Issue 44 winter 24 - Journal - Page 31
No glass is impervious to breakage so it is important to
remember that roof glazing can and often does sit high
above common areas in a home or office, so there will
always be an element of concern if a unit were to break.
Thermal Stress is created when one area of a glass pane
gets hotter than an adjacent area.If the stress is too
great,then the glass will crack.The stress level at which
the glass will break is governed by several factors.Toughened glass is very resilient and not prone to failing due to
thermal stress.Laminated glass and annealed glass behave
in a similar way and the thicker the glass the less tolerant
it becomes, which is an important factor for larger
rooflights.
Certain industry bodies are calling for all rooflight glazing
to include a laminate dinner pane to provide greater
security in the event that the glass breaks. Much the same
as how car windscreens are required by law to be laminated to protect passengers from shattered glass in the
event of an accident. There can be no dispute that laminated glass is safer because it forms a net when broken,
which remains in one piece, whereas toughened glass
breaks into little cubes and will fall down onto whatever
is below.
The temperature difference for a location can be
calculated and the risk of breakage due to thermal reasons
reduced.However,to assess the thermal risk you will need
to take the following factors into consideration:
Type of glass being specified for the insulating glass units
Where the building is located
Orientation of the rooflight
Size of any glazing bars (if required)
Details of any internal shading such as blinds or louvres
The framing material and powder coat colour
The window size and if it opens as this will change the
angle to the sun
Whether any radiators are located directly below the
rooflight
Any other details like other buildings or trees casting
a shadow onto the glass.
As laminated glass is more expensive,the industry perhaps
see this as a way of combatting the cohort of cheap flat
rooflight products that have flooded the market with
potentially dangerous products.
However, laminating rooflight glass creates its own unique
set of problems,as annealed laminate is prone to thermal
fracture and heat stress.This risk can be reduced by
polishing edges,however the low-e coating is not practical
for polished edges as there is a risk of damaging the coating. Swapping the low-e coating for a laminated glass with
treated edges is likely to have a negative impact on the
thermal performance of the unit. Using products such as
SGG Cool-Lite on the outer pane can help as this has
both solar reflective and thermal coatings but these
do not have a self-clean coating which is an important
consideration for pitched rooflights as they are usually
positioned out of reach.
The risk of thermal cracking and heat stress changes
throughout the year with the highest risk seasons being
spring and autumn due to the low angle of the sun and
the lower evening temperatures.
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Conservation & Heritage Journal
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