Structural performance
The design professional must evaluate the existing skylight support system and building structure, as replacing the skylight may trigger alterations to, or replacement of, existing gravity or lateral load carrying elements in accordance with Sections 706 and 805 of the International Existing Building Code (IEBC) depending on the level of alteration. To determine if such an analysis is required, the design professional must compare the current code-imposed loads to those relevant at the time the building was constructed. Additionally, if the existing skylight curb is not a structural curb, it may need to be redesigned to accommodate the thrust load of the new skylight (depending on the skylight system design) and provide a load path to the existing structure.
Replacement skylight glazing must be designed in accordance with the strength and safety glazing requirements outlined in Chapter 24 of the IBC. Many factors outside the scope of this article affect glass strength design. It may not be sufficient to match the existing glazing, as the replacement glass must meet the present-day code requirements, including those outlined in Section 2405 of the IBC. It is worth noting, with some exceptions, the building code allows monolithic glazing, either by itself or as the inner lite of an IGU, if glass breakage retention screens are installed below the glazing material and designed in accordance with Section 2405.3 of the IBC. However, it is highly advisable to provide a laminated inner lite, as the glass will more likely be retained within the frame if it breaks. Moreover, it can be advantageous, especially for larger openings and those with relatively low slope, to use a stiffer interlayer (e.g. ionoplast in lieu of polyvinyl butyral [PVB]) to limit post-breakage flexibility and reduce the risk of the entire sheet falling from the frame.
Condensation and energy performance
Design professionals must consider interior space conditions with regards to the condensation resistance of the replacement skylight, particularly if the space will be repurposed and the ambient conditions change. Newer skylight technologies allow for condensation control and management through intricate glass and framing design, including integral condensate gutters. Skylights can be particularly condensation prone in large, tall spaces, where air circulation near the interior surface of the glass and frame is limited, or in such high-humidity interior spaces as fitness centers, hospitals, museums, and natatoria. In such instances, condensation control should be a priority for the design professional. The design should incorporate continuous insulation outboard of the existing skylight curb, which may need to be added, depending on the existing construction, and aligned with the thermal break of the replacement skylight (if the frame is thermally broken). In high-risk spaces, the design professional should consider performing computational modeling to evaluate the risk of interior surface condensation on the skylight and surrounding surfaces and to determine whether additional measures, such as providing localized heaters and/or dedicated airflow to wash warm air over the skylight interior, are required to manage condensation potential. Computational modeling could include two-dimensional and three-dimensional thermal modeling of components and computational fluid dynamics for interior space conditions such as airflows and local temperatures, and should include the effects of the skylight slope and night sky radiation on performance (Figure 4, page 36). The replacement skylight basis of design should include requirements for condensation performance based on either project-specific condensation modeling in accordance with NRFC 500, Procedure for Determining Fenestration Product Condensation Resistance Values, or a tested condensation performance, such as a condensation resistance factor (CRF) per AAMA 1503,Voluntary Test Method for Thermal Transmittance and Condensation Resistance of Windows, Doors and Glazed Wall Sections.
The skylight’s contribution to whole-building energy performance is based on the overall thermal and optical performance of the skylight system and relative size of the system in relation to the building enclosure surface area. Thermal performance of the skylight system (U-factor) directly contributes to building heat loss and gain. Factors that can affect thermal performance include overall glazing build-up and number of lites (IGUs have improved thermal performance over monolithic glazing), width and makeup of the glazing air gap (argon has improved thermal performance within air gaps), glazing spacer bar material (e.g. steel, aluminum, non-metal), frame materials, mullion profile, and provision of thermally broken framing members. Optical and solar performance of the skylight system (solar heat gain coefficient [SHGC] and visible light transmittance [VLT]) can help mitigate uncontrolled daylight, excessive heat gain, glare, and occupant discomfort. Factors that can affect optical and solar performance include glass tints, low-emissivity (low-E) coatings, and frits.
When determining the thermal and optical properties of the replacement skylight, the design professional must consider the use, configuration, mechanical design, and electrical system layout of the enclosed space below. Overall thermal performance and solar heat gain directly affect the demand on the building’s mechanical systems, and the visible transmittance directly affects the demand on the lighting systems (e.g. by allowing use of daylight controls). Note the overall U-factor, SHGC, and VLT of the replacement skylight assembly must comply with Chapter 5 of the International Energy and Conservation Code (IECC) and jurisdictional requirements.
