Overheating Risk & TM59 Explained

TM59 itself is not legislation—it's a CIBSE methodology. However, Part O of the Building Regulations requires new residential buildings to address overheating risk, and TM59 is referenced as the accepted method for dynamic thermal modelling compliance. So while TM59 isn't mandatory, it's often the required route when the simplified method isn't suitable.

Part O applies to all new residential buildings in England from June 2022 onwards. This includes new-build houses, flats, conversions creating new dwellings, and extensions that create new habitable rooms. It does not apply to existing buildings unless you're creating new residential units.

Yes, Part O allows compliance via either route. However, the simplified method has limitations and may not be appropriate where there are noise constraints, security concerns, or other factors affecting window usability. In such cases, dynamic thermal modelling (TM59) provides more flexibility and realistic assessment.

Failing isn't the end—it identifies areas needing improvement. Options include design modifications (reduced glazing, improved shading, better ventilation), exploring different unit layouts, or in some cases mechanical cooling. Early assessment helps avoid costly late-stage changes.

Part O currently applies to England only. Scotland, Wales, and Northern Ireland have their own building regulations, though overheating is increasingly being considered. Projects in these regions may still need TM59 assessments for planning conditions or BREEAM requirements.

TM52 applies to non-domestic buildings (offices, schools) using daytime occupancy criteria. TM59 applies to residential buildings and includes specific night-time bedroom criteria. If your building has bedrooms, TM59 is typically required.

Timescales depend on project complexity. A typical single dwelling might take 1-2 weeks. Larger developments with multiple unit types may require 3-4 weeks. Early engagement allows for design iteration and reduces overall timeline.

For a TM59 assessment, we typically need:

• Floor plans with room uses identified (living, bedroom, kitchen, etc.)
• Elevations showing glazing areas, positions, and window types
• Sections showing floor-to-ceiling heights
• Site location and building orientation
• Window and glazing specifications (frame type, opening mechanism, g-value if known)
• Envelope/construction details – all materials and their layers for walls, roof, floors, and glazing (U-values are helpful)
• Proposed ventilation strategy (natural, System 3, MVHR, etc.)

We build fresh 3D thermal models in specialist simulation software, so CAD exports aren't essential—but accurate drawings are.

TM59 uses Design Summer Year (DSY) weather files representing future climate scenarios. DSY1 is the standard assessment (moderately warm summer). DSY2 and DSY3 represent more extreme conditions and may be required for vulnerable occupant groups or high-risk locations.

No. Part F addresses air quality and moisture control, not thermal comfort. You can fully comply with Part F and still have significant overheating risk. Both assessments may be required, and they serve different purposes.

Ideally at RIBA Stage 2-3 (concept/developed design) when massing, orientation, and unit layouts are being finalised but changes are still feasible. Commissioning too late (Stage 4+) can result in costly redesigns or reliance on mechanical cooling.

Absolutely. We regularly collaborate with architects, M&E engineers, and planning consultants throughout the design process. This includes design workshops, iterative testing of options, and coordination with other environmental requirements like daylight assessments.

The two are closely linked—larger windows improve daylight but increase solar gains and overheating risk. Shading solutions affect both. We recommend coordinating overheating and daylight assessments together to find balanced solutions that work for both requirements.

Common effective strategies include:

• Redistributing glazing or reducing glazed area
• Adding external shading (brise soleil, overhangs)
• Ensuring openings on opposing facades for cross-ventilation
• Increasing ceiling heights for better thermal stratification
• Specifying solar control glazing (low g-value)

Costs vary by project scale and complexity. A typical assessment for a residential development might range from £800 for a single dwelling to several thousand pounds for larger schemes with multiple unit types. We provide fixed-fee quotes based on your specific requirements.

Yes. A quick screening review can identify high-risk areas before committing to full modelling. This helps prioritise design focus and can save time and cost by addressing issues early. We can look at floor plans and identify likely problem areas within a few days.

