Frequently Asked Questions About Windows

What is a U-factor?
The U-factor is a mea­sure of heat flow or con­duc­tiv­ity through a mate­r­ial, the rec­i­p­ro­cal of R-value. Although R-values are used as for mea­sures of the resis­tance to heat flow for indi­vid­ual build­ing mate­ri­als, U-factor is always used to mea­sure the con­duc­tive energy of build­ing enclosures.

What is a Design Pres­sure Rat­ing?
Design pres­sure, or also referred to as DP, expresses a numer­i­cal value that defines the struc­tural wind load­ing require­ments (in pounds per square foot) for a build­ing and the com­po­nents and cladding of a build­ing. For win­dows and patio doors, the higher the DP rat­ing indi­cates bet­ter per­for­mance under wind load (e.g.: a DP-50 win­dow is struc­turally more sound than a win­dow rated DP-35). Coastal regions often require higher DP rat­ings by code to antic­i­pate higher wind veloc­i­ties that can be encoun­tered in prox­im­ity to the coast line.

What is meant by “Solar Heat Gain Coef­fi­cient” (or, some­times expressed as “SHGC”)?
The num­ber to know when select­ing win­dows and patio doors – it mea­sures how much of the sun’s heat is trans­mit­ted through these fix­tures, expressed in a num­ber from zero to one. A win­dow that has a SHGC of .30 will allow 30 per­cent of the sun’s heat to pass through. Whether you want a higher or lower num­ber will depend on your goal. Espe­cially in the South, you will be pri­mar­ily inter­ested in a win­dow or patio door with a low SHGC that will help to block solar heat gain inside your home, thus reduc­ing cool­ing loads in hot weather.  North­ern cli­mates often look for higher SHGC per­for­mance to har­ness pas­sive solar warmth on cold, sunny win­ter days.

What is insu­lated glass? 
Insu­lated glass con­sists of two pieces of glass her­met­i­cally sealed to a spacer. This cre­ates a sealed, insu­lated air space between the two pieces of glass, result­ing in bet­ter ther­mal insu­la­tion per­for­mance. Insu­lated glass also helps reduce con­den­sa­tion while keep­ing the heat in dur­ing the win­ter, and heat out dur­ing the summer. 

What is low-E glass?
Low-E stands for low-emissivity glass – this is a nearly invis­i­ble coat­ing on the glass sur­face that are micro­scop­i­cally thin metal­lic oxide lay­ers pri­mar­ily to reduce the U-factor by sup­press­ing radia­tive heat flow. The prin­ci­pal mech­a­nism of heat trans­fer in mul­ti­layer glaz­ing is ther­mal radi­a­tion from a warm pane of glass to a cooler pane. Coat­ing a glass sur­face with a low-emittance mate­r­ial and fac­ing that coat­ing into the gap between the glass lay­ers blocks a sig­nif­i­cant amount of this radi­ant heat trans­fer, thus low­er­ing the total heat flow through the win­dow. Low-E coat­ings are nearly trans­par­ent to vis­i­ble light. 

What is argon gas? How does it work? 
Added inside an insu­lated glass unit air space, argon gas is an invis­i­ble, insu­lat­ing gas with lower ther­mal con­duc­tiv­ity than atmos­pheric air. Dur­ing the man­u­fac­tur­ing process, the atmos­pheric air is dis­placed when argon gas is pumped into the glass unit air­space. When com­bined with Low-E glass the Low-E glass helps reflect heat away, while the argon gas helps reduce ther­mal trans­fer to enhance the glass unit insu­lat­ing performance. 

What is a good source for win­dow instal­la­tion infor­ma­tion?
We rec­om­mend that you refer to the Amer­i­can Soci­ety of Test­ing and Mate­ri­als (ASTM) guide enti­tled:  ASTM E 2112 “Stan­dard Prac­tice for Instal­la­tion of Exte­rior Win­dows, Doors and Sky­lights” for com­pre­hen­sive instal­la­tion guid­ance and best prac­tices:

For coastal region instal­la­tion con­sid­er­a­tion, you can also refer to the Amer­i­can Archi­tec­tural Man­u­fac­tur­ers Asso­ci­a­tion (AAMA) guide 100–07:

How should I eval­u­ate the energy per­for­mance of a win­dow or patio door? 
Look for the National Fen­es­tra­tion Rat­ings Coun­cil (NFRC) label on the win­dow or patio door. This label shows the U-Value, Solar Heat Gain Coef­fi­cient, and Vis­i­ble Light Trans­mit­tance val­ues. All val­ues are backed by inde­pen­dent lab test reports on file with every win­dow and door manufacturer. 

