Shared Learnings: U-Factors

by Tom Minnon, LEED® AP, CDT, Eastern Region Sales Manager for Tubelite Inc.

Energy efficiency and occupant comfort are two key criteria in designing green buildings, especially for projects seeking LEED® certification. Contributing to these goals, thermal performance will continue to be an essential factor in the upcoming International Green Construction Code (IgCC) and the 2012 International Energy Conservation Code (IECC), as well as the Architecture 2030 Challenge, which calls for the operation of all new buildings and major renovations to be carbon neutral by 2030.

According to Architecture 2030, the non-residential/or commercial building sector is currently responsible for almost half of the energy – consumption (49%) and green house gas emissions (47%) in the U.S. The greatest percentage of the energy consumption and their associated emissions is attributed to buildings’ operations, such as heating, cooling and lighting.

Windows in these buildings are major contributors to the nation’s gross energy consumption. Energy-efficient building designs must comply with codes and meet owner expectations. Design professionals are concerned with misrepresentation of energy savings. Understanding thermal transmittance in window systems will help to correctly report energy savings.

U-Factor is a measure of:
* Conduction – heat transfer through a solid, liquid or gaseous material via molecular contact. For example, touching a hot stove. To reduce conduction in windows, add frame thermal barriers.
* Convection – the transfer of heat through the movement of liquids or gases. For example, facing into a cold, north wind. To reduce convection in windows, add enclosed air spaces.
* Radiation – the transfer of heat through space without relying on an intervening medium. For example, the heat of the sun on your face. To reduce radiation in windows, add low-e glass coatings.

U-Factor = BTUs / Square Foot / °F Differential / Hour
BTUs are British Thermal Units, the approximate heat required to raise 1 lb. of water 1 degree Fahrenheit, such as from 59°F to 60°F.

U-Factor allows the HVAC engineer to calculate peak loads, as well as energy consumption, for any size window, in any climate. When comparing U-Factors, remember that lower is better. For example, 0.18 BTU/ft2-hr-ºF is better than 0.34 BTU/ft2-hr-ºF.

For an apples-to-apples comparison of commercial window systems’ U-Factors, be aware that there are two U.S. thermal testing protocols: American Architectural Manufacturers Association (AAMA) and National Fenestration Ratings Council (NFRC). The two programs yield similar, but not identical, thermal performance results. NFRC U-Factors are typically 10% lower than AAMA U-Factors. NFRC was residentially-oriented, so NFRC operable window test sizes are small. Window area and configuration can significantly affect the overall window assembly U-Factor. Smaller test sizes make a huge difference in U-Factor for aluminum windows.

Aluminum remains the framing material of choice for non-residential applications, when all design requirements are considered in balance. Be certain to consider ALL factors – from structural integrity to longevity to stiffness to heat build-up – before deciding on alternative materials.

The three components used to calculate U-Factor for window systems are:
* Center of Glass (COG) – typical value is 0.29 BTU/ft2-hr-ºF (low-e IG)
* Edge of Glass (EOG) – typical value is 0.34 BTU/ft2-hr-ºF (aluminum spacer)
* Frame – typical value is 0.90 BTU/ft2-hr-ºF (thermal break). A frame’s U-Factor includes heat transfer through surfaces perpendicular to the glass plane.

COG is different than “whole window” U-Factor. Whole window U-Factor, which includes frame and EOG areas, usually is required by codes. To calculate total system U-Factor:
* First, we must determine what percentage of glass versus aluminum there is in the opening. Most storefront and curtainwall elevations have between 85% and 90% glass.
* Second, we need to know what the COG. This information is obtained from the glass fabricator.

In the following graph, assuming 85% glass and a glass COG of 0.24, the total system U-Factor will be 0.45. In addition to noting total system U-Factor vs. COG, be cautious not to confuse U-Factor with R-Value. R-Value is the inverse of U-Factor and commonly used in residential applications. For instance, a U-Factor of 0.50 would equate to an R-value of 2.0. If commercial fenestration can get down to a U-Factor of 0.25, that is still only R-4. Equal to about 1 inch of fiberglass insulation.

The commercial glazing industry is faced with some real challenges in meeting the 2012 IECC. Most of the U.S. Climate Zones will require Total System U-Factors of 0.38 or lower. These new U-Factor requirements cannot be met by a standard, thermally-broken, aluminum-framed storefront system, even one with a glass COG of 0.24. Some manufacturers are responding to this new code by developing new technology, including “double” pour and debridge thermal breaks. (Thermal barriers will be the topic of my next blog.)

In addition to reviewing Climate Zones’ criteria, remember to check that the specifications for U-Factor match the glass and glazing specifications, code requirements and building permit values. Clear specifications and accurate data are essential to optimizing window systems’ thermal performance and to achieving the intended results.

**

Tom Minnon, LEED® AP, CDT, is the eastern region sales manager for Tubelite Inc., serving clients from Maine to Georgia. With nearly four decades of industry experience and many professional accreditations, he regularly provides educational and consultative support to architects, buildings owners and glazing contractors regarding storefront, curtainwall, entrances and daylight control systems.

...[View full article]

Shared Learnings: Lessons for a New Year

by Tom Minnon, LEED® AP, CDT, Eastern Region Sales Manager for Tubelite Inc.

As we begin 2012, I add blogging to my list of New Year’s resolutions and new skills. In our industry, “What’s new?” is a question we’re constantly asked by architects. Staying current, accessible, relevant and knowledgeable are expectations. These are especially important qualities when discussing sustainable design and environmentally responsible construction.

The challenges, codes and opportunities pertaining to green building will continue to increase in the coming months. For the last several years, sustainability issues have gained momentum and the conversations have gained depth. In my upcoming blogs, I’ll explore considerations and requirements of green attributes in commercial fenestration. Topics are likely to include daylighting, views, natural ventilation, indoor air quality and VOCs, recycled content, maintenance and durability, re-use and restoration, energy-efficiency and thermal performance, as well as specific applications such as in high-security projects or hurricane zones.

For me, the green building conversation and its practical implementation are a way of life. I’ve been working with architectural glazing systems for almost four decades. I earned my Leadership in Energy and Environmental Design (LEED®) professional accreditation when this green building rating system was still on version 1.0.

I also wake up each morning with the knowledge it takes to renovate a 30-year-old house into a water-efficient, energy-efficient home using sustainable materials. I understand what it’s like to sort through a myriad of manufacturers’ claims regarding recycled content, regionally sourced materials, energy efficiency, eco-friendly manufacturing and life cycle analysis (LCA) when selecting building products, interior finishes, sustainable landscaping practices, and renewable energy sources.

Whether prescribing green goals to residential, commercial or governmental project, the complexity will remain, even as established organizations and institutions attempt to simplify it. Two approaching milestones will be the USGBC’s release of LEED 2012, the International Green Construction Code (IgCC), which will be available this Spring, and the 2012 International Energy Conservation Code.

I’m looking forward to sharing the lessons that I’ve learned, and those that I’ve yet to learn, with you. Feel free to add your comments and questions. It’s through our individual experiences that we find the insight in collective wisdom and enrich the value of our industry for a brighter future.

**

Tom Minnon, LEED® AP, CDT, is the eastern region sales manager for Tubelite Inc., serving clients from Maine to Georgia. With nearly four decades of industry experience and many professional accreditations, he regularly provides educational and consultative support to architects, buildings owners and glazing contractors regarding storefront, curtainwall, entrances and daylight control systems.

**

 

 

 

 

 

...[View full article]