Hygrothermal analysis models the combined movement of heat ('thermo') and moisture ('hygro') through a construction over time, to predict whether moisture will accumulate within it and whether it can dry out again. It is how a designer demonstrates, in advance, that a wall, roof or floor build-up — particularly one with added insulation — won't quietly rot from the inside. There are two levels of method.
The Glaser method (BS EN ISO 13788)
The Glaser method is the classic, steady-state condensation-risk calculation, standardised in BS EN ISO 13788. For each month of the year it computes the temperature gradient and the vapour-pressure gradient through the build-up, finds whether and where the vapour pressure reaches saturation (the condensation plane), estimates how much condensate forms in the cold months, and checks whether it evaporates again in the warm months. If annual condensation exceeds annual drying — or if it lands on a moisture-sensitive material — the build-up fails the assessment.
The limitations of Glaser
- Steady-state and vapour-only: it considers diffusion alone, ignoring liquid (capillary) transport that can redistribute and dry moisture.
- No rain: it doesn't model wind-driven rain absorbed by the outer leaf — often the dominant moisture load on a solid wall.
- No sorption / moisture storage: it treats materials as not buffering moisture, whereas real hygroscopic materials store and release it.
- No solar drying or real weather: it uses simplified monthly averages, not the actual transient climate.
- No built-in moisture or air leakage: both significant in reality.
Because of these simplifications, Glaser can be misleading for moisture-sensitive, rain-exposed or capillary-active build-ups — sometimes flagging a safe vapour-open wall as risky, or (more dangerously) missing a rain-driven failure it simply doesn't model. That's where transient simulation comes in.
Transient hygrothermal simulation (WUFI)
WUFI (Wärme und Feuchte instationär — 'heat and moisture, transient') is the leading transient hygrothermal simulation tool. Rather than monthly averages, it models heat and moisture movement hour by hour over multiple years, using real climate data and validated material datasets that include liquid (capillary) transport and moisture storage. It accounts for the things Glaser ignores:
| Factor | Glaser (ISO 13788) | WUFI (transient) |
|---|---|---|
| Time basis | Steady-state, monthly | Transient, hourly over years |
| Liquid (capillary) transport | Ignored | Modelled |
| Moisture storage / sorption | Ignored | Modelled |
| Wind-driven rain | Ignored | Modelled (with exposure) |
| Solar gain & real climate | Simplified | Real hourly weather data |
| Best for | Simple screening; diffusion-dominated walls | Solid-wall IWI, heritage, rain-exposed, high-risk |
Interpreting the results
A hygrothermal assessment isn't just 'pass/fail'. The useful output is the trend in total moisture content of the assembly and of its sensitive components (e.g. embedded timber, the masonry behind IWI) over several simulated years. The questions are: does moisture reach a stable equilibrium, or keep climbing year on year (a slow failure)? Does any component exceed the moisture level at which decay or mould becomes likely? Does the wall dry over summer? Good practice judges results against recognised assessment criteria (such as those for timber moisture content and mould-growth risk), not against a single dew-point line.
When to use which
- Simple, diffusion-dominated, low-risk build-ups (e.g. a well-detailed new cold roof or a cavity wall): Glaser is usually sufficient.
- Solid-wall internal wall insulation, heritage and traditional buildings, highly rain-exposed elevations, moisture-sensitive materials, or anything where a wrong answer is costly: transient (WUFI) analysis.
- Either way: pair the calculation with realistic inputs (local climate and rain exposure, accurate material data, sensible airtightness assumptions) — a model is only as good as what you feed it.
Worked example — a Glaser month-by-month check
A Glaser assessment is, at heart, a bookkeeping exercise repeated for each of the twelve months. For each month it takes the average internal and external temperature and humidity, computes the temperature profile through the build-up (from the layers' thermal resistances) and the saturation vapour pressure at each interface, then overlays the actual vapour-pressure profile (from the layers' vapour resistances). Wherever the actual vapour pressure would exceed saturation, condensation is predicted at that interface, and the method tallies how many grams per square metre accumulate. Through the cold months condensate builds up; through the warm months the gradient allows it to evaporate. The build-up passes if the annual total that condenses is fully re-evaporated within the year and never exceeds the amount a moisture-sensitive layer can tolerate. A typical failure looks like steadily accumulating condensate at the cold face of insulation that does not fully clear over summer — a slow, year-on-year wetting that the monthly ledger reveals before a single brick is laid.
Setting up a WUFI model — the inputs that decide the answer
A transient simulation is only as trustworthy as its inputs, and three matter most. First, the external climate: WUFI uses real hourly weather files, and the choice of reference year (including a deliberately wet, cold 'design' year rather than an average one) materially changes the verdict. Second, the driving-rain exposure: the proportion of rain striking the elevation that the outer surface actually absorbs depends on orientation, exposure and the rain-shedding quality of the finish — get this wrong and a rain-driven failure is either invented or missed. Third, the material data: validated datasets that include liquid-transport and sorption curves, not just a vapour-resistance number, because it is precisely the liquid redistribution and moisture storage that Glaser omits and WUFI captures. Garbage in, garbage out applies with full force to hygrothermal modelling.
Assessment criteria — what counts as a pass
Modern practice does not judge a result against a single dew-point line; it judges the simulated moisture content of each component against recognised performance criteria over several years. The questions are concrete:
- Does the total moisture in the assembly reach a stable equilibrium, or climb year on year (a slow failure)?
- Does any embedded timber exceed roughly 20% moisture content, the threshold above which decay fungi become active?
- Does the surface relative humidity at any sensitive interface stay long enough above the mould-growth isopleth to support growth?
- Does the wall demonstrably dry over the summer, recovering any winter gain?
Guidance such as the WTA criteria and the principles behind BS 5250 frame these judgements. A build-up that equilibrates at a safe moisture content, keeps timber below the decay threshold and dries each summer is assessed as moisture-safe; one that creeps upward year on year fails, even if no single month looks alarming.
Driving-rain exposure and orientation
Because wind-driven rain is the load Glaser cannot see and WUFI can, exposure is often the variable that flips a solid-wall internal-insulation decision. The same wall build-up may be safe on a sheltered, south-facing elevation and unsafe on a severely-exposed, south-westerly one that catches the prevailing wet weather, because the latter absorbs far more rain and has less solar drying. This is why a responsible assessment is elevation-specific: it is entirely normal to clear internal insulation on the sheltered elevations of a building while recommending that the exposed elevation is first made rain-resistant (repointing, a breathable water-repellent, or render) or insulated externally instead. A single whole-house verdict that ignores orientation is a red flag.
