Skylight Design Lets Glass Take the Spotlight

Good skylight design and project integration can mean a product not only provides light and possible ventilation — it also can make a statement as a strong aesthetic component.
Photo: Wasco

Skylights continue to gain recognition as energy-efficient daylight harvesting devices. When properly specified, proportioned, located and installed, skylights can meet the latest editions of national model energy conservation and green building codes and rating systems. Beyond the concerns of daylighting and thermal performance, skylights also must serve as a viable element of the building envelope.

Consequently, given the growing use of large, complex sloped glazing systems, design criteria for skylights and sloped glazing  are undergoing rapid creative evolution, as are the codes — primarily the International Building Code (IBC) — governing their application. In some cases, best practice can be to consider requirements in excess of those in the codes. Sloped glazing is defined in building codes as those where glass is inclined 15 degrees or more from vertical.

Potential Breakage is Key

Proper glass selection and system design is intended to meet specified design load(s), with the primary goal of reducing the probability of glass breakage, which can pose risks to people and property.

Breakage may occur due to several factors, either alone or in combination, some of which are noted below:

  • Loads in excess of the specified design loads
  • Large thermal stresses
  • Damage to the glass during handling or installation
  • Forces exerted by the framing system
  • Vandalism
  • Wind-borne gravel or other debris
  • Large hailstones
  • Impurities in the glass causing spontaneous fracture
Proper glass selection and system design must meet specified design load(s), with the primary goal of reducing the probability of glass breakage, which can pose risks to people and property. Photo: CrystaLite

The differences in design considerations between vertical and sloped glazing must be considered. For example, sloped glass is more susceptible to impact from falling objects than vertical glass. Sloped glazing is also more likely to fall from its opening when it breaks than vertical glass.

Typically, the preferred practice for glass selection in skylights and sloped glazing is to provide firm support for all edges of the glass for both inward (positive) and outward (negative) loads. This is mandatory for insulating glass units. The support may be by conventional channel glazing or by structural retention with a silicone sealant.

Design Considerations

Glazed systems require special glass design considerations. Designers and architects must orchestrate the use of such industry and regulatory standards and guidelines, as ASTM E1300-16,Standard Practice for Determining Load Resistance of Glass in Buildings,” ASCE/SEI 7,Minimum Design Loads for Buildings and Other Structures,” and others, as well as the IBC and International Residential Code (IRC).

Glazed systems of skylights often require special glass design considerations when designing for things like structure, thermal design and control of solar heat gain. Designers and architects must orchestrate the use of industry and regulatory standards and guidelines. Photos: FGIA

Once the 2021 edition of the IBC is adopted, new code language in IBC Section 2405.1, 2405.3 will clarify that screens are not required below skylights and sloped glazing when 30-mil interlayer laminated glass is used. The use of 30 mil-laminated glass in skylights improves daylighting, aesthetics, and helps protect building occupants, along with eliminating the need for screens.

Other design considerations are outlined below.

Strength

At base, the selection of glass for skylights and sloped glazing begins with the use of ASTM E1300, which uses a failure prediction model with the glass strength based on weathered glass. This takes into account a rational reduction in glass strength from initial production to in-service use. The procedure determines if the proposed glass type (annealed, heat-strengthened, fully tempered or laminated) will meet the specified load, allowing it to be determined whether to consider either a thinner or thicker glass.

A skylight is an integral part of the building envelope, controlling the movement of moisture and air. Photos: FGIA

ASTM E1300 supplies load resistance charts for a glass probability of breakage of eight per 1000, as this is considered practical and reasonable for most glass applications. The designer should aim for a low probability of breakage, but if breakage does occur, the consequences must be acceptable.

ASCE/SEI 7 lists formulas for calculating the equivalent combined pressure due to a combination of dead, wind, snow and other loads, as does Chapter 24 of the IBC. For common shapes of buildings, background guidance on design wind velocities may be found in ASCE/SEI 7 — with a caveat: buildings of unusual shape or geometry may render that standard inadequate for defining loads on sloped glazing and skylights.

