Stone wall insulation

[joatex]

Three coats of lime plaster to exterior rubble walls walls 0.56M thick should reduce/seal against wind and rain driven into the wall but would the plaster (harling) also measurably improve the thermal resistance or would there be a need for additonal exterior insulation ? Is fixing insulation to the exterior wall common and successful practice, it would be easier to do so than fix insulation to the interior surfaces.

 

[Gervase]

It would improve the thermal resistance, but I don't think you will be able to find any accurate figures for U values which will satisfy a zealous BCO attempting to implement the new Part L guidelines.
External insulation is possible - if can be installed under corrugated iron or slate cladding - but would probably mean sacrificing the look of the building and, if listed, you will probably have difficulty getting consent for such an drastic addition.

 

[worms]

External timber cladding using local timber is also an option and if done in the right way on the right building, can still look pretty good. All sorts of issues though on how you alter the eaves and how you avoid cold bridging to the inside.

 

[joatex]

Gervase

probably mean sacrificing the look of the building

a few years ago I repointed the church walls, took a couple of summers. Tight under the eaves was a small section of hair reinforced lime plaster approx.10mm thick. I handled it gently in case of anthrax.

The original church was tumbling down 1760 and rebuilt so I presume that at that time the stonework was all-over lime plastered and limewater washed. I wondered if that was the norm for all stone buildings and a means of protection from wind and rain cooling together with a additional insulation.

 

[Gervase]

It's certainly common here in West Wales on the windward sides of buildings as a preventative to avoid penetrating damp. Traditionally houses here had large overhanging eaves, but the winds can be so strong as to force the rain against the walls under the eaves, hence the roughcast on vulnerable sides.
When I do roughcast I put plenty of hair in the first two coats, but that's really to give flexibility and strength rather than for any insulation value.
The three layers and dash coat probably amount to just over an inch of stuff on top of the stone, following the contours of the wall rather than ruled plumb and square as a render would be. There must be some insulation provided, admittedly, but I can't imagine it would be hugely significant - and certainly not a lot compared to the 'fill' in the rubble-stone wall.

 

[Nemesis]

Yes, render was common once - much of it removed by 'improvers' later.

 

[worms]

There must be some insulation provided, admittedly, but I can't imagine it would be hugely significant

Highly significant, though, to the overall insulating properties of the wall if it made the difference between the "fill" being wet or dry!

 

[Gervase]

Very true - that's a good point.