Construction Materials-Windows

Windows, sometimes referred to as glazing or fenestration, have been an important part of our homes for centuries.  They let light in and still keep the weather out.  Most are designed to open and let fresh air in and, in an emergency, allow someone to get out.  Sometimes that includes a sneaking teenager.

In this weeks blog, I’ll be discussing window labeling and some of the building codes relating to windows.

Let’s start with the National Fenestration Rating Council, who according to their website, is an independent non-profit organization that establishes objective window, door and skylight energy performance ratings to help compare products and make informed purchase decisions by communicating the energy performance of the product.   All of the NFRC’s certified products are independently tested, certified and labeled.  The image below is an example of a label required on all new windows.

What is the labeling saying?  The U-factor is the first rating on the window information label.  A U-factor is the heat transfer coefficient of a building product, or the rate at which heat moves through the assembly.  U-value is actually the inverse of an R-value (1 / R-value = U-value) and is used in calculating the insulating performance of the various materials used in the construction of the window.  Minnesota energy code states that all new homes must be constructed with fenestration U-factor of  U-.32 or better. (U-.55 is allowed for skylight applications).  A lower U-value means a better insulation value. U-.32 equates to an R-3.125, not a very good R-value when the insulation value of most new wall insulation in our climate is R-21.  A good performing window for our climate will have a U-.25 or less energy performance.

Single pane window with storm. Around R-2.

The second rating is the solar heat gain coefficient, or SHGC.  This is the heat produced from sunlight passing through the entire window assembly, commonly referred to as solar energy.  Solar heat gain coefficient is expressed between 0 and 1, the lower the number, the less heat that is transmitted.  Most southern states have codes requiring less solar heat gain.  The state of Minnesota does not have a code requirement for the SHGC, although sometimes it might be a good idea to include windows with a higher SHGC, which will allow more heat through our windows during the heating months.

The Additional Performance Ratings section on the window labeling includes visible transmittance and air leakage values.  Visible transmittance is the amount of visible light that is allowed through the window. The number range is 0-1, with higher numbers allowing more light through the glass.

The air leakage rating is the amount of air that leaks through the assembly. The number range for this rating is 0.1 to 0.3 with the lower number meaning less leakage.  Building codes limit the amount of air leakage allowed through a window assembly to less than 0.3 CFM per square foot and swinging doors to less than 0.5 CFM per square foot.  Testing must be conducted by an accredited, independent laboratory and listed by the manufacturer.

An optional listing for windows is the condensation rating of the window assembly.  This information is important in our climate because windows typically do condensate during cold weather.  The higher the number, the more resistance to condensation.  The number scale for condensation rating is between 1 and 100, with the higher number more resistant to condensation. Window condensation is one of the bigger complaints I receive about windows when conducting energy audits is my area.  I will have a future blog addressing these concerns.

Additional code requirements with regards to fenestration in Minnesota. Up to 15 square feet of glazed fenestration is permitted to be exempt from the U-factor code and one hinge-side opaque door assembly up to 24 square feet is also exempt.  These exemptions apply only to the prescriptive path requirements.  Log type construction requires a minimum U-.29 window rating for the U-factor.

OK, we have discussed the window labels and a little about window code requirements, now some terminology describing window assemblies.

Argon gas-an inert gas used between two or three panes of glass. Argon increases the insulation performance of Low E glass.
Double pane-a window assembly consisting of two panes of glass, used to reduce heat loss through the window.
Glazing-the glass installed into a window.
Krypton gas-an inert gas used between two panes of glass. Similar to Argon gas, use for insulating between two or three panes.
Low E-Low E or low emissivity is a very thin glass coating made of metal or metallic oxide used to reduce the radiative heat flow through the glass. This process increases the thermal resistance of the window.
Spacer-used to separate two or three pieces of glass in a window assembly.
Triple pane-a window assembly consisting of three panes of glass.  Mostly used in high performance homes, these windows are substantially more expensive than standard two pane windows, but also provide some of the best thermal properties of all window types.

Installing a window into a wall isn’t particularly difficult, insuring the window is attached to both the air barrier and drainage plane, and properly insulating around the unit can be more challenging.  Another subject for a future blog.

Most windows being installed today are double pane with R-values between 3 and 4.  Not very efficient considering average wall R-values of over R-15 and exterior wall insulation values of R-21 are common in my northern climate. Triple pane windows are available, many with insulation levels of R-8 or more, but because of expense, weight, and thickness, only account for around 2% of the window market.  There is a new glass option that hopefully will be available soon, a triple pane using a very thin piece of glass for the center pane.  This technology was invented in the 1970’s, but thin glass was difficult to obtain in the quantities needed back then.  Thanks to the flat screen TV and smart phone markets, this thin glass is now being massed produced.  R-8 to R-10, or about twice the insulation values of standard double pane windows are being achieved.  The payback in energy savings is expected to be 5 to 7 years, less than normal triple pane windows.  Two window manufacturers are currently testing the technology, Anderson Corporation and Alpen High Performance Products. Hopefully we will be seeing this “super window” available in the next few years.

Two great resources dealing with windows are:

See you next week.

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