Posted by: Ken Mariotti
Most people know about the
National Fenestration Rating Council (NFRC) rating system and label that shows consumers how a window performs in several areas, such as, thermal
insulation, solar heat gain, visible light transfer and air leakage. What most
people don’t know is that the NFRC also rates a window’s condensation resistance
(CR). While, the U-Factor, which rates thermal insulation or how well
a window prevents heat from escaping, is most important. For cold climate like Illinois, another important
rating, maybe CR. Since this rating is
optional for manufacturers to include on their
Energy Star label, it may not
be available, however it is important to consider when purchasing new or
replacement windows.
Condensation on your windows can
be worse in winter when the outside temperature is below the dew point of the
indoor air. Water or frost appears when warm moist air comes in contact with
colder dry air.
Maybe your windows have a single pane of glass, which
provides no insulation. Since condensation is more likely to form on cold
surfaces than warm surfaces, there are water droplets or ice on the glass. If
this describes your windows, be aware that upgrading your windows to today’s
technologies will make a significant difference.
Early double-pane wood windows with aluminum spacers can experience
condensation. The aluminum spacer separating the two panes of glass is very
conductive, thereby transferring in the cold from the outside. Spacers are
located around the glass edges and so conduct heat from the inside pane
to the outside pane. The area where the wood meets the glass can develop
condensation, freeze, thaw, and eventually rot the wood.
Today’s windows are engineered differently. Frankly, the
most important part of the window to resist condensation is that spacer that
keeps the panes of glass apart and seals in the gas. Today’s spacers are made
of less conductive stainless steel, U-shaped tin steel, and foam polymers.
Low-E and gas filled glass panes reduce temperature transfer between the inside
and outside. Add the frame, to this list of improved materials, and you
essentially have the features that are measured in the NFRC U-factor ratings. The NFRC recognizes three parts to a
window: the center-of-glass, the edge-of-glass, and the frame. The lower the
U-factor, the better the window is at keeping heat in – particularly important
during winter’s heating season.
Some condensation is expected and can result from any number
of variables, including a homeowner’s lifestyle. Dryers that are not vented to
the outside, gas cooking without using the vent, room humidifiers or forgetting
a boiling teapot, our lifestyle choices all contribute. However, sloppy, wet,
sweaty windows are unsightly and left unchecked, the constant moisture can lead
to serious damage of the windows, window coverings and walls.
With so many window choices out there, it would be nice to
have a simple rating system, like the U-factor to compare windows for
condensation resistance.
The NFRC and others, have attempted to articulate a common measurement for manufacturer’s
to use, which is called the CR scale of 1-100, with a higher number
representing more resistance to the formation of condensation. A single pane
wood window might have a rating of 15, while a triple-pane wood window may
achieve a rating of 75. A cold climate, like Illinois, ideally would opt for a
rating over 50.
So why aren’t window manufacturer’s required to include the
CR rating on their windows? In a nutshell, while it’s a valuable metric to
obtain, there are still multiple ways to measure condensation resistance and
limitations to either rating method. It may come back to the homeowner’s lifestyle choices
contributing to indoor condensation. For example, shades, curtains, and blinds
tend to reduce the amount of airflow across the window. This allows the window
to cool and increase the likelihood of condensation.
Woodland Windows and Doors have done the research
about condensation resistance and have discovered that a single rating probably
isn’t a particularly useful number. However, selecting new or replacement windows
that reduce the relative risk that you will have condensation is a very
worthwhile pursuit. If you need to maintain a higher interior humidity during
the winter months, for health or comfort reasons, you want to pay attention to
the edge-of-glass details. Or maybe you just want to be sure you understand the
important features and specifications of the windows you buy.
Ask Woodland -Woodland knows.
