[Zeke]

I sell and install windows for a living. I'm an independent. Windows are rated in U-factors so you don't see how bad they are. 2 x 4 walls with stucco and fiberglass are R-13. 12" of insulation in the attic rates R-30

A dual pane window is an R-2 and the best you can buy is an R-3+

Now, there are 3 types of energy loss (or gain), conduction, convection and radiation. Let's start with radiation. Most radiant heat enters through light transmissible surfaces such as glass or skylights. The warmth is felt locally on the floor or on your skin if you are in the light. Easy to stop that and the UV damage that comes with it. Cover things up. Oh, you don't like a dark house? Then get the LowE which stands for low emissitivity. Blocks 86.7 % of the UV and helps considerably on the longer wave lengths that heat.

But, that's just part of the story. First, it's only one 3 energy sources. And, add up the total square footage of all your glass and compare that to the total SF of all you ceilings that have an attic overhead (or roof) and all exterior walls. Pretty small percentage, eh? I thought so.

Conduction: The sun is out and the walls and windows are getting warm. This in turn warms the interior. That's why we don't put the fridge next to the stove. Insulation slows this process down. That's all, slows it down. So you can keep up with it using less energy.

Last one, convection, the biggest factor and my favorite. I can change the feel of your home with a roll of plastic and some tape. I'll wrap that puppy up and no air will move in or out. No more hot spots or cold drafts. Just a nice even temp with the fan on. No way? Yes, way. Any window you buy will be better weatherstripped than products of 20 years ago. They have to be, even the cheapest one to get a certified label for air and water infiltration. But what about the rest of the house? Under doors, pet doors, range hoods, bath fans, electrical outlets (as small as they are) contribute a little leakage and there's plenty of them. The list goes on.

So, yes, I sell and install replacement windows and I have Volkswagens, Porsches and Maybach's. Spend what you like. At R 2-3, they all pretty much do the same thing. As I mentioned above, to keep you from comparing fenestration with built up wall and roof systems, they give you a U value. A window with LowE is .35. Dual pane clear is .50 You can get down to .25 with that blue stuff on the office buildings.

Even though I sell windows, I think overall weatherstripping and adequate heating and cooling go a long way toward saving energy. Nice windows improve the look of the house, increase the selling price of the house too. So, looking at actual dollars saved by installing windows is missing the point, me thinks.

I like to call them furniture for walls.