Load Duration

The strength of glass is a function of load duration. Long duration loads, or any load lasting approximately 30 days, such as snow loads, must be treated differently than short duration loads, defined as any load lasting three seconds or less, such as wind loads.

Surface Damage

Mechanical damage to the surface of glass, as opposed to weathering, can cause a significant reduction in glass strength.

Thermal stress happens where there is a mix of heavy sunlight and shade. Glass must accommodate these changes. Photo: CrystaLite

Flat glass surfaces inherently have numerous, randomly occurring, microscopic flaws, resulting in widely varying strengths among otherwise identical lites. (A lite is a pane of glass or an insulating glass unit used in a window, door, tubular daylighting device, roof window, secondary storm product or unit skylight.)

So, the strength of glass exposed to transient and static loads must be analyzed on a statistical basis. This may be expressed in various ways, one of which is the coefficient of variation, a measure of the distribution of the glass strength for a large number of lites. It is influenced by the degree of heat treatment of the glass, being highest (0.25) for annealed and lowest for fully tempered glass (0.10) due to surface compression of the latter. This minimizes the tendency of surface flaws to propagate under load and cause glass breakage.

Impact From Wind-Borne Items

Limiting deflection of the frame is important. Care should be taken not to bow or distort the frame due to over-compaction of insulation. Photos: FGIA

The ability of fenestration of all types to resist such impacts is especially important in areas where high wind events, such as hurricanes, regularly occur. Building codes or other regulations in these areas frequently require that fenestration products either be rated as impact-resistant or be protected by impact-resistant devices. Resistance to hail impact — especially applicable to skylights — is a special case of impact resistance. Here, FM 4431, “Approval Standard for Skylights,” is often the governing standard.

Thermal Stress

Differential thermal expansion between framing and glazing, as well as between exposed and shaded areas of a given lite, must be accommodated through appropriate glass bite dimensions and selection of proper sealant, as well as glass type. For most orientations, the temperature that sloped glass may reach is usually higher than for vertical glazing due to the sun’s radiation being oriented more directly to the glass surface. Consequently, thermal stresses created in the glass most often require heat treated glass (heat-strengthened or fully tempered).

Edge Strength

Both the design of the skylight system and the integration into the structure of the building take careful consideration to ensure water is controlled and drained away properly. It is imperative in all glazing systems that water infiltration and condensation be drained or weeped away from edges of the glass and away from the skylight system.

The quality of the glass cutting and the edge finish are critical variables. For example, good quality, clean cut glass edges have an average strength of about 4650 psi (32 MPa) and a predicted failure of 1 percent at about 2,400 psi (16 MPa). For very poorly cut, nipped or damaged edges, the average strength may be in the range of 1,200-1,500 psi (8-10 MPa).

Frame Deflection Limits

A supported glass edge should have an edge deflection limited by the framing member to no greater than L/175 where “L” is the length of the glass edge and the deflection is determined by the displacement of the framing member along the edge.

Water Drainage

It is imperative in all glazing systems that water infiltration and condensation be drained or weeped away from the edges of the glass. This is to prevent detrimental freezing of the water or deleterious effects of moisture on edge seals of insulating glass, or possible debonding of interlayer material in laminated glass. The framing system must always drain the water from the lowest point of the glazing channel and the lowest point of the framing system.

All these design considerations and more, as well as guidance in applying them, are detailed in AAMA GDSG-1, Glass Design Guide for Sloped Glazing and Skylights, published by the Fenestration and Glass Industry Alliance (FGIA). Other published FGIA resources include the following.

  • AAMA SDGS-1-89, “Structural Design Guidelines for Aluminum Framed Skylights”
  • AAMA TIR-A7-11, “Sloped Glazing Guidelines”
  • AAMA TIR-A11-15, “Maximum Allowable Deflection of Framing Systems for Building Cladding Components at Design Wind Loads”
  • IGMA TB-3001, “Guidelines for Sloped Glazing”

All are available at aamanet.org/store.

