Typically, the thermal performance of fenestration is a prime consideration for buildings in cold, heating-dominated climates, but it takes a back seat when designing buildings or setting their energy codes in cooling-dominated climates. This can be seen clearly when looking at U-factor requirements in building energy codes across the globe.
Fenestration U-factor requirements in hot climates are less stringent than in colder climes and the reverse is true for solar heat gain coefficient (SHGC). The use of coated insulating glass in hot climate zones, such as Florida and Singapore, is driven mostly by the requirement to meet a low SHGC rather than a U-factor. Often, reducing the SHGC of the centre of glass is the main lever used for managing fenestration SHGC.
However, the thermal performance of a window’s frame and edge of glass is a major factor in its solar heat gain performance and can have a significant impact on energy use as well as thermal comfort in buildings. Here we dive into the extent to which different regions of the world recognize the role that these opaque elements of fenestration play in managing solar heat gain, building energy performance and thermal comfort.
Thermally breaking the frame matters
Figure 1 shows a thermal image capture of the interior of an installed non-thermally broken fenestration system in Singapore, demonstrating that the hottest part of the system is the aluminium framing members.
Opaque framing members absorb the sun’s impinging radiation. That energy is then turned into heat in the frame, typically increasing its temperature. If the frame and edge of glass are not thermally broken, this absorbed heat will transfer unhindered from the exterior surface to the room-side surface of the fenestration, increasing both the room-side surface temperatures and the cooling load in the building. This increases energy usage, creates uncomfortable spaces next to the windows and produces hot surfaces that can be harmful to touch.
Consider the case of the relatively new (2009) Pan Pacific Hotel situated in the mild, temperate climate of Melbourne, Australia. In the summer, when exterior temperatures are 40oC, the interior temperature of the non-thermally broken interior mullions of the north-east-facing, dual-pane glazed curtain wall reaches as high as 80oC. The HVAC system runs at a 100% capacity starting at 7:30 a.m. during the summer months so that when new guests arrive at 2 p.m. their rooms are not as hot as ovens. This is all due to the aluminium components of the glazing acting like heat sinks, radiating the heat load into the rooms.
In a Melbourne office building, which was glazed with a non-thermally broken fenestration system with dual-pane insulating glass, a room-side mullion temperature of 32oC was recorded at 11:30 a.m. on a winter’s morning when the outside ambient air temperature was a cool 14oC (57oF). An area of approximately 150 sq.m. (1500 sq.ft.) on the eastern and western facades on each floor of this 25-storey building was unusable in summer months, equating to an area equivalent to one whole floor! That’s a huge cost in wasted floor space, unhappy tenants and lost revenue. Assuming a typical gross rent of AU$690 per sq.m. in Melbourne, that equates to over $100,000 in lost rent per floor, per year! Two large tenants, who rented 10 floors of the building between them moved out because of thermal comfort issues.
A study of the impact of thermally broken framing systems by the Solar Energy Research Institute of Singapore showed that a high-performance thermally broken frame could reduce the mean daily heat gain by 61% compared to a non-thermally broken frame. It also reduced the interior frame temperature by 16oC (30oF) for a dark-coloured frame and 13oC (23oF) for a light-coloured frame. The results of this study are described in more detail here.
The solar heat gain through the frame is directly proportional to its U-factor. Thus, the lower the U-factor, the better the SHGC of the frame and; therefore, the whole fenestration assembly. As an example, a good thermal barrier can reduce an assembly U-factor from say, 3.9 to 2.1 W/m2K by 46%, and concurrently reduce its SHGC by 8% from 0.162 to 0.149.
Singapore – Metrics drive centre of glass focus
Singapore has been leading the way in terms of adopting building metrics to manage solar heat gain. Since 1979, the Singapore building code has used the concept of overall thermal transfer value (OTTV) in the design of the building envelope to improve energy efficiency. OTTV is a measure of the heat gain through the building envelope – a weighted average of the conduction through the opaque wall and window glass, and the solar radiation through the glass.
This metric has been adopted in the building codes of several other Southeast Asian countries. However, there are limitations in using OTTV to measure the performance of a building envelope as it focuses on conduction through the centre of glass and ignores the impact of the frame. As such, it has driven the market to focus on improving centre of glass solar heat gain performance, neglecting the frame and edge of glass. Typically, frames installed in Singapore are not thermally broken.
In 2004, Singapore introduced a new metric – Envelope Thermal Transfer Value (ETTV) – to replace OTTV, which provides a more accurate measure of the thermal performance of building envelope by better reflecting the relative performance of the different elements in an envelope system. Specifically, the ETTV metric considers the heat conductance through the whole fenestration assembly (frame and glass), the heat conduction through the opaque walls and the solar radiation through the whole fenestration assembly, representing the full extent of heat gain through the envelope.
