How to draught proof a sash window which really doesn't fit the frame

[expandabandband]

I need lateral thinking help please...

Our house is old and at the front we have big sash windows. The problem is that age has made the frames wonky and one in the sitting room is ridiculous. There's a gap where the window just doesn't meet the frame at the top - on one side it's just a few mm, on the other it is a 1.5 cm gap.

How do I fill this to stop draughts? In a way which doesn't look stupid?

Our best idea at the moment is white plasticine, but am I missing something here?

We've filled the gap between the two sashes with a kind of rubberised draught proofing which works really well (gapseal, really recommend it) but that's too narrow for the vast space at the top). And yes, we have thermal triple lined curtains.

Expanding foam?

I'd drop the sash down and make a little wooden wedge from an offcut out of a skip or similar & glue/nail it on the bad bit. You could then gapseal around that. Also, my friend has made magnetic perspex 2ndry glazing. You can get the perspex cut to size on ebay and then use sticky magnetic strip around the edges to grip it to magnetic strip around the window frame. Easy to put up for winter & remove any time for cleaning. Mag strip comes in white so is discreet. That should sort your draughts.

Yep, put a perspex layer of seconds glazing over the whole thing.

As students, rolled up newspaper filled sash window gaps quite nicely!

In the past I've used those small spongy packing bits (used in delivery boxes). They squash down easily & can fill those gaps. Easily removable as well.

As a temporary/annual measure we have used rolled up newspaper before but the wedge of wood trick sounds good. Also agree with secondary magnetic glazing. We have just ordered some after a post on here from last week.

A lot of great ideas, thank you.

I am wondering about a wedge of packing material with plasticine/ putty as the final layer of draught proofing.

Perspex glazing is likely to be difficult as it's an odd window, which is much wider than it is high, but we are doing this in other smaller windows in the house (and in fact I can recommend a site called the Perspex Man who will cut really odd shapes of perspex for you - I did say I live in an old, wobbly house - and are cheap).

For the one that is just a few mm, I would route a thin draught strip into the sash (this is worth doing with all sashes and beading TBH). The 15mm gap will need a wedge-shaped timber pieced in as a PP also suggested, although I would buy a new length to cut it from rather than trying to salvage something from a skip.

We've found some spare insulation fabric in the garage and have stuffed that in there. Looks like we've filled it with cat fur but seems to be doing the job.

For all other small gaps, though, gapseal is the business.