarding including shutters with childproof fixed non climbable guarding.

[edit] Protection from entrapment - key points

Louvered shutters, window railings and ventilation grills should: Not allow the passage of a 100mm diameter sphere. Holes allowing 8mm diameter rod should also allow 25mm diameter rod, not tapered in a way that allows finger entrapment. Looped cords with child safety devices.

[edit] Providing information

Whilst provision of information will be discussed with the approving body, below is a list of key points and data elements that designers should know, track and communicate with their team for the application of the simple method.

• Every unit needs to be assessed (overlap with SAP information?)
• Gathering the correct data is key, suggest using a template.
• Location; High risk (predominantly London and Manchester) or a moderate risk location suggest consider including both as high risk.
• Cross-ventilation; means specifically windows on opposite facades, which excludes corner window units.
• Orientation; of the most glazed façade, if similar on each take the worst case.
• Unit floor area; as targets are based on percentages of this, so a key element.
• Glazing areas; are higher in high-risk locations than moderate-risk locations for West facing units, puzzling but more glazing outside London so long as shaded.
• Note that if majority is West oriented glazing facade very low glazing is allowed.
• Total glazing area (not window area); detailed measures for each unit, in each room or apply a frame factor. Don’t underestimate the amount of glazing.
• Largest total glazed area façade; in m2 not % of floor area + the orientation.
• Room with largest total area of glazing, or the most glazed room.
• Dual activity rooms; i.e. kitchen / living rooms or kitchen / dining rooms deeper than 4.5 meters then specific floor area is capped at a 4.5m. Don't discover too late.
• Equivalent areas; of ventilation from each window is needed, defined using the term free area but evaluated by equivalent areas against targets. Know difference and the 3 methods to work this out.

If you are unsure of anything you should refer directly to Document O or discuss details with your building control officer.

[edit] Related articles on Designing Buildings

• Approved Document O.
• Better prediction of overheating in new homes.
• Comfort in low energy buildings.
• Good homes alliance overheating tool
• Human comfort in buildings.
• ‎Maximum and minimum workplace temperatures.
• Overheating - assessment protocol.
• Overheating in residential properties.
• Preventing overheating.
• Temperature.
• Thermal comfort.
• Thermal indices.

[edit] External Links

Future Homes Hub for interim guidance document ‘Part O (England) Technical Guidance” July 2022 – Final Draft.

Good Homes Alliance for Overheating guidance in new and retrofit homes.