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Skylights and energy efficiency

Skylights have a high potential to provide the amount, type and distribution of daylight needed to meet the specifications of a building. At the same time, they make it possible to reduce the energy used for artificial lighting and heating.

Chief among its many benefits is that natural light represents a source of heat at no cost. In fact, buildings that enjoy high levels of evenly distributed natural light are naturally warm for much of the year. In this case, significant reductions in heating costs can be achieved by maximizing window and skylight openings on the sunnier, southern sides of buildings and minimizing these openings on the colder, northern sides.

Type and amount of light

Skylights are the most effective method of letting natural light into a building, as they also allow the type and amount of natural light to be regulated. Essentially, light can be of two types:

  • Direct light: light enters the building through the skylight as a direct beam, without obstacles or interference; therefore, light is concentrated on a limited area. This type of light is obtained using polycarbonate and PVC, both clear and most of the available tints;
  • Diffused light: light passes through the skylight, illuminating the structure below more evenly. Obtaining this type of light can be useful in cases where a very large area needs to be illuminated with minimal shadow formation. This lighting is obtained using materials such as fiberglass, polycarbonate and PVC, in a structured form and with opal colors.

The designer of a building must dimension the skylights in such a way that they regulate the internal illumination according to the most appropriate levels for the intended use of the building, must choose the appropriate materials and must determine the best area to be occupied by the skylights. During the design phase of a building, it is also necessary to dimension the area occupied by the skylights, taking into account the possibility that the final use of the building may change. This ensures, in any case, adequate levels of natural light. In any case, it is generally considered that the area occupied by skylights in the roof of a building should not be less than 10%. In some situations, the amount of light entering a building must be controlled to avoid overheating the lower area.

Skylight configurations

The main factors to consider during the design of a skylight are:

  • The need for lighting that creates a welcoming and suitable environment for the activities that will take place in it;
  • Special lighting requirements in specific areas of the building;
  • The relationship between the height of the building and the ability of the skylight to provide adequate illumination at ground level;
  • Frequency of coverage maintenance and ease of access to coverage.

The following skylight configurations are possible:

Skylights in mosaic

This configuration allows the installation of separate individual skylights and ensures the most uniform distribution of light. The skylight is fixed to the roof on all four sides, so it is very well anchored to the roof. This configuration provides for the highest number of brass elements and therefore requires maximum care during the application of the sealants, with a corresponding increase in installation costs.

Rounded skylight in the roof line

The rounded skylight along the roof line has a favorable aesthetic impact on the coverage and ensures a relatively uniform light distribution, provided that the slope of the coverage is low. Compared to the checkerboard arrangement, the number of joints to be fixed and sealed is less. In addition, this type of skylight represents a safer solution in case of frequent access to the roof. In any case, this configuration is subject to high wind loads.

Skylights from roofline to gutter, flat or rounded

Skylights with both flat and rounded profiles; they can be installed from the roof line downwards, in some cases reaching up to the gutter. They minimize the number of joints required, improving reliability and performance over the life of the skylight. In any case, a configuration of the roof line to the gutter limits the possibilities of access to coverage.

Half-water skylights

This configuration represents a middle ground between the checkerboard configuration and the roofline-to-gutter configuration, both in terms of light distribution and simplicity of use.

Flat skylights with a continuous path

This type of skylight provides good levels of illumination, but is less commonly used in modern architecture.

Special care must be taken with the tolerances of production and coupling of the skylights with the covering elements.

Sawtooth flat skylights

This type of skylight could be considered a continuous path skylight, although it is not subject to the same problems related to tolerances.

Rounded skylights or skylights with casual arrangement on flat roofing

Used on horizontal or low-slope roofs, skylights are placed on specially designed supports depending on the different needs and roof design.

Rounded skylights on curved roof

In this solution, the rounded skylights, placed on a support that has the same curvature as the roof, run along the entire cross-section of the roof, stopping in the middle of the roof.

Stained glass structure

These structures can have almost any shape and design and are usually built with aluminum or steel sections and polycarbonate or glass units, which allow infinite freedom during the conception and design phase.

Materials used

The materials used for skylights must not only allow light to pass through, but must also meet all the expected durability, thermal, safety and fire performance requirements. The materials normally used are fiberglass, polycarbonate, glass and, to a much lesser extent, PVC.


