If Building Control, an approved inspector, or a warranty provider has asked you for a "condensation risk analysis" and you are not sure which method they mean, you are not alone. Two methods dominate UK practice: the Glaser method and WUFI. They are not interchangeable, and using the wrong one can mean a rejected submission, a delayed warranty, or a construction that looks fine on paper but fails in the wall.

This article sets out the regulatory basis for condensation risk analysis, the standards behind each method, what each one actually does, and a clear framework for deciding which one your project needs.

The short answer

Condensation risk analysis under UK Building Regulations usually means one of two things: a Glaser calculation to BS EN ISO 13788, or a dynamic hygrothermal simulation to BS EN 15026, commonly run in software called WUFI. BS 5250:2021 sets out when each applies. Standard vapour-closed constructions such as masonry cavity walls and most pitched roofs are usually fine with a Glaser calculation. Higher-risk or more complex builds, including internally insulated solid walls, historic fabric, and constructions with an impervious render, usually need the WUFI method. If Building Control or your warranty provider has flagged a solid wall, a retrofit, or anything outside a standard cavity or timber-frame build-up, assume they want the more detailed method until you confirm otherwise.

The regulatory standpoint: why you have been asked for this at all

Condensation risk analysis exists because of Building Regulation C2, part of Approved Document C: Site preparation and resistance to contaminants and moisture. The regulation requires that walls, floors, and roofs adequately protect the building and its occupants from harmful effects caused by interstitial and surface condensation, alongside ground moisture, precipitation, and water spillage. Approved Document C is still the 2004 edition incorporating the 2013 amendments, and this remains the current statutory guidance for England.

Approved Document C does not set out its own calculation method in detail. Instead, it points to BS 5250 as an accepted route to compliance, stating that a roof will meet Requirement C2 if it is designed and constructed in accordance with that standard. This is why almost every condensation risk analysis you see referenced in the UK ultimately traces back to BS 5250, whichever calculation method it points you toward.

Building Control is not the only party asking. New-build warranty providers and PAS 2035 retrofit coordinators apply similar scrutiny, particularly on solid wall, internal wall insulation, and non-standard projects, where the standard prescriptive guidance in BS 5250 does not automatically apply. If you are running professional due diligence on a construction before committing to a spec, checking which method it was assessed against is a fast way to spot a weak or outdated report.

The British Standards and ISO behind the two methods

Three documents sit behind almost every condensation risk analysis produced in the UK:

BS EN ISO 13788:2012
is the standard behind the Glaser method. It gives calculation methods for internal surface temperature (to assess mould risk) and for the risk of interstitial condensation due to water vapour diffusion. This edition was last reviewed and confirmed in 2023, so it remains the current standard.

BS EN 15026:2023
is the standard behind dynamic hygrothermal simulation, the method most people mean when they say WUFI. This edition superseded BS EN 15026:2007 on 31 July 2023, so any report citing the 2007 number is now working from a withdrawn standard.

BS 5250:2021
the Code of Practice for the Management of Moisture in Buildings, is the master document. It takes a whole-building view and tells you, construction by construction, whether no analysis is needed, whether a Glaser calculation is sufficient, or whether a BS EN 15026 assessment is required.

Worth noting: WUFI itself is not a British Standard. It is proprietary software developed by the Fraunhofer Institute for Building Physics in Germany (the name is short for Wärme und Feuchte Instationär, meaning transient heat and moisture). It has simply become the industry shorthand for numerical simulation to BS EN 15026, in much the same way "Hoover" became shorthand for a vacuum cleaner.

Method one: the Glaser method

The Glaser method, sometimes called the dew point method, works layer by layer through a construction. For each interface between materials, it calculates a temperature and a vapour pressure, using monthly average external climate data and a fixed internal temperature and humidity for the building's occupancy type. If the calculated vapour pressure at any interface exceeds the saturation vapour pressure (the dew point) for that temperature, the method flags a risk of interstitial condensation at that point.

It only needs two material properties per layer: thermal conductivity and vapour resistivity. That is why it slots so easily into the same online tools already used for U-value calculations, and why it has been the default method in UK practice for decades.

Where it holds up:

• Fast and inexpensive
• Well understood by Building Control, warranty providers, and manufacturers alike
• More than adequate for typical, vapour-closed constructions

Where it falls down:

• Ignores capillary action and liquid moisture transport entirely
• Takes no account of driving rain
• Assumes any built-in construction moisture has already dried out
• Uses monthly averages rather than real, variable weather
• Cannot assess junctions or thermal bridges, where many real failures start
• Not valid for hygroscopic materials in significant quantities, or for elements exposed to rain or freeze-thaw cycles

The standard is explicit about its own limits. BS EN ISO 13788 itself lists these simplifications and states plainly that the method is less suitable for certain building components and climates as a result.

