Part O Overheating — Future Homes Standard Guide

Part O is the overheating section of the Building Regulations for England, introduced in June 2022 to address a growing risk in modern housing: homes that are too hot in summer. As the Future Homes Standard pushes new homes towards dramatically higher levels of insulation and airtightness, the risk of overheating increases significantly. Part O requires designers to control unwanted solar gains and provide adequate means of removing excess heat — making it one of the critical regulatory hurdles for FHS homes.

Why Overheating Matters in FHS Homes

Overheating in homes is not just a comfort issue — it is a health risk. The UK has experienced increasing summer temperatures, and excess heat in homes has been linked to heat-related illness and mortality, particularly among elderly and vulnerable residents. The problem is amplified in well-insulated, airtight buildings because the very features that keep heat in during winter also trap heat in summer.

FHS homes will have substantially better fabric performance than current homes — lower U-values, better airtightness, and triple glazing. While this is excellent for reducing heating demand, it creates a thermal envelope that retains unwanted heat gains from:

• Solar radiation through windows — the dominant source of overheating in most homes, especially with south-facing glazing
• Internal gains from occupants, cooking, appliances, and lighting
• Reduced ability to lose heat through the fabric — well-insulated walls and roofs slow the rate at which excess heat can dissipate to the outside

In a conventionally built home, heat escapes relatively quickly through the building fabric. In an FHS home, it stays trapped unless the design actively addresses how to remove it.

What Part O Requires

Approved Document O sets out two key requirements for new residential buildings:

1. Limiting unwanted solar gains — controlling the amount of solar energy entering the dwelling through windows, rooflights, and other glazed elements
2. Removing excess heat — providing adequate ventilation (primarily through opening windows and cross-ventilation) to purge excess heat when internal temperatures rise

Designers can demonstrate compliance through one of two routes:

The Simplified Method

The simplified method uses prescriptive limits and checks. It assesses:

• Glazing limits: Maximum glazing area as a percentage of floor area, varying by orientation (south-facing windows have the tightest limits) and by location (higher-risk areas such as London have stricter limits)
• Cross-ventilation: Whether the dwelling can achieve effective cross-ventilation through openable windows on opposite or adjacent facades
• Shading: Whether permanent external shading (overhangs, brise-soleil, recessed windows) is provided on south-facing glazing

The simplified method is straightforward to apply but conservative. Designs that genuinely perform well in summer may fail because the prescriptive rules cannot account for the full complexity of thermal behaviour.

Dynamic Thermal Modelling

The alternative is to use dynamic thermal simulation to demonstrate that the dwelling does not overheat. This typically follows the CIBSE TM59 methodology, which assesses overheating risk using:

• Design Summer Year (DSY) weather data — representative of a warm summer for the specific location
• Adaptive thermal comfort criteria from CIBSE TM52 — based on running mean outdoor temperature rather than fixed temperature thresholds
• Bedroom temperature criteria — bedrooms must not exceed 26°C for more than 1% of annual occupied hours (typically around 32 hours per year)

Dynamic modelling is more expensive and requires specialist software, but it provides greater design flexibility and can demonstrate compliance for buildings that fail the simplified method.

Overheating Assessment: SAP vs HEM

The shift from SAP to the Home Energy Model transforms how overheating is assessed at the energy calculation stage.

The key advantage of HEM is that its half-hourly timestep and dynamic thermal modelling can capture periods of intense overheating that would be invisible to SAP's monthly averaging. A room that reaches 35°C for several afternoons in July but averages an acceptable temperature over the month would pass SAP's check but would be flagged by HEM.

Design Strategies to Reduce Overheating Risk

Addressing overheating effectively requires a hierarchy of strategies, applied in order of priority:

1. Reduce Solar Gains

• Orientation: Minimise south and west-facing glazing area where possible. West-facing glazing is particularly problematic because low-angle afternoon sun is difficult to shade
• Glazing area: Limit the total glazed area, especially on sun-exposed facades. The simplified method sets specific limits, but even under dynamic modelling, less glazing generally means less overheating risk
• Solar control glazing: Low g-value (solar factor) glass reduces solar heat gain. However, this also reduces useful winter solar gains and daylight — a trade-off that HEM can model explicitly
• External shading: Overhangs, brise-soleil, deep window reveals, and external shutters or blinds are the most effective way to block direct solar radiation. External shading is far more effective than internal blinds because it intercepts the heat before it enters the building

