Thermal Sash Window and Door Systems is your source for one of the most advanced vinyl window systems available today, Envirostar. With "U Values" as low as .20 (this is equivilent to an "R Value" of 5 !) and air infiltration ratings at .01 cubic feet per minute, this window is the most energy efficient vinyl window on the market today. While industry minimum requirements for double hung windows call for an air infiltration rating of .30 cubic feet per minute or less, Envirostar beats this by a factor of 30! Most window sold today actually have a better rating than .30 ... the average window is rated at .20 cubic feet per minute. That may not sound like much, but if you think of a balloon being about one cubic foot in volume, a window rated at .20 cfm would fill a balloon up with air in 5 minutes. Thats 12 balloons every hour. Thats 288 baloons a day! That is a lot of cold air to reheat in the winter, or to cool in the summer. Envirostar is rated at .01 cfm, which means it is 20 times better (than a .20 factor) at keeping air infiltration from costing you money. Another way to look at it is that the average window will leak the same amount of air in one year as an Envirostar window will leak in 20 YEARS! Why not find out how Envirostar windows can vastly improve your living comfort and save you enough to pay for themselves? Just click on the link below to get a no obligation energy inspection of your home.