If this is a concern, I would probably just replace the transom as well. They already have a larger glass area than the casements already. Or maybe remove the muntins out of the transoms , since they appear to be simulated anyway.

That transom will have to be custom with lots of notes to have that irregular grid pattern. I’ve never seen that before- I’ve only ever seen regular spacing of the grids which results in a slight misalignment.

How about no grids ?

Yes, no grids would be the easiest. But this is the front of a colonial house, the bedrooms on the 2nd floor and the garage window facing the same side all have grids. I have no problem going gridless on the backside of the house.

No mullets, be consistent throughout the house.

Please consider replacing the windows with new ones the exact size of the original. Your house deserves correct, if somewhat more pricey, windows.

I'd ditch the casements if it were and go with a full height twin double hung. You will need to re-trim out the interior, but more colonial looking and casements that large are a PITB to operate anyway.

It sounds like there’s no way to guarantee grids alignment between the transom and the window beneath it without a “full frame” replacement. In the “ pocket” installation, even with new transom, if it has grids, misalignment can occur.

I do not think casements are the right choice for your home so i would do tall double hung with the grids BTEW I do not like grids but they suit the house

Patricia Colwell Consulting, you mean no more transom, just tall double hung? What is grids BTEW?

She meant by the way. And yes many are suggesting full height paired double hungs but if you actually want advice we need a full front photo and dimensions. To answer your first question your grids will NEVER align with a pocket window replacement and keeping the existing transom. Honestly I’m surprised a window company would suggest it since that transom was likely factory mulled to the casements.

" Honestly I’m surprised a window company would suggest it since that transom was likely factory mulled to the casements. "

Not "likely", it is factory mulled. Terrible idea. I would also replace the entire window. If the casement is old enough to have rot, don't send more money after bad, replace the whole magilla.

Yes, the transom is factory mulled to the casement. I have 16 such transoms when counting in the sunroom. Sunroom has 3 triple casements and 2 single casements with transoms on top. To keep the budget lower, I want to explore the options of not replacing them, only changing casement windows since they have some degree of rottenness. In sunroom, I plan to have no grids in the new casement windows. For the living room and dining room, I want to have some grids, like 1v3h(2x4 grids). If I replace the transom on the top with four evenly spaced out vertical lines, will they align with the vertical lines on the new casement window(see the last photo)? I am talking about the "pocket" installation here. The casement width is about 26.5"x70", transom size is 54.5"x14".

Not going to look correct on more than the grids. The sightlines will look awful as well. You will have much less glass on the lower units. The grids not lining up will be the least of the optical issues.