One Job, Done
Properly
Every feature below exists to answer one question: will this dwelling pass Approved Document O? There is nothing here you will pay for and never open.
The Simplified Method, Without the Spreadsheet
Approved Document O's Simplified Method sets maximum glazing areas and minimum openable free areas, varying by orientation, dwelling type, and whether your postcode sits in a high or moderate risk location. It is arithmetic. It is also fiddly, repetitive, and easy to get wrong at eleven o'clock on a Thursday.
AutoTHERMAL applies the limits from the geometry you have already entered. You get a pass or fail per facade, the number you missed by, and a straight answer on whether this dwelling needs to go to dynamic modelling.
It is the fastest sanity check there is on a house type. For a lot of plots, it is the only evidence you need.
TM59 Dynamic Modelling, Powered by EnergyPlus
When the Simplified Method will not carry a dwelling (a single-aspect flat, a corner unit that does not count as cross-ventilated, too much west-facing glass), Part O's other route is dynamic thermal modelling to CIBSE TM59.
AutoTHERMAL runs that simulation on EnergyPlus, the US Department of Energy's building simulation engine and the reference much of the industry is built on. The model steps through every hour using CIBSE Design Summer Year weather data, with the occupancy, internal gains and window-opening profiles TM59 prescribes already configured.
You get a clear pass or fail, the margin on each test, and the room that is driving the failure.
What You Get
All of it, on every plan. No modules to unlock.
Bring your own weather files
Upload the CIBSE DSY or TRY file your assessment needs and AutoTHERMAL runs against it. Drag it in once and reuse it across every dwelling on the site.
Realistic openable areas
Noise, security, fall protection and restrictors all reduce free area, and are all accounted for, including bedrooms that must be modelled closed.
Shading and overshadowing
Overhangs, fins, balconies, external blinds and the building next door.
Batch the whole site
Every plot, every orientation, one run, one schedule of results.
Scenario comparison
Change one variable, keep both answers, and show the client why you chose what you chose.
EnergyPlus engine
Results rest on the same engine the rest of the industry validates against, not a calculation of our own.
Bespoke client reporting
Your logo, your practice name, your standard wording. Issue it as your own document, not ours.
House type library
Save a type once. Reuse it across every site you ever build.
Shared projects
The architect, the assessor and the client can all open the same model.
Frequently Asked Questions About Part O & TM59
What is an Approved Document O overheating assessment?
Approved Document O provides guidance on meeting Part O of the Building Regulations in England. It applies to new residential buildings and is intended to reduce the risk of overheating by limiting unwanted solar gains in summer and providing an adequate means of removing excess heat.
There are two routes described in Approved Document O: the Simplified Method and dynamic thermal modelling using CIBSE TM59.
AutoTHERMAL supports both routes in one workflow.
Which Part O route should I use?
The Simplified Method is often the quickest place to start. It checks the dwelling against limits for glazing and minimum free areas for removing excess heat, taking account of factors including location, orientation and cross-ventilation.
Dynamic thermal modelling can provide greater design flexibility and is particularly useful where the Simplified Method cannot be satisfied or does not adequately represent the building or its site conditions.
AutoTHERMAL lets you assess both routes from the same building model.
What is the Part O Simplified Method?
The Simplified Method is one of the two routes set out in Approved Document O for demonstrating compliance with Requirement O1.
It limits unwanted summer solar gains through glazing limits and, in high-risk locations, additional shading requirements. It also sets minimum free-area requirements for removing excess heat.
The applicable limits depend on factors including location, orientation and whether the dwelling or space has cross-ventilation.
What is CIBSE TM59?
CIBSE TM59 is a methodology for assessing overheating risk in homes using dynamic thermal modelling.
For the dynamic modelling route under Approved Document O, the TM59 methodology is used together with additional limits and requirements set out in Approved Document O.
AutoTHERMAL runs the thermal simulation using the EnergyPlus simulation engine and applies the Part O assessment workflow around the model.
Do I need to know EnergyPlus or traditional thermal modelling software?
No. AutoTHERMAL is designed so that you do not need to operate EnergyPlus directly or become proficient in a general-purpose building simulation package.
You still need to understand the building and the overheating assessment: how the dwelling is ventilated, the assumptions being made and what the results mean.
AutoTHERMAL handles the simulation workflow so you can concentrate on the assessment and the design decisions behind it.
What weather files are used for a Part O dynamic thermal assessment?
Dynamic thermal modelling under Approved Document O follows the CIBSE TM59 methodology, together with the additional requirements of Approved Document O.
AutoTHERMAL allows the appropriate CIBSE weather file for the assessment to be used in the EnergyPlus simulation.
The appropriate weather file should reflect the requirements of the assessment and the building's location and circumstances.
Do I need to submit a TM59 report to Building Control?
Yes. Approved Document O states that the building control body should be provided with a report demonstrating that the residential building passes CIBSE's TM59 overheating assessment.
The report should contain the information required by section 2.3 of TM59. The Approved Document O Appendix B compliance checklist should also be provided, and additional information may be required.
AutoTHERMAL is designed to produce the assessment information and results needed to support this reporting process.
Will Building Control accept an AutoTHERMAL report?
Compliance is determined by the relevant building control body rather than by the software used to perform the assessment.
AutoTHERMAL is designed around the Approved Document O dynamic thermal modelling route, including CIBSE TM59 and the additional modelling requirements specified by Approved Document O, and provides the assessment inputs and results required to support the compliance report.
Does Part O apply to extensions or conversions?
Part O applies to new residential buildings in England.
It does not apply to extensions or conservatories added to an existing residential building after it has been built, and it does not apply to buildings undergoing a change of use.
A conservatory forming part of a new residential building must meet the requirements of Part O.
Can I use mechanical cooling to pass Part O?
Mechanical cooling should only be used where insufficient heat can be removed without it.
Approved Document O requires buildings to meet Requirement O1 using passive means as far as reasonably practicable. Where mechanical cooling is proposed, it should be demonstrated to Building Control that practicable passive measures for limiting unwanted solar gains and removing excess heat have been considered first.
Installing mechanical cooling can also affect compliance with Part L and must be reflected in the relevant SAP calculation.
How do noise and security affect a Part O assessment?
An overheating strategy needs to be usable in practice.
Approved Document O therefore requires consideration of factors including noise, pollution, security, protection from falling and entrapment.
For example, if occupants cannot reasonably be expected to leave a window open because of external noise or security risk, the assessment should not simply assume that the full opening is available for removing excess heat.
AutoTHERMAL allows these practical constraints to be reflected in the assessment.
Can restricted windows be used for Part O?
Yes, but the usable opening needs to reflect the restriction.
For the Simplified Method, the equivalent area should be calculated with the restrictor in place. Approved Document O also contains specific requirements for openings used as part of the overheating strategy, including protection from falling where windows can open wider than 100mm.
What is the difference between TM52 and TM59?
CIBSE TM59 provides a methodology for assessing overheating risk in homes and is the methodology referenced by Approved Document O for its dynamic thermal modelling route.
CIBSE TM52 provides guidance for assessing thermal comfort and overheating in buildings and is commonly associated with non-domestic buildings.
AutoTHERMAL's Approved Document O workflow is focused on residential overheating assessment using the Part O Simplified Method and CIBSE TM59 dynamic thermal modelling.
What does it cost?
AutoTHERMAL is in early access and pricing is not published yet. Join the waitlist and you will hear it first.
Bring us a plot that keeps failing
Send us the dwelling that keeps failing and we will walk through it with you.