Thanks for your input. Yes, the current window construction is the standard dual pane builder grade. It's the original windows the house came with and the house is around 22 years old. What's a good "tight air infiltration number" for laminated glass and how should I ask for it? Also, I know that having varied glass thickness helps with STC, should I ask for it as well?

Good air infiltration number is 0.05 or below. A good double pane, with double strength glass, is going to be an upgrade already. Laminated is that much better. If the noise is bothersome, I would probably just do it in the sleeping rooms.

As WoW pointed out, air infiltration is your number one concern. The window and install must be as airtight as possible.

I am a bit surprised at an STC 33 in that configuration, I would have expected something a bit better than that. Fixed and casement will be a bit (or more than a bit) better than double hung or sliders simply because they close tighter and have better air infiltration numbers.

Also, outside of infiltration, the width of the gap between the inner and outer lites is the most important consideration. Wide = good, narrow = bad, and anything 3/8" airspace or less = very bad.

And while different width glass in a dual pane will improve sound performance, my experience it that only applies when using non-laminated glass. I have never seen evidence that proves that using two different glass thicknesses in a lami unit is any better than simply using two of the same thickness....in fact while a single laminated unit consisting of two lites at 5mm each ( 7/32"x2) and a .030" interlayer has an STC 36, a laminated unit using different thickness lites at 3mm (1/8") and 6mm (1/4') with a .030" interlayer will have an STC of...36.

@Windows on Washington Ltd @oberon476

Very valuable information, thank you much. The Simonton I was quoted has 3/4'' IGU and 2x1/8 thickness. If my calculations are correct, this leaves me with a 1/2'' gap. Is that not good enough?

Anlin and Milgard are another 2 options in Southern CA. You think I'll get better windows for my need with any of these 2 brands?

Ask for the air infiltration numbers on the windows and post that back up.

The Simonton I was quoted has 3/4'' IGU and 2x1/8 thickness. If my calculations are correct, this leaves me with a 1/2'' gap. Is that not good enough?

Maybe?

The information that you posted is a bit confusing because it states that you have a laminated glass package but that your IGU has two lites of 1/8" glass, or at least that's how I am reading it.

That doesn't really make sense if you have laminated glass since the lami should be at least 1/4" rather than 1/8" unless 1/8 in - 1/8 out DS means that the laminated is 1/8" / .030" interlayer / 1/8", and the DS stands for the non-lami monolithic lite (DS means double strength which also 1happens to be 1/8").

If that is true then your 1/2" has become 3/8"and that might be at least part of why the STC is lower than I would expect. Of course just speculation without more information.