Hygrothermal and moisture risk.
Add insulation and the dew point moves into the wall. We run condensation risk analysis across a full year of real weather, so internal insulation, retrofit and flat roofs stay dry instead of rotting two winters in.
What hygrothermal analysis delivers.
What it isInsulate an airtight envelope and the cold layers move inward, the dew point moves with them, and condensation that once dried out harmlessly now collects inside the build-up. The wall no longer behaves the way intuition says it should. This is exactly where it bites: internal wall insulation to solid masonry and stone, heritage and protected fabric, inverted and warm flat roofs, timber frame and hybrid masonry-insulation build-ups, the assemblies where the cold side and the vapour-open side are no longer where you would guess.
A moisture risk assessment de-risks the build-up before anything is specified or ordered, not after a wall starts to fail. We simulate each critical assembly across a full year of real weather, so the result tracks the genuine seasonal cycle of wetting and drying rather than a single design-day snapshot. Every assembly comes back with a verdict: the moisture content over time, whether the construction settles to a stable equilibrium or wets up year on year, and the specified fix that keeps the fabric dry.
A Glaser steady-state screen to BS EN ISO 13788 sorts the simple, clearly safe assemblies fast. Transient WUFI simulation to BS EN 15026, run over several years on real materials, real weather and real moisture loads, tells the truth about anything that buffers water, absorbs driving rain or insulates from the inside.
Internal insulation, deep retrofit of solid walls, heritage and protected structures, flat roofs and timber frame. These are the places where getting the moisture balance wrong rots the structure or grows mould a few winters later. We assess mould-growth risk at the critical surface and flag any build-up that cannot reach a safe annual equilibrium for redesign before a detail is fixed.
Where moisture risk bites.
Critical situationsMost build-ups are fine and a Glaser screen says so in an afternoon. A handful are not, and they are always the same handful. These are the situations where the moisture balance is genuinely on a knife edge and a transient WUFI run earns its fee.
The single highest-risk retrofit detail. Insulating inside leaves the masonry colder and damper and pushes the dew point into the original wall. Vapour control, insulation choice and driving rain decide whether it survives.
Pairs with airtightness strategy →Deep retrofit of solid stone or brick to the EnerPHit standard. Certification expects the moisture-critical details proven safe, and we model them before the insulation is ordered, not after damp appears.
See EnerPHit retrofit →Warm and inverted flat roofs and CLT decks dry slowly and hide moisture above the ceiling. Get the vapour control or the drying path wrong and the deck holds water it cannot shed, often unseen until the structure has gone.
Timber tolerates very little sustained moisture before it decays. In a frame wall the sheathing and the structural studs sit close to the cold side, so the build-up has to be shown to dry faster than it wets.
Protected structures rarely allow external insulation, which forces the higher-risk internal route on walls that were never designed to be sealed. The original fabric has to be kept inside safe moisture limits.
Designed with a Passivhaus designer →Natural and vapour-open insulants change the moisture behaviour of a wall as much as its carbon footprint. The two decisions are best made together, with the moisture model and the carbon count open side by side.
Linked to whole-life carbon →Method: how we work.
MethodWe read the construction details and pull out the moisture-critical assemblies and the worst-case rooms: internal wall insulation to solid walls, warm and inverted flat roofs, CLT decks, timber frame, hybrid build-ups, and the high-humidity spaces (bathrooms, kitchens, plant rooms) that set the internal moisture load. That list is the scope for modelling. Everything else gets a screen and a clean bill.
Each candidate runs through a Glaser steady-state calculation to BS EN ISO 13788. The screen clears the build-ups that are plainly safe and flags the borderline ones, and any assembly with materials that buffer water or absorb rain, to carry forward into full transient simulation. It keeps the expensive modelling pointed only at the walls that need it.
The flagged assemblies go into WUFI to BS EN 15026, run over several years on the right local weather file and a realistic internal moisture load for that room. The transient model tracks heat and moisture hour by hour, with material storage, rain absorption and drying, so the result shows whether the wall settles to a stable equilibrium or wets up a little more every winter.
Where the simulation shows a risk, we specify the way out: a vapour control layer, an intelligent variable-permeability membrane, a breather layer, or a different insulant altogether. The report sets out the moisture results, the mould-growth verdict at the critical surface, and the specified assembly the contractor builds from. The fix lands while the build-up can still change, which is the whole point.
Screen a build-up for condensation.
Run a first-pass surface and interstitial condensation check on a wall before you commit to full WUFI modelling. It tells you in seconds whether the assembly needs a closer look.
Common questions.
FAQWhat is interstitial condensation?
Interstitial condensation forms inside a construction, not on a visible surface. Warm, moist internal air pushes water vapour into the assembly, and where the temperature falls below the dew point that vapour condenses inside the build-up: in the insulation, behind a board, against a cold structural layer. It is hidden, so it can saturate insulation, rot timber and feed mould for several winters before anything shows at the surface. Airtight, heavily insulated envelopes move the cold layers, so condensation risk analysis has to model where the dew point now sits rather than assume the wall still dries the way it used to.
What is the difference between Glaser and WUFI, and when is each enough?
Glaser is a steady-state calculation to BS EN ISO 13788. It uses fixed monthly conditions and ignores moisture storage, rain absorption and liquid transport, which makes it a fast first screen for simple vapour-open assemblies and nothing more. WUFI is a transient simulation to BS EN 15026 that tracks heat and moisture hour by hour over several years, on real materials, real weather files and a real internal moisture load. For internal wall insulation, solid-wall retrofit, flat roofs, timber frame and any layer that buffers water, Glaser reads too optimistic and a multi-year WUFI run is the only way to see whether the build-up dries out or wets up year on year.
Can you internally insulate a solid stone or brick wall without causing damp?
Usually yes, but internal wall insulation is the highest-risk detail in any solid-wall retrofit, so it has to be modelled, not guessed. Insulating on the inside leaves the masonry colder and wetter, because the wall no longer gets warmed from within, so driving rain exposure, the insulation choice and the vapour control strategy decide whether it stays sound. We simulate the specific wall in WUFI to BS EN 15026, with the right local weather file and a realistic internal moisture load, then specify a build-up that holds the masonry inside safe moisture limits. We do that before the insulation is specified or ordered, not after a failure.
Do we need hygrothermal analysis for an EnerPHit or heritage retrofit?
For any deep retrofit that adds insulation to an existing solid wall, yes. EnerPHit certification expects moisture-critical details such as internal wall insulation to be demonstrated as moisture-safe, and heritage and protected structures rarely permit external insulation, which forces you down the higher-risk internal route. A WUFI simulation to BS EN 15026 confirms the insulation and vapour strategy while the build-up can still change, which is far cheaper than tearing out hidden damp two winters after completion. The moisture risk assessment pairs directly with our EnerPHit retrofit and airtightness work.
Prove it dry before you build it.
Screen a wall with the condensation tool, or send us the build-ups and we will scope a transient WUFI simulation for the moisture-critical details. Early, while the detail can still change.