About the author: Glenn Ferris is the Fenestration and Glass Industry Alliance’s (FGIA’s) Fenestration Standards Specialist. He began his career with the association in 2018. He has extensive experience in the fenestration industry dating back to 1992. Ferris is a liaison for many councils, committees and study/work/task groups guiding them in the completion of the scope of each group.

Designing Thermally Efficient Roof Systems

Photo 1. Designing and installing thermal insulation in two layers with offset and staggered joints prevents vertical heat loss through the insulation butt joints. Images: Hutchinson Design Group Ltd.

“Energy efficiency,” “energy conservation,” and “reduction of energy use” are terms that are often used interchangeably, but do they mean the same thing? Let’s look at some definitions courtesy of Messrs. Merriam and Webster, along with my interpretation and comment:

· Energy efficiency: Preventing the wasteful use of a particular resource. (Funny thing, though — when you type in “energy efficiency” in search engines, you sometimes get the definition for “energy conservation.”

· Energy conservation: The total energy of an isolated system remains constant irrespective of whatever internal changes may take place, with energy disappearing in one form reappearing in another. (Think internal condensation due to air leaking, reducing thermal R-value of the system.)

· Reduction: The action of making a specific item (in this case energy use) smaller or less in amount. (Think cost savings.)

· Conservation: Prevention of the wasteful use of a resource.

So, looking at this article’s title, what does “designing a thermally efficient roof system” imply?

Photo 2. Rigid insulation is often cut short of penetrations, in this case the roof curb. To prevent heat loss around the perimeter of the curb, the void has been sprayed with spray polyurethane foam insulation. Open joints in the insulation have also been filled with spray foam insulation. Note too, the vapor retarder beyond the insulation.

I conducted an informal survey of architects, building managers, roof consultants and building owners in Chicago, and they revealed that the goals of a thermally efficient roof system include:

  • Ensuring energy efficiency, thus preventing the wasteful use of energy.
  • Reducing energy use, thus conserving a resource.
  • Being energy conservative so that outside forces do not reduce the energy-saving capabilities of the roof system.

Unfortunately, I would hazard a guess and say that most new roof systems being designed do not achieve energy conservation.

Why is this important? The past decade has seen the world building committee strive to ensure the energy efficiency of our built environment.

A building’s roof is often the most effective part of the envelope in conserving energy. The roof system, if designed properly, can mitigate energy loss or gain and allow the building’s mechanical systems to function properly for occupant comfort.

Photo 3. Rigid insulation is often not tight to perimeter walls or roof edges. Here the roofing crew is spraying polyurethane foam insulation into the void to seal it from air and heat transfer. Once the foam rises it will be trimmed flush with the surface of the insulation.

Energy conservation is increasingly being viewed as an important performance objective for governmental, educational, commercial and industrial construction. Interest in the conservation of energy is high and is being actively discussed at all levels of the building industry, including federal and local governments; bodies that govern codes and standards; and trade organizations.

As with many systems, it is the details that are the difference between success and failure on the roof. This article will be based on the author’s 35 years of roof system design and in-field empirical experience and will review key design elements in the detailing of energy-conserving roof systems. Best design and detail practices for roofing to achieve energy conservation will be delineated, in-field examples reviewed and details provided.  

Advocacy for Improvement

In the past decade, American codes and standard associations have increased the required thermal values every updating cycle. They have realized the importance of energy conservation and the value of an effective thermal layer at the roof plane. They have done this by prescribing thermal R-values by various climatic zones defined by the American Society of Heating and Air-Conditioning Engineers, now better known by its acronym ASHRAE. Additionally, two layers of insulation with offset joints are now prescribed in the IECC (International Energy Conservation Code). Furthermore, the American Institute of Architects (AIA) has also realized the importance of conserving energy and defined an energy conservation goal called the 2030 Challenge, in which they challenge architects, owners and builders to achieve “zero energy” consuming buildings by 2030.

