With new and shifting standards pushing window performance ratings higher, insulating glass unit (IGU) fabricators and window manufacturers need to determine how to best meet market demands. That process may involve rethinking their selection of window components to include more a...
With new and shifting standards pushing window performance ratings higher, insulating glass unit (IGU) fabricators and window manufacturers need to determine how to best meet market demands. That process may involve rethinking their selection of window components to include more advanced technologies that offer higher thermal performance. When making component and design decisions, these manufacturers ultimately have one goal in mind: to build units in the most efficient manner possible to achieve a desired performance rating, such as R-5. That’s a tall order, considering the wide array of component options – spacers, glass, framing systems, gas filling and more – and how they interact to affect total window performance. Manufacturers can produce R-5 triples using a mixture of performance-rated components, scaling back the efficiency of certain components in favour of others that provide maximum U-value improvements. Alternatively, they may employ a combination of high-performance components to achieve R- 5 ratings in a double. Doing so may be more economical in the long run compared to retooling traditional manufacturing operations to accommodate triples. However, manufacturers also need to consider the likelihood that future energy efficiency standards and consumer demand will go beyond R-5, including ratings that are harder to achieve economically in a double, if at all. Therefore, manufacturers opting to meet today’s requirements in a double may be delaying an inevitable transition to triples production in the future. Let’s examine what goes into each design to achieve R-5 ratings.
Reaching the R-5 target for triples is relatively easy. The extra airspace alone creates an additional barrier to heat transfer and enables improved thermal performance. It also gives manufacturers another option for gas filling. In addition, the extra glass lite provides additional surfaces for low-E coatings.
By specifying low-conductivity spacers, gas filling, thermally enhanced frames and two lites of very low-E glass, manufacturers can produce triples with U-values as low as 0.15 (R-7). Each component adds cost, but choosing the optimal combination can reduce the total manufactured cost. For example, a triple with high-performance spacer and frame systems can meet the 0.22 Uvalue target without using krypton, as long as manufacturers adhere to the 3-millimetre grid proximity rule. Triples should have a minimum air space of 3 millimetres between internal muntin bars and the glass to minimize heat transfer from one lite through the grid to the opposite lite. Because of frame width limitations, manufacturers may need to employ an offset triple design in which one glazing cavity is wider than the other. Neglecting the 3- millimetre grid proximity rule may force manufacturers to use krypton to achieve R-5 values, which significantly increases costs.
Even when using krypton, manufacturers are not able to realize R-5 values in a traditional double. Figure 1 shows that the best possible centre-of-glass (COG) U-value is just above 0.20 in a double featuring a 0.018 low-E coating on glass Surface 2, krypton gas filling and high-performance spacer and frame systems. That COG U-value results in a total window Uvalue of 0.23 – or R-4.
However, by applying low-E coatings to two of the glass surfaces in a double (the same 0.018 low-E coating on Surface 2 and a pyrolytic low-E coating on Surface 4), manufacturers are able to achieve R-5 values using lesser expensive argon gas.
The design reduces COG U-values to 0.20, resulting in an R-5 (0.22 U-value) window. Applying this same concept to argon-filled triples – with 0.018 low-E coatings on Surfaces 2 and 4 and a pyrolytic coating on Surface 6 – drops COG U-values to 0.10, yielding an R-7 (0.15 U-value) window. Naturally, krypton provides further COG U-value reductions, but with added cost. One potential trade-off in this design relates to surface temperature. The coating on Surface 4 reflects heat back into the home, minimizing the amount of radiant heat passing through to warm the interior glass surface. The cooler interior surface may increase the potential for condensation inside the home.
The anatomy differences between R-5 triples and doubles introduce a variety of factors manufacturers must consider when determining which design will work best for their operations. They need to compare the cumulative costs of added material and processing requirements between the two designs, some of which include:
Obviously, triples require an extra lite of glass compared to doubles. That means extra raw materials, storage, handling, washing and processing, each of which represents potential cost increases. Manufacturing R-5 doubles also adds cost in terms of coatings. Therefore, a manufacturer needs to compare the cost differential between the two options. Another consideration with triples is that the extra lite gives manufacturers two additional coating surfaces, which enables room for further improved thermal efficiency – although, at an added expense.
With triples, manufacturers essentially double their spacer costs compared to double-pane windows. In addition, the extra spacer profile in a triple creates an extra moisture vapour transmission path, which is an additional point of potential failure in each window. Whether producing doubles or triples, manufacturers should consider the thermal efficiency advantages of using spacer systems that do not contain metal. Moving from the least efficient metal spacer to the most efficient spacer in a double can yield about a 0.04 U-value improvement.
Filling airspaces with gas is a necessity in a double to push it over the R-5 threshold, but not necessarily in a triple. However, the low cost-tothermal-benefit ratio of argon makes it a desirable addition to most production lines. Argon may enable a 0.04 U-value improvement in a double for as little as one per cent of the total raw material cost of a window. Krypton and xenon offer even greater U-value improvements, but their high cost makes them harder to justify. An advantage of gas filling in a triple is that manufacturers can fill both airspaces for optimal thermal performance.
High-performance vinyl framing systems, including those with integral insulated air-cell cores or foam filling, offer potential U-value savings up to 0.04 at a price premium. However, with vinyl windows, producing a double with a high-performance frame may be less expensive than manufacturing a triple hollow-framed window due to potential retooling costs associated with redesigning the frame glazing pocket to accommodate a thicker IGU. Triples may also require additional frame material and reinforcements to support the extra weight of the heavier IGU. In wood frames, hybrid technologies that combine wood with higher-performing foamfilled PVC or pultruded fiberglass help to improve thermal performance. However, it remains to be seen if R-5 values can be achieved in a wood double.
Because of their added weight, triples often require heavier-grade hardware compared to doubles. For example, manufacturers may need to specify heavier-duty spring balances for double- hung triples and stronger arms and hinges for triple casement windows. Transportation Manufacturers must also consider any extra packaging and shipping costs associated with the heavier weight of triples. Heavier units may require more robust packaging, and manufacturers may endure higher fuel costs due to the added weight and the potential to fit fewer units on a truck. Production Finally, manufacturers may need to reconfigure their operations to accommodate triples production as the added raw materials create new staging and workflow challenges. For example, the extra glass inventory required at the beginning of the line and the reduction in the number of IGUs that fit on a cart may require additional carts and storage space throughout a facility.
It may take years for R-5 windows to become the standard for energy-efficient windows. However, their growing acceptance in the marketplace is driving more manufacturers to add these high-performance designs to their product lines. Doing so creates a point of differentiation that could potentially increase one’s success rate and market share. Today’s homeowners are realizing that highperformance windows are a worthwhile investment to ensure energy savings and reduced utility bills. Some may have the perception that a triple is necessary to achieve high efficiency, and manufacturers and retailers may be able to use that opinion to their advantage when developing marketing programs. However, sellers should primarily focus on a window’s performance rating. How manufacturers arrive at the best performance ratings in the most efficient manner possible is up for debate. Both R-5 triples and R-5 doubles are viable options. It’s up to manufacturers to determine which design – or both – is right for their operations.