A comprehensive TM59 report typically includes:

• Project description and methodology
• Modelling assumptions and inputs
• Results for each room type assessed
• Pass/fail summary against TM59 criteria
• Recommendations for mitigation if needed
• Supporting graphics and data tables

Yes. Our TM59 assessments are prepared to meet Part O requirements and are suitable for Building Control submission. Reports clearly set out the compliance route, assumptions, and evidence required for sign-off.

Yes, we work on projects across England and can assess schemes in any location. While London has specific high-risk zones with stricter requirements, overheating risk exists nationwide and Part O applies to all new residential buildings in England.

Prolonged exposure to high indoor temperatures can affect health in several ways:

• Sleep disruption: Bedroom temperatures above 24°C significantly impair sleep quality
• Heat stress: Can cause dehydration, heat exhaustion, and in severe cases heat stroke
• Vulnerable groups: Elderly, infants, and those with respiratory or cardiovascular conditions are at higher risk
• Productivity: Cognitive function and concentration decline in overheated environments

These concerns are why Part O was introduced—to address overheating as a health issue, not just comfort.

Yes. Vulnerable groups include:

• Elderly residents (reduced thermoregulation ability)
• Young children and infants (higher metabolic rate, less body awareness)
• People with chronic health conditions (cardiovascular, respiratory)
• Those on certain medications affecting temperature regulation
• Residents with limited mobility who cannot easily move or seek cooler areas

Care homes and sheltered housing projects often require more stringent overheating criteria or additional testing with extreme weather files (DSY2/DSY3).

Climate projections indicate UK summers will become hotter and more unpredictable. TM59 assessments use future climate scenarios (2020s, 2050s, 2080s weather files) to account for this. Buildings designed today need to remain comfortable for their 60+ year lifespan, which is why forward-looking weather data is used rather than just historical records.

Yes. Research links high indoor temperatures to:

• Increased stress and irritability
• Lower mood and reduced sense of wellbeing
• Sleep deprivation effects on mental health
• Reduced productivity and concentration in home workspaces

Good thermal comfort design supports both physical and mental health outcomes.

There's no single answer—the most effective strategy depends on the specific design. However, external shading (brise soleil, overhangs, shutters) is often the most impactful single intervention, as it stops solar heat before it enters the building. Other key strategies include reducing excessive glazing, ensuring cross-ventilation potential, and specifying solar control glass.

Yes, but only as a last resort. Part O explicitly requires passive measures to be prioritised first. Mechanical cooling can only be proposed where it can be demonstrated that sufficient heat cannot reasonably be removed by passive means. If cooling is used, the system must be capable of maintaining bedroom temperatures below 26°C during sleeping hours.

External shading is significantly more effective than internal blinds:

• Fixed overhangs: Block high summer sun while allowing low winter sun—good for south-facing glazing
• Brise soleil: Horizontal or vertical louvres that filter light and heat
• External shutters: Highly effective, occupant-controlled
• Balconies: Can provide shading to units below if designed correctly

Internal blinds reduce glare but allow most solar heat to enter the room. Solar control glass (low g-value) is a middle ground solution.

Absolutely. Key glazing properties include:

• g-value (solar factor): Lower values (0.3-0.4) reduce solar heat gain
• Light transmittance: Affects daylight levels—balance with g-value needed
• Glass type: Triple glazing retains more heat at night; consider ventilation strategy

Very low g-values can harm daylight performance, so specification requires careful balance.

Ventilation is crucial for removing accumulated heat. Key considerations:

• Cross-ventilation: Openings on opposing facades create airflow—much more effective than single-aspect
• Night-time purge: Cooling the building overnight when external temperatures drop
• Opening sizes: Part O specifies minimum free areas for heat removal
• Window type: Casement and tilt-turn openers typically provide larger free areas than sliding windows

Yes, though it's often fixed by site constraints. In general:

• South-facing: High summer sun is easier to shade with overhangs
• West-facing: Receives low-angle afternoon sun (hardest to shade) and warmer air temperatures—highest risk orientation
• East-facing: Morning sun before peak temperatures—lower risk than west
• North-facing: Lowest solar gain—rarely problematic for overheating

Where west-facing glazing is unavoidable, reduced window sizes or enhanced shading is essential.

