" I found Pella Impervia to have the best combination of energy efficiency".

that is the most ridiculous statement i ever heard. The pella impervia is one of the most flmsy windows i have ever seen. Ask pella what the air infiltration rating is on the impervia double hung. I hope you are'nt afraid of heights because that number is going to be very high.

I will defer to what my guys say when they put them in. I know the PG ratings on them are okay and suitable for most construction applications, but my installers say they are literally the most flimsy window they have ever handled.

Thanks for everyone's comments. We're a little limited, we live in a mid-century modern "historic district" and are limited by covenants and an architectural review committee. Can't have anything too ornate, thus the Marvins are frowned on. They really want aluminum but even the thermally broken get pretty poor energy efficiency (we're northern mid atlantic so get enough cold and snow for it to matter). I know the Pella's aren't the overall "best" window, but given our constraints, seem better than aluminum and other brands of fiberglass are much wider, so the review committee isn't happy with them. So maybe my question should have been: Is Pella Impervia better than thermally broken aluminum (ie gerkin) or does anyone have recommendations on simple, thin profiled fiberglass (Pella was the thinnest fiberglass I could find and have yet to find a thin profile vinyl).

Pella is probably the thinnest fiberglass out there. You are correct about that and it definitely does fit that more modern look of the extruded aluminum.

Depending on where you are in the Mid-Atlantic, thermally improved aluminum may not be a terrible choice. The glass is far and away the largest and most significant thermal weak point in the window. Thermally broken aluminum will have lesser CR number than fiberglass, but not necessarily by much. That is the number, along with U-Factor, that you need to be comparing.

If you don't have any massive sweating issues with the current windows, there is no reason to think that a newer window with better frame and glass thermal numbers, would start sweating.

If you are really reaching for that modern look, its hard to match metal (aluminum or steel).

What kind of operators are we talking about here?

Gerkin has very low air infiltration numbers across all there lines, even there sliders. Last I checked they are limited to only dual pane Cardinal 366 glass. There u- values are not that good nor are the condensation numbers. May want to check building code, we need a 0.35 u-value to meet residential building code in MN with exceptions for some historic.