Method two: WUFI (dynamic hygrothermal simulation)

Dynamic hygrothermal simulation, run through software such as WUFI in accordance with BS EN 15026, models heat, air, and moisture transfer through a construction on an hourly basis, over a period of one or more real years. Rather than a single pass or fail snapshot, it produces a time-based picture of temperature, relative humidity, and moisture content at any point in the build-up.

Because it works on real, hour-by-hour data rather than monthly averages, it can account for the physical processes Glaser ignores: capillary suction and liquid moisture movement, moisture storage within hygroscopic materials, driving rain, solar radiation on the outer surface, and realistic starting conditions rather than an assumption that everything has already dried out.

Where it holds up:

• Captures the physical processes that cause real-world failures in solid wall, retrofit, and historic construction
• Gives a much richer output than a single pass or fail line
• Suited to exactly the cases where Glaser is known to be unreliable

Where it falls down:

• Needs detailed material property data (moisture storage functions, liquid transport coefficients) that is often unavailable or unreliable for UK building materials, particularly historic ones
• Needs good local weather data, which is not always readily available
• Remains a one-dimensional model, so it cannot represent junctions or interfaces, which is exactly where many condensation problems actually occur
• There is no established background guidance for its correct use in the way there is for Glaser, so results depend heavily on the competence of whoever is running the model
• Takes longer, needs more specialist input, and costs more than a Glaser calculation

WUFI is not a magic fix. Run with poor material data or an inexperienced modeller, it can produce results that look precise but are not actually reliable. The method is only as good as the inputs.

Condensation Risk Analysis comparison at a glance

Which one should you use, and when?

BS 5250:2021 sets this out as a three-tier decision, not a straight choice between two options.

No formal analysis needed. Some well-proven, low-risk constructions are already covered by prescriptive guidance in BS 5250 itself, so no calculation is required.

Glaser is enough. This covers most standard, vapour-closed construction: masonry cavity walls, most pitched roofs, and lightweight, well-ventilated timber frame. Many predictable flat roof build-ups also fall into this category.

WUFI is expected. This applies to constructions facing particular risk or subject to more complex moisture movement, including:

• Internal wall insulation on solid masonry
• Historic and traditional solid wall construction
• Any build-up with an impervious external render
• PAS 2035 retrofit projects assessed as higher risk
• Constructions using significant quantities of hygroscopic materials
• Non-standard build-ups that BS 5250 does not already describe

The Condensation Risk Analysis mistake that gets projects rejected

A Glaser calculation can technically be produced for almost any build-up. That does not mean it is the right one. If Building Control or a warranty provider has specifically asked for an assessment on a solid wall, a retrofit, or anything outside a standard cavity or timber-frame construction, a Glaser printout is likely to come back with a request for the proper method, at the cost of another round of correspondence and delay.

This scenario is only becoming more common. As Part L pushes fabric performance toward higher insulation levels and tighter airtightness, more projects involve internal wall insulation, solid wall retrofit, and other constructions that sit outside the standard, low-risk categories BS 5250 was written around. A compliant SAP or SBEM model tells you the building meets its energy target. It does not tell you whether the wall build-up is safe from condensation, and increasingly, Building Control and warranty providers are asking for that answer separately.

Frequently asked Condensation Risk Analysis questions

Do I need a Glaser calculation or WUFI for Building Control?
It depends on the construction. Standard cavity walls, most pitched roofs, and ventilated timber frame usually only need a Glaser calculation. Solid wall insulation, historic fabric, and non-standard build-ups usually need a WUFI assessment to BS EN 15026.

What is the actual difference between the Glaser method and WUFI?
Glaser is a steady-state calculation using monthly average weather data and vapour diffusion only. WUFI is a dynamic simulation that also accounts for capillary action, moisture storage, and driving rain.

Is BS EN ISO 13788 still current?
Yes. The 2012 edition was reviewed and confirmed in 2023 and remains the current standard behind the Glaser method.

Does Building Regulations Part C require a condensation risk analysis?
Requirement C2 of Approved Document C requires that walls, floors, and roofs are protected from interstitial and surface condensation, and points to BS 5250 as an accepted route to demonstrating this.

Can I use the Glaser method for internal wall insulation on a solid wall?
Generally no. BS EN ISO 13788 itself states the method does not account for capillary action or liquid moisture transport, which are the dominant risks in solid wall construction. BS 5250:2021 points these cases toward BS EN 15026 assessment instead.

Energy Digest carries out condensation risk analysis for architects, developers, and self-builders across the UK, with fixed pricing on most projects and free phone and email support before you commit to any work. For support with Condensation Risk Analysis, please go here: Condensation Risk Analysis For Buildings & Construction Projects | Energy Digest