Why modern insulated buildings can still get condensation, mould, and rot. Interstitial condensation, dew point, vapour control, BS 5250, and how the Glaser method works under ISO 13788.

Insulation keeps heat in. It also, inconveniently, changes where moisture collects inside a wall. A wall that was fine for eighty years as a ventilated solid brick construction can develop condensation and mould within a couple of winters once you internally insulate it, unless the detailing gets the vapour physics right.

This guide covers the two kinds of construction-related condensation (surface and interstitial), what causes them, how to assess risk, and what good practice looks like in both new build and retrofit. It's aimed at designers, architects, surveyors, and retrofit coordinators.

Two kinds of condensation

Surface condensation

Moisture condensing on the room-side surface of a building element — typically a cold corner, a window, or a patch of wall directly behind furniture. You can see it, wipe it, and it leads to visible mould within a few months of being allowed to persist.

Cause: the surface temperature drops below the dew point of the room air.

Primary risk factor: high humidity indoors combined with cold surfaces — usually poor insulation, thermal bridging at junctions, or bathrooms/kitchens with inadequate extract ventilation.

Interstitial condensation

Moisture condensing within the structure of a wall, roof, or floor — inside the build-up where you can't see it. It rots timber, corrodes metal, degrades insulation performance, and can accumulate for years before presenting externally (bulging render, stained plaster, musty smells, frost damage).

Cause: water vapour diffuses through the construction and hits a cold layer where it drops below dew point. The vapour condenses to liquid water inside the wall.

Primary risk factor: a vapour-open material placed outboard of a vapour-open cavity, or insulation added internally without addressing the vapour path.

Interstitial condensation is the harder problem, the more expensive one to fix, and the one that new insulated retrofits routinely get wrong.

The physics, fast

Air holds water vapour up to a maximum — the saturation point — which depends on temperature. Warm air holds more vapour than cold air. When air is cooled, its capacity to hold vapour decreases; at the temperature where the existing vapour load equals the saturation point, the air is at 100% relative humidity, and any further cooling causes water to condense. That temperature is the dew point.

A room at 20°C and 50% relative humidity has a dew point of 9.3°C. So any surface (or point inside a wall) colder than 9.3°C will see water condense on or within it.

Two conditions drive condensation risk:

Indoor humidity — set by occupant behaviour, cooking, washing, drying clothes, and ventilation rate
Material temperature — set by the outdoor temperature, the U-value of the construction, and the vapour resistance of each layer

Fix either, and the condensation risk drops. Ideally, fix both.

Surface condensation and the temperature factor f_Rsi

The official UK metric for surface condensation risk is the temperature factor (f_Rsi), defined in BRE IP 1/06 (Assessing the effects of thermal bridging at junctions and around openings).

f_Rsi = (T_si − T_e) / (T_i − T_e)

Where:

T_si = internal surface temperature (°C)
T_i = internal air temperature (°C)
T_e = external air temperature (°C)

For a dwelling, f_Rsi must be greater than 0.75 at all junctions to avoid mould growth at 20°C internal, 5°C external, 50% RH internal — the design condition.

A good insulated wall with well-detailed junctions achieves f_Rsi > 0.85 even at cold corners. A bad thermal bridge (uninsulated concrete lintel, cantilevered steel balcony) can drop f_Rsi to 0.60 — well below the 0.75 threshold, and the surface will see mould within a few winters.

This is why Psi values and thermal bridging matter as much as U-values: they control where the cold spots are that drive surface condensation.

Interstitial condensation — how to assess it

There are two standard methods:

1. The Glaser method (ISO 13788)

A steady-state method where you calculate the vapour pressure at each layer interface in a wall build-up, and compare it to the saturation vapour pressure at that interface temperature. Where vapour pressure exceeds saturation, condensation occurs.

The method treats one month at a time (typically the mid-winter month for the location), assumes steady-state diffusion with no air leakage and no moisture buffering, and reports whether condensation forms, at which interface, and whether the accumulated moisture dries out during summer.

Strengths: simple, spreadsheet-able, widely understood, underpins BS 5250 and most UK compliance work.

Weaknesses: ignores air leakage (often the dominant moisture transport mechanism in real buildings), ignores material moisture buffering, ignores solar-driven vapour redistribution, and isn't suitable for buildings with high moisture load (swimming pools, laundries) or unusual exposure.

For standard residential construction in the UK, the Glaser method is the regulatory baseline — good enough for most purposes, but worth knowing its limits.

2. WUFI / dynamic hygrothermal modelling

WUFI (Wärme Und Feuchte Instationär — "heat and moisture, transient") runs a full time-stepping simulation of heat and moisture transport through a construction, accounting for liquid water transport, moisture buffering, and real hourly weather data. It's the gold standard for complex or unusual constructions and for retrofit of existing walls.

Specialist tool. Expensive to licence, time-consuming to set up. But for internal insulation of solid walls, specialist retrofit projects, or anywhere you're pushing the envelope of the Glaser method, WUFI is the right answer.

Vapour resistance and where to put the vapour control layer

A wall has a vapour resistance at every layer, measured in MNs/g (meganewton-seconds per gram) or sometimes the inverse, sd-values in metres of equivalent still-air thickness. These describe how hard it is for water vapour to pass through that layer.

The general rule for vapour control in cold climates (like the UK):

Vapour resistance must decrease from inside to outside.

