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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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It is critical to use the right type of adhesive to correctly and securely bond mirrors to their backings, because of the multi-layer structure of the mirror. The reality is that most bonding occurs on a mirror’s back coating, which means the adhesive has to bond the structure to that back coating. Only a handful of adhesives can perform well on this kind of multi-layer structure. To prevent mirror delamination the manufacturer must select the appropriate adhesive – otherwise it’s only a matter of time before the product fails.

Organic Solvents and the Smell Test

Most strong organic solvents, including solvents such as toluene, xylene, MEK and acetone generally have a strong chemical odor due to their tendency to evaporate rapidly (think model airplane adhesive or Liquid Nails™). This property which makes them smell also makes them very useful in common paints and adhesives but unfortunately can weaken bonds in multi-layer structures such as mirrors, causing the entire structure to delaminate. For those who aren’t experts with chemical data sheets, there is a rudimentary, yet surprisingly effective test to rule out some candidate adhesives. Solvent-based adhesives give off a powerful, unmistakable smell. If the adhesive has a strong organic chemical odor, it probably won’t work well on a mirror.

Urethane and Silicone: Built for Bonding with Mirrors

Two types of adhesives that get the best results with mirrors are urethanes and silicones, neither of which are solvent-based adhesive systems.

Urethane adhesives are usually as sold as two-part adhesives. When part A is mixed with part B, this initiates a chemical reaction that polymerizes the constituents and creates an amazing adhesive bond. Since there are little or no solvents to evaporate, there is no strong odor and there is no organic solvent to extract that can cause the mirror to delaminate. Depending on the application, urethane adhesives can be formulated to cure rapidly or to give the assembler some work time. In either case, once the adhesive is cured, it creates a permanent and irreversible bond. Urethane adhesives are fairly rigid once cured, which is good in some applications and bad in others. Unfortunately some small percentage of people are hypersensitive to the constituents of urethane adhesives, so read the MSDS sheets and follow appropriate safety and hygiene guidelines.

Silicones can also be formulated as either two-part or single part adhesives. Single-part silicones typically generate acetic acid as a byproduct of the chemical reaction (which gives off the smell of vinegar). Acetic acid isn’t a very strong solvent and doesn’t generally contribute to the delamination of mirrors. Two-part silicones are completely free of acetic acid, and therefore give off no vinegar odor. Much like the description above for urethanes, mixing part A and part B initiates the chemical reaction that polymerizes the constituents and creates the bond. Once fully cured, silicones are permanent (often with an expected lifetime of over 30 years in all weather), can tolerate a wide temperature range, are waterproof, and importantly – for some applications – are extremely pliable, tolerating a huge amount of strain prior to failure.

Common Adhesives That Won’t Work on Mirrors

Many people experiment with common adhesives without understanding that these products will probably delaminate the mirror. These products include:

Liquid Nails ™, or other construction adhesives suitable for wood
Methacrylate adhesives (such as WeldOn™ or Acryfix ™)
Any cyanoacrylate adhesive such as Super Glue™
Silicone VS Urethane: Which One is Superior?

Both products have their advantages and disadvantages. Silicone may be the superior product – it’s not uncommon for it to last up to 30 years, and it is much more resistant to UV light. The drawback is that silicone is messy in small applications and requires a greater degree of skill. Silicone is often used in industrial applications and is available in five-gallon pails for dispensing with automated equipment. It is also available in smaller quantities, usually in the one-part variety, for dispensing from a caulking gun. So, for large industrial applications with an experienced person, silicone is likely the most appropriate choice.

Urethanes work extremely well, too, and are usually more user-friendly for small jobs in limited quantities because they are often sold with convenient application tools. One special type of urethane, polyurethane foam, is an excellent adhesive, but it is not at all easy to dispense. This is an exception to the statement above.

Choosing the Right Adhesive To Prevent Delamination

Often, manufacturers experiment with different adhesives that they’ve picked up from the local hardware store, without realizing that due to the multi-layer structure of mirrors, adhesives loaded with organic solvents will cause the structure to delaminate entirely. To prevent failure, start your experiments with a silicone or urethane adhesive – these options will more likely create a strong bond that will stay secure far longer than the other common adhesives out there.

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There are two predominant methods of thermoforming that can be used to create shapes out of plastic substrate: free blowing and molding. The overwhelming majority of Replex mirrors are created by free blowing.

