Bathstone Bay Window advice

[pumerw]

We're in the middle of having the double glazing replaced. We've been advised that the insulating foam in the picture shouldn't be used to fill the gap entirely due to condensation/damp moving through the bathstone. Any advice if this is correct and whether we should fill the gap with a type of insulation so it's as warm as possible?
The old windows had wood screwed to the bathstone which has been removed. The new windows have a small void where the wood once was, the fitter when we suggested insulation was wary of too much insulation preventing the bathstone breathing.
We're after a second opinion, I've included an image of inside, outside after installation and one as old windows removed.

Thanks
Gareth

 

[Swwils]

Bath stone is porous, meaning it absorbs moisture. If this moisture can't escape, it can lead to damp and mould issues.

Your window fitter's caution around using insulating foam to fill the entire gap is justified, as this could create a vapour barrier, trapping moisture inside the stone and leading to damp problems over time.

However, you can still insulate the gap. You'd just need to use a material that is both insulating and breathable, like sheep wool insulation or some types of mineral wool. This will help keep the heat in, while still allowing the stone to 'breathe'.

Remember, double glazing itself is a very effective insulator. The trapped gas between the two panes of glass helps reduce heat loss. The key is to ensure that the windows are well-sealed, but still allow for the natural dissipation of moisture.

For the best advice tailored to your specific property, consider consulting with a building surveyor or conservation architect