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From +160F to 40F Below Zero: 4 Steps To Prevent Skylight Leaks Caused By Thermal Expansion And Contraction

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Leaks: they’re one of the biggest concerns for building owners when they consider skylights. Why do skylights leak and how can this be prevented? Thermal expansion and contraction is one of the primary causes of skylight leaking. Thankfully, there are four steps to prevent this failure by preserving the integrity of the bond between the dome and the frame. The result? Leak-free skylights.

How Does Thermal Expansion And Contraction Cause Leaks?

Skylight manufacturers use adhesives to bond acrylic or polycarbonate domes to aluminum or other metallic frames. That adhesive bond is usually created in a climate-controlled factory, where the adhesive undergoes no stress during cure. However, once installed, an outdoor skylight is subjected to inevitable temperature swings. Thermal expansion and contraction can cause enough movement of the plastic relative to the frame so as to cause the adhesive to fail. When the adhesive fails, the skylight leaks.

Though it’s by no means easy, there are four steps to take to build skylights that can resist material separation caused by thermal expansion and contraction.

Step 1: Determine the length change of different materials caused by thermal contraction and expansion.

First, use some simple but precise mathematical calculations to understand the relationship between temperature change and thermal expansion and contraction.

The calculations in the chart below show that acrylic contracts when it’s cold and expands when it’s hot, and does so much more dramatically than aluminum does. When manufacturers create an eight-foot section of skylight in a 70-degree factory, the bond line measures exactly 96 inches. Once installed, however, the skylight heats up and the plastic outstretches the aluminum. At 160 degrees, the plastic expands to 96.363 inches, but the aluminum expands to just 96.106 inches. The plastic now measures .257 inches longer than the aluminum.

Things get even worse when the temperature drops.

Again, the bond line is an even 96 inches in the 70-degree factory. But as the temperature falls to -40 degrees, which is the traditional limit of testing, the acrylic shrinks by nearly half an inch to 95.556 inches. At -40, the aluminum shrinks to 95.870 inches. The aluminum is now .314 inches longer than the plastic. Therefore, .314 inches is the amount of length change that the manufacturer must design for.

Chart 1: Length Difference between Acrylic and Aluminum at Various Temperatures (for a 96 inch bond length)

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Note: Cut these figures in half for a four-foot bond length.

Without a specific bond design that accounts for this expansion and contraction, the adhesive bond between these dissimilar materials would surely break, causing leaks. With a few more calculations, the manufacturer can determine an appropriate design that will not break.

Step 2: Determine how much the shear the adhesive bond can tolerate without adhesive failure.

With the materials contracting and expanding at different rates, the adhesive between them is stretched and distorted like a parallelogram at some shear angle. As seen in the diagram below, the shear angle depends on how much the original bond length has been altered by thermal contraction or expansion.

Calculate the shear angle for the longest bond length (for an eight by four inch skylight, for example, one would focus on the eight foot side). At room temperature, the materials are the same length and there is no stress, with the same bond thickness across the connection points. When temperature differences are introduced, the bond can get stretched or compressed.

Diagram 1: Shear Angle Configuration at Room Temperature vs. -40 deg. F

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Different adhesives can withstand different shear angles. High-quality silicone, for example, can tolerate a shear angle of about 45 degrees repeatedly without failure. Other adhesive materials are typically less tolerant of shear stresses, so manufacturers need to consult with their adhesive suppliers for more information on specific products.

Two possible (and common) failures arise when the shear angle is excessive:

The adhesive can peel off the acrylic or the metal
The adhesive can fracture
Either way, the adhesive would no longer provide a water-tight bond.

Step 3: Determine the Required Bond Line Thickness.

The shear angle directly impacts the necessary bond line thickness. For instance, an inch of adhesive could tolerate virtually any foreseeable contraction and expansion, but a millimeter of adhesive wouldn’t even be able to tolerate even a small temperature change. The calculation of minimum bond line thickness is based on the shear angle, the bond length, and how much shear the specific adhesive can withstand. This is a simple geometry problem.

In our example, with a shear angle limited to 45 degrees and a predicted length change of .314 inches, the bond line needs a thickness of .157 minimum (see diagram). Should the manufacturer use a thinner bond line, then it is likely that the adhesive would initially be water tight, but would eventually fail during an extreme hot or an extreme cold weather event.

Step 4: Perform Temperature Cycle Tests, Identify Causes of Failure.

Do all those calculation guarantee a leak-free skylight? It’s worth testing the theory to see if it’s accurate. The final step is to build the unit at full scale, place it in a temperature chamber and heat it up to 160 degrees and then cool it down to -40 degrees. This process should be repeated at least 10 times. Finally, the manufacturer should dismantle the unit to see if the bond has failed. There are four reasons for failure:

The manufacturer chose the wrong adhesive. Some adhesives, like epoxy, are incredibly strong, but not flexible enough to handle shearing.
The manufacturer chose materials that become brittle at extremely low temperatures, unlike silicone, which remains pliable in extreme cold.
The bond line wasn’t thick enough. If the shear angle required a bond thickness of .157 inches, and the bond was only 0.04 inches (1 mm) thick, then the bond was four times too thin and was doomed to fail.
The adhesive came loose or fractured. This happens when the adhesive wasn’t applied correctly or the substrate wasn’t properly prepared. The best way to avoid this is to assemble the units in a controlled, factory environment — not in the field.
Skylight manufacturers who are committed to preventing leaks from thermal expansion and contraction use conservative design to account for the most dramatic temperature changes. This design includes a generous bond line thickness and close attention to detail when selecting the bonding agents. The four-step process, which culminates with rigorous temperature testing, ensures that the dome will stay bonded to the frame, even through repeated cycles of cold winters and hot summers. This way, building owners, workers, and residents can enjoy the benefits of all-season daylighting, without the fear of leaks.

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