> For the complete documentation index, see [llms.txt](https://docs.betterbuilding.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.betterbuilding.io/sign-up-and-plans/news-and-updates/2026/we-built-our-own-hygrothermal-solver.md). # We built our own hygrothermal solver For roughly the last six months we've been building something that scratches an itch I've had for years and provided a few headaches and sleepless nights along the way... the [Better Building](https://www.betterbuilding.io/) heat and moisture transfer solver. An obvious question might be why? There are already strong tested software solutions for this, and there have been for a while. Interest in hygrothermal (hygro = moisture, thermal = heat) modelling solvers appear to have built to a peak in the early 2000s, and since then the field has settled around a handful of established names. TU Dresden gave us Delphin. The Fraunhofer Institute gave us WUFI. Anyone working in this space is likely to know both of them, and knows they earn their reputation that has built on empirical evidence and careful verification. Since then, hygrothermal modelling has increased in both popularity as a way to validate design decisions and also to tick the boxes of compliance. The later is not always a good thing! As such, we thought it was time to put hygrothermal modelling in front of a much wider audience, sitting inside the Better Building platform ready to use for our clients, and ***we didn't bolt a surcharge onto it either.*** > [Register For Early Access to the Better Building HAMT Solver](https://forms.cloud.microsoft/r/AVZkByxN7z) #### What's new? For a long time your weather file was inherited from your project information on Better Building. Our hygrothermal modelling now lets you override it and upload your own weather file, which sounds like a small thing but its not. If you can find a [Moisture Design Reference Year](https://www.linkedin.com/pulse/moisture-design-refernce-years-mdrys-tim-law-7x9zc/) file, you can pop it in here!

Article content — Our weather file processing has had a big lift!

Setup is quick. In theory you can have a project defined in under 5 mins, from a blank screen to a built wall or roof. And that 5 mins covers the simulation runtime for a multi-year study, which still feels slightly unreasonable to type out. But its fact... this is a very quick process if you are confident about your inputs and overall assumptions (the much harder bit). Ogh, and that 5 mins also covers your reporting! ### The adventures of driving rain The genuinely fiddly part was better accountability of precipitation, or driving rain. Getting driving rain (and air infiltration) modelled properly, as sources rather than workarounds in EnergyPlus, took a lot of care, and the results correlate very closely to what you'd expect. Water is where hygrothermal models tend to quietly fall apart, so this is the bit we sweated over most.

#### The testing A hygrothermal solver you can't check against something is just a confident guess. We held ours to two industry benchmarks. The first was HAMSTAD, and there's a test report in our documentation if you want to see the numbers. The second was EN 15026, another hygrothermal performance benchmark.

Read on - Between them they allow us to review solver drying, air pressure, rapid transient responses, and capillary active behaviour, amongst other bits. The point of all of it was to confirm the solver responds the way other established software responds using the same tests, rather than the way we'd like to believe it does. The good news... all tests passed that laid the ground work for tests outside of there scope, such as driving rain, suction and redistribution, explicit radiation and a hose of other important consideration for hygrothermal design. #### Cavity ventilation with a little less hand-waving Many tools in this domain let you assume a fixed air change rate for an externally ventilated cavity and move on. We kept that option, but we also added a validated dynamic method for when it isn't.

If you opt in, the solver reuses the local wind direction, speed and solar loading already acting on your outer wall finish to estimate the air velocity through a drained and ventilated cavity, and from there the air exchange happening inside it. That gives you a far more honest read on the cavity's drying capacity than a single assumed figure ever could, and the reasoning is laid out in detail in the post-processing so you can see how it got there. #### Summarised reporting Results come with extensive reporting and two ways to explore the data, low resolution for the overview and high resolution when you need to drill into a single minute of the study. That matters more than it sounds. It lets you confirm for yourself whether the hygrothermal behaviour matches what you believe it should be, and whether the system you're looking at is genuinely safe and durable, instead of trusting a summary chart and hoping

It's been a big build, and it's ready to go. We're [opening early access first](https://forms.cloud.microsoft/r/AVZkByxN7z). We've already collected plenty of names from people wanting in, and there's a form above if you'd like to be one of the last few to get on the list and learn a bit more before it goes live. Early access will close on Wednesday 29th of July. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://docs.betterbuilding.io/sign-up-and-plans/news-and-updates/2026/we-built-our-own-hygrothermal-solver.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.