Insulated Window Board

I think the reality is that an insulator has a higher surface temperature despite 'being at the same temperature'

I've only ever routed one thing before, but it was easy and came out very nicely:

1. Get a router bit with a bearing on the top. I have this one in a 1/4" router:
   formatting link
   but it might be better to use a 1/2" router for speed (although you could totally do this in a 1/4", it'll just take longer).
2. Make up a template of where you don't want to route. In your case you might screw some battens to the underside of your cill to make a U-shape
3. Make sure you have dust extraction, as it'll make a lot of wood chippings, and ear/eye protection
4. Route several passes, using the top bearing to follow your template. Start shallow and gradually increase the depth as needed.

It sounds like at the very least you should put <something> under the cill board as a thermal break so you don't have a conductive path straight through to the stone. The multifoil suggested upthread won't meet its spec, but will be a thermal break and block radiation and moisture. Even the foiled bubble wrap will be better than nothing. (often can be found cheap in Aldi and Lidl at this time of year)

Sound similar to the conditions for cloud formation - you can't just have damp air, you need airborne dust to act as nucleation sites. The right kind of surface coating might reduce nucleation, but as soon as it gets any kind of dust or dirt on it you've lost the game (and once it's got started, mould growth will likely provide your nucleation).

Theo