Understanding Windows

Windows

When buying windows, the more you understand how the window has been ratted and its application of use, the greener your windows choices will be.

When talking with a contractor, sales staff, or just researching windows as your own consumer, you will see a NFRC label on the bottom right of the window. The National Fenestration Rating Council (NFRC) is a 501(c)3, non-profit organization that provides performance ratings on windows, doors, and skylights. NFRC administers a certification and labeling program for the energy efficiency of windows, doors, and skylights.

Four items minimum should be listed on this label. All four items when evaluated, understood and incorporated correctly into the design or retrofit of a building can greatly increase the green efficiency and resource use reductions of the buildings operations.

Please inform yourself with:
Thermal Transmittance,
Solar Heat Gain Co-efficient,
Visible Transmittance, and
Air Leakage.

Thermal transmittance (U-factor):

The lower the value of a U-factor a window has the slower the rate of heat loss or heat gain the window’s specific properties are cable of. One-way to possibly understand this better is that the U-value is the inverse of the R-value, (or that the R-value is the reciprocal of the U-value. Now remember in terms of building envelope and when purchasing insulation, the higher the R-value the better.

Lets do an example:

Say a window has a U-value (thermal transmittance) of 0.17. Take out a calculator and type in 1 / 0.17 = 5.88 approx. This is the R-value of that window. So if the building has incredible R-50 insulation but also has 25% exterior wall space containing windows at an R-value of 6 the building is losing a lot of heat in the area of 25% glazing.

Notes: U-factor ratings generally fall between 0.20 and 1.20. In the United States, the U-factor is usually expressed in Btu/hr./ ft²/ F.

SHGC : The Solar Heat Gain Co-efficient

The Solar Heat Gain Coefficient (SHGC) measures how well a
window blocks heat from sunlight.

The SHGC is the fraction of the heat from the sun that enters through a window. SHGC is expressed as a number between 0 and 1. The lower a window’s SHGC, the less solar heat it transmits. This is really important when designing a building that should require little heating in the winter and minimal cooling in the summer, ideally. This can be done in Humboldt more easily than other climates due to smaller flections of temperature in more populated areas of the county.

Tip: If the building is in a geographical area of the county where it gets cold in the winter (snow level), a possibly higher SHGC might be intriguing. This can be accomplished with longer roof eves that will block the unwanted heat gains of the summer and allow them in during the winter when the sun is at a long angle of declination (below the eave).

Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

Note: it can be better to have windows with a higher VT on the north sides as this allows more diffused and less glare visible light into the building. Glare issues should be discussed with your architect.

The more correctly designed visible light into the building the less conventional lighting backup needs to be on. This saves electricity, which saves energy and reduces emissions and demand for resources. Green Operations of the building are equally as important as the building mediums.

Manufactures of windows can choose not to incorporate the following to ratings.

Note: Air leakage is extremely important and should be known when purchasing a window.

Air Leakage Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

Condensation Resistance (CR)

measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation.

While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.

“Greener” window purchasing suggestions:

Buy a recycled window. Arcata Recycling Center
Craigslist
Urban Ore ” basically a Home Depot for recycled building
http://www.dhwsalvage.com/windows.htm
http://www.selectsalvage.com.au/
http://www.greenconcepts.com/producttips/buildingcomponents/window.html

Tax Credits and Rebates for your window choices. Go to the site listed below and scroll down the windows section and click on more details.

http://www.energysavers.gov/financial/70010.html

Local contractors might have clients that are getting new windows.

Great resources for designing how many windows and where on a house or building should have.

Request that your architect read and understands the tips given by
these engineers:

Great Suggested Readings…
The Passive Solar House Book, The Solar House Book.