Condensation Risk Analysis: Glaser Method vs WUFI

Condensation risk analysis — Glaser vs WUFI hygrothermal modelling.

You should never have to wait for a wall to fail to find out whether it's moisture-safe. Condensation risk is predicted before construction by hygrothermal calculation. This article explains the two methods used — the traditional Glaser method and the transient WUFI simulation — what each can and can't tell you, and when a serious retrofit needs the more rigorous one.

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:

Glaser vs WUFI

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.

Common questions from clients.

Do I need a condensation risk assessment for my retrofit?

For anything that adds insulation to an existing wall or roof — especially internal wall insulation on solid masonry — yes, strongly. It's how you find out before building whether the build-up will stay moisture-safe. A simple, low-risk detail may only need a Glaser check; a solid-wall IWI or heritage build-up warrants transient (WUFI) analysis, because the dominant risks (rain, capillarity, drying) are exactly what Glaser can't see.

Is the Glaser method just wrong, then?

No — it's a valid, standardised screening tool that's perfectly adequate for many straightforward, diffusion-dominated build-ups, and it's conservative and transparent. The point is to know its limits: it ignores liquid transport, rain, sorption and real weather, so for moisture-sensitive or rain-exposed constructions it can mislead, and a transient simulation is needed.

Can modelling guarantee my wall won't have condensation?

No model is a guarantee — results depend on the inputs (climate, rain exposure, material data, workmanship and airtightness assumptions) and on the build being executed as modelled. What hygrothermal analysis does is identify and quantify the risk, let you compare safer options, and design the moisture defences in before construction. It hugely reduces the chance of a hidden failure, but it must be matched by good detailing and airtight, well-ventilated execution on site.