Unusual design bottom rail of lower sash and how to draught seal?

[BarumMike]

We have just renovated and draught proofed a large sash window in our Victorian house. This included fitting a staff bead with a slide in brush seal to contact the inner face of the bottom rail of the lower sash. Unfortunately the underside of the bottom rail slopes downwards to the outside at around 10-11 degrees. The cill only slopes at around 3 degrees and forms a step about 6mm short of the front face of the sash. So, when the sash is closed the front edge of the sash overhangs the edge of the step of the cill (which helps prevent rain running back up the cill) but means that there is a significant gap at the inner edge of the bottom rail, so that the staff bead only overlaps the bottom of the inner face of the sash by a few mms. This in turn means that the brush seal doesn't make enough contact with the inner face of the bottom rail and cold air whistles through. Has anyone come across this design before? It seems odd that the cill and the bottom of the sash don't have the same slope. Should I build up the bottom face of the lower sash to more closely follow the three degree slope of the cill and provide more contact surface for the brush seal? Not easy to do without taking out the sash again. Perhaps easier would be to insert a ?8mm fillet under the bottom staff bead to improve contact with the brush seal.
And a related observation - there is no draught seal or parting bead at the top rail, so there is still a significant draught. When we had other sash windows refurbed by a prestigious local firm they did not provide either protection at the top rail, and I followed their example which I now regret!

 

[CliffordPope]

I don't think that's unusual. I've often noticed the slope of the bottom of the sash is greater than that of the cill. I assumed it was a clever way of avoiding trapping a film of water which would never dry out, as would be liable to happen if the two surfaces were parallel.

 

[Feltwell]

I've always found them to be parallel, or near enough. As a "belt & braces", on the top of the top sash and the bottom of the bottom sash, I always fit a bubble seal - I fit one that goes into a vary narrow routed groove, but if you wanted a quick & easy then a self adhesive one is available, should stay stuck in that location:-

https://www.reddiseals.com/product/standard-bubble-seal/

Old thread detailing what I do is here:-

https://www.periodproperty.co.uk/fo...-repair-dont-replace.16963/page-2#post-229756

 

[CliffordPope]

As a "belt & braces", on the top of the top sash and the bottom of the bottom sash, I always fit a bubble seal - I fit one that goes into a vary narrow routed groove, but if you wanted a quick & easy then a self adhesive one is available, should stay stuck in that location:-

Surely any kind of seal that takes up space will mean that the two sashes overlap slightly in the middle?
Even a thick layer of old paint was enough to stop the catches working because the two surfaces were not level.

 

[Feltwell]

Surely any kind of seal that takes up space will mean that the two sashes overlap slightly in the middle?
Even a thick layer of old paint was enough to stop the catches working because the two surfaces were not level.

You just compensate by taking a little off the sashes. In this case if the sash and cill surface are not parallel anyway and the bubble seal sits in a gap then they probably won't need to bother.

 

[fernicarry]

I have parting beads across the top of the frame. I think this is a Scottish thing.

These usually need thicker brushes in them since there is nothing pulling the top of the top sash into the bead. I've had to trim and/or add wedges to some top sashes to get them to actually overlap the brush due to frames being out of square.

Ventrolla had fitted brush seals to the bottom edge of the bottom sashes that were here when I moved in and those proved to be rot magnets. The bottom rails rotted from the inside out due to water being drawn up into the groove. I haven't fitted seals to the bottom of the ones I've replaced for this reason.

I do find that I get a film of water trapped between the bottom sash and the cill unfortunately. Maybe they fit together too well, ha, how ironic. To try to counter this the bottom edge of the sash gets the paint absolutely ladled on especially on the end grain of the stiles. I suppose I could open each a crack every few days to let them dry out, but...

For the OP, the staff bead profile I buy (which is also Scottish specific) has the bead channel offset slightly from the centre, so make sure you've got the wider gap towards the frame. I often find that again with out of square frames the gap between the side of the sash and the frame varies from quite wide to virtually touching (with the opposite run on the opposite side) so the extra couple of mill this offset gives you is important.