Polycarbonate is a very versatile material whose key properties are basically the following:

  • exceptional impact resistance;
  • high levels of light transmission;
  • good processing capacity;
  • good fire performance classification;
  • high resistance to adverse weather conditions;
  • lightness.

All polycarbonate panels are normally available in clear and colored versions, to provide direct light, or in structured and opal colored versions, to produce diffused light. Coextruded UV protection eliminates up to 99% of ultraviolet radiation, protecting both materials and people inside the building.

Substantially, polycarbonate can be of three different types:

  • multiwall: allows to achieve the highest thermal insulation performance. As with compact polycarbonate, it can be cold-folded (although to a lesser extent) and has a very high strength-to-weight ratio, making it particularly suitable for the construction of glazing structures;
  • profiled: used if the skylight to be installed is flat and must exactly reproduce the profile of the adjacent roof cladding elements;
  • compact: good transparency properties. It can be cold bent, is suitable for use in structures of a certain complexity and can be folded into different shapes, such as domes or pyramids.


Fiberglass is the most versatile and commonly used profiled material. Ideal for the construction of rounded skylights, it demonstrates excellent performance and provides high levels of diffused light within a building.

High quality fiberglass sheets are now available on the market. These incorporate a surface protection against ultraviolet rays, capable of virtually eliminating the color fading that generally occurs in the long term.

Safety glass

Glass, which is widely used in the supermarket and commercial sectors, has:

  • excellent fire performance;
  • good impact resistance;
  • high light transmission characteristics;
  • exceptional transparency characteristics.

The glass can be folded for the construction of rounded skylights.

In addition, they are delivered with different coatings and surface treatments that allow diffused light to pass through, control the amount of transmitted sunlight and provide total protection against UV rays.


PVC has a low resistance to impact and environmental conditions compared to alternative solutions. PVC does not satisfy the non-fragility requirement if other safety measures, to be implemented during skylight construction, are not added.

Flat skylights


PanelSandwich.ORG allows its customers to install flat skylights on sloped roofs, by coupling a plastic sheet with A38 profile with the sandwich panel Cover A38 P1000 G4.

Depending on the material used, it is possible to obtain the following integrated systems:

double-sheet skylight assembled at the construction site.

Composed of a profiled fiberglass sheet, available in lengths up to eight meters, to be used together with a corrugated roll on the inner side;

single-sheet skylight assembled at the construction site:

based on the use of a honeycomb-shaped profiled polycarbonate sheet, with the following characteristics;

  • length of up to twelve meters and width of one meter;
  • excellent thermal insulation properties (K = 2.8 kcal/m2 h °C);
  • resistance to temperature variations (from -40 to 120 °C);

single-sheet skylight assembled at the construction site:

composed of a flat honeycomb polycarbonate panel and four walls, with the following characteristics:

  • length of up to twelve meters and width of 0.93 meters;
  • excellent thermal insulation properties (K = 1.39÷1.43 kcal/m2 h °C);
  • resistance to temperature variations (from -30 to 120 °C);

double-sheet skylight assembled at the construction site:

composed of a profiled sheet of compact polycarbonate. It is available in lengths up to eight meters, to be installed together with flat honeycomb polycarbonate sheet on the inner side;

Installation guidelines

When a trapezoidal profile sheet is installed as a flat skylight, it is necessary to ensure that its lateral projections overlap the corresponding projections of the adjacent metal covering elements. In addition, as shown in Fig. 8.15, the sheeting should be installed so that its upper side is below the metal sheeting of the cover panel above, while its lower side should be above the metal sheeting of the cover panel below.

Special attention should be paid when installing the four-wall honeycomb polycarbonate sheet, as it can only be coupled with the Cover panel using a suitable brass element system.

All the skylights described above must be installed with a safety net, as prescribed by current legislation. The safety net should be located directly below the plastic sheeting.

The assembly is carried out using fasteners that anchor the skylight to the elements of the structure without damage due to wind loads, ensuring the tightness of the fastening holes in adverse weather conditions (even in extreme conditions) and, at the same time, allowing considerable play for thermal expansion. The main fasteners must be applied in correspondence of the lower part of the profile and a sealing washer must be mounted. The fasteners must be narrow enough to adequately compress the sealing washer. To prevent rainwater accumulation in the area of the fasteners, the diameter of the washer should be at least 10 mm less than the width of the fastening area.