Who is the NFRC?
The National Fen­es­tra­tion Rat­ing Coun­cil (NFRC) is a non-profit, public/private orga­ni­za­tion cre­ated by the win­dow, door and sky­light indus­try. It is com­prised of man­u­fac­tur­ers, sup­pli­ers, builders, archi­tects and design­ers, spec­i­fiers, code offi­cials, util­i­ties and gov­ern­ment agen­cies. NFRC has estab­lished a vol­un­tary national energy per­for­mance rat­ing and label­ing sys­tem for win­dows, doors and sky­lights. For more infor­ma­tion, visit their web­site:

What is the Energy Star® pro­gram?
Energy Star is a vol­un­tary part­ner­ship among the U.S. Depart­ment of Energy, the U.S. Envi­ron­men­tal Pro­tec­tion Agency, prod­uct man­u­fac­tur­ers, local util­i­ties, and retail­ers. Part­ners help pro­mote effi­cient prod­ucts by label­ing with the Energy Star logo and edu­cat­ing con­sumers about the ben­e­fits of energy effi­ciency. By choos­ing Energy Star-labeled prod­ucts, you’ll help to keep your util­ity bills lower, and help the envi­ron­ment at the same time. For more infor­ma­tion, visit their web­site:    (Energy Star® is a reg­is­tered trade­mark of the U.S. Depart­ment of Energy).

What about win­dow con­den­sa­tion?
Con­den­sa­tion is a direct result of inte­rior humid­ity and the dif­fer­ence between indoor and out­door air tem­per­a­ture. If you keep the humid­ity in your house low, then the like­li­hood of expe­ri­enc­ing con­den­sa­tion is also low. How­ever, the effi­ciency of your win­dow will also impact the tem­per­a­ture and humid­ity level at which con­den­sa­tion occurs. Energy effi­cient win­dows will help reduce con­den­sa­tion. Here’s why – high per­for­mance win­dows with low U-factors result in inside glass sur­face tem­per­a­tures much closer to the room air tem­per­a­ture. Win­dows with non-metal frames and more thermally-efficient spac­ers in the dual-pane glass units are also less likely to have con­den­sa­tion on the frame or at the edge of the glass.  Also, real­ize that in cer­tain con­di­tions (such as humid morn­ings after a clear night sky), some highly insu­la­tive win­dows may have dew on their out­side sur­face. These win­dows are such good insu­la­tors, that dew is con­dens­ing there just like it does on an insu­lated wall.

Why does con­den­sa­tion occur? 
Con­den­sa­tion (water vapor or mois­ture in the air) is a sign of excess humid­ity inside the home. Con­den­sa­tion occurs when moist air comes in con­tact with the colder sur­face, such as a win­dow or mir­ror. Although the sur­face of a win­dow may be the first place you notice con­den­sa­tion form­ing, the win­dow is not the prob­lem. Win­dows, in this case, merely pro­vide a vis­i­ble sign that excess humid­ity or mois­ture is present in the house.  Warm air holds more mois­ture than cool air. When that warm, moist air comes in con­tact with a cooler sur­face, the mois­ture sus­pended in the warm air trans­fers to the cooler sur­face as condensation. 

What causes mois­ture inside the home? 
Indoor mois­ture in the air is caused by a vari­ety of fac­tors. Com­mon house­hold activ­i­ties such as cook­ing, show­er­ing, using the wash­ing machine or dish­washer and other activ­i­ties that use hot water all add mois­ture to the air. 
Newer homes are often more sub­ject to con­den­sa­tion because they are con­structed with bet­ter weather tight mate­ri­als than older homes. Weather strip­ping, improved insu­la­tion, vapor bar­ri­ers and mod­ern con­struc­tion tech­niques are designed to reduce air leak­age. But at the same time, these mate­ri­als and tech­niques can also seal mois­ture inside the home. In newer, more weather tight homes, it is impor­tant to be aware of humid­ity lev­els and to pro­vide ade­quate ven­ti­la­tion to reduce humid­ity levels.

Energy Star® win­dows 
The Depart­ment of Energy (DOE) and the Envi­ron­men­tal Pro­tec­tion Agency (EPA) have devel­oped the Energy Star des­ig­na­tion for prod­ucts meet­ing cer­tain energy per­for­mance cri­te­ria. Most Atrium win­dows and patio doors avail­able in your mar­ket area can be spec­i­fied to meet Energy Star cri­te­ria for your cli­mate zone. 

Please refer to the Energy Star web­site to access com­plete pro­gram infor­ma­tion. Energy Star is a reg­is­tered trade­mark of the U.S. Depart­ment of Energy.

Can you pro­vide me with an inde­pen­dent, infor­ma­tive web­site where I can learn more about energy effi­cient win­dows?
For exten­sive infor­ma­tion, backed with details pro­vided by the U.S. Depart­ment of Energy, refer to:

Lawrence Media Group