PROUD SPONSOR OF GLASS & SEALED UNITS EXTRA
Glass & Sealed Units Extra
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 .
Certain industry bodies are calling for all rooflight glazing to include a laminated inner 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 .
As laminated glass is more expensive , the industry perhaps sees 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 .
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 .
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 .
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 .
Solar control glass either reflects energy or absorbs it to reradiate the heat outwards . By its nature it gets hotter than clear glass and glass that is designed for thermal efficiency alone . Whilst the majority of installations are within the operating tolerance , in some cases fluctuation in heat can put the stress beyond the limits .
Laminated glass is also heavier which needs to be remembered when planning lifting schedules and structural requirements .
Another issue experienced with laminated glass is a phenomenon called lensing , where images become distorted . This doesn ’ t necessarily cause such an issue with flat rooflight glazing where the view is a simple sky backdrop , but on pitched rooflights with a view of a landscape , this distortion will be a problem .
Clearly there is so much more to specifying large rooflights than meets the eye and while your rooflight manufacturer will be able to provide recommendations , ultimately having a better understanding of what glazing is required and involving a qualified structural engineer in the early phases will resolve any issues you may have further down the line .
www . stellarooflight . co . uk
CLEARVIEW-UK . COM » DEC 2020 » 51