Free Blowing: Vacuum Pressure vs. Air Pressure

During the free-blowing process, substrate sheets are heated to a point of malleability. A bubble is then created and regulated by stretching the sheet through the application of either compressed air or a vacuum to one side of the heated sheet.

We most often create the necessary pressure differential through the use of a vacuum chamber, which allows for very effective process control. The main reason we recommend vaccuum pressure instead of air pressure is safety.

The danger lies in ...

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In order for plastic to be reflective, it has to be coated with a layer of metal. That metal layer, however, is so thin that it is extremely vulnerable — and it needs to be protected. Without it, the metal can easily be scratched, chipped or chemically attacked, leading to spots in the mirror that are no longer reflective. The protection comes from a layer of paint called back coating. The type of back coating that mirror manufacturers apply depends on the conditions that the mirror will have to endure. There are two kinds of back coating that offer two different levels of protection.

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This article integrates forward thinking in efficiency with renewable energy generation to illuminate a large commercial or industrial building with zero net energy use, which of course means zero net CO2 emissions from lighting. For a more thorough exposition, you may download our white paper here.

The Four Critical Subsystems

To achieve net zero energy use over one calendar year, one must integrate four critical subsystems: luminaires (electric lighting), daylighting, photovoltaics (PV), and system controls. We will touch on each subsystem below and show how they can work together to provide a beautiful, well-lit building that consumes a net of zero energy.

Luminaires: For net zero buildings, LED technology is superior to T5 or T8 flu...

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Any manufacturer can boast about the level of their quality management system — and many do. But when a company has ISO 9001 certification, they don’t need to boast. This certification means that an accredited third party has audited their entire operation and determined that the manufacturer has developed internal controls and systems in line with internationally recognized best practices, codified in the ISO 9001 standard. Certification ensures that the manufacturer follows through on its documented promises and procedures while working toward continual improvement. Thus, suppliers and consumers are likely to see better systems, higher accountability, and even better products and services when they opt to work with ISO 900...

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How can building owners achieve code-compliant energy performance and enjoy the benefits of daylighting without spending a fortune on roof deck insulation? By installing skylights with better R-Value. Understanding and investing in better R-Value skylights leads to greater energy efficiency – and therefore makes it easier to meet energy codes, without excessive insulation and the additional costs associated. Here’s how it works:

R-Value and the 3 Fundamentals of Heat Loss

R-Value is the universal measurement to gauge an object’s ability to resist heat transfer. Objects with a higher R-Value transfer less heat. By multiplying the following three factors and dividing by R-Value, one can determine the amount of heat that a building transfers to the surrounding atmosphere (heat loss).

Area: Heat loss increases with the size of the object that heat flows through. Therefore, a 5×6 foot skylight (30 sq ft area) loses nearly twice the heat as a 4×4 foot skylight (16 sq ft) if all other factors were the same.
Temperature Difference: Heat loss increases as the temperature difference widens. The colder it is outside, the more heat is transferred from inside. Conversely in the summer, the hotter it is outside, the more heat load is imposed on the air conditioning unit.
Time: Heat loss increases with time. This factor is especially important for buildings in northern climates with longer winters or for buildings in southern climates with long hot summers. Buildings in temperate areas with minimal heating nor air conditioning loads will not benefit as much by increasing R-value.
replex table

Remember that heat flow is additive, meaning that a person must calculate parallel paths of heat loss individually, and then add them together. For example: Q1 (roof heat loss) + Q2 (heat loss through windows) + Q3 (heat loss through walls) + Q4 (heat loss through skylights) = Qtotal. Note: Heat loss is measured in British thermal units (BTU) or in Watt-hours (W-hr).

Good Skylights VS Bad Skylights VS No Skylights: An Example of Heat Transfer

Builders insulate roof decks with stacked rigid foam polyisocyanurate (polyiso) boards. Generally, every inch of polyiso achieves an R-Value of 5The thicker the layers of polyiso, the more insulated the roof.

Let’s examine the role R-Value plays in minimizing heat loss. The following example uses a 50,000 square-foot building for 24 hours. It is 65 degrees (F) inside and 20 degrees outside, resulting in a 45-degree temperature difference.

In Case 1, the base example, the building has no skylights.
In Case 2, the roof has common skylights with an R-Value of 1.2.
In Case 3, the roof has superior skylights with an R-Value of 4.0.
In cases 2 and 3, skylights cover 2,000 feet, or 4% of the roof area (63 common 4×8 units or 135 Replex units).

