Fall into that trap and you'll quickly find yourself purchasing low quality junk for what is an inflated price.
Working in the home improvement industry, I have seen carpenters suggest junk windows with a short term life to customers. I have demoed high quality windows to homeowners with million dollar houses on the northshore only to see a sign out in front of their house weeks later from the lowest price seller in our market. That's a bit like putting ketchup on your porterhouse while wearing a robe and big fluffy slippers at a high end steak house. I've often wondered who the dimwit yokels are inside and what they do for living-- thinking to myself, that's a doctor or an attorney I would never want to hire.
Some homeowners who know nothing about windows think they're going to learn by sitting through 6 demos. Usually after 3 presentations your brain starts to rot and all the facts from the presentations you've sat through have blended together, leaving you to decide on the one fact you do recall -- price. That's a recipe for an incompetent purchase.
So if you want to make an educated decision here's the scoop:
You need to know about 3 agencies to make a decision on what you want. NFRC, (nfrc.org) AAMA and Energy Star.
NFRC, also known as the National Fenestrations Rating Council, measures heat transferrance and reports their findings in u-factors.
AAMA measures air infiltration.
Energy Star doesn't measure anything. Energy Star is a government agency that uses ratings primarily from NFRC to decide whether a window is energy efficient.
The better the window the lower the u-factor. Most companies will try to sell you a low-e and argon window. Unfortunately the best u-factor you're going to get with a double pane plus low-e and argon window will be a u-factor right around .29.
Your really superior windows are going to be in the .17 to .19. These windows generally pay for themselves with energy savings in 5 to 10 years.
AAMA has a rating you should pay attention to. AAMA Gold Label certification is a distinction that only 10% of all windows qualify for. If a window bears the AAMA Gold Label, it means that window leaks less than .30 CFM (cubic feet per minute) when there is a 25 mile per hour wind. There are some windows that leak as little as .02 cfm. Less air leakage is better. Some windows to bear an AAMA label but not the Gold label. This means that AAMA has not tested those windows because the window company has chosen not to submit their window to AAMA for testing. If it bears a non-gold AAMA label, that window was tested by a private lab of the manufacturers choosing under that manufacturer chose; meaning the manufacturer could have added clips, screws and caulk to seal a window which had air infiltration problems.
All the same, u-factor is King in the window industry. Make your decision based upon u-factor or don't bother replacing your windows.
The new Obama stimulus plan gives homeowners a tax credit of up to $1500 to help pay for new energy efficient windows. To qualify the windows have to have an NFRC u-factor of .30 or less. You can get 30% of the cost of the new windows, up to $1500. The savvy homeowner won't settle for a u-factor as high as .30 but that is a personal choice. My recommendation is to not even bother replacing your windows if the u-factor is higher than .31.
All windows to not qualify for this tax credit. When your windows are installed they should have an NFRC sticker on them. That sticker should tell you whether they have less than .30 and therefore qualify.