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;

  • A slim framed sliding system
  • A frameless fixed window
  • A frameless window seat (oriel window)
  • A glass box extension
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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