The U-factor And The R-value

Both U-factor and R-value represent sort of rating of the energy-efficiency of the window, skylight, door, etc. Here are some more detailed explanations:

The U-factor

In order to have the perfect, most energy-efficient windows or skylights these days you shall buy those with the lowest possible U-factor.
Just for your comparison here are some usual U-factors:

- the old-era metal sash windows have the U-factor of 1.5 and more;
- the single pane windows can reach the U-factor of 1.1;
- the double pane windows with low-E coating have (best) 0.4 U-factor;

The U-factor value also tells you somewhat more about the materials the windows/skylight manufacturers did use. The quality materials (usually more expensive) offer lower U-factor; but the cheaper and low-quality materials can never achieve a really low U-factor.

The R-value

The R-Value refers to the ability of the glass to resist to heat gain or loss (both in and out). The higher the R-value, the more recommended product you are buying.

The R-Value, in fact, is a reciprocal measure to the U-factor, as it represents the measure of the rate of heat loss through a material. The best U-factor or R-value can be achieved with the glasses treated with low-E coating or Argon (AG) glazing.

The ENERGY STAR products

For those of you living in USA, there is the National Fenestration Rating Council (NFRC) operating a voluntary program testing and certifying windows, doors, and skylights based on their energy-efficiency. The NFRC label is a reliable sign of your window´s or skylight´s energy-efficiency.

Usually the energy-efficient products are also labeled as the ENERGY STAR products.