CIBSE TM59

CIBSE TM59 utilises dynamic thermal analysis to predict overheating risk in residential building designs, whether new-build or undergoing major refurbishment. While primarily tested on flats due to their heightened overheating risk, it's designed to be applicable to houses as well. The objective is to encourage well-balanced design that ensures comfort without overly relying on mechanical cooling. The test must be straightforward for widespread adoption.

This document offers usage profiles reflecting typical home behaviors to evaluate overheating risk, drawing from CIBSE guidance where feasible. These profiles assess building design rather than covering all usage scenarios, necessitating certain assumptions. While further refinement is needed, this methodology fills a crucial gap in providing a consistent approach for interim use.

Intended for designers, it influences building design positively and can be employed during planning or later design stages to assess risk effectively.

Download a copy of TM59 Design methodology for the assessment of overheating risk in homes (2017). More.

CIBSE TM59 Design Comfort Criteria

The basis for design comfort criteria relies on CIBSE TM52: Limits of Thermal Comfort (2013), outlining principles to avoid overheating in European buildings. Additional guidance can be found in CIBSE Guide A: Environmental Design (2015), which advises that temperatures above 24°C may compromise sleep quality and sets a peak bedroom temperature threshold of 26°C.

CIBSE TM59 Methodology

This methodology, relying on dynamic thermal modeling, is recommended for all residential buildings, particularly: large developments, urban projects, blocks of flats, highly insulated and airtight dwellings, and single-aspect flats.

Individual houses or low-risk developments may not necessitate dynamic thermal modeling, but professional judgment is crucial. The decision should involve the client, design team, and planners, considering project context.

Simulation Steps

CIBSE TM59 sets the following these steps to undertake an assessment via hourly dynamic simulation modelling, which includes all the relevant features of the building

1. Select a suitable sample of units within the development.

2. Zone all sample units into separate rooms (kitchens, living rooms, bedrooms, bathrooms, halls).

3. Model building constructions accurately, reflecting thermal properties like mass, insulation, and glazing solar transmittance.

4. Apply standard profiles for occupancy, lighting, and equipment gains.

5. Follow guidance for treatment of communal corridors.

6. Model pipework and equipment gains .

7. Include operable windows in the model.

8. Incorporate internal or external shading provisions.

9. Include additional mechanical ventilation (e.g., mvhr or extract systems).

10. Base air speed assumptions.

11. Use the DSY1 weather file for the 2020s, high emissions, 50% percentile scenario.

12. Conduct assessment using hourly dynamic simulation modeling, encompassing all relevant building features.

Guidance

Sample Size

A CIBSE TM59 assessment aims to identify dwellings at risk of overheating, likely those with:

(a) Large glazing areas,

(b) Topmost floors,

(c) Less shading,

(d) Large sun-facing windows,

(e) Single aspects, or

(f) Limited opening windows.

The CIBSE TM59 report justifies the chosen sample units, considering the development's scale, location, and emerging modeling results. Lower-risk dwellings may be included for performance illustration. If corridors contain community heating distribution pipework, at least one corridor should be assessed.

Weather Files

Developments must adhere to the latest CIBSE Design Summer Year (DSY) weather files, as applicable, to represent typical summer weather conditions for a specific location. Any variation from from using DSY weather files should be agreed with the relevant consent authority or amongst the design team.

Window and Door Openings

Windows in each room should be individually controlled and modeled as open when the internal temperature exceeds 22°C and the room is occupied. If additional security and rain protection details are included, opening hours during the night could be extended. For instance, patio doors should only be open in unoccupied rooms or at night if securely locked open, with the locked percentage of free area used in the model. Opening areas should reflect architecturally designed windows, including any restrictors required. Follow guidance from CIBSE Guide A (2015) and CIBSE AM10 (2005) for free area calculations, considering security, acoustic, or air quality limitations.

If blinds are included, ensure they don't obstruct window opening, or account for reduced free area when in use. Internal doors can be left open during the day but assumed closed at night. The compliance report must justify all assumptions.

Exposure Type

Models should be configured with the suitable exposure type for the site location and façade orientation, as per the software definitions, with justification provided in the compliance report.

Infiltration and Mechanical Ventilation

The infiltration and mechanical ventilation rate for each zone should be determined based on the design specifications for normal, acoustically compliant modes of operation. For more details on infiltration rates and noise design limits, refer to CIBSE Guide A (2015) or your local building code or design reference. Mechanical boost mode, intended for occasional use with louder fan noise, should not be considered in the overheating risk analysis.

Air Speed

Operative temperature calculations, as utilised within CIBSE TM52 (2013), necessitate assumptions regarding air speed. The modeled air speed in a space should be set at 0.1 m/s where the software allows, unless a ceiling fan or other method reliably generates air movement. If fixed ceiling fans are installed as part of the new build or refurbishment, any elevated air speed assumptions must be disclosed, typically not exceeding 0.8 m/s.

Blinds and Shading Devices

Blinds and shading devices may be included in the analysis only if they are part of the design, provided in the base build, and explained in associated home user guidance. Blinds should not obstruct window openings. If blinds are used to pass the overheating test, the report must demonstrate no conflicts with window openings or calculate and include the reduction in airflow due to any conflicts in the model, with explanations in the compliance report.

