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

> Condensation forms on any surface colder than the dew point of the air touching it. In air-conditioned offices in humid Indian climates it often appears on the outside of the glass, which is usually harmless. Condensation on the inside face of glass or frames, or mould on walls, means a cold surface such as an unbroken aluminium frame is meeting humid indoor air, and needs a fix.

## 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.

| Where you see it | What it means | Usually a defect? | Who looks at it |
| --- | --- | --- | --- |
| Outside face of the glass, mornings or monsoon | Cool glass, humid outdoor air | No; it clears as the glass warms | Nobody, unless it is persistent |
| Inside face of single glazing | Glass colder than room dew point | Often a design limitation | Facade consultant, MEP engineer |
| Inside face of aluminium frames | Frame acting as a thermal bridge, or very humid room air | Yes, if frequent | Facade consultant |
| Around diffusers, on ducts or pipes | Cold surfaces, broken insulation or vapour seal | Yes | MEP engineer |
| Behind wallcovering on external walls | Moisture driven inward, trapped at a vapour-tight finish | Yes | Envelope consultant, interior designer |
| Inside the wall or roof build-up | Interstitial condensation | Yes | Envelope consultant |

## 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](/glossary/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](/blog/office-co2-and-fresh-air-ashrae-62-1)).
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](/compare/dgu-vs-single-glazing-commercial).

## 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](/glossary/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

| Cause | Typical fix |
| --- | --- |
| Frame without thermal break | Replace or upgrade frames in a facade project; meanwhile reduce humidity and redirect cold air |
| Cold air directed at glass | Relocate or redirect diffusers and split units |
| High indoor humidity | Improve dehumidification of outdoor air, extract from wet areas, control door openings |
| Short-cycling or oversized AC | Rebalance or re-control the system so it removes moisture as well as heat |
| AC off overnight in humid seasons | Review start-up sequence and humidity control with the MEP engineer |
| Vapour-tight finish on external wall | Change the finish or the wall build-up after a condensation check |
| Broken insulation on ducts and pipes | Repair insulation and seal the vapour barrier continuously |

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](/services/mep/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.

## Frequently asked questions

### Why is there water on the outside of our office glass in the morning?

Because the air conditioning keeps the glass cool, and on a humid morning or during the monsoon the outdoor air touching that cool glass is above its dew point. Moisture condenses on the outer surface. It usually clears as the sun warms the glass and is a sign the glass is separating a cool inside from a humid outside. It is not a leak and not usually a defect.

### Condensation is forming on the inside of the window frames. What does that mean?

It means the inside surface of the frame is colder than the dew point of the room air. The usual causes are aluminium frames without a thermal break, which conduct cold from cooled glass or outdoor air, cold supply air blowing directly onto the frame, or indoor air that is too humid. Each has a different fix, so the cause should be established first.

### Is mould in an office a health problem?

Mould growth indicates persistent dampness, and dampness and mould indoors are associated with respiratory complaints. Treat it as a building problem to be fixed at the cause, not a cleaning task. Remove affected porous materials such as gypsum board or wallcovering where growth is established, and find out why the surface stayed damp long enough for mould to grow.

### Why does mould appear behind vinyl wallcovering on external walls?

In hot, humid climates with air-conditioned interiors, moisture moves from the humid outside inward through the wall. A vinyl wallcovering on the inside face is close to vapour-tight, so the moisture is trapped at the cool back of the wallcovering, where it condenses and mould grows unseen. The wall build-up and the finish need to be checked together by the envelope consultant.

### Does turning the AC off at night cause condensation?

It can contribute. When the air conditioning is off overnight or at weekends, humid outdoor air leaks into the building and the materials absorb moisture. When the system restarts, surfaces cool quickly while the air is still humid, and condensation can form on frames, diffusers and cold pipes. Start-up sequences and humidity control are questions for the MEP engineer.

### Will double glazing stop condensation?

It reduces condensation on the inside of the glass, because the inner pane stays closer to room temperature than single glazing does. It does not help if the frame itself is the cold surface, if indoor humidity is too high, or if the condensation is on the outside face. The frame, the glass and humidity control all need to be considered together.

## Sources

- [ISO 13788](https://www.iso.org/standards.html) — Hygrothermal performance of building components - internal surface temperature to avoid critical surface humidity and interstitial condensation
- [ISO 10211](https://www.iso.org/standards.html) — Thermal bridges in building construction - heat flows and surface temperatures
- [ASHRAE 55](https://www.ashrae.org/technical-resources/standards-and-guidelines) — Thermal environmental conditions for human occupancy
- [ASHRAE 62.1](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2) — Ventilation and acceptable indoor air quality
- [NBC 2016, Part 8](https://www.bis.gov.in/standards/national-building-code/) — Building services - air conditioning, heating and mechanical ventilation

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Source: https://hagerstone.com/blog/office-window-condensation-and-mould