Due to the thermal expansion coefficient of PVC and polycarbonate, it is necessary to provide fixing holes with large tolerances around the main fasteners; this will allow for possible changes related to thermal expansion without causing unwanted stresses. Considering that these changes can be considerable, the length of PVC and polycarbonate sheets used in skylights should not exceed six meters. Fiberglass skylights, in general, do not require special measures related to these thermal changes.

Great care must be taken in the application of the fasteners and sealant to obtain a weatherproof coating. At least one strip of sealant must be applied on both sides (top and bottom) of the sheet, between the sheet itself and the metal covering element, on the underside of the corresponding fasteners. Preferably, the sealant should be applied close to the fasteners to take advantage of the compression produced by the fasteners. Additional strips of sealant should be applied where necessary, depending on the slope of the roof and the degree of exposure. Generally, the lower the roof slope, the greater the sealant to be applied.

The sealant strips can be applied to the sides of the sheet, corresponding to the highest part of the overlapping overhangs, as shown in Fig. 8.21, although it is often preferred to put two sealant strips, one on each side of the overhang.

When installing a corrugated profile sheet for flat skylights, the main fasteners must be applied in correspondence of the highest point of the profile. In this case, it is necessary to use self-sealing fasteners with synthetic rubber stem and sealing cap.

The sealant should always be light-colored, preferably white, as a dark-colored sealant can cause distortions in the skylights due to solar heating.

Out-of-plane skylights

Installation details and guidelines

PanelSandwich.ORG offers the possibility to install two different types of out-of-plane skylights: rounded skylights at the roofline, fixed or opening, and dome skylights, fixed or opening.

The rounded skylight in the roof line:

is constructed from a self-supporting curved honeycomb polycarbonate sheet, with a thickness of 10-16 mm and a width of about 650 mm.

The film is suitable for curved roofs with a fixed radius of about 3,500 mm. It comes with UV protection and is available in clear, pearl, green and smoke colors. The load-bearing structure of the skylight is built by assembling self-supporting curved transversal supports, in galvanized and plasticized steel, with a square cross-section and open joint, with two small U-shaped profiled sheets at the ends.

The square supports are placed at a distance of 650 mm. Installation should be carried out by inserting the profiled edges of the polycarbonate sheet into the square supports, as shown in the sectional view in Fig. 8.28, and the enlarged view in Fig. 8.29.

The heat-sealed ends of the polycarbonate sheet, together with other accessories, provide exceptional system tightness.

In any case, it is necessary to carefully follow the manufacturer’s instructions regarding the application of the fasteners and sealant to ensure that the requirements for weather tightness, impact resistance, durability and thermal insulation are maintained.

Dome skylights can be fixed or openable, either manually or electrically.

They generally consist of a dome, which can be made of polycarbonate or acrylic material, and a support, whose function is to raise the dome by at least 150 mm from the surface of the covering.

The support can be made of preformed metal, plastic material or fiberglass. In addition, it can have an appropriate side profile with protrusions to facilitate its coupling with the sandwich panel A38 P1000 cover.

The bracket must be fastened to the roof structure by means of suitable fasteners. When the domes are sold together with their corresponding preformed brackets, they are generally also delivered with the necessary fixing holes.

If these fixing holes are to be drilled during installation, they must be made larger to allow for possible thermal expansion without stresses.

A special solution, suitable for the installation of a dome skylight, without the aid of preformed supports, over a Hipertec Roof sandwich panel roof, is shown in the .

In this case, an insulating pad, used as a support, must be fixed to the panel using special brass elements suitably profiled. Prior to the installation of the dome, a continuous strip of sealant should be applied to the top of the support.

Rules to be observed during the installation of a skylight

When installing a skylight, it is a good rule to apply the following rules, which should be followed:

  • the skylight must be resistant to the impacts produced by violent hailstorms. In general, polycarbonate and fiberglass sheets do not show significant signs of damage when exposed to hail, as recent tests have shown. These tests are performed using a small compressed air cannon capable of firing polyamide projectiles (polyamide has a specific mass similar to that of hail) against the sheets to be tested, at a speed of about 21 m/s. PVC sheets have proven to be less resistant, and can seriously deteriorate after prolonged exposure to hailstorm impacts;
  • When installing a skylight, it is important to choose a system that provides thermal insulation similar to that of the roof. In fact, in cases where the thermal insulation of the skylight is lower than that of the surrounding roof, a thermal bridge is created in correspondence with the skylight, which leads to the formation of condensation;
  • The static values of skylight systems can vary greatly depending on the materials and installation methods used. Therefore, in order to design a skylight with certain safety features, it is the designer’s responsibility to check with the distributor whether the chosen solutions are adequate.