Solar transmittance/reflectance properties and usage profiles for blinds must be justified and described in the compliance report. Results without blinds should also be provided for reference when blinds are used to enable a pass.

Pipework, HIU and Heat Maintenance Tape Heat Loss

Heat losses from pipework, HIUs, and heat maintenance tape should be considered in community heating systems. HIUs and connecting pipework remain charged with hot water year-round. The assessment should factor in heating pipework distribution gains and losses from the HIU. Default values for pipework are available if specific data isn't provided. Standing gains within the home should be based on primary side pipework length up to the HIU. Heat maintenance tape, if used, should be modeled as 8 W/m or as per design calculations.

Communal corridors

The inclusion of corridors in the overheating analysis is mandatory if community heating pipework runs through them. The analysis should determine the number of annual hours exceeding an operative temperature of 28°C. Communal corridor heat gains should be modeled based on calculated losses from pipework or a simplified method from relevant guides. Calculated values based on design temperatures and insulation performance may be used if justified.

Corridor ventilation should reflect the design specifications. While there's no mandatory target, if the operative temperature exceeds 28°C for over 3% of total annual hours, it should be flagged as a significant risk in the report.

Compliance Criteria

Definitions

Homes predominantly relying on natural ventilation, including those with mechanical ventilation with heat recovery (MVHR), and ample summer ventilation opportunities, should evaluate overheating using the adaptive method outlined in CIBSE TM52 (2013).

Each habitable room requires operable windows meeting purge ventilation criteria, ensuring a minimum window opening area relative to the room's floor area. Effective control may require accessible, secure, and quiet ventilation.

Homes primarily relying on mechanical ventilation, with limited or no window opening opportunities due to noise or air quality concerns, should assess overheating using the fixed temperature method outlined in CIBSE Guide A (2015).

Criteria for Homes Predominantly Naturally Ventilated

Compliance hinges on meeting both of the following criteria:

(a) For living rooms, kitchens, and bedrooms: the number of hours during which the difference in temperature (DT) is greater than or equal to one degree (K) from May to September inclusive should not exceed 3% of occupied hours (CIBSE TM52 Criterion 1: Hours of exceedance).

(b) For bedrooms only: to ensure comfort during sleeping hours, the operative temperature in the bedroom from 10 pm to 7 am should not surpass 26°C for more than 1% of annual hours. (Note: 1% of the annual hours between 22:00 and 07:00 for bedrooms is 32 hours, so 33 or more hours above 26°C will result in a fail). Both criteria (a) and (b) must be met for all relevant rooms, even if Criteria 2 and 3 of CIBSE TM52 are not fulfilled.

Criteria for Homes Predominantly Mechanically Ventilated

Homes with limited window openings must adhere to the CIBSE fixed temperature test, ensuring that all occupied rooms do not surpass an operative temperature of 26°C for more than 3% of the annual occupied hours (CIBSE Guide A, 2015).

Adjustments for Homes with Vulnerable Occupants

Care homes and accommodations for vulnerable occupants, primarily relying on natural ventilation, should adhere to criteria (a) and (b) from section 4.2 above, assuming Type I occupancy as described in CIBSE TM52 (2013). If predominantly mechanically ventilated, they should use the fixed temperature method outlined in section 4.3.

In cases where there is a heightened risk of overheating, a heatwave strategy should also be developed using additional weather files to explore performance and demonstrate mitigation options under extreme events, such as heatwaves.

Corridors

The overheating assessment for corridors should consider the number of annual hours where the operative temperature exceeds 28°C. Although there is no mandatory target, exceeding an operative temperature of 28°C for more than 3% of total annual hours should be highlighted as a significant risk in the report.

Internal Gains Profiles

The provided occupancy and equipment gains and profiles serve as a comprehensive test to capture key aspects of overheating. They address high-risk periods during the middle of the day and early afternoon, as well as night-time hours crucial for ensuring uninterrupted sleep. While individual homes may have varying occupancy patterns, this standardized test ensures tested units perform adequately throughout the day and night. It's imperative to use these profiles consistently for all assessments following this methodology.

Occupancy and Equipment

Following CIBSE Guide A (2015a), living spaces assume a maximum sensible heat gain of 75 W/person and a maximum latent heat gain of 55 W/person. Additionally, a 30% reduction in gain during sleeping is allowed based on Addendum g to ANSI/ASHRAE Standard 55-2010: Thermal environmental conditions for human occupancy (ASHRAE, 2013), Table 5.2.1.2 'Metabolic rates for typical tasks'.

Lighting

For the assessment, lighting energy is assumed to correlate with floor area, with lighting loads measured in W/m². Between 6 pm and 11 pm, a default of 2 W/m² is assumed for an efficient new-build home, considering adequate daylight levels (noting assessment within CIBSE TM52 is limited to May to September). For existing buildings or specialized lighting designs, a calculated higher value should be applied. For communal corridors, use 2 W/m²; this may be assumed as zero if passive infrared (PIR) sensors are present.