Thermal Shock – Breaking Glass How?
By Richard Keen
Caused by Thermal Shock ?
I paraphrase a statement from a leading Architect Practise’s guide document to its staff as to what should be considered in the specification of shading (paraphrased to prevent identification, retribution and possible punishment – mine ).
‘…with any window blind…approval must be obtained, as the type and colour of the blind may cause heat build up, leading to the failure of the glass due to thermal shock..’
Now I’m on the technical committee of the BBSA (British Blinds and Shutters Association) so I should know the science behind this, particularly as I have heard it referred to a number of times before. Sometimes thermal shock, sometimes solar shock, I have come across it with glazing/façade manufacturers stipulations in respect of blinds.
I had been only mildly interested before, but then I hadn’t just had installed nearly £10,000 of double glazing on which I want to put some blinds…some blinds in a black screen fabric….very close to the glass. So I started to ask around the industry, many willing to make suggestions, add comments, but categorical statements, no.
Then I started to read technical papers from the glazing manufacturers and the science doesn’t persuade me…Is ‘Thermal Shock’ the industry’s Yeti or Big Foot ? how can the colour or type of the blind cause the glass in front of it to break ? Should Double Glazing manufacturers be able get away with saying that the warranty is invalidated.
If a material with a significantly different heating /cooling /expansion /contraction rate is fixed to an area of glass – I can understand how stress and consequent glass failure might arise. But when is the blind ever fixed to the glass….
I can understand how an incorrect installation of a blackout blind with side guides and a bottom strip to exclude any light could cause air to be trapped between the fabric and the glass. If that air is warmed and expands, with no means of escape, increasing pressure on the glass and the fabric, then something has to give, and I suppose it could be the glass. But that is an incorrect installation – with ventilation /exhaust slots in the headbox the warm air can escape – in fact I have even seen proposals to ‘harvest’ this warmed air. If its not that. Then what else could it possibly be….?
Part 2 next month, when I’ll start on short wave, long wave radiation, visible light, reflectance, absorbance etc then move on to some of the various coatings that can be applied to the glass and how they work…. in the meantime, if anybody has any input that can help me get to the bottom of this I would be massively grateful, Mrs. Keen is getting desperate for her blinds.
Tags: blackout, glazing, shock, thermal, thermal shock