Exterior / Interior Farmhouse Stonewalls - insulation

[DavidKilburn]

All,  I'm starting to renovate a farmhouse in the UK.  The external walls are solid stone - no cavity.  The downstairs walls are also solid, upstairs are lathe and plaster, as are all ceilings.  No damp as such, but it's been empty a while, and just cold.

There is absolutely no chance of externally insulating the property, so internally insulating with stud boxes in each rooms, well insulated with PIR, vapour barriers, non-ventilated air and service voids etc. etc., and then a PB layer with a coat of plaster on top; kind of like a box within a box approach.  I know I'll lose room space / dimensions, but it will be worth it for a warm insulated house.

 

Question is, all the external walls, and the internals downstairs which are stone, have a thick cement/plaster (could be lime but I'm not sure) skim on them, of varying quality.  Some is sound, other parts crumbling away and very dry.  Do I strip the lot off, and go back to stone and start over - allowing the stone to breathe, or does it matter if I just build my box in a box and leave the good skim where it is on the stone work and stud over it?  Obviously I'll remove the loose / crumbling stuff.   My thinking is take the lot off, but that's a big job, not to mention filthy work, but I think it will be worth it - if it's necessary.  It must weigh a tonne too!#

 

Thoughts?

[saveasteading]

Welcome. 

Yes knock off the loose stuff. When it gets silly hard work, you can assume that it is well enough stuck for a few decades more.

 

If it was to crumble off, then it would fill your gap inside the masonry and might cause a damp issue so you're right to take this seriously.

 

It takes days per m2 to remove hard and sound render, and then you have exposed soft mortar that may need repair.

 

It seems to me you've got this thought out, but keep asking.

 

What is the wall makeup? what stone, dressed or rough, 600 thick?  how deep are your footings? Beware undermining.

 

It surprised me to find that a 3 layer stone wall had a worthwhile u-value. In addition to that, a stone like granite will shed the rain quickly (unlike brick) so the wind isn't sucking the heat out of the walls.

 

The surprise  downside we found was just how much internal space is lost due to the roughness of the internal walls. Making a line for your internal box is dependent on the furthest-in stones in both length and height.

[DavidKilburn]

@saveasteading Thanks for the swift response.    I take your point re - if it gets hard to chip off, it's probably sound - so I'll leave where it is!  Not sure about the stone - in the final throws of legal negotiations before getting the keys.  From the looks of things it's dressed, and well pointed (not sure what with).  The external walls are hefty, certainly between 300-500mm deep and sound.  Footings I'm not sure about - survey will tell is that hopefully.  The place is at least 250 years old - the attached barn dating back 400 years maybe.  Not a crack in sight, the windows and door apertures are square and true (well as good as they were in the 1800s), no cracked glass, and the sash windows still run smooth.

The property is on the Lancashire/Yorkshire border, so if its a vernacular construction it will be either Yorkshire sandstone or possibly a grey limestone, but not a limestone you'd find say in the Lakes.  My money is on a hard yorkshire sandstone - it's not grey like a granite, more a yellow / pain colour.  It's in an exposed location though and will get plenty of weather - rain and wind.  Again, I think we're prepared for the trade off in room size, but we'll have to see just how much we lose.  A warm slightly smaller room has got to trump a slightly larger cold one right?

[saveasteading]

16 minutes ago, DavidKilburn said:

A warm slightly smaller room has got to trump a slightly larger cold one right?

Absolutely.

We were concerned how narrow it might feel, starting at under 5m width in some places, but it is fine. Avoiding corridors helped a lot.

 

The best advice I got early (ie something I hadn't thought about) was from an experienced renovating builder.....it was about the constraints of existing lintels and roof heights, when considered against the footing depths, assuming you want a new, insulated floor.. Sometimes there isn't enough height, if you can't dig without destabilising  the walls.

 

And one more fundamental....a services void in all the walls.

Edited August 31, 2023 by saveasteading

[AlSmith]

Just finished renovating a 140 yr old farmhouse with similar stone walls. We went for breathability throughout, cosy wool, membrane and lime plaster (internally) for  all external walls.  UFH via a ground source heat pump and very pleased with outcome. Decent heating bills and internal humidity is 40% average. Thought about tanking the house but very glad we let it breathe again after many years. 

[Mike]

On 31/08/2023 at 16:29, DavidKilburn said:

a thick cement/plaster (could be lime but I'm not sure) skim on them, of varying quality.  Some is sound, other parts crumbling away and very dry.  Do I strip the lot off, and go back to stone and start over

If it's cement-based, I'd remove at least the lower 1m to promote moisture evaporation. I'd also remove any gypsum finish anywhere. And anything loose, crazed or hollow sounding.