2. Remove Excess Heat

• Cross-ventilation: Openable windows on opposite or adjacent facades allow air to flow through the dwelling, removing excess heat. Single-aspect dwellings (such as many flats) are at much higher overheating risk because cross-ventilation is impossible
• Purge ventilation: Large openable windows or doors that can rapidly ventilate a room when temperatures peak. Part F requires this capability in all habitable rooms
• MVHR summer bypass: MVHR systems should include a summer bypass mode that routes incoming air around the heat exchanger when outdoor air is cooler than indoor air, avoiding unwanted heat recovery in summer
• Night-time purge cooling: Opening windows at night when outdoor temperatures drop can flush stored heat from the building fabric, resetting thermal mass for the following day

3. Use Thermal Mass Effectively

Thermal mass (heavyweight materials such as concrete floors, masonry walls, and plaster ceilings) absorbs heat during the day, slowing the rise in internal temperature, and releases it at night when it can be purged through ventilation. This is particularly effective in combination with night-time purge cooling.

HEM models thermal mass in detail using the methodology in BS EN ISO 52016-1, calculating heat storage and release at each half-hourly timestep. This means dwellings with good thermal mass and effective night ventilation can demonstrate a genuine overheating benefit in the compliance calculation — something SAP's simplified thermal mass parameter could not capture accurately.

4. Mechanical Cooling (Last Resort)

Active cooling (air conditioning) can be used to prevent overheating, but it comes with significant energy and carbon penalties. HEM calculates the energy consumption of cooling systems explicitly, and this energy use counts against the dwelling's overall energy performance. In most cases, good passive design should eliminate the need for mechanical cooling in UK homes.

How HEM Models Overheating

HEM's approach to overheating is fundamentally different from SAP. The model calculates space heating and cooling demand using a dynamic heat balance at each half-hourly timestep, based on BS EN ISO 52016-1. This means it can capture the full complexity of overheating behaviour:

• Solar gains per window: HEM calculates direct and diffuse solar radiation through each glazed element at every timestep, accounting for orientation, tilt, shading from overhangs, and glass properties (g-value)
• Thermal mass dynamics: Heat storage in floors, walls, and ceilings is modelled explicitly, so the beneficial effect of thermal mass in smoothing temperature peaks is properly credited
• Ventilation interaction: The pressure-driven ventilation model accounts for natural ventilation through open windows, MVHR summer bypass behaviour, and the interaction between wind conditions and ventilation rates
• Internal gains: Realistic occupancy profiles and internal heat gains from appliances, cooking, and lighting are applied at each timestep
• Cooling demand output: Where internal temperatures exceed the cooling setpoint, HEM quantifies the cooling demand in kWh, providing a clear measure of overheating severity

Practical Implications for Design Teams

Balancing energy efficiency with overheating control is one of the central design challenges of the Future Homes Standard. Practical considerations include:

• Early-stage orientation analysis: Site layout should consider overheating from the outset. South-facing living areas with large glazing may be desirable for passive solar heating in winter but problematic for overheating in summer. Orientation decisions are essentially irreversible once planning is secured
• Regional variation: Overheating risk varies significantly across England. London and the south-east are highest risk due to the urban heat island effect and higher summer temperatures. Northern and western regions have lower risk but are not immune, particularly for upper-floor, south-facing rooms
• Cost of mitigation: External shading, solar control glazing, and enhanced thermal mass all add cost. However, they are typically cheaper than mechanical cooling and avoid its ongoing energy consumption. Design teams should model overheating risk early to avoid expensive redesign later
• Single-aspect dwellings: Flats with windows on only one facade are at the highest overheating risk because cross-ventilation is impossible. The simplified method in Approved Document O is particularly restrictive for single-aspect designs. Dynamic modelling may be essential to demonstrate compliance
• Compliance interaction: Part O compliance is separate from the Part L / FHS energy calculation, but the design decisions interact. Reducing glazing to address overheating also reduces useful solar gains in winter. Increasing thermal mass helps with overheating but may not be reflected in the simplified Part L calculation. HEM's detailed modelling captures these trade-offs more accurately

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