Renovating a stone walled house

Carnmore

The house is single storey and circa 100 years old with solid stone and lime mortar unplastered walls about 450mm thick.

It consists of just two existing external walls (40m2) , a party wall, an extension to be rebuilt at the back and a roof to be renovated (slates removed, timbers repaired where necessary, felted, new roof battens and reslated using original slates.

The finished floor has been poured - rigid insulation board and cement.

For insulating the external walls, my architect and builder are recommending installing a steel stud wall to create an air gap and then dry lining with insulated plasterboard.

I'm on a budget and the cheapest method is insulated plasterboard as described above which would cost about €1k. Despite this cost and advice, I'm concerned about the breathability of the walls.

I would also prefer to use non-toxic materials where possible.

Would Calsitherm or Gutex be a better option or is insulated plasterboard okay?

The cost of Calsitherm including a lime scratch coat would be circa €10k..

Thanks in advance for any advice re this or in general.

Doozergirl

It's the air gap that is important. The walls are beathing because there's nothing touching them. 450mm solid walls aren't going to be going anywhere fast and they provide thermal mass themselves.

Do you really need to insulate those walls anyway? Just leaving them would probably be the cheaper, more attractive, least invasive option.

Camster

I did some internal insulation on my house last year. The house is over 100 years old and I found some useful information on the historic Scotland website.

The have a number of case studies of various projects, which might be helpful to look at:

https://www.historicenvironment.scot/about-us/what-we-do/conservation/refurbishment-case-studies/

the_lunatic_is_in_my_head

We insulated our external stone walls by having an air gap, Thermafleece insulation, stud wall lined with wood wall boards finished with lime and breathable paint.

Our walls have an inner and outer wall filled with rubble inbetween and aren't good at holding in heat, the above made a big difference to keeping the rooms warm.

If the existing unplastered walls have any gaps in them I'd recommend filling them (or a scratch coat of lime over the whole wall) to make sure mice can't find their way through the walls and set up camp in your lovely warm insulation.

There are other types of natural insulation such as hemp or wood fiber boards (like the Gutex) or thermal renders (like Bauwer). I've not heard of Calsitherm before.

At some point one wall had 2by1 attached with a plastic sheet and plasterboard, the result was dry rot and I don't think it's wise to use modern non-breathable material on external stone walls. There are loads of such houses here and builders are filling them with cement, plasterboard and Celotex boards, all trapping moisture.

All that said 10K sounds like a heck of a lot of money for 40m2, do you have a breakdown of costs for materials and labour? I'd say you can do breathable at much lower cost.