With vinyl, you can add much more weather stripping and incorporate various engineering features that really help prevent air infiltration opposed to wood or fiberglass. This is because vinyl is an extruded product and it can be moulded to each respective manufacturers preference. There are so many different designs and that is why quality amongst vinyl can vary so widely. A low quality vinyl offering will have minimal amounts of weather stripping as well as just the most basic features. some companies can even add structural reinforcement in their vinyl extrusions that can really add to its strength.

you can also add foam insulation in a vinyl extrusion which also makes the frame more insulating.

finally, and this is very important, there are different qualities of the vinyl itself. Higher end vinyl is made to be much more durable than the lower end vinyl you may see in a building supply store. The vinyl formulation in one vinyl window can be of a much higher grade than another. Different additives and different amounts of additives play an important role with that.

if you ever saw a commercial or newspaper add showing a vinyl window that has warped, they are always showing a low grade vinyl window, never a higher quality vinyl window. The new higher end vinyl are made extremely well and also look very nice opposed to the cheaper looking vinyl offerings most are familiar with.

Wood windows aren’t purchased for performance first and foremost, they’re purchased for aesthetics, versatility and to match the original windows. Additionally, there are hundreds of vinyl window manufacturers across the U.S. and one of the best ways to stand out is with superior performance ratings.

Well said ultra windows.

another thing that stands out in higher end vinyl is the actual look. The sash may have some sort of design or a rounded edge opposed to the standard squared edges. The exterior usually has a much nicer design as well.

I say it comes down to the spacer and the foam filling of the frames and sash for U-value. None of the wood window companies seem to care about air infiltration numbers even if they have good numbers.