Windows of Opportunity
By Anthony D. Cinnamon - July 2002 - Windows & Exterior Walls
By providing daylight while offering protection from the elements, windows give facility occupants a view of the world and keep out the cold. Whether used in new construction or as part of a building rehabilitation, windows can represent a sizeable portion of a project’s cost, while having a large impact on facility energy efficiency and operating costs. As a result managers specifying a window system must keep in mind such factors as durability, maintainability, window technology, and long-term serviceability.
Whether windows are fixed or operable, double-hung or casement, maintenance considerations play a major role during the specification process.
Window performance standards. One of the most important window performance criteria is resistance to water penetration. A leaking window can cause unseen damage to interior and exterior materials and lead to high repair costs.
All windows leak some amount of air, and the energy efficiency of a particular facility or building is directly related to a window’s air tightness. Windows that leak large amounts of air can increase demand on HVAC systems and lead to higher operating costs.
Energy performance refers to a window’s thermal efficiency — how much heat is gained through it during warm weather and how much heat is lost during cold weather. This characteristic is known as thermal transmittance, or U-value, which is a measured rate of heat flow through a body, such as a wall or window unit, and it depends upon the materials and construction of the window unit.
Condensation forms on window glazing and frames when warm air with moisture in a vapor state meets a colder surface, causing the vapor to liquify on the cooler surface. Condensation affects a window’s thermal performance by lowering its thermal capacity. Condensation also can damage interior surfaces and materials.
If a window’s structural capacity cannot support wind and gravity loads imposed on it, a variety of problems can arise. If members of the window system become permanently deflected or are simply too flexible, air and water infiltration can occur, as can damage to exterior finishes, leading to deterioration of the overall window.
Seals and sealants. The seals within the window system and the sealant between the window system and the surrounding substrate are key factors in window maintenance. They have a direct impact on air and water infiltration into the window system and into the building.
Seals within the system include the glazing seals and those of the insulated glass units. Glazing methods can be categorized by window frame material and range from putties and sealants to rubber, neoprene and PVC gaskets. If an incorrect type of glazing material is specified, it can quickly deteriorate and allow water to infiltrate the window itself, causing further deterioration of the window and leading to higher maintenance and repair costs.
The perimeter sealant around the window maintains a weathertight seal between the window and the building envelope. Silicone and polyurethane are the two main types of sealant used at window perimeters. Silicone sealants are mainly used for aluminum and vinyl windows and aluminum curtain wall systems, while polyurethane sealants are used for most others.
A high-quality sealant should be specified and detailed properly to ensure water cannot infiltrate into the building envelope and cause deterioration to the building’s exterior and interior.
Exterior finishes. The finish on the exterior of a window plays a crucial role in its maintenance and long-term performance. A durable exterior finish protects a window from wind, rain, snow, and ultraviolet rays. The greater the protection afforded by the finish, the less need there is for repairs in the future.
Materials and Technology
Managers need to consider a range of factors when specifying windows, including materials used to construct the windows, means of assembly, compatibility with existing building elements and materials, types of hardware, performance for water and air-tightness, finishes, glazing types and treatments, and cost.
Wood windows. When properly maintained, wood windows have a very long service life. They offer a high degree of thermal comfort and have a warm, natural appearance. But they require a high level of maintenance, and their operation is affected by climatological changes. Wood windows clad with vinyl or aluminum, which were developed to address this maintenance concern, are almost ubiquitous today.
Aluminum windows. The principal advantage of aluminum windows is low maintenance requirements. If properly finished, aluminum stands up to the exterior environment for a very long time. The relative cost of aluminum windows has been favorable relative to wood windows.
Most aluminum window systems today are thermally broken, meaning system members exposed the exterior and interior separated by a continuous joint filled with a polyurethane sealant. This improves the thermal performance of the window by greatly reducing transmittance of heat and cold from the interior and exterior, which prevents condensation on the inside face of the frames.
Vinyl windows. Windows made of polyvinyl chloride (PVC) have come into use recently, largely in residential and low-rise commercial construction, but they do not have an extensive track record. The advantage of vinyl windows is low initial cost and subsequent maintenance. But because PVC cannot be painted, window elements are exposed directly to the environment.
Glass. Glass makes up the largest portion of the window, although it seems to be the least complicated part of the whole system. Annealed or tempered glass can have reflective and low-emittance (low-e) coatings applied to the surface, or tints can be dissolved into the glass to reduce the transmission of heat through the glass or otherwise improve its thermal property.
Insulated glass (IG) window units employ two or three layers of glass that are separated by a spacer around the perimeter of the glass. Air trapped between layers act as an insulator to reduce heat transmission.
Two significant developments in glass products in the past decade have been the use of low-e coatings and the introduction of different types of gasses into the air space between panes of glass in an IG unit.
Low-e coatings are applied to one interior surface of double-glazed windows and permit maximum transmission of visible light while preventing transmission of solar radiant heat. As a result, radiant heat is reflected away from the building in the summer and to the interior of the building in cold weather.
Insulating glass uses trapped air as an insulator to reduce heat transmission through the glass area and, therefore, heat loss in cold weather and heat gain in warm weather are significantly lessened.
No matter the type of window or the material, post-installation inspection and maintenance is essential for longevity. Managers should schedule regular visual inspections of each window to determine any required maintenance or repairs.
Inspectors should determine the condition of all parts that make up the window, including glazing, frame material, finishes, hardware, weatherstripping, screens, glass, glazing, and sealant joints. Inspectors also should check the operability of windows, and they should observe the window and surrounding interior finishes for any signs of water leaks or damage.
Selecting windows for a project can be fraught with confusion for architects, contractors and building owners. Maintenance and engineering managers must address maintainability issues during the specification process and develop a plan to address window maintenance issues.
Managers will have to consider the materials used to construct the windows, means of assembly, compatibility with existing building elements and materials, types of hardware, performance for water and air-tightness, finishes, and glazing types and treatments.