Thermal stress breakage of glass How to minimise the risk
Thermal stress breakage What does a thermal stress breakage look like? A typical low energy thermal stress breakage forms right angles within 10-20mm of the glass edge and then branches out into one or more directions. The number of branches is dependent on the amount of stress in the glass.
GLASS SURFACE FRACTURE LINE
10-2
0mm
90° 90°
low stress GLASS EDGE
High stress breakage pattern
What causes thermal stress breakage? The sun warms the exposed part of the glass, causing this area to expand. The glass edge is covered by the frame, keeping it cool and preventing it from expanding at the same rate. The glass breaks when the tensile stress developed at the edge exceeds the strength of the glass - a bit like a balloon bursting when overstretched.
Exposed glass warming up and expanding as it absorbs solar energy
Solar Energy
Tensile stress developing at edge and breaking the glass
Covered edge remaining cold and resisting expansion
Frame partially removed for illustration
Factors that contribute to thermal fracture
Eaves & overhangs
Shelled or poorly cut edge
Shades & louvres
Warm air trapped in ceiling recesses
Blinds, curtains, posters, objects or window films
Deep columns, transoms or mullions
Heaters (or HVAC ducting in the ceiling directed at the glass)
Stress breakage A damaged edge may be enough to trigger off thermal stress breakage even in clear glass. See illustration below of high stress concentration due to shelled edge of a laminated glass
Thermal tensile stress
High stress
High stress concentration due to shelled edge
Low stress
How to reduce the risk of thermal fracture of glass Choose dark coloured window frames so that glass edges can warm up quicker Install aluminium window frames that are thermally broken Ventilate curtains/blinds with 50 mm wide gap at the top and bottom, or at the sides and bottom Avoid unventilated black-out blinds Keep heaters away from glass Keep objects away from glass (bean bags, cats & dogs etc) Do not apply stickers or window films on annealed glass Where possible, remove external objects that cast shadows on glass Avoid annealed laminated glass IGUs for sliders and sidelites, especially those with solar control glass. These become quadruple glazing when the sliders are open Avoid placing highly reflective external objects or surfaces that can reflect solar energy to the glass Get a thermal assessment done when selecting glass. If annealed glass is not thermally safe, choose heat strengthened or toughened glass Be careful of IGUs with annealed Low E and tinted glass, as these are highly susceptible to thermal breaks Ensure glass edges are not damaged before glazing, especially laminated glass. Did you know that timber window frames, being poor conductor of heat, increase the risk of thermal fracture? This is because the glass edges take longer to warm up in the sun.
AGP offers a free thermal assessment service when selecting AGP glass products. If annealed or annealed laminated glass is not thermally safe, we recommend heat strengthened or toughened glass. Please note that thermal breakage of glass is not covered by warranty unless a thermal assessment has been carried out by AGP prior to ordering glass. If this is desired please fill in the following form and email to Techsupport@AGPL.co.nz
Risk of thermal stress INFORMATION FORM 1. Project Information Project name: Address: Company:
2. Glazing A. Glass Description:
❒ Single glazing
Pane 1 (outer pane):
………………………………………………………………
Thickness: ………… mm
Spacer:
………………………………………………………………
Thickness: ………… mm
Pane 2:
………………………………………………………………
Thickness: ………… mm
Spacer:
………………………………………………………………
Thickness: ………… mm
Pane 3:
………………………………………………………………
Thickness: ………… mm
B) Orientation:
❒ All Orientations
❒ North
❒ North-East
❒ East
❒ North-West
❒ South
❒ South-East
❒ West
❒ South-West
❒ Double glazing
C) Dimensions: Maximum sizes of the glazing (width x height): ……………… x ……………… mm Minimum sizes of the glazing (width x height): ……………… x ……………… mm D) Sloping: ❒ Vertical glazing ❒ Horizontal glazing ❒ Sloping (if sloping, write slope from horizontal): ………………
❒ Triple glazing
Risk of thermal stress INFORMATION FORM 3. Frame A. Type : ❒ Fixed
❒ Opening
❒ Sliding
B. Material : ❒ PVC
❒ Aluminium with thermal break
❒ Wood
❒ Aluminium without thermal break
❒ Steel
❒ Concrete
C. Colour: ❒ Light
❒ Dark
4. Interior Environment : ❒ BLIND / CURTAINS A: TYPE ❒ Internal blind
❒ Curtain
B: Ventilation ❒ Natural
❒ Forced
C: Colour ❒ Light ❒ RAL ……… D: Dimensions A = ……… mm B = ……… mm C = ……… mm D = ……… mm
❒ Dark
RAL ………
Risk of thermal stress INFORMATION FORM
❒ OBSTACLE (an object behind the glazing) A: TYPE
B: Dimensions A = ……… mm B = ……… mm C = ……… mm D = ……… mm C. Colour: ❒ Light
❒ Dark
RAL ………
D. Material: Layer 1:
…………………………………………………………………………………
Thickness: ………… mm
Layer 2:
…………………………………………………………………………………
Thickness: ………… mm
Layer 3:
…………………………………………………………………………………
Thickness: ………… mm
Layer 4:
…………………………………………………………………………………
Thickness: ………… mm
E. Drawing Of The Detail And Description Of Material: ❒ Dwg file
❒ Dxf file
Risk of thermal stress INFORMATION FORM 5. Exterior Environment ❒ Cast Shadow ❒ Yes
❒ No
❒ Blind / Screens A. Type : ❒ External Blind
A
❒ Screen
B. Ventilation: ❒ Natural
❒ Force
OUTSIDE
B
INSIDE
C. Colour: ❒ Light
❒ Dark C
❒ RAL ……… D: Dimensions A = ……… mm B = ……… mm
INSIDE
C = ……… mm D = ……… mm
OUTSIDE
D
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M04/21
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