CIBSE weather file updates

What is a Weather File 🌞⛈️🌪️, how is that linked to EPCs & the Future Homes Standard, and why has overheating compliance just become more complicated?

A new publication from CIBSE prompted me to dig into these questions...

https://go.cibse.org/weather-data-technical-briefing-document

The new package of weather files includes more options, and doesn't rely on the assessor picking the nearest weather station to the development.

Starting with the ‘What’:

When an overheating assessor builds a 3D model of a development, they apply a Weather File to simulate a years’ worth of sunlight, wind and humidity.

This climate data interacts with the building’s solar gains, air flow and thermal mass to calculate hourly internal temperatures, which ultimately gives us a Pass or a Fail for Approved Document Part O and TM59 assessments.

There are hundreds of weather files that can be loaded into any building model. Each one creates different results because they hold unique climatic data. For example, some feature a summer with an intense heatwave, some include historical data, and others are predictions for weather scenarios in future decades.

Weather files are also location specific. The assessor should use a weather file that is both relevant to the site’s location, and is also an approved scenario for checking compliance.

Sounds simple enough 🤪, so why is it becoming more complicated?

CIBSE has just published a new series of weather files.

🟢 They’ve replaced the old locations with 28 new zones. These should NOT be applied based on geographical proximity, but instead based on areas that experience similar weather patterns (see map).

🟡 For each region, there are new Test Reference Year models (TRY) to simulate weather conditions in the 2030s, 2050s and 2080s.

🔵 For each region, there are new Design Summer Year models (DSY) which are summer specific files that test building performance under extreme heat.

In recent years, weather in the UK has become more extreme, and these new weather files reflect this. The below chart compares old and new weather data, showing most of the UK is now hotter, so passing TM59 using these weather files will be more difficult.

The only exception is the Glasgow region, which has got a bit colder. Answers on a postcard!

Annual mean temperatures, comparing 2016 and 2025 data for the UK's main cities. The vast majority show an increase in temperature.

What’s this got to do with the Future Homes Standard and Energy Performance Certificates?

As part of the FHS transition, SAP is being replaced with the Home Energy Model. One of the (many) differences is that SAP uses fixed, UK-based weather data, but HEM uses CIBSE Weather Files.

The HEM assessor can apply different weather files to their model, which will impact the dwelling results.

I should clarify here, new Building Regulations and EPC guidance will define which weather files can be used, but if you’re using HEM for other purposes, any TRY weather file can be applied.

We don't yet know which weather files will be used Part L / Section 6 compliance or for EPC generation, so we can’t guess how/if this will impact results.

But for TM59 overheating assessments, these new weather files are available to use now.

UPDATE - I've had it confirmed from Government officials that compliance with AD-O can continue to use the older (2016) weather files. An assessor can choose to use the newer 2025 weather files, but they are under no obligation to do so. This will remain Government policy until the text of Approved Document Part O is revised to require the use of the newer weather files (I'm not aware of any timeframe for that happening).