In this initial ETTV formulation, the shading coefficient (SC) – a centre of glass value – is considered rather than the SHGC of the entire fenestration unit. As a result, solutions meeting the ETTV requirement of 50 W/m2 continued to place overwhelming focus on increasing the solar control performance of the glass, with fenestrations solutions utilising single-pane solar-control glazing and non-thermally broken aluminium frames.
A Green Building Innovation Cluster (GBIC) study by the Singapore Building and Construction Authority (BCA) recently found ETTV to underestimate the true envelope heat gain in Singapore by as much as 78%. The BCA has proposed an enhanced ETTV calculation using SHGC of the full fenestration assembly, and its study concluded that incorporating thermal breaks systems into aluminium frames “can result in substantial reduction in the (adjusted) ETTV of between 7.5% and 36% translating to a reduction in cooling energy consumption of between 1.5% and 18.0%.”
In recent years, Singapore has ramped up its drive towards more sustainable buildings, with Super-Low Energy Building (SLEB) and energy building and zero energy building targets (ZEB), along with stricter requirements for the building envelope. The Code for Environmental Sustainability of Buildings is currently undergoing review and feedback is being sought from industry stakeholders. As a result, there have been various changes proposed which are likely to result in the tightening of the mandatory ETTV requirement to 45 W/m2, with above-code BCA Greenmark program requiring an ETTV of 42 W/m2 or less and 40 W/m2 or less to achieve for GoldPlus and Platinum level performance respectively. These revised ETTV levels may drive the adoption of dual-pane glazing with double or triple silver low-e coatings with a centre of glass U-factor of 1.5 W/m2K (0.26 BTU/oF.hr.ft2). BCA also provided a glimpse of a possible alternative in meeting the more stringent ETTV standard if thermal bridging can be reduced under criteria NRBE01-1 Tropical Building Envelope Performance, although details have not yet been specified.
Southern Europe, U.S., China
The Mediterranean areas of Southern Europe are considered hot climates, although they typically don’t reach the temperatures experienced in the Middle East. Unlike central and northern parts of Europe, thermal performance of fenestration is not typically a focus. In the very south of Spain, the fenestration U-factor required by code is only 2.7 W/m2K (0.48 BTU/oF.hr.ft2), but more stringent values of 2.3 and 2.1 W/m2K are required in other southern and coastal Mediterranean areas.
Acoustical and solar heat gain requirements drive the use of dual-pane low-e glazing in southern Europe, which also help meet the window U-factor requirement, but small thermal barriers (e.g. 14.8mm) are still necessary in the frame for code compliance (Figure 5). Indeed, while the small 14.8mm barriers are still the most popular in the south of Spain, they are being rapidly replaced by systems with wider barriers of 24 to 25mm.
In southern Italy, the typical window installed has a U-factor of 1.8 to 2.0 W/m2K (0.32-0.35 BTU/oF.hr.ft2), which generally means dual-pane low-e glazing, thermally broken frames with a 20-24mm insulating polyamide barrier and aluminium spacer. Some small areas in the very south of the country allow less stringent U-factors of 2.6 W/m2K, but since the market does not design unique lower performance systems for these minor geographies, the higher performance systems used elsewhere in are typically installed.
In the U.S., the relatively more progressive codes in the southern states of Texas and Florida have similar requirements (2.7-2.8 /m2K – NFRC 100) to that of the upper end of range required in southern Spain. (Note that actual values of U-factor are not directly comparable across the Atlantic). Typically, in these states, dual-pane IGUs must be used for solar heat gain compliance, so the assembly U-factor is achieved with minimally thermally broken aluminium frames. In several southern U.S. states, the allowable U-factors are much higher (3.2-3.8 W/m2K – NFRC), and non-thermally broken frames and aluminium spacer are ubiquitous.
In the hottest regions of southern China, most typical requirements are for fenestration U-factors of 2.7 to 3.2 W/m2K, similar to the southern U.S. Non-thermally broken systems with dual-pane low-e IGUs are most common with some movement towards systems with small (14.8mm) thermal barriers.
Middle East is driving frame performance
When thinking of hot climates, the Middle East comes immediately to mind. However, it isn’t always thought of as the most progressive in building energy codes because of the ubiquity of cheap oil and gas-fired power. However, because the economies of the countries in the region are structured around oil, they can be volatile. Also, their electrical grids become overloaded in the summer months, and electricity often must be cut to homes during the day to manage the loads.
Energy use by citizens is also highly subsidized by governments, so home and building owners do not prioritize energy savings. Governments are therefore actively diversifying their economies and driving energy-efficiency programs to save money and improve grid stability. This strategy also increases their revenue because it leaves more gas and oil for export sales.