These codes, standards and laudable goals have gone a long way to improving energy conservation, but they are short on the details that are needed to achieve the vision.

Energy Conservation Is More Than Insulation

Roofs are systems and act as a whole. Thus, a holistic view of the system needs to be undertaken to achieve a greater good. Roof system parameters such as the following need to be considered:

  • Air and/or vapor barriers and their transitions at walls, penetrations and various roof edges.
  • Multiple layers of insulation with offset joints.
  • Preventing open voids in the thermal layers at perimeters and penetrations.
  • Protection of the thermal layer from physical damage above and warm moist air from below.
Photo 4. The mechanical fasteners below the roof membrane used to secure the insulation conduct heat through them to the fastening plate. The resultant heat loss can be observed in heavy frost and snowfall.

Air intrusion into the roof system from the interior can have extremely detrimental consequences. In fact, Oak Ridge National Laboratory research has found that air leakage is the most important aspect in reducing energy consumption. Interior air is most often conditioned, and when it moves into a roof system, especially in the northern two-thirds of the country where the potential for condensation exists, the results can include wet insulation, deteriorating insulation facers, mold growth and rendering the roof system vulnerable to wind uplift. Preventing air intrusion into the roof system from the interior of the building needs to be considered in the design when energy efficiency is a goal. Thus, vapor retarders should be considered for many reasons, as they add quality and resiliency to the roof system (refer to my September/October 2014 Roofing article, “Vapor Retarders: You Must Prevent Air and Vapor Transport from a Building’s Interior into the Roof System”). The transition of the roof vapor/air barrier and the wall air barrier should be detailed and the contractors responsible for sealing and terminations noted on the details.

One layer of insulation results in joints that are often open or could open over time, allowing heat to move from the interior to the exterior — a thermal short. Energy high to energy low is a law of physics that can be severe. Thus, the International Code Council now prescribes two layers of insulation with offset joints. (See Photo 1.)

When rigid insulation is cut to conform around penetrations, roof edges and rooftop items, the cuts in the insulation are often rough. This results in voids, often from the top surface of the roof down to the roof deck. With the penetration at the roof deck also being rough, heat loss can be substantial. Thus, we specify and require that these gaps be filled with spray foam insulation. (See Photos 2 and 3.)

Insulation Material Characteristics and Energy Conservation

In addition to the system components’ influence on energy loss, the insulation material characteristics should also be considered. The main insulation type in the United States is polyisocyanurate. Specifiers need to know the various material characteristics in order to specify the correct material. Characteristics to consider are:

Photo 5. Heat loss through the single layer insulation and the mechanical fasteners was so great that it melted the snow, and when temperatures dropped to well below freezing, the melted snow froze. This is a great visual to understand the high loss of heat through mechanical fasteners.
  • Density: 18, 20, 22 or 25 psi; nominal or minimum.
  • Facer type: Fiber reinforced paper or coated fiberglass.
  • Dimensional stability: Will the material change with influences from moisture, heat or foot traffic.
  • Thermal R-value.

In Europe, a popular insulation is mineral wool, which is high in fire resistance, but as with polyisocyanurate, knowledge of physical characteristic is required:

  • Density: If you don’t specify the density of the insulation board, you get 18 psi nominal. Options include 18, 20 and 25 psi; the higher number is more dimensionally stable. We specify 25 psi minimum.
  • Protection required: Cover board or integral cover board.
  • Thermal R-value.

Protecting the Thermal Layer

It is not uncommon for unknowledgeable roof system designers or builders looking to reduce costs to omit or remove the cover board. The cover board, in addition to providing an enhanced surface for the roof cover adhesion, provides a protective layer on the top of the insulation, preventing physical damage to the insulation from construction activities, owner foot traffic and acts of God.

