How Thermal Barrier Systems Can Improve Aluminum Windows, Doors and Curtainwall Systems
Among building owners and managers energy costs rank as the number one concern. While energy management is also important to tenants’
bottom lines, occupant comfort and indoor temperature control are the occupants’ top concerns. Improving thermal performance of windows,
doors and other exterior openings presents a great opportunity to address both audiences’ interests and influence long-term savings
for the life of the building.
Aluminum stands out as a favored choice in window and wall systems for commercial buildings because of its structural longevity and its high resistance against corrosion, deflection and wind load. Aluminum is the most stable construction material available; it does not absorb moisture or support mold growth. It does not shrink, split, crack or rust. Climate and temperature extremes have no effect on aluminum. This outstanding material is lightweight and is quick and simple to extrude, machine and fabricate into virtually any form. These characteristics contribute to lower costs of finished parts, as well as lower shipping and handling expenses.
Aluminum offers limitless design options; it provides an ideal finish base for a variety of paint and anodize coatings. Polyvinylidene fluoride paint such as Kynar® and anodize coatings can be securely and permanently applied with environmental protection inherent in the finish. The vast selections of Kynar paint and anodize finishes allow the building owner to render nearly any color or combination of colors imaginable.
An important aspect of energy conservation and energy efficient products is recycling. Aluminum is one of the most recycled products in use today. It is, however, a highly thermal conductive material that will rapidly transfer heat unless something is done to stop thermal conduction. A thermal barrier system will improve thermal performance, while maintaining structural integrity and long-term durability. The fit and ability to maintain structural properties in all climatic conditions gives aluminum thermal barrier systems long-term energy performance that will not lose efficiency over time.
Optimizing thermal performance contributes to energy efficiency and helps reduce associated heating and cooling costs. When high-performance window, door and curtainwall systems are specified as part of a whole building design process, they also may influence smaller sizing and reduced first-costs of HVAC equipment and contribute to comfortable indoor temperatures.
Choosing glazing systems with thermal barriers can contribute to LEED credit 7.1 Indoor Air Quality: Thermal Comfort when the design of the building envelope provides a thermally comfortable environment that maintains temperature and humidity comfort levels.
There are three ways heat transfers through fenestration:
• Convection is heat transfer through a fluid medium such as the air space of insulating glass or dual glazing.
• Radiation is heat transfer that does not require an intervening material. This can be reduced by the addition of low emissivity (low-E) coatings on glass surfaces.
• Conduction is heat transfer through a solid medium, which can be controlled by the addition of low-conductance thermal barrier materials.
A thermal barrier can be achieved with a number of different methods: Two of these methods are commonly referred to as “structural” thermal barriers. The first is a two-part urethane that is poured into and cured within a “dog bone” structural cavity in the aluminum extrusion. The other thermal system involves pre-formed engineered glass reinforced polyamide inserts, which are placed between two aluminum frame components. Read more about the pour and debridge and thermal insulating strut process.
|Azon's pour and debridge thermal barrier system||Technoform's Insulating Strut (I-Strut) system|
When a thermal barrier process is completed, there is no aluminum contact from the exterior to interior. Thus, transfer of heat is interrupted, resulting in an energy-efficient, insulating thermal barrier.
When specifying high thermal performance, two properties describe frame performance, U-Factor, and Condensation Resistance Factor (CRF). The overall U-factor (U-value) measures the rate of heat loss or how well a product prevents heat from escaping. It includes the thermal properties of the frame as well as the glazing. The insulating value is indicated by the R-value, which is the inverse of the U-factor. U-factor ratings generally fall between 0.20 and 1.20.
Overall U-Factors range from 1.20 BTU/hr-sqft-˚F (6.8 W/sqm-˚K) for single glazing in non-thermal barrier frames, to 0.60 BTU/hr-sqft-˚F (3.4 W/sqm-˚K) for uncoated insulating glass, to as low as 0.20 BTU/hr-sqft-˚F (1.1 W/sqm-˚K) for state-of-the-art multi-thermal barrier systems with triple insulating glass. The lower the U-factor, the greater a product’s resistance to heat flow and the better its insulating value.
We remind designers and specifiers that smaller U-Factors are better, but take the time to understand how the performance is described:
- Don’t confuse the “center-of-glass” (COG) U-Factors published by glass manufacturers with “whole window” overall U-Factors tested and required by codes. Local edge-of-glass and frame U-Factors can be considerably higher than the COG values, which are calculated from solar-optical properties, and assume two infinite pieces of glass separated by an air space. Size, configuration and sightline all affect overall U-Factor.
- NFRC “Standardized” U-Factors referenced in many codes are determined per NFRC 102, not AAMA 1503. Differences in test interpretation algorithms make NFRC 102 U-Factors about 10% lower (better) than the more-commonly cited AAMA 1503 U-Factors.
- Spandrel area U-Factors do not “count” in determining compliance with energy codes. Required spandrel insulation R-Value is covered separately, and codes require at least 1-inch-thick mullion wraps in cold climates.
- Also, remember to check that Division 8 Metal Windows specifications for U-Factor match Glass and Glazing specifications, and
that both match requirements for Climate Zones and local energy codes. The best products for Phoenix are unlikely to be the best
products for Minneapolis.
Whereas U-Factor is “averaged” over all areas of a test specimen, condensation is a local phenomenon. It will occur when any point on the interior surface falls below the dew point of the interior air. Therefore, it is important to look at specific cold points in addition to the average surface temperature from guarded hot box test results. The second basic thermal performance parameter, the Condensation Resistance Factor (CRF), addresses the local nature of condensation.
Condensation is of particular concern in occupancies where humidity can be relatively high in cold weather months, such as hospital patient rooms, condominiums, apartments, and laboratories.
Thermal barrier systems not only match a full range of geographic specifications, they also fit a breadth of commercial building applications. Whether designing and specifying an office in Omaha, a school in Seattle, or a mall in Miami, thermal barrier systems enhance window, door and curtainwall systems’ performance to tame energy costs and improve occupant comfort. Choosing the system to suit the specific climate and application is best delivered by a whole building design approach guided through close collaboration of owners, architects, contractors and suppliers.