Lowes is currently offering ThermaStar windows by Pella (20 series)
vinyl replacement windows. Does anyone know how these fair? They are
argon filled. Does the argon break down over time? Will the windows
lose their ability to keep cold out? Or is the argon breaking down a myth?

When we purchased our house last year, we had all new windows (9) and
doors & slider put in for only $4,200.00. All windows were
virgin vinly gold star energy with argon/double/double sash/tilt in
out with hurricaine glass and they have a lifetime free replacement
of glass, gas, and screens. This holds true with the slider as well.
The doors were energy star as well and the storm door was mid-range
Larson and upgrade to brass with brush nickle finish. Shop around,
there are better products at lower cost than what you can find at a
home improvement store.