Thermal fracture is the name given to glass breakages that occur due to high ranges of temperature variation within the glass unit. When areas within a pane of glass heat up faster, and to a higher temperature than other areas within the same unit then it will lead to increased stresses inside the pane and the chance of the glass cracking increases.
Thermal shock should be assessed as early as possible on a project to identify if there are potential issues, and what can be done to mitigate them as far as possible.
Factors that affect the way the glass heats up and therefore the likelihood of thermal fracture are:
- Orientation of the unit
- Any projecting features of the building or other nearby features likely to cast a shadow on the glass (feature mullion caps, brise soleil, overhanging soffits, adjacent buildings etc)
- Make-up of the glass including coating
- Type of frame (timber, aluminium, thermally broken etc),
- Frame colour
- Any internal back up to the glass (walls / ceilings close behind the glass unit)
- Details of any internal heat sources directly behind the unit (trench heaters etc) including temperature and direction of the heat flow
- Details of internal blinds including:
- Position of blind in relation to glass unit – including spaces around the edge of the blind.
- Type of blind. (Blackout, ventilated, unventilated)
It is obvious from the above list that some of the items are relatively easy to determine at an early design stage on a project. Factors 1 through to 6 are reasonably straightforward to establish in the early design phases, however factors 7 and 8 are more difficult to resolve until later in the project. The effect of internal blinds is the most difficult to determine at an early stage. Architectural drawings may show an indicative blind detail but often enough there are no finite details for these as they are often left to a fit-out stage – potentially even under another contract. This means that any assumptions need to be agreed with the client team and clearly recorded for the client in the O & M manual in order to ensure that nothing is introduced at a later stage that will potentially cause issues with the glass.
Thermal stress is analysed preferably before tender stage (if there is sufficient detail), but at the latest in the detailed design stage of a project to ensure that the best product selection is made for the client. Analysis will produce a maximum temperature differential for the glass, this is then compared against a list of maximum allowable temperatures for each glass type. The maximum allowable temperature differentials for different glass types are given in the table opposite.
There are actions that can be taken to reduce the risk of thermal fracture. Probably the most common of these is to heat treat the glass either through heat strengthening or toughening, although this does bring certain visual challenges and comes with an increased cost. Edge treatments to glass, especially polished edges, will reduce the risk of thermal fracture as the fracture will always spread from a minor defect at the edge of the glass. The highlighted values in the table opposite show that with the edge treatment of the glass, the maximum allowable temperature differential for annealed laminated glass is increased from 35 to 42. You can also see the greatly increased figures for the heat-treated glasses in this table.
It is possible to have issues after the handover of a project with post fit items, we have gone over the situations with blinds above, but post applied window films also have the potential to cause issues as do some more subtle items such as school windows that are used to display work of students or an improvised blind by way of a piece of paper stuck to a pane. The area of glass behind such an item will heat up at a considerably quicker rate than the surrounding glass which can then cause an issue.
All of the above show the need for us to record details of thermal stress analysis and to ensure that the client is aware of the parameters that have been agreed in relation to this to ensure that they enjoy trouble free use of their building.