Condensation and Glass Designs

Condensation occurs when moist, or humid air comes in contact with a colder surface and ‘condensates’, the water molecules in the air turning from a gaseous form to liquid collected on the face of that cold surface. As glass has a low thermal mass it does not hold heat energy meaning the surface of a glass design is almost always slightly cold to touch. Modern glazing design projects must consider ventilation as a key part of the build to remove the natural build up of condensation that occurs in a space occupied by people.

External Condensation on Glass Designs

A modern issue, for modern thermally efficient windows.

The glass units used in architectural glazing and structural glass installations are so thermally efficient that very little heat is able to pass through the glass to the outside; therefore the external pane of glass always remains cold.

Cold surfaces are prime surfaces for condensation to form and some structural glass designs will experience condensation building up on the outside face of the architectural glass installation at certain times of day, especially the morning when the air is more humid.

The condensation build-up is a by-product of thermally efficient architectural glazing and not a defect in the glazing or something to be worried about if it occurs. If you are worried about the possibility of condensation build-up on the external face of your glass installation you could specify Anti Condensation Glass.

In some instances, this external condensation could make the invisible sucker marks on the glass visible. These are invisible elements of residue on the external face of the glass unit left over from the glass lifting equipment. This residue is invisible in normal circumstances and will disappear naturally.

frameless glass wall to house in cotswolds
highly glazed side infill extension in london

Internal Condensation on Glass Designs

The build-up of condensation on the inside of a modern glass design can sometimes occur, although this happens much less frequently with modern glazing than it would do on older windows.

There can be many reasons why this minor condensation build-up occurs. When humans occupy spaces they breathe out moisture constantly. This moisture must be removed from the building through your designed ventilation solutions or it will settle somewhere. This moisture will condensate on the coldest surfaces which (due to the low thermal mass of glazing) could be the glass design or associated fixings.

It is important that the architect designs adequate ventilation as part of the wider building project. The requirements for ventilation in building projects are dictated by Building Regulations Approved document F.

There is likely to be a high moisture content within any buildings after construction / renovation works. The likelihood of this showing will be higher if the clients are now in residence and the central heating has been activated. All of the wet trades carried out in recent months will be drying out which will generate potentially high volumes of moisture on the coldest item in the room.

Warmer air and good ventilation are a requirement in any new build / re-developed areas. If the glass panels are condensating on the outside face of the glass it’s a clear sign the glass is acting efficiently. If there is condensation internally its usually a heat / air circulation issue.

Dan, IQ Contracts Director

Ventilation in Highly Glazed Designs

There are two things to consider regarding condensation when designing a house or extension; you must use a thermally broken glazing system and the architect must design in adequate ventilation into the design.

If you do not use a thermally broken and insulated window system then the inside of the glass and frame will always be cold. This will be a key area where natural condensation will build up causing damage to internal building finishes.

The ventilation required for a space is a complex calculation determined by buildings regulations approved document F.

One easy way to incorporate background ventilation in a typical window or door is to include trickle vents within the frame. These are typically located in the fixed frame at the top of the window or door. The downside of including trickle vents in windows and doors is that the fixed frame has to be exposed.

modern house with lots of minimal glazing
minimally framed sliding doors to london courtyard

Ventilation with Frameless Glazing Design

Lots of modern or contemporary designs want minimal/frameless glazing where you hide the framing. In these cases, you cannot provide background ventilation in the glazing and the architect has to design the ventilation elsewhere. This could be through ventilation panels or a mechanical ventilation system.

Examples of these types of windows would be;

Suction Marks on Glass

Sometimes the formation of condensation on the outside of a glass unit will highlight irregularities in the external glass surface. This includes residue left over from the vacuum suction equipment used to lift and move the glass during manufacture and install.

These marks are not defects in the glazing and will fade over time. They can also be removed with the use of a glass polishing agent.

Glass appears as a smooth even surface however, if you view glass microscopically you can see its surface has peaks and valleys in it. In the process of manufacturing and installing the glass suction cups are used to safely move and lift the glass. Very small particles from the rubber suction cup may be deposited on the glass during use and will settle into these valleys.

Normally, these particles are not visible, but water droplets from condensation will settle on these surfaces differently than it does to the rest of the glass. This causes the suction marks to become visible under normal condensation conditions.

Over time, with normal exposure to the elements, the suction cup marks will diminish or disappear. In the meantime, they can sometimes be minimised or removed with the use of glass cleaners or by using special glass polishing agents.

glass lifting with glass lifting machine

… when water would bead up on the glass surface from either condensation, cleaning or rain, the outline of the processing contact may become visible. The different water beading patterns can create an outline with distinct lines of demarcation which can take the shape of the device that had previously contacted the glass surface in this area.

While this appearance would be noticeable under certain conditions, it does not affect the functionality, performance, or longevity of the glass. It is possible that this condition will dissipate over time with normal exposure to the elements and regular glass cleaning. It may be possible to immediately minimize or remove the surface differences present by following cleaning procedure C. Use caution with this cleaning method, test a small area first, as glass scratching may occur if light  pressure is not used.

Glass Technical Document TD-107, PPG Glass Technology

Rebecca Clayton
Technical Sales
Rebecca is Head of Marketing at the IQ Group and has worked in glazing specification for many years. She has a broad range of technical knowledge about all our glazing products and offers technical advice and guidance to architects for specification. Her easy to digest technical advice is often quoted in magazines and publications. You might also recognise her as one of the IQ Glass CPD presenters.
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