Absence of long-term fragility

  • The skylights are designed and manufactured to be completely non-fragile once installed, provided they are installed according to the manufacturer’s recommendations.
  • The absence of long-term frailty can be influenced by many external factors, such as:
  • incorrect initial installation;
  • the number and position of the fixings;
  • corrosion of fasteners or supporting materials;
  • unsuitably sized fasteners;
  • insufficient number of fasteners in areas subject to high wind loads;
  • the size of the washers;
  • hardened or deteriorated sealing gaskets;
  • inadequate inspection and maintenance operations.
  • Duration


All skylights, especially those made of PVC, are subject to gradual deterioration which, as a consequence, leads to color fading and weakening. Skylights are generally resistant to the usual atmospheric pollution conditions, provided they are protected with ultraviolet light inhibitors and appropriate surface protections have been implemented. The most commonly used materials are:


Most fiberglass skylights remain structurally sound for thirty years or more. The combined action of UV rays and atmospheric agents may cause color fading and surface erosion, but does not weaken or make the material more brittle.

Unprotected films may begin to fade within five years. On the other hand, good quality films, protected superficially against UV rays, eliminate fading problems during their entire life cycle, especially if they are subjected to an adequate maintenance program.


These products ensure a useful life of more than fifteen years, with a slow deterioration of mechanical properties and light transmission. Special attention should be paid to the compatibility of this material with those in direct contact with it. Some surface finishes (such as plastisol), over time, can affect the mechanical characteristics of the product, so a suitable insulation system must be designed and applied.


Most PVC products tend to darken and weaken under the action of UV rays, guaranteeing a useful life of between five and ten years. PVC products are available on the market, with special formulations and protections that can last for more than twenty years. PVC, especially at low temperatures, should always be treated as a brittle material and therefore should not be used in industrial and commercial buildings, unless additional systems are provided during the design phase to prevent falls through skylights.

When talking about flat skylights, it is generally preferred to choose light-colored materials, as these tend to absorb less sunlight compared to darker materials and therefore tend to exert lower stresses in the areas where the covering panels overlap; these tend to deteriorate more rapidly at elevated temperatures. Similarly, sealants and other light-colored components should be used.

Out-of-plane skylights are generally not influenced by surrounding and adjacent materials, as they are separated from them by elevations, supports and other insulation systems. In any case, they must also be subject to the same rules applied to sealants and other components.


The frequency of inspection and maintenance operations must be scheduled taking into account the local environmental conditions to which it is subjected: more aggressive atmospheres require more frequent inspections. In general, maintenance operations can be described as follows:


Skylights should be cleaned regularly to remove surface dirt. In this way, light transmission and heat absorption levels will always remain high and long-term physical and optical properties will not be altered. The cleaning process generally consists of rinsing with hot water and a mild detergent (abrasive, caustic and chemical treatments should be avoided at all costs). Stubborn dirt should be removed with a cloth or soft bristle brush. Paint or tar splashes should be removed by wiping with a soft cloth soaked in liquor or alcohol. The process is completed by rinsing with water only.

The use of high-pressure pumps is not recommended, as the high-pressure water jet can penetrate the sealing gaskets;


Skylights should be inspected at least once a year. It is preferable to associate this procedure with a cleaning operation. The surface of the skylights should be checked for possible damage because, if it penetrates the surface protection, it influences the ability of the skylight to resist impact over time.

Finally, all fasteners should be checked both for signs of corrosion and to ensure that they are properly tightened to the correct torque. Any fixture that does not correspond to the installation requirements should be replaced with speed.

It is a good rule, every two or three inspections, to include a check of the sealing system: seals showing signs of deterioration should be replaced.

It is important to remember that, although the degradation of the skylight can be minimized by taking special care of the manufacturing details with frequent cleaning, inspection and repair operations, it will not be easy for its useful life to exceed 20-25 years. Therefore, in the building maintenance program, it is necessary to foresee a replacement of all skylights.