In Dubai, condensation on aluminium windows almost always forms on the outside of the glass, not the inside — the reverse of what happens in a cold climate. Outdoor air here spends the summer carrying a dew point well above 24 °C, so any surface chilled below that temperature collects water, and the outer face of a high-performance insulating glass unit is exactly such a surface on a still night. Exterior condensation is therefore evidence that the glazing is insulating properly. The condensation that signals a genuine problem is the kind that forms on the room-side face, or between the panes.
The three look alike through a window and mean entirely different things. This guide sets out where each comes from, and why condensation and unwanted heat gain are the same engineering problem seen from two directions.
Why does condensation form on the outside of the glass in Dubai?
Condensation has one cause: a surface sitting below the dew point of the air touching it.
Dubai supplies the humidity generously. Using Weather Spark’s dew-point comfort bands — muggy begins at a 21 °C dew point and oppressive at 24 °C — July records 27.0 days that are muggy or worse, against 0.6 days in January. Through August, the chance that any given day is muggy falls only from 88% to 84%, while daily lows sit at 29–30 °C. Overnight, outdoor air is warm, nearly saturated, and barely cools.
Meanwhile the outer pane of a well-insulated unit is thermally disconnected from the room behind it. It loses heat to the night sky by radiation with little warmth arriving from indoors to replace it, so it drifts below the surrounding air temperature — and below the dew point. Vitro Architectural Glass is explicit that better glazing makes this more likely: “Low-e coatings help insulate the airspace between glass lites in an IGU and improve its overall thermal performance which makes the occurrence of condensation more likely.” Morning exterior condensation, the same source concludes, “can actually be evidence that your window units are performing well.”
It clears within an hour of sunrise. No action is required, and no specification change removes it without also removing the thermal performance you paid for.
When does condensation form on the inside instead?
Interior condensation needs two things at once: a room-side surface below the dew point, and humid air actually reaching it.
In a sealed, properly conditioned villa neither condition is met. Indoor air at 24 °C and 50% relative humidity has a dew point near 13 °C, and in summer every internal surface is warmer than the room, because heat flows inward from a 40 °C exterior. There is nothing cold enough to wet.
The failure mode is humid outdoor air leaking in — through window and door perimeters, unsealed service penetrations, or doors left open — and meeting surfaces in an over-cooled room. Set a bedroom to 20 °C, admit air at a 27 °C dew point, and the arithmetic changes.
Worked example: room-side surface temperature
The internal surface temperature of a glazed element can be estimated from its U-value and the internal surface resistance. Taking Rsi = 0.13 m²K/W (the standard internal surface resistance for horizontal heat flow in EN ISO 6946, and the value used for windows in EN ISO 10077-1), an outdoor air temperature of 40 °C, and infiltrating air at a 27 °C dew point:
| Element U-value (W/m²K) | Room-side surface at 20 °C setpoint | Room-side surface at 24 °C setpoint |
|---|---|---|
| 1.4 — thermally broken frame, double-glazed low-E | 23.6 °C — below dew point | 26.9 °C — marginal |
| 2.0 — moderate thermally broken assembly | 25.2 °C — below dew point | 28.2 °C — clear |
| 5.5 — unbroken aluminium frame | 34.3 °C — clear, but 14 K above the room | 35.4 °C — clear, but 11 K above the room |
Illustrative calculation on the stated assumptions; actual surface temperatures vary with frame geometry, glazing build-up, air movement and solar exposure.
The result is counterintuitive and worth stating plainly: the better the thermal performance, the cooler the room-side surface, and the more vulnerable it is to humid air that should never have been in the room. An unbroken aluminium frame never suffers interior condensation because it runs 14 K hotter than the air around it — which is precisely the defect you replaced it to fix.
The conclusion is not to specify worse glazing. It is to stop the humid air arriving, and to stop over-cooling.
What do the three kinds of condensation mean?
| Where it appears | What it means | What to do |
|---|---|---|
| Outer face of the outer pane | The unit is insulating well; the outer glass has cooled below the outdoor dew point overnight | Nothing. It clears after sunrise. |
| Room-side face of the inner pane, or on the frame | Humid air is reaching a surface below its dew point — air leakage, an open door, or an over-cooled room | Address air-tightness, raise the setpoint, control humidity at source |
| Inside the sealed cavity, between the panes | The edge seal has failed and the desiccant is saturated | Replace the insulating glass unit. It cannot be dried out or repaired. |
Only the third is a product fault, and it is the only one that permanently degrades performance — a breached cavity loses its gas fill and its insulating value along with its clarity.
How does a thermal break change the picture?
Aluminium conducts heat readily — roughly 160 W/mK. Polyamide PA66, the material used for thermal break strips, sits near 0.3 W/mK, some 500 times lower, while sharing a similar coefficient of thermal expansion so the composite profile behaves predictably as temperatures swing.
Without that barrier the two halves of the frame form a single conductive path and the room-side face tracks the outdoor temperature: a frame surface in the mid-thirties radiating at whoever sits beside it, and a continuous heat gain the air conditioning must remove. With a break, the halves are decoupled, the internal face stays close to room temperature, and the frame stops being the weakest thermal point in the opening.
EN ISO 13788 formalises that last point, setting out calculation methods for the internal surface temperature of a building element — expressed as a temperature factor — so designers can find where a junction runs closest to the dew point. In glazed openings that weak spot is almost always the frame and the glass edge, not the centre of the pane, which is why centre-pane U-values tell you little about condensation behaviour. Our thermal break specification guide covers the profile construction in more detail.
