Condensation on Office Windows and Mould on Walls: Causes and Fixes

Three different problems that look the same

A facilities manager gets a photo of a wet window and a stained sill, and a second complaint about a musty smell and black spots behind a cabinet. Both get filed as "condensation", and both get the same response: wipe it, repaint it, ask the AC contractor to check.

They are not the same problem. Condensation on the outside of the glass, condensation on the inside of the glass or frame, and mould on walls have different causes and different owners, and only some of them are defects. The common misconception, carried over from cold-climate advice, is that condensation means the building is too cold inside and warmer air will fix it. In an air-conditioned building in a humid Indian city the physics often runs the other way.

The costs are real when it is the wrong kind: water damage to sills, carpets and joinery, corroded fixings, stained ceilings, mould on gypsum board and wallcoverings that has to be cut out, and staff complaints about smell and air quality.

Dew point: the one idea that explains all of it

Air holds water vapour. The dew point is the temperature at which that air becomes saturated and starts to release liquid water. Humid air has a high dew point; dry air has a low one. Any surface colder than the dew point of the air touching it will collect condensation.

So condensation is never just about the surface or just about the air. It is about a cold surface meeting air humid enough to condense on it. Every fix works by doing one of two things: warming the surface, or drying the air that reaches it.

Water on the outside of the glass

In a humid climate the outdoor air on a monsoon day or an early morning can have a dew point above the temperature of glass that air conditioning is keeping cool from inside. Moisture then forms on the outer surface. It often appears on one elevation first, and on high-performance insulating glass it can be more noticeable, because the outer pane is better separated from the cool inside.

This is usually not a defect. It does not mean water is getting in, and it clears as the sun and outdoor air warm the glass. It becomes a question only if it is persistent, or if water from it is tracking somewhere it should not.

Condensation on the inside of glass and frames

Inside condensation means the inner surface of the glass or frame is colder than the dew point of the room air. There are three usual causes.

The frame is a thermal bridge. Aluminium conducts heat readily. A frame without a thermal break connects the inside face directly to the outside face and the glass edge, so the inside of the frame runs much colder than the wall or the glass centre. A thermal bridge is any path where heat flows much more easily than through the surrounding construction. ISO 10211 sets out how heat flows and minimum surface temperatures at thermal bridges are calculated, so the condensation risk at a frame or slab edge can be assessed rather than guessed. The way a polyamide break interrupts that path is explained under thermal break aluminium.

Cold air is blowing at the glass. A supply diffuser or a split unit directing cold air onto the window chills the glass and frame well below room temperature. Moving or redirecting the supply is a cheap and often overlooked fix.

The room air is too humid. Pantries, washrooms with poor extract, wet cleaning, large numbers of people, doors left open to humid corridors or the outside, and outdoor air supplied without enough dehumidification all raise indoor humidity. ASHRAE 55 treats humidity as one of the factors in thermal comfort, and ASHRAE 62.1 covers ventilation and indoor air quality; how outdoor air is treated before supply is part of that design (see office fresh air and CO2). Oversized AC that cools quickly and switches off before it has removed moisture can leave a room cool but clammy.

Single glazing makes inside condensation far more likely than insulating glass; the difference is set out in DGU versus single glazing.

Mould on walls, and the moisture you cannot see

Mould needs a damp surface for long enough. In offices it appears most often on external walls, behind furniture pushed against them, around window reveals, and behind finishes that stop the wall drying.

A pattern specific to hot, humid climates is mould behind vinyl wallcovering or impermeable paint on the inside of external walls. With air conditioning inside and humid air outside, moisture moves inward through the wall. A vapour-tight finish on the inside face stops it at the coolest point, where it condenses behind the finish and mould grows unseen until the wallcovering bubbles or stains. The same mechanism inside a wall or roof build-up is interstitial condensation.

ISO 13788 gives the calculation methods for the internal surface temperature needed to avoid critical surface humidity and for interstitial condensation, so a proposed build-up and finish can be checked before it is built.

Before treating any of this as condensation, rule out the alternatives: a roof or terrace leak, a plumbing leak, a blocked AC condensate drain, or a facade leak. Each produces damp patches that look similar.

Fixes by cause

Air conditioning, ventilation and humidity control provisions sit in NBC 2016, Part 8. How the system should be changed in a particular building is covered under HVAC systems design and is for the MEP engineer to decide.

Common mistakes

• Treating outside condensation as a leak and resealing joints that are not leaking.
• Repainting mould without finding why the surface was damp.
• Using vinyl wallcovering on external walls in humid, air-conditioned buildings without a condensation check.
• Lowering the set point to deal with clamminess, which can make cold surfaces colder.
• Blowing cold air straight at the glass from a split unit or diffuser.
• Replacing glass when the frame is the cold surface.
• Assuming every damp patch is condensation when it is a plumbing or roof leak.

What to ask before fixing it

• Is the condensation on the outside or the inside, and when does it appear?
• Are the window frames thermally broken?
• What is the indoor humidity during occupied hours and at start-up?
• How is outdoor air dehumidified before it is supplied?
• Are supply diffusers or split units directed at glass or frames?
• What finish is on the inside of external walls, and was a condensation check done for it?
• Have leaks from the roof, plumbing, facade and condensate drains been ruled out?

Standards referenced

Internal surface temperature to avoid critical surface humidity, and interstitial condensation, in ISO 13788; heat flows and surface temperatures at thermal bridges in ISO 10211; thermal comfort, including humidity, in ASHRAE 55; ventilation and indoor air quality in ASHRAE 62.1; air conditioning and ventilation in NBC 2016, Part 8. Diagnosis, condensation calculations, facade upgrades and changes to air conditioning and humidity control for a specific building must be established by the project's facade or envelope consultant and MEP engineer.