Ewi in stone cottage

MegamanBoo

I'm currently looking at the option of going for a small rural cottage 60-70sqm, with a view to extending it in a few years time.

In the meantime I'd like to retrofit, get it as warm and energy efficient as possible while keeping some of the character.

The one I'm looking at, and this seems to be typical of the location, is rendered externally with a sand/cement mix and plastered internally with what looks to me like a gypsum plaster.

I like the idea of stripping the outer walls back to stone internally, going ewi and adding underfloor heating.

With an mvhr system does this sound like a realistic proposal?

If I bring the insulation under the soffits and up under the joist will I get a decent airtightness or is that something that would be difficult with the outer walls exposed internally?

If I have to go with a breathable ewi does this bring the cost up significantly?

Also, would it better just to extend it now? If so, how would I approach matching a newer construction to the old, without loosing any continuity of insulation in the process. To me that seems safer with ewi than dry-lining.

I appreciate I will need a specialist to evaluate fully and oversee the work, when the time comes, I just want to see if I'm broadly on the right track.

MicktheMan

Some things to consider when dealing with a stonewalled building.
First wrt the ewi; it's best practice to continue the ewi down to the foundations to help address the thermal bridge at the base of the wall. Many old stone walled buildings don't have foundations so you're limited here and I don't have a solution.
Breathable ewi, for instance rockwool batts, does not afaik increase costs but am open to correction on this.
On your air tightness question, insulation is separate to air tightness and, contrary to what you may be told (mainly by sales folk) ewi does not materially improve airtightness. You address airtightness generally from inside, not outside.
If you get your air tightness done well, them mvhr is an excellent solution for ventilation especially when you take into account the probable higher moisture load from the walls.

Calahonda52

I agree with MtM so what follows is just some other stuff to think about

• How thick are the walls?
• is there a hard path all the way around the cottage?
• how exposed in the cottage?
• Is there a dpc in the walls?
• (for UFH you will add a dpc so thats sorted)
• The internal gypsum would bother me as its does allow moisture to pass through it so good it is coming off.
• Are you putting in new windows/doors?

I would think about planning the extension now and if you are knocking out an ope how will that fit with ewi?
What I have seen done is building a thermal broken connection between the old and the new, both with full EWI as well as the connection .
Works better than trying to link new walls with old walls etc etc
The other consideration is to make the perimeter insulation in the floor maybe 50 mm then put calcitherm or a lime cork plaster on the inside with a breathable natural paint

I would do this and leave the outside alone, except for a breathable water repellant application