Thanks for the input! You are correct in your perception of this photo being from the interior with the window on the left and wall on the right. I've added this clarification to the question. The gap is going to be too small for the nozzle of a can of spray foam, would I just 1/2 fill the gap between the window sill and drywall instead? So no need to remove the sill? I will have to hop on the roof and check the status of the exterior caulking.

I've continued removing the sill on the right side and posted a wider photo. The sill/framing is very tight so I can't get to the failing caulk unless I remove the sill. That caulked gap is quite slim, hence the caulk not foam. But I could potentially cut back the drywall and try to fill between sill/framing rather than window/framing.

Thanks for the input. I updated my photo again, as my area of concern isn't the area you circled, but the air seal between the window and the framing. I have no concerns about being able to caulk a small gap. That's definitely doable. Rather, my question is what is the best way to air seal a small gap between the window and the framing. My understanding is that your suggestion is not to re-seal around the window and the framing, and instead seal between the sill (which I have partially removed) and the framing instead. Is that correct?

@pirateforhire the main suggestion is to use expanding foam insulation to fill all the gaps to the exterior, wait for it to fill the space up, cut off the excess, then finish it with caulk (or here in the wet UK we'd more likely use silicone sealant). The bit about the sill just being caulked over is an additional suggestion and is because it is interior trim only i.e. not the source of your drafts :) (this is how I understood it anyway!)

@LukeBriggs Please point out where I can better clarify the fact that the gap between the window and the framing is 1/8th of an inch and not likely a suitable gap for spray foam. I mentioned in the initial question that this does not seem to be a good situation for spray foam, which is probably why the original installer used caulking between the window and the frame instead. The goal of this question was to focus on alternate air sealing methods that are often suggested, from caulking, backer rod/caulking and specialized air sealing tape.

@pirateforhire can definitely get a spray foam nozzle into a gap like that - the spray foam gun applicators (in the UK, these things are <£10) have very tiny nozzles on the end and quite the throw - the foam then also expands too so you'd have no issue filling it up :)

@pirateforhire also an extra thought to avoid wasting the rest of the can as one of those canisters goes for ages and can't really be left around once they're opened - maybe check other windows as well and fill other little gaps etc while you're at it

@LukeBriggs I've used spray foam on windows and doors (as well as other places) before and the nozzles on the cans they sell here in the US will not fit in a gap this small. If I attempt to spray into the gap without being able to insert the nozzle, more foam will expand out onto my window sill and won't expand into the gap. That's where at least with a backer rod, I can force it into the gap and ensure I get a tight seal.

@pirateforhire there's 2 kinds and they're definitely available worldwide - if you're getting the canisters with a little straw, yes those things have pretty big nozzles and wouldn't fit at all. The gun applicator version uses a screw-in can and the hole on the end is absolutely tiny :)

Their biggest problem is because of their size, you have to clean them out when you're done to avoid the actual gun part getting ruined too. There's a separate applicator cleaner which handles that - unlike the foam though the cleaner can be shelved for a while.

@LukeBriggs I will look into the gun applicators available near me. I appreciate this suggestion, but there are also other reasons to avoid spray foam, such as evidence of it disintegrating quickly as well as holding moisture more than a closed cell foam backer rod. I'm gathering maybe I should have asked my question in a green building forum where people have more interest in discussing alternate methods. Thank you!

@pirateforhire I follow passivehaus rules here - If you're concerned about water ingress, that's all about how you seal it (i.e. silicone sealant is better than caulk given it flexes more as your window and wall heat up/ cool down)

That's fair, obviously the outside seal should prevent moisture penetration. But sometimes seals fail before you notice and moisture gets in anyways. That's why many passive house advocates here in the US prefer the backer rod method.

The thing about anything like that though is it really needs to be continous - in your case that's going to be easier with something you can squirt when doing a retrofit

Also, fair, but that's why I was asking about the tapes that are so popular in places like germany. That I can definitely do continuously. If I combined the two methods, in theory I could get the best of both worlds. And then why I re-side my house (someday) I can be sure all the exterior flashing also meets passive house standards and I'm set.

Yeah the tapes are also very impressive - I'm definitely a fan of those; I think your bigger problem is more likely to be the frames themselves as they're probably not insulated or airtight (just general degredation of the seals around glass on an older pvc frame)

It's possible. The windows aren't actually that old so the seals are pretty good. I'm not sure how I'd go about determining if the frames themselves are insulated.

They sound different when you knock on them but unless you have a reference of "this one is definitely insulated" then I doubt it would be that obvious

The alternative is to pop a screw out, assuming they're screwed in and the screw is visible through an opening window

Good to know. I've got screws I can poke around.

Obviously I can't get anything perfect retroactively. I'm trying to do all the things I can to airseal and reduce a blower door number. Which, going back to the spray foam, I've read plenty about how that stuff disintegrates quickly so even if you can reduce your blower door number initially, the results don't last, maybe even less than the caulking that has failed now.

The screws themselves should have sealant on them too by the way if you want to get full marks on the window detailing, but yep definitely at least solving the "oh I can see light through that!" hole would go a long way in your case

And I do see light. Good call on sealing screws, I definitely don't think that has been done. I gotta run, but thanks for all the input! Greatly aprpeciated.

No problem, enjoy the upgrading! :)