The underside of the thermal layers should be protected as well from the effects of interior building air infiltration. An effective air barrier or vapor retarder, in which all the penetrations, terminations, transitions and material laps are detailed and sealed, performs this feat. If a fire rating is required, the use of gypsum and gypsum-based boards on roof decks such as steel, wood, cementitious wood fiber can help achieve the rating required.

Insulation Attachment and Energy Efficiency

The method in which the insulation is attached to the roof deck can influence the energy-saving potential of the roof system in a major way. This fact is just not acknowledged, as I see some mechanically attached systems being described as energy efficient when they are far from it. Attaching the insulation with asphalt and/or full cover spray polyurethane adhesive can — when properly installed — provide a nearly monolithic thermal layer from roof deck to roof membrane as intended by the codes.

Figure 1. Roof details should be drawn large with all components delineated. Air and vapor retarders should be clearly shown and noted and any special instructions called out. Project-specific roof assembly details go a long way to moving toward ensuring energy conservation is achieved. Here the air and vapor retarder are highlighted and definitively delineated. Voids at perimeters are called out to be filled with spray foam and methods of attachment are noted.

Another very popular method of attaching insulation to the roof deck and each other is the use of bead polyurethane foam adhesive. The beads are typically applied at 6 inches (15.24 cm), 8 inches (20.32 cm), 9 inches (22.86 cm) or 12 inches (30.48 cm).

The insulation needs to be compressed into the beads and weighted to ensure the board does not rise up off the foam. Even when well compressed and installed, there will be a ±3/16-inch void between the compressed beads, as full compression of the adhesive is not possible. This void allows air transport, which can be very detrimental if the air is laden with moisture in cold regions. The linear void below the insulation also interrupts the vertical thermal insulation section.

The most detrimental method of insulation attachment in regard to energy loss is when the insulation is mechanically fastened with the fasteners below the roof cover. Thermal bridging takes place from the conditioned interior to the exterior along the steel fastener. This can readily be observed on roofs with heavy frost and light snowfall, as the metal stress plates below the roof cover transfer heat from the interior to the membrane, which in turn melts the frost or snow above. (See Photo 4.)

The thermal values of roofs are compromised even more when a mechanically attached roof cover is installed. The volume of mechanical fasteners increases, as does the heat loss, which is not insignificant. Singh, Gulati, Srinivasan, and Bhandari in their study “Three-Dimensional Heat Transfer Analysis of Metal Fasteners in Roofing Assemblies”found an effective drop in thermal value of up to 48 percent when mechanical fasteners are used to attach roof covers. (See Photo 5). This research would suggest that for these types of roof systems, in order to meet the code-required effective thermal R-value, the designer needs to increase the required thermal R-value by 50 percent.

Recommendations to Increase Energy Savings

Code and standard bodies as well as governments around the world all agree that energy conservation is a laudable goal. Energy loss through the roof can be substantial, and an obvious location to focus on to prevent energy loss and thus create energy savings. The thermal layer works 24 hours a day, 7 days a week, 52 weeks a year. Compromises in the thermal layer will affect the performance of the insulation and decrease energy savings for years to come. Attention to installation methods and detailing transitions at roof edges, penetrations, walls and drains needs to be given in order to optimize the energy conservation potential of the roof system.

Based on empirical field observation of roof installations and forensic investigations, the following recommendations are made to increase the energy-saving potential of roof systems.

  • Vapor and air barriers are often required or beneficial and should be specifically detailed at laps, penetrations, terminations and transitions to wall air barriers. (See Figure 1.) Call out on the drawings the contractor responsible for material termination so that this is clearly understood.
  • The thermal layer (consisting of multiple layers of insulation) needs to be continuous without breaks or voids. Seal all voids at penetrations and perimeters with closed cell polyurethane sealant.
  • Design insulation layers to be a minimum of two with offset joints.
  • Select quality insulation materials. For polyisocyanurate, that would mean coated fiberglass facers. For mineral wool, that would mean high density.
  • Attach insulation layers to the roof deck in a manner to eliminate thermal breaks. If mechanically fastening the insulation, the fasteners should be covered with another layer of insulation, cover board or both.
  • Design roof covers that do not require mechanical fasteners below the membrane as an attachment method.
  • Protect the thermal layer on top with cover boards and below with appropriate air and vapor barriers.

