Greengauge Director Toby Cambray looks at a new study that warns of shifting weather patterns that will test the resilience of our buildings to wind-driven rain.
Rain and wind often come together. When they do, rain usually doesn’t just fall straight down, but gets hurled at angles against our buildings, sometimes approaching the proverbial horizontal stair rods. This slanting, building-battering moisture has a name in building physics circles: Wind-Driven Rain, or WDR.
Leave it to the British to have specialised terminology for rain types. While the myth about Inuit having fifty words for snow might be exaggerated, the British weather obsession has genuinely produced a rich vocabulary for describing various forms of getting wet – from drizzle to downpour, deluge to dreich.
A significant recent study from the UK Department for Energy Security and Net Zero examines projected changes in wind-driven rain patterns precited by UKCP18. The findings deserve attention for both new buildings and retrofits, Passivhaus or otherwise.
Same volume, different delivery
While the research suggests minimal changes in overall annual wind-driven rain exposure when averaged across the UK, this headline figure masks more significant shifts that matter for building performance.
It’s not about how much rain we’ll get, but when, where, and how intensely it will arrive.
Most notably, we might expect greater seasonal variation – winter months will see increases in WDR from southerly and westerly directions (up to 25% more in some regions under the most extreme warming), while summer months will experience marked decreases.
Buildings will face more concentrated moisture exposure during winter months, precisely when drying potential is already at its lowest.
On the other hand, north, northeast, and east-facing walls may actually see decreases of 15-25%.
Short and sweet (or rather, short and wet)
Perhaps most significantly for building durability, the research indicates in the future, rain is likely arrive in fewer, shorter, more intense spells. The number of medium and long spells (<10 hours) will decrease, while short, intense spells will either remain similarly prevalent or increase.
However, for most areas, the intensity of extreme events – specifically those used to determine exposure zones in Approved Document C – will likely decrease. However, banking on this potential reduction would be foolhardy given the increased intensity of shorter spells. Crucially for some, this is not country wide. The regions already classified as having “severe” or “very severe” exposure to wind-driven rain – typically western coastal areas – are precisely those projected to experience the greatest increases in WDR. This compounding effect makes robust detailing in these locations even more critical than it already is.
A further complication is that wind driven rain is not just about what hits the surface; wind induced pressure is a crucial factor in the volume of water that penetrates through cracks and compromises in the façade.
The AMOC wild card
These projections are based upon the prevailing climate models, however, as I discussed in my previous column, these models may overestimate the stability of the Atlantic Meridional Overturning Circulation (AMOC). Recent research suggests a potential AMOC collapse could dramatically alter northern European climate patterns.
An AMOC collapse scenario would introduce a different meteorological vocabulary to the UK and Ireland. Perhaps those (mythical) fifty Inuit snow words would become more relevant than our extensive rain lexicon. Our buildings would face significantly colder temperatures and different precipitation patterns than those predicted by the WDR study – less rainfall perhaps, but a whole new (to the UK) class of moisture issues from snow and ice. The next great American export could be the term “Ice dam”.
Designing for uncertain futures
For conscientious designers and builders, these findings reinforce the need for climate-resilient approaches.
- We should be adopting more robust details, like drainage cavities with external wall insulation (EWI) rather than relying on perfect installation and maintenance of ‘face-sealed’ systems.
- We must not underestimate the importance of roof overhangs and other architectural features that can reduce WDR exposure.
- We must ensure adequate drying potential in wall assemblies to handle more concentrated winter moisture loads, and develop quantitative and qualitative assessment methods to do this.
- While all design decisions ultimately end up as OK/not OK, we must get more comfortable with communicating with our clients in terms of risk, and insurers need to recognise this too.
We must acknowledge and adjust the risk profiles we use (usually implicitly) in our work, including the fact that there are climate scenarios within the realms of probability that will upend the way our buildings have to respond to their environments, including colder cold times, hotter hots, and wetter wets.
The weather obsession isn’t just a British cultural quirk – it’s becoming an essential design consideration for buildings that will need to perform through increasingly variable conditions for decades to come.
Toby Cambray is Director of Moisture at Greengauge.
This article first appeared in Passive House + magazine, Issue 50, 2025.
