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: