It can get confusing! Here is what you need to know.
- U and R values basically represent the same thing (for simplification purposes). They are simply the inverse of each other.
- Both numbers refer to how well a material or assembly of materials (like a wall or a window) keep things insulated.
- That is, how well they keep hot or cold, in or out -- whichever you are looking for.
For windows we generally talk 'U Value' and the smaller the value the better the window is at insulating.
A window for Arizona
In Arizona you are in a cooling climate. Which means you generally want to keep cool in - and heat out - during the day. But we also know it can get cold there at night, so at night you want to keep heat in - and cold out.
- Remember that insulating is not just for heating.
- Insulating also keeps things cool.
So you DO want a well insulated window. You may not need a window that is as well insulated as you would for a house in Northern Minnesota, but a good U value will definitely reduce your cooling bills.
I would shoot for a U value that meets or surpasses Energy Star minimum requirements for starters. Right now that is a U value of .30 or less. Check this chart to get specific recommendations for your exact area.
Solar Heat Gain Coefficient
SHGC measures how much solar radiation is transmitted through the glass.In Arizona you want that number to be low.
In general - in a cooling climate - you want to keep the sun out. So shoot for a low SHGC. I would say start also at .30 or less. It should be easy to achieve significantly less SHGC than .30.
Generally adding films, coatings and gasses to windows to get the U value low also means that the windows become a bit tinted and the SHGC is automatically reduced. In fact up here in the north where we want more solar heat gain that can be a bit harder to get through many window companies - because we also want a good U value. But in Arizona, you should have it easy.
A good start
Shoot for less than .30 in both U and SHGC and you will be off to a good start. Get your SHGC down to .25 or less and you will be doing even more toward reducing your cooling load.
All of the coatings, gasses and films vary from company to company but basically are there to achieve one of the above goals:
- better U value and/ or
- better SHGC.
Another factor to be concerned with when picking windows is Visible Transmittance.
This refers to how much light passes through the window,meaning the lower the VT the more 'tinted' the windows look.
- The lower the rating the less visible light passes through.
- This usually goes hand in hand with Solar Heat Gain but it is not always that simple.
- Some films, etc that cut down on solar heat gain do not reduce visible light that dramatically.
The best thing for VT is to see windows in person and look at the glass before you buy. So - go to a window company, tell them that you want to achieve certain goals for U and SHGC and then ask to see windows that do this.
I hope this help!
There are many other factors to consider - like air infiltration and condensation resistance and spacer bars etc, but as a consumer - if you just want a decent window - start here and you should be fine!
For more information:
Read "Can you recommend suppliers of low U-value solid wood windows?" a Q&A answered by Florian Speier.