As seen in the chart below, when the roof in the base example is insulated with three inches of polyiso, it has an R-Value of 15 and loses 3.6 million BTUs during that 24-hour day. When 1.2 R-Value skylights are added, the roof suffers a 46% increase in heat loss (5.256 million BTUs). To negate the 46% increase in heat loss from the common skylights and still meet energy code regulations, the builder would have to increase the polyiso thickness to six inches, an increase of 2.5 inches of polyiso.

However, with only 3 inches of polyiso and 4.0 R-Value skylights, the roof loses 3.996 million BTUs, just 11% more than the base example. With this minimal change, there would be no need to install additional roof deck insulation. Therefore using R-4 skylights avoids the cost of an extra 2.5 inches of polyiso that will be needed if R-1.2 skylights are used. Builders in some states (e.g. Florida) often use six inches of polyiso to achieve the State required R-Value of 30. In those cases, installing 1.2 R-Value skylights would nearly double (96% change) the level of heat loss in winter (or heat gain in summer), as opposed to the 26% increase spurred by the installation of 4.0 R-Value skylights.


The need to compensate with ever-increasing layers of polyiso insulation can discourage builders from investing in skylights, preventing companies from reaping the benefits of daylighting and long-term electrical energy savings otherwise needed for lighting. The good news is that higher R-value skylights can minimize the heat gain or loss, without towering inches of polyiso. But what’s more cost effective?

Cost of Heating and Insulating: How BTUs Translate to Dollars Spent

For building owners who want to simply meet energy code regulations for the minimal cost, it is important to examine the cost of wasted fuel through heat loss. To compensate for the heat loss of the low R-Value skylights, the building owner would have to purchase and burn natural gas.

As seen in the chart below, 1.2 R-Value skylights require six inches of insulation to achieve the same energy efficiency that 4.0 R-Value skylights could achieve with just 3.5 inches of insulation.


How does this affect costs? Polyiso insulation costs roughly 37 cents per square foot per inch of thickness. Thus, to achieve the same performance with the extra 2.5 inches of insulation, the owner pays an extra 93 cents per square foot. For a building with a 50,000 square foot roof, he or she could save $46,000 in avoided insulation cost by installing superior, R-4.0 skylights.

High-quality, 4.0 R-Value skylights enable building owners to meet energy codes with far less insulation and heating fuel than common, 1.2 R-Value skylights. Though installing no skylights at all presents up-front cost-savings, this solution reaps none of the long-term benefits and ROI on lighting costs that come with high-quality daylighting solutions. As daylighting requirements and insulation value are increasingly mandated in building energy code regulations, it’s worth understanding the impact of skylight R-Values on overall heat loss. More importantly, it’s worth it to examine the cost-friendly options to achieve the required heat transfer levels and still enjoy the many positive impacts of daylighting.


Two types of coatings can be applied to acrylic to change the way water behaves when it comes in contact with the plastic: hydrophobic and hydrophilic. Without a coating, acrylic is neither hydrophobic nor hydrophilic. Replex doesn’t apply such coatings in-house, but we occasionally contract third-party contract coaters to apply scratch-resistant coatings.

Hydrophobic Coatings

Hydrophobic coatings have extremely low surface tension. When water touches a surface coated with a hydrophobic coating, it beads into little balls. A hydrophobic commercial product that many are familiar with is the windshield product, Rain-X™. Even in heavy rain, windshields coated with Rain-X™ don’t require wipers because incoming water beads up and rolls off the windshield.

Hydrophilic Coatings

Hydrophilic coatings manipulate water in exactly the opposite way as hydrophobic coatings. They have very high surface tension, so water physically can’t form drops. Instead of beading, water spreads out in a thin, consistent layer. These are often called “anti-fog” coatings because foggy mirrors are the result of small, round drops of water forming and clinging to the surface. Hydrophilic coatings prevent this process. Instead, water forms a thin sheet, making the surface appear dry, even though it is completely covered with water.

The Two Coatings In Nature And Industry

Both hydrophobic and hydrophilic coatings are found throughout nature, from the water-repellent lotus leaf to the bumps on the desert stenocara beetle, which attract condensation. Industries of all kinds attempt to mimic these properties. When water flows over a surface to cool it, for example, hydrophilic coatings assure maximum contact between water and surface. On the surfaces of condensers in desalination plants, on the other hand, hydrophobic coatings allow all droplets to slide off and be replaced with new ones.

