The gaps are currently being covered with tape 😅 we’ll have another situation to fix when we pull it off I’m sure… but I can feel the heat from outside when I hold my hand up to it.

I would weatherstrip the window to prevent air from leaking around the sash. Videos:

• Method to Weather Strip Casement Windows with Spring Bronze
• Weatherstripping Video (weatherstripping an awning window but casement should be similar)

I suppose you could caulk in the gap between the sash and the jam. I like DAP Dynaflex 230 caulk.which remains flexible.

The heat is coming just from the glass. You could put weatherstripping in but you still gonna feel the heat.

@toddinmn: "but you still gonna feel the heat."

Oh yeah. Those are old, single-pane windows with none of the features of modern windows that help prevent sunlight from overheating a room. If those are west-facing windows, the problem is made even worse. Applying window film or shading the windows would help.

Yes, there is for sure heat coming from the glass. Those blinds are typically pulled down but we plan to put solar shades there. Still, filling the gaps around the windows (sealing them shut) that are letting the actual air conditioning out of the house will help even more.

Also, sealing the windows will help during the winter when we lose heating through them in the winter.

And yes, those are west facing and gorgeous windows. We get great light through them when it is temperate and we pull the blinds open. But we never open the actual windows. They are casements, they open out, there aren’t screens, and we need a ladder to open them (they’re in the stairwell).

Window film would help to prevent overheating the room in the summer. West-facing windows are the worst as unobstructed sun shines through them from noon until dusk. I have a skylight which made the condo unbearable until low-SHGC tint was applied.

You could use cheap window insulation (example) in the winter to sorta-kinda simulate a double-pane window and reduce the loss of heat through the glass.

Ah, that just gave me an idea, maybe after sealing, I could have window inserts put there as a more attractive solution. We had the film there for the winter and it helped some but we do want something that looks better than that especially if it’s going to be up year round.

I'd consider converting the window from a casement to a non-operable picture window. I'd replace the sash and other window parts as necessary to convert the existing inefficient, single-pane, leaky casement window to an efficient picture one. I'd include the correct Insulated Glass Unit (IGU) for your climate that would reduce summertime heat buildup and reduce wintertime heat loss. I'd do it once and properly, not mess around with trying to "fix" the current one. One and done.

Guide to Energy-Efficient Windows

I like this idea. I will run it by the historic commission and see what they say. Typically they don’t approve any sort of window replacement if the windows are in good condition. Unfortunately they don’t care about our comfort, just preserving the history of a historic landmark.

I too live in a historic district. Our condo building was not historic -- too new -- but it still had to follow certain rules. Most significantly we could not replace our windows with vinyl so we used fiberglass which was okay. Here is a summary of the Window Repair and Replacement rules for our historic district. If yours are similar, I would think you could upgrade the sash, install an IGU, and convert the window to a picture window as long as its appearance didn't change (much).

What's ironic that while new vinyl windows cannot be installed, old, energy-inefficient, single-pane, rotting wood windows with peeling paint are fine and there a lot of them in the neighborhood.

Hi, Jamille. If replacement windows aren't an option then I would recommend interior storm windows. I never recommend storm windows but based on the comments in this thread, that seems like the best solution at this point. https://indowwindows.com/storm-windows/