How to Measure Performance
: a measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value.
the lower the number the more comfortable the window in the summer heat, while a high number can be used on certain elevations for passive winter-time heating. The fraction of solar radiation admitted through a window or skylight, both directly transmitted, and absorbed and subsequently released inward. The solar heat gain coefficient is a standard indicator of a window's shading ability. It is expressed as a number between 0 and 1.
The lower a window's solar heat gain coefficient, the less solar heat it transmits, and the greater its shading ability. For instance, to mitigate summer heat, a windows with a lower SHGC would be used, while a high SHGC window can be used on certain elevations for passive winter-time heating. It is important to understand the concept of directionally tuning windows and glass package to optimize solar control, and this very much depends on climate, location, orientation and placement of a building in relation to the sun.
STC Rating: the higher the number the better the window reduces external noise transmission.
Weight per Square Foot:
the lighter the insulating glass, the lighter the sash, and the longer operating hardware and air-tight weatherseals will last. Heavier windows can also carry in higher installation costs and may require additional structural framing to support the window in the opening.
Types of Glazing
There are four basic glass options available to choose for your window: single pane, double pane, triple pane and suspended film. Dual pane windows are commonly used but lack the insulating performance found in triple pane windows and suspended film windows. Though triple pane windows exceed the performance levels of dual pane windows, they still can’t offer the thermal performance of suspended film glazing and can be more than three times heavier. This excess weight causes multiple limitations and issues when it comes to operable windows.
Single pane windows
– These windows are very light and low cost. They are composed of a single pane of glass surrounded by a frame. Initially they are the least expensive glass option, but can end up costing more in heating and cooling bills because they do not insulate well.
Double pane windows
– Double pane windows consist of two layers of glass with one layer of gas in the middle. They are slightly more expensive than single pane glass but offer a significant improvement over single pane glass in insulating a home and can reduce a home’s cooling and heating bill.
Suspended film (SF)
– Suspended film technology is one of the best advancements in improving insulating performance of window and glass design. SF has a low-e coated polyester film suspended inside an insulating glass unit creating two to four air chambers that provide exceptional insulation performance that reduces conductive heat and radiated heat, but without the weight and size restrictions imposed by triple-pane insulating glass.
Triple pane windows
- Triple pane windows consist of three layers of glass - the exterior glass, a middle layer of gas, and the interior layer glass. Each cavity can be filled with different types of inert gas. Because of dual air chambers this provides, triple pane windows also reduce conductive heat flow better than dual pane windows; however, they do not insulate against radiated heat flow. In addition, because of the extra pane of glass, triple pane windows are much heavier than Alpen Windows. Additional weight puts added stress on the window frame and will compromise the operability and functionality of window fixtures over time.