Trial / Demo
The release of TM59:2026 introduces significant updates to the assessment of overheating risk, including a revised methodology, new weather file requirements, and a staged approach to compliance assessment.
To help the industry navigate these changes, we recently hosted a webinar exploring the new guidance, the practical implications for modelling workflows, and the launch of the new TM59:2026 Navigator for IESVE.
Following the session, we've compiled answers to the most common questions we've received about TM59:2026, the Navigator, and recommended modelling approaches within IESVE.
Where can I get the TM59:2026 navigator and how much does it cost?
The navigator is free to download from the Content Store and is compatible with VE 2026.
Which IES VE package do I need to use the TM59:2026 navigator?
You'll need at least the Professional pack to be able to access the required applications for running a TM59:2026 assessment.
Is a user guide provided with the TM59:2026 navigator?
Yes, the user guide is included in the navigator and is accessed by clicking on the “Help” step.
What weather files should I be using, and where can I get them?
CIBSE guidance states that you should use the latest version of the DSY1 file appropriate to the site location for the 2050s, RCP8.5, 50th percentile scenario. Such files are labelled as <zone reference>_DSY1_2050s_HIGH50_CIBSE_v1.1. These files can be purchased on our website, where our Sales team will be able to assist you.
For TM59:2026 assessments in Ireland, we offer several Irish locations, including future climate files. Contact our Sales team who will be happy to help.
Could the TM59:2026 workflow become more complicated if a project passes Stage 1 but fails Stage 2, and a passive measure such as increasing the openable window area is introduced to achieve compliance? In such cases, will this then need to be reassessed in Stage 1 given it is a passive measure?
If a project passes Stage 1 but fails Stage 2, and increasing the openable window area is used to achieve compliance, Stage 1 should be rerun with the revised design change. While such a measure would be expected to improve Stage 1 performance, the revised design should be documented and used consistently throughout the TM59 assessment.
Which version of TM59 should I be using: 2017 or 2026?
TM59:2017 is still the required methodology for Part O compliance. CIBSE has confirmed it is discussing incorporating TM59:2026 and the updated weather files into the approved documents and recommends using TM59:2026 to align with the current industry best practice. Therefore, you should assess against TM59:2017 for Part O and assess to TM59:2026 if you want to reflect best practice and anticipate required overheating performance.
Other regulations such as Irish TGD-L and Scottish Section 3 reference TM59 without explicitly stating a version. We recommend checking with the relevant authority for which version or versions satisfy compliance.
If rooms pass Stage 1, why are Stage 2 and Stage 3 required?
Stage 2 is still required to assess dwellings under actual site constraints. Stage 1 assesses dwellings under ideal conditions, where security and acoustic constraints are assumed to have no impact on ventilation openings. Stage 2 then considers how the dwellings perform when these site constraints are applied. If there are no relevant site constraints, Stage 2 and Stage 3 assessments are not required. Stage 3 assesses the use of cooling and is only required where dwellings do not pass Stage 2, including after enhanced mechanical ventilation has been considered.
If the site the dwelling is being assessed in has no site constraints (security, acoustics or air quality), will further assessment be required beyond Stage 1?
If the dwellings being assessed show compliance against applicable criteria in Stage 1, no further assessment is required.
How does thermal bridging affect the overheating assessment at each stage?
Thermal bridging is typically considered a heat loss mechanism; however, it can also contribute to heat gain, for example where a building element is exposed to direct solar radiation. Detailed thermal bridging functionality was introduced in IES VE 2025, and we recommend referring to the relevant documentation for further guidance. If additional support is required, please contact our Customer Success team.
Does criterion D for communal areas apply to communal corridors between apartments, circulation within a single dwelling, or both?
Criterion D applies only to communal areas between apartments. This includes spaces such as stair wells, corridors, lift and entrance lobbies. We still recommend checking circulation spaces inside individual dwellings for overheating.
How should infiltration be modelled for TM59:2026?
Infiltration should be 0 for new builds and for retrofits where airtightness is expected to meet new build standards. Infiltration in existing builds should be modelled based on expected airtightness levels.
Can TM59 compliance be demonstrated when assessing only sample dwellings?
Yes, an overall pass of the TM59 assessment is achieved if all occupied spaces (including communal areas in multi-dwelling buildings) within every sampled dwelling passes all the applicable criteria.
If Peak Lopping is to be introduced with Mechanical Ventilation, will this be assessed in Stage 2 or Stage 3? And when is Stage 3 required?