What specification prevents it?
If humid air is the mechanism, air-tightness is the control. EN 12207 classifies window and door air permeability from Class 1 to Class 4, with Class 4 the most airtight, tested under pressures up to 600 Pa to the EN 1026 method. Across the systems we fabricate, Class 4 air permeability is published as standard — the Schüco AWS 90 BS.SI+, Cortizo COR 80 Industrial, Alumil SMARTIA S77 and Altest WDS70 all carry it, alongside Class 9A watertightness and multi-level EPDM gasket lines.
On the thermal side, the published figures across our aluminium window range show what a deep break delivers: the Cortizo COR 80 Industrial uses a 45 mm tubular polyamide break for a Uw from 0.71 W/m²K, the Alumil SMARTIA S77 uses 40 mm anti-distortion polyamides for a Uf from 0.85 W/m²K, and the Schüco AWS 90 BS.SI+ reaches a Uf of 0.96 W/m²K in a 77 mm depth.
Three things then decide whether that performance survives contact with the building:
- The perimeter seal. A Class 4 window in an unsealed structural opening leaks around the frame rather than through it. The installation detail matters as much as the system.
- Opening joints. Every operable sash is an air path. Large fixed lights with fewer, well-sealed opening vents outperform a facade of opening units — a trade-off that also shapes the choice between sliding door systems and pivot configurations.
- Setpoint discipline. Moving from 20 °C to 24 °C lifts the room-side glass surface by more than 3 K, taking a well-specified window clear of the dew point.
Why condensation and heat gain are the same problem
Both are governed by surface temperature and air leakage. A frame that runs hot is gaining heat; a surface that runs cold while humid air reaches it is condensing. Fix the envelope and both improve together.
Air leakage carries a cost beyond comfort. Infiltrating summer air arrives hot and loaded with moisture, and the cooling plant must remove both — dehumidifying that moisture is real work, paid for at DEWA’s published residential slab tariff of 23, 28, 32 and 38 fils/kWh as monthly consumption crosses 2,000, 4,000 and 6,000 kWh. A large air-conditioned villa sits in the upper slabs through the summer, so every avoidable kilowatt-hour is charged at the highest marginal rate. Dubai Municipality’s Al Sa’fat green building system sets minimum thermal performance requirements for the building envelope, including limits on glazing heat transfer and solar gain, for the same reason.
What should you check first in an existing villa?
- Identify which surface is wet. Outer pane, room-side pane, or between the panes — the answer determines everything else.
- Look for the air path. Condensation concentrated at one corner, jamb or threshold marks a leak rather than a glazing shortfall.
- Check the setpoint. Rooms held far below 22 °C in August are the commonest cause of room-side condensation in otherwise sound buildings.
- Check the drainage. Blocked frame drainage slots hold water against the glazing rebate and are often mistaken for a seal failure.
Frequently asked questions
Is condensation on the outside of my windows a fault?
No. Exterior condensation on the outer pane means the insulating glass unit is doing its job — the outer glass is thermally separated from the conditioned room and has cooled below the outdoor dew point overnight. It clears shortly after sunrise and requires no action.
Why do I get condensation indoors in summer but not in winter?
Because Dubai’s summer air is far more humid. Outdoor dew points sit in the oppressive band above 24 °C through July and August, so humid air leaking into an air-conditioned room can condense on surfaces that stay perfectly dry in winter, when muggy days are almost absent.
Does better glazing increase the risk of interior condensation?
It can, in a leaky building. A lower U-value keeps the room-side surface closer to room temperature, so it sits further below the dew point of any infiltrating humid air. The answer is not weaker glazing but better air-tightness and a sensible cooling setpoint — an unbroken aluminium frame avoids condensation only by running dangerously hot.
What does condensation between the panes mean?
The edge seal of the insulating glass unit has failed and the desiccant is saturated. The cavity has lost its gas fill along with its clarity, so thermal performance is permanently degraded. The unit must be replaced; it cannot be dried out or resealed in situ.
What air permeability class should I specify in Dubai?
Class 4 to EN 12207, the most airtight classification, tested under pressures up to 600 Pa using the EN 1026 method. Class 4 is published as standard across the Schüco, Cortizo, Alumil and Altest window systems we fabricate. The perimeter installation seal must match it, or the window’s class is irrelevant.
Will raising my air conditioning setpoint stop condensation?
Often, yes. On the worked example in this guide — 40 °C outdoors, infiltrating air at a 27 °C dew point — moving from a 20 °C to a 24 °C setpoint raises the room-side surface of a 1.4 W/m²K assembly from 23.6 °C to 26.9 °C, taking it clear of the dew point. It also reduces the cooling load.
A specification conversation
Condensation is diagnostic: read correctly it tells you whether the glazing is performing, whether the envelope is sealed and whether the cooling strategy is sensible; read incorrectly it prompts people to replace units that were working exactly as intended.
We fabricate thermally broken aluminium windows and doors at our Dubai facility using Schüco, Cortizo, Alumil and Altest systems, with published air permeability, watertightness and thermal data for every profile we supply. If you are specifying glazing for a villa, or diagnosing condensation in an existing one, our technical team can review the opening details with you.
Speak to our specification team about a site survey or a glazing review.