Put the highest-resistance layer (vapour control layer, VCL) on the warm side of the insulation. Keep the external layers vapour-open so any moisture that does get through can escape to the outside.

If you reverse this — say by putting a low-permeability render over breathable brickwork — vapour will diffuse inward-to-outward, hit the impermeable render, and condense inside the wall. This is exactly what happens with many badly-specified external insulation systems on breathing older buildings.

Rough vapour-resistance ordering in a modern timber-frame wall (inside to outside):

Layer	Vapour resistance (MNs/g)
Plasterboard	0.6
Vapour control layer (polythene or AVCL)	200 – 1,000
Mineral wool insulation	0.1 per mm
OSB sheathing	5 – 15
Breather membrane	0.01
Ventilated cavity	~0
Brick outer leaf	2 – 6

Vapour resistance drops monotonically from the VCL outwards. Good.

Retrofit — where condensation risk explodes

Adding insulation to an existing wall changes where the dew point sits. Solid-wall Victorian housing is the classic problem:

Original wall: 225mm solid brick, uninsulated. Dew point sits in the middle of the brick or just inside the outer leaf. Brick is hygroscopic and tolerant — moisture comes and goes with weather.
Add 80mm internal PIR: dew point shifts to the interface between the PIR and the original internal plaster. PIR is vapour-tight. If even a tiny amount of internal humidity gets behind the PIR (via a poorly-sealed junction), it condenses at the cold brick face and has nowhere to go.
Within 2–3 winters, you have black mould behind the insulation, wet plaster, and decaying internal finishes.

The same physics applies to:

Internal insulation of older cavity-fill walls with cavity trays in the wrong place
Room-in-roof conversions with rafter-level PIR and no vapour control
Floor-to-wall junctions at a damp-bridging DPC level
Cold roofs ventilated to the loft where the insulation extends into the eaves

Good retrofit practice:

Do a hygrothermal assessment — Glaser method minimum, WUFI for high-risk retrofits
Pick vapour-open insulation where possible (wood fibre, sheep wool, mineral wool with AVCL) rather than PIR for breathable walls
Continuous VCL on the warm side with taped joints and careful detailing around every penetration
Maintain ventilation — internal humidity levels rise after insulating, so mechanical ventilation (MVHR, continuous extract) is more important, not less
Assume some rain penetration — external walls occasionally get wet. Design for drying.

Ventilation: the other half of the equation

Everything above addresses where moisture condenses. The other lever is how much moisture is in the room air to begin with.

Modern airtight buildings produce roughly the same moisture as leaky old buildings (cooking, bathing, breathing, clothes drying — all temperature- and occupant-dependent, not fabric-dependent). But the airtight ones trap it.

UK regulations now require continuous mechanical ventilation in new-build dwellings:

Natural ventilation with extract fans — acceptable for smaller dwellings with moderate airtightness
Continuous mechanical extract ventilation (MEV) — better for tight houses
Mechanical ventilation with heat recovery (MVHR) — best for Passivhaus-grade airtightness (< 1 ach @ 50 Pa)

If you insulate heavily and tighten the building envelope but don't upgrade ventilation, you will see condensation on every cold surface and mould within a year. The ventilation is not optional.

Frequently asked questions

My cavity wall has black mould on internal bedroom corners — what do I do? It's almost always surface condensation from a combination of thermal bridging at the corner and high internal humidity. Fixes: check the temperature factor (f_Rsi) of that corner with a thermal camera; improve ventilation (replace a failed extract fan, upgrade to MEV/MVHR); internally insulate the wall if the corner is a thermal bridge. Don't just wipe the mould off with bleach — you're treating the symptom.

Can I insulate a solid stone cottage internally? Yes but carefully. Solid stone walls often have high wind-driven rain loading and are breathable. Use vapour-open insulation (wood fibre, lime plaster + hemp), do a WUFI assessment, and assume a long drying-out period. PIR with a polythene VCL is the wrong answer for most heritage solid walls.

Is a polythene vapour barrier required in a timber-frame wall? In cold UK climates, yes — either a full polythene VCL on the internal face of the insulation or an airtight and vapour-controlling (AVCL) membrane layer. Newer designs use smart variable-permeability membranes (Pro Clima Intello, Partel Vara) that tighten in winter (preventing condensation) and open in summer (letting any trapped moisture escape).

What's BS 5250? BS 5250:2021 is Management of moisture in buildings — Code of practice. The main UK standard for moisture risk in residential construction. It codifies the Glaser method, ventilation requirements, retrofit assessments, and standard detailing for cold-bridge avoidance. Always worth having to hand for any project where moisture matters.

Does our U-value calculator check condensation risk? Not in the current version. U-value and condensation risk are linked (the U-value affects internal surface temperatures and thus f_Rsi) but they're separate calculations. We flag high-risk build-ups when we see them, but a proper condensation assessment requires its own ISO 13788 or WUFI analysis. Planned for a future release.

The short version

If you take away three things:

Surface condensation is caused by cold internal surfaces — fix thermal bridging and ventilation.
Interstitial condensation is caused by vapour finding a cold layer within the wall — fix by putting the VCL on the warm side and keeping external layers vapour-open.
Ventilation is not optional. Airtight insulated buildings produce the same moisture as draughty old ones — if it doesn't leak out through the fabric, it has to be ventilated out mechanically.

Condensation risk in construction: causes, prevention, and assessment