Usually yes. Single-aspect dwellings (windows on one facade only) have limited natural ventilation potential and are recognised as having considerable overheating risk. They typically cannot use the simplified method and require dynamic thermal modelling via TM59 to demonstrate Part O compliance.

No. Part O explicitly defines cross-ventilation as having openings on opposite facades. Corner flats with windows on two adjoining walls (e.g., south and west) do not qualify as cross-ventilated. This is a common misunderstanding that can lead to compliance failures.

Rooflights present a challenge. Under the simplified method, rooflights and roof windows are not explicitly included in the standard calculations. For rooms primarily lit by rooflights, dynamic thermal modelling is often necessary. Rooflights can contribute significant solar gains and are typically harder to shade than vertical glazing.

Part O recognises this issue. If external noise levels exceed defined limits (typically 45dB LAeq for bedrooms at night), the assessment must assume windows cannot be relied upon for ventilation during those hours. This significantly affects compliance strategy and may require:

• Mechanical ventilation with summer bypass
• Acoustic ventilators that allow airflow with windows closed
• Stricter limits on glazing to reduce heat gains

No. Internal blinds and curtains cannot be included as part of the overheating mitigation strategy for Part O compliance or in TM59 calculations. This is because they allow solar radiation to enter the building before providing any shading effect. Only external shading devices or solar control glazing can be credited.

The simplified method is not suitable for buildings with multiple residential units that use communal heating or hot water systems with significant horizontal pipework. These buildings typically require TM59 dynamic modelling because the heat gains from distribution pipework can significantly affect overheating risk.

Currently no for most extensions and conservatories added to existing buildings. Part O applies to new residential buildings. However, conservatories on new-build properties must comply, and a 2024 consultation explored extending requirements to buildings undergoing material change of use (e.g., office-to-residential conversions). This may change in future.

Part O compliance can affect and be affected by several other regulations:

• Part F (Ventilation): Window opening requirements for air quality overlap with overheating ventilation
• Part L (Energy): If mechanical cooling is used, it affects SAP calculations
• Part K (Falls): Restricts how windows with low cills can open, limiting their use for ventilation
• Part B (Fire): Escape window requirements may conflict with acoustic or security restrictions
• Part M (Access): Accessibility requirements for window controls

Early coordination between these requirements prevents conflicts at later stages.

Part O categorises England into two risk zones:

• High-risk: Primarily urban and suburban London postcodes, plus some central Manchester areas. Stricter glazing limits and mandatory shading on certain orientations.
• Moderate-risk: The rest of England. More relaxed thresholds than high-risk areas.

The location classification affects both simplified method limits and the weather data used in TM59 modelling.

Only as a last resort. Approved Document O explicitly states that mechanical cooling should only be considered where passive measures cannot reasonably remove sufficient heat. If cooling is used:

• The system must maintain bedroom temperatures below 26 degrees C at night
• SAP calculations must include the cooling energy use (affecting Part L compliance)
• You must demonstrate why passive measures were insufficient

For overheating mitigation, glazing with g-value of 0.4 or below is typically considered solar control. However, lower g-values reduce daylight transmission:

• g-value 0.5-0.6: Standard double glazing
• g-value 0.3-0.4: Solar control glazing (good for south/west facades)
• g-value below 0.3: High-performance solar control (may impact daylight compliance)

For Part O simplified method, shading credits require glazing with maximum g-value of 0.4 and minimum light transmittance of 0.7.

Thermal mass (heavyweight construction materials like concrete, masonry) affects overheating in complex ways:

• Benefit: Absorbs heat during the day, reducing peak temperatures
• Challenge: Releases stored heat slowly, which can be problematic if night-time ventilation is limited

Heavyweight construction generally performs better where good night-time ventilation is available. Lightweight construction (timber frame) may be more suitable where night ventilation is restricted, as it cools faster once heat input stops.

Common scenario. Often it is the worst-case units that fail—typically top-floor, west-facing apartments with high glazing ratios. Options include:

• Targeted design changes to failing unit types only
• Different glazing specifications by orientation
• Additional shading to specific facades
• Layout modifications to move bedrooms away from critical orientations

Building Control requires all units to demonstrate compliance.