(Originally published in the August 2009 Housing Journal)

Double Check Those Windows – The Game Has Changed

It Used To Be Easy

The days when it was relatively easy to select appropriate windows for your project are over. Our current code mandates highly efficient windows, with few exceptions, in all climate zones of New Mexico and the new code will ratchet everything up yet again. Energy efficiency experts who travel the state looking at construction projects report many builders do not pay close enough attention to the windows on their projects. It’s easy to understand how that happens, given the complexity of the subject these days. To help builders solve this problem, the Housing Journal is providing the following basic information on windows.


It’s All About The Numbers

New window units and fixed-pane glass are given ratings that measure their ability to provide insulation and reflect heat back from the window’s surface. All materials absorb, reflect and emit radiant energy. The window’s U-Factor relates to the insulating properties of the window. The SHGC number is the Solar Heat Gain Coefficient, which is a measure of the window’s ability to reflect heat away from the window surface. Specific values for both, by climate zone, are mandated in our existing code, as well as in the new code. While we are accustomed to using R-Values for insulation requirements (and these are usually stated as “minimums”), U-Factors and SHGC are maximum numbers, therefore lower numbers indicate greater energy efficiency. Emissivity is the value given to materials based on the ratio of heat emitted compared to a black body, on a scale of 0 to 1. A black body would have an emissivity of 1 and a perfect reflector would have a value of 0. This is where the term “Low-E Glass” comes from – it means the glass surface has been treated (usually with a metal oxide coating) to allow less heat to be emitted into a building from a warmer and sunny outside. Low-E doesn’t usually come with a number; windows are marketed as either having Low-E or not, so the SHGC is used to indicate how low the Low-E is. Our code mandates the values for new construction. Here is what is required by the 2006 NM Energy Code now in effect:

Climate Zone 3 Counties 
Dona Ana

Climate Zone 4 Counties

Climate Zone 5 Counties
Los Alamos
Rio Arrib

U-Factor 0.65 / 0.40 SHGC

U-Factor 0.65 / 0.40 SHGC

U-Factor 0.35 / 0.40 SHGC
San Juan
San Miguel
Santa Fe

Code allows for up to 15 sq. ft. of window area to be exempted from the U-factor/SHGC requirements. This provision allows builders to use that old stained glass window that Mrs. Homeowner wants built into her new den. However, the average of all windows in the home must meet or exceed the U-factor and SHGC requirements for the climate zone. And remodel projects are not exempt from compliance. Where some or all of an existing window unit is replaced with a new window unit, including sash and glazing, the replacement window unit is required to comply with current code.


Don’t Just Assume The Window Is Code Compliant

An informal recent survey has shown that approximately 90% of the windows found on the shelves of national home center chain stores do not meet the current code requirements. While this does not represent dedicated window suppliers, it is indicative of the problem facing builders. To make matters worse, representatives of several window manufacturers have warned that not all Low-E windows will meet the existing requirements because the window frame may not have a low enough U-factor. A few manufacturers have developed proprietary higher-efficiency Low-E glass, while others are now including “standard” Low-E glass. Because of our hot climate zones, energy experts have said virtually all windows in New Mexico must include Low-E glass at a minimum in order to meet existing code SHGC requirements. Some cold climate area builders want to encourage heat gain into their buildings, which flies in the face of the Low-E theory, but that is another story for another day. Meanwhile, such builders should talk to a knowledgeable designer about performance based methods of compliance. Also, double check the stickers on your windows when they are delivered to the job site. The National Fenestration Rating Council (NFRC) rates all window assemblies and a sticker is placed on each window that indicates the U-factor and SHGC. Windows vary tremendously by type, style, glazing and manufacturer. The rating on your windows should match the Energy Code compliance report you provided with your plans when they were submitted for a building permit.


2009 Code Changes Anticipated

The 2009 New Mexico energy code being reviewed now in anticipation of a 2010 implementation will include slightly more stringent requirements. The proposed (be sure to check the final document later this year) requirements for the new code are:

Climate Zone 3
Climate  Zone 5 & 5

U-Factor 0.65 / 0.40 SHGC
U-Factor 0.35 / 0.40 SHGC

Only Climate Zone 4 will see slightly more stringent window requirement, with the (maximum) U-factor going down to 0.35 from today’s requirement of 0.40.

Energy efficiency features of building construction are attracting unprecedented political pressure at all levels, so for builders and code compliance officials the game has changed. Windows are a big part of the energy efficiency equation so today’s prudent builder will double check plans and specs and installed window packages.

Need more information on energy efficiency building science? Consider attending the next NAHB International Builders Show in Las Vegas, Nevada. That opportunity is the very best home builder “university” available anywhere and will include numerous technical seminars on energy efficiency techniques.