Saving limited fossil fuels and reducing carbon emissions is a worldwide goal. Designing and installing roof systems with a well thought out, detailed and executed thermal layer will move the building industry to a higher plane. Are you ready for the challenge?

About the author: Thomas W. Hutchinson, AIA, FRCI, RRC, CRP, CSI, is a principal of Hutchinson Design Group Ltd. in Barrington, Illinois. For more information, visit www.hutchinsondesigngroup.com.

Composite Shake Is the Answer for Home in British Columbia

The Siebert residence was originally built in 1991. Its original cedar shake roof was replaced with a new roof system featuring composite shakes. Photos: DaVinci Roofscapes

Myrtle Siebert grew up in the logging industry as the granddaughter of a hand logger in British Columbia, Canada. She married a man who dreamt of having his own logging company and saw that dream come true. But when it came time to replace the cedar shake roof on her own home near Victoria, she decided to go with composite shake shingles because of their durability, fire resistance, and ease of maintenance.

Siebert and her son did their homework. They visited local builder supply businesses and then struck gold when they hiredVictoria-based Custom Roofing Inc.to do the job. Siebert worked closely with Caleb Friesen, owner of Custom Roofing, to make sure she got the roof system she wanted.

“Caleb and his team confirmed what we already knew … that composite shake from DaVinci was the product for my home,” says Siebert. “I chose the style and color of the composite shakes carefully so that the new fake cedar shakes would look like the real cedar roof we had previously. Mission accomplished.”

“She definitely wanted to maintain the look and feel of the thick wood shakes that the house had on it previously,” Friesen notes. “The idea of longevity and consistent appearance truly appealed to this homeowner. The selection of Bellaforté Shake in the Tahoe color blend really complements the design of this house.”

Photos: DaVinci Roofscapes

Made of pure virgin resin, UV and thermal stabilizers plus a highly-specialized fire retardant, Bellaforté products are created to resemble natural slate and shake products. The composite roofs are designed to resist fading, rotting, cracking and pests, plus high winds, hail and fire. The realistic-looking roofing tiles stand up to weather challenges while requiring no maintenance.

Siebert was intimately involved in the re-roofing process. “I had been directly involved in building this home back in 1991, plus several others over the years,” she says. “I love doing the planning and design work. Caleb was delightfully communicative and his team of workers was fabulous.”

Despite interruptions of pelting rain, snow obliterating the drawn lines and slippery conditions causing work stoppages, the team from Custom Roofing was careful and dedicated to the re-roofing process. “This roof gives me confidence,” Siebert says. “As much as I love wood, I no longer have to worry about maintaining a real cedar shake roof. The DaVinci composite shake is the best possible option I could find for staying as close to real wood on our roof.”

NRCA’s Roof Calculator Has Been Updated to Include ICC’s IECC and IgCC, ASHRAE Standard 90.1, and More

NRCA’s EnergyWise Roof Calculator Online has been updated to include information from the 2015 versions of the International Code Council’s IECC and IgCC, as well as the 2013 version of ASHRAE Standard 90.1. Revised minimum long-term thermal resistance values and NRCA’s latest recommendations for minimum R-values for polyisocyanurate insulation have been included in the application. The application also will determine the temperature gradient through a roof assembly and present the information graphically on a report.

Users will find this beneficial when evaluating the effectiveness of a vapor retarder. The EnergyWise Roof Calculator Online is available for free on NRCA’s EnergyWise Roof Calculator page.