Many regions rely primarily on the centre of glass to control solar heat gains, leaving the frame and edge of glass thermally unbroken. In contrast, several countries in the Middle East have approached the solar heat gain problem by thermally breaking the aluminium fenestration and now are moving to adopt warm-edge spacer. As a result, the Middle East leads the way in terms of adoption of high thermal performance fenestration in hot climates.
The focus on thermally broken fenestration was in some sense serendipitous. In the mid-2000s, when these markets were first developing and local stakeholders were asking for support, the European fenestration system manufacturers answered the call. With the European focus on thermal performance and the local market dependent on local glass supply, which was focused mainly on delivering the desired exterior aesthetic and was not yet set up to deliver complex solar control coatings, adoption of thermally broken frames was a natural result.
In addition, because of the local market dynamics, achieving total fenestration performance through improving centre of glass performance also is much more expensive than improving the aluminium framing or adding a warm-edge spacer. For example, a glazing with a centre of glass U-factor of 1.3 W/m2K is 40% less expensive than one with a U-factor of 1.1 W/m2K.
The United Arab Emirates (UAE) has led the way in building energy codes. In 2007, the Emirate of Sharjah (90% of the people who work in Dubai live in Sharjah) introduced a requirement that all aluminium fenestration be thermally broken in order to receive a building permit.
In 2009, the emirate of Abu Dhabi introduced Estidama, a non-mandatory sustainable building rating standard similar to the U.S. Green Building Council’s LEED program. It is now mandatory to meet the lowest level of performance in the Estidama standard (Pearl rating level 1). This typically means achieving a fenestration assembly U-factor of 2.2 W/m2K (0.39 BTU/oF.hr.ft2) for small buildings (less than four storeys) or 2.1 W/m2K (0.37 BTU/oF.hr.ft2) for larger buildings; and therefore, fenestration must be thermally broken and have a dual pane low-e coated glazing infill.
Thermal barriers of 20 to 40mm are typically used in the region. This is the same as, and in some cases greater than, the size of barriers used in the much colder northern areas of the U.S. Adoption of warm-edge spacer is just starting, too, since it is often a less expensive route to achieving the Estidama lower level than improving the glass or frame.
Note that while the UAE codes originally referenced NFRC calculation methods for U-factor, this has been removed and systems meeting the U-factor requirements using either NFRC or the European U-factor standard are acceptable.
Table 1: The fenestration performance requirements of the Estidama energy code of the Emirate of Abu Dhabi
Similarly in the Emirate of Dubai, their Al Sa’fat energy code requires fenestration U-factors of 1.9 to 2.1 W/m2K (0.33-0.37 BTU/oF.hr.ft2) depending on the glazed area. It is therefore not surprising that 85% of all fenestration is thermally broken in the UAE.
Table 2: Fenestration performance requirements for the Emirate of Dubai’s Al Sa’fat energy code
Kuwait is just about to leapfrog the UAE with a new code for government buildings that goes into effect this year (2021) which requires fenestration U-factors (NFRC) to achieve 1.14 W/m2K (0.20 BTU/oF.hr.ft2) or lower. This requires a very well thermally broken frame and triple-pane glazing, and will likely need a warm-edge spacer.
Table 3 summarizes the U-factor requirements and typical business-as-usual aluminium fenestration installed in commercial buildings in hot climates across the globe, focusing on the extent to which the thermal performance of the frame is emphasized.
Table 3: Commercial fenestration U-factor requirements for a representative sample of the world’s hot climate areas
A review of this global landscape illustrates that the key drivers of whether the frame (and edge of glass) thermal performance is improved in the typical fenestration assembly to manage energy and comfort performance are:
- The type of metrics used in the code. Are metrics focused on just centre of glass measures? Or do they specifically call out fenestration assembly measures, including both U-factor and SHGC? Singapore exemplifies the situation where up until now, there has been a focus on centre of glass metrics like shading coefficient.
- Code stringency – as in the UAE, U-factor is not just a required metric, the stringency is greater than in hot climates elsewhere in the world, like southern parts of the U.S., China and Europe.
- Market development history and cost structures are key determinants. The relative cost and availability of glass versus aluminium frames have a significant influence over how the assembly SHGC and U-factors are obtained. Where high-performance glass is easily and cost-effectively obtained, and where codes don’t require particularly stringent U-factors, as in the U.S. and Australia, the centre of glass is used to deliver the SHGC performance, and thermally breaking the frame is largely ignored.
To deliver fenestration that provides both building energy efficiency and thermal comfort, it is important from a policy perspective to focus code requirements on metrics that drive both frame and centre of glass performance. And from a design perspective – regardless of which climate zone the building is located – don’t forget to require high thermal performance in the frame and edge of glass: Spec the edge!
Authors: Helen Sanders and Alexandra Blakeslee, Technoform North America; Jose Del Toro and Cristian David, Technoform Europe, Middle East, India and Africa; Amos Seah and Vincent Wardill, Technoform Asia Pacific.