Hydrophobic and hydrophilic coatings can be expensive and difficult to apply. Before any manufacturer orders one or the other, it’s worth spending some time understanding what the coatings do, and what applications require their presence.

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Many people assume that plastic is plastic, but saying all plastic is the same is much like saying all wood is the same or that all metal is the same. Replex primarily uses two different types of transparent polymers (clear plastics). Each polymer has different properties, different benefits and drawbacks, and are generally used for different applications. There are several other transparent polymers available in the industry; but they are generally not used in mirrors very often, if at all. We will explain why.

The two polymers primarily used by Replex are represented by complicated chemical names that few can remember or spell, so the result is an alphabet soup of acronyms:

Polymethylmethacrylate acrylic is just called “acrylic” or “PMMA;”

Polycarbonate often becomes “PC;”

Just as balsa wood and mahogany are totally different, these two classes of polymers are radically different from one another. Furthermore, in the case of acrylic these is a big difference between extruded acrylic versus cell cast acrylic. Both are forms of PMMA, but are made by totally different manufacturing processes that result in some important property differences, leading to different end uses.

The following chart offers a glimpse into the differences, uses and applications of these materials.

Extruded Acrylic (PMMA)

Cell Cast Acrylic (PMMA)

Polycarbonate (PC)

Typical End Uses:

Convex mirrors,

dome mirrors &

skylight domes

Camera domes

Playground domes & some skylights

Beneficial Material Features:

Very clear, UV stable, great optics, uniform thickness, scratch resistant versus PC due to hardness. Low cost. Hard and clear

Has unique forming properties suitable for camera domes.

Highest resistance to breakage due to high impact strength. Highest maximum service temperature and best fire rating.

Negative Features or Disadvantages:

Hardness results in brittleness compared to PC.

Poor thickness uniformity & more expensive than extruded acrylic.

Needs protection for UV stability. Softness renders PC very susceptible to scratching unless coated. Highest cost.

Impact Resistance:

Much better than glass, but inferior to PC. Does not stretch much before breakage. Can be improved by adding impact modifier.

Similar to extruded acrylic in this respect, but is not available with impact modifiers.

Highest impact strength, often considered “unbreakable” in most applications.

UV Resistance:

Excellent: typically, durable 10-20 years in outdoor applications under full sun, or longer if given extra UV protection.

Depends highly on the manufacturer, grade, and UV protection. Can be good or bad for outdoor uses.

Not as naturally UV stable as extruded acrylic. Typically lasts 3-5 years outdoor unprotected, 10 years with a UV protection layer.

Fire Performance:

Burns thoroughly and cleanly with minimal smoke. Adds fuel to any fire.

Same as extruded acrylic.

Somewhat fire retardant; more so with special additives. If burned, smoke is very toxic.

Indoor/Outdoor Usages:

Our most commonly used polymer. Its optics, UV stability, scratch resistance make it excellent for use in mirrors, both indoors and outdoors. Very safe compared to glass. Also, very lightweight compared to glass.

Typically limited to camera domes at Replex due to higher cost, and poor thickness uniformity compared to extruded acrylic.

Used anywhere that safety is a prime consideration, such as playgrounds, eyeglasses, safety goggles and hockey visors. High impact strength makes it good for vandalism-prone mirrors, such as those found in prisons. Used for skylights in hurricane prone areas.

Known Brands:



Acrylite ™




Lowest cost among these three material types, both to purchase and to manufacture into domes.

Manufacturing process makes this more expensive than extruded acrylic.

Most expensive of the three material types, and more expensive to process as well. Must be dried before forming.

Some other polymers that are transparent include PETG (glycol modified polyester), PS (polystyrene), and some grades of PVC (polyvinylchloride).

PETG is sometimes used in mirror with low optical requirements (such as inexpensive toys) but is not UV stable whatsoever, and this greatly reduces the application range for this polymer. Polystyrene (PS) is quite brittle and therefore cracks too easily for most durable goods. PS is more commonly used in disposable applications where long-life is not required. PVC is very tough and durable but is a bit too cloudy to make a high-quality mirror.

Hence the decision generally boils down to finding the best balance of properties between extruded acrylic, cell cast acrylic, or polycarbonate. The requirements of the end use will dictate which polymer is the best choice.

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