Mechanical Ventilation with Peak Lopping would be assessed in Stage 3. In Stage 3, mechanical cooling or air tempering is assessed if the spaces cannot comply in Stage 2 without cooling.
When is a room assessed against criteria A and B (predominantly naturally ventilated) instead of B and C (predominantly mechanically ventilated)?
Criteria A and B are assessed when opening constraints allow for ventilation devices (openable windows) to open for at least 50% of the occupied hours. Otherwise, the room is assessed against criteria B and C.
Is CMEV or MVHR considered as a passive ventilation and can it be assessed at Stage 1?
Continuous background ventilation from CMEV and MVHR systems can be included in Stage 1. Such systems must provide flow rates that meet only whole-dwelling air requirements.
What format is the automatically generated report available in?
The report is generated in PDF format and is available in the Content Manager. You can copy the data from the report to a Word document or Excel spreadsheet as required.
Circulation spaces assessable under criterion D are unoccupied — what are the occupied hours used for such spaces?
Communal areas are considered unoccupied for the purposes of modelling internal gains but may be occupied at any time of day or night, so they are assessed over 3672 occupied hours during May to September.
What Apache simulation settings should I use?
Details on recommended simulation settings can be found in the user guide. We recommend these settings:
• Suncast: on (to account for solar gains)
• MacroFlo: on (to account for airflow through external openings and between rooms)
• Auxiliary ventilation air exchange: off (mechanical ventilation is accounted for with system outside air supply in the ready-made templates)
• Natural ventilation air exchange: off (natural ventilation via openings or internal airflow should be accounted for with MacroFlo)
• Simulation period: 1st January to 31st December
• Simulation timestep: 6 minutes
• Reporting interval: 60 minutes
• Preconditioning period: 10 days
Should Apache Systems or Apache HVAC be used for modelling the HVAC systems?
You can use either application for modelling HVAC systems, however, keep in mind that the automatically generated report shows mechanical ventilation and cooling modelled via Apache Systems only.
Can blinds be included within the assessment?
Blinds can be included in TM59 assessment as part of the passive design strategy, provided their operation is clearly defined and any reduction in the effective openable area of the window is appropriately reflected in the model. Stage 1 should include an assessment with and without blinds.
How should trickle vents be accounted for if present?
Whether or not trickle vents should be accounted for depends on the stage of assessment, to achieve a balance of speed and detail. Generally, we recommend using MacroFlo to model the opening and assign the control strategy with a formula profile. The trickle vent can also be incorporated into the window opening to make model management easier.
Where can I find the ready-made thermal templates?
These are bundled in the navigator, and you’ll import them to your model as part of the navigator workflow.
Are DHW gains included in the ready-made thermal templates?
We have included thermal templates for spaces where DHW gains are likely to be required, including cupboards with HIUs, risers with communal pipework, and circulation ceiling voids with pipework. The heat gains should be edited based on the project to meet TM59:2026 Section 3.7.2 requirements.
We have seen very low g values (0.24) used as a mitigation measure — is there guidance on a reasonable lower limit given the impact on occupant wellbeing?
If products available on the market offer a low window g value of 0.24, this can be justified as a mitigation measure. However, the use of a low g value should also be considered in terms of its trade-offs with energy use, including both seasonal heating and cooling demand, as well as the potential impact on daylight performance.
How should the effects of ceiling fans be considered?
The TM59:2026 navigator report dialog allows the input of an elevated airspeed for the criteria A & C temperature uplift. Heat gains from ceiling fans are not accounted for as this is not required for TM59:2026.
Does TM59:2026 require scripted profiles for openings left open during the night?
The ready-made MacroFlo opening types include formula profiles to modulate the opening and closing of windows during the day and night according to TM59:2026 requirements. Scripted profiles are not required to meet these requirements.
How should the ready-made MacroFlo opening types be used?
The ready-made MacroFlo opening types provide a starting point for modelling airflow from external openings. Edit the opening category, opening area percentage, and proportions to match the opening design. For openings that open during the day and night, but with a reduced secure opening, edit the appropriate formula profile.
Each MacroFlo opening type’s intended room type and opening/closing configuration is described in its name:
The TM59:2026 Navigator is available now as a free download from the Content Store and is designed to simplify the process of assessing against the latest guidance. The Navigator includes:
Please note that the Navigator requires IESVE 2026 and is available to users with the appropriate IESVE licence package.
