Plastering Over Granite

[schiehallion]

Hi all.
I am renovating an old granite property to live in. it was neglected for some years and was badly damaged with damp. I have had the roof re done and outside re pointed and am now in the middle of removing the old plaster and lath internal wall/ ceiling coverings. What i was wondering is should i re point the internal granite walls, or would it be easier to either fill the gaps between the blocks with expanding foam or just build a new platerboard wall over the granite and fill the cavity with expanding foam.
Cheers in advance,
John.

[The Builder]

For a solid stone wall, it really depends on how attractive the stonework is. Perhaps leave a few small areas exposed for "features". The problem is that the wall will be uninsulated. Quite likely to attract condensation as well as the obvious heat losses.

For any exposed areas, re-point with a lime mix. That will allow the masonry to breathe, thus alleviating a lot of the condensation problem.

I usually tackle the solid masonry by setting an insulated timber stud wall 50mm away from the inner face of the masonry.

A clear cavity will prevent moisture transfer from the cold stone to the warm studwork.

[KARL]

Regarding stone wall and cavity versus insulation, just to clarify/explain and prevent misunderstanding:

Cold stone will cool down any warmer air coming close to it and bring that air closer to dew point when it will shed/dump its moisture onto the stone's surface (condensation), just like happens with a container fresh out of the refrigerator. If the air in contact with the stone is already at more or less the same temperature then nothing much is going to happen unless the air is at that point already at its dew point (as for example when fog brushes up against outside stonework).

If insulation is applied to the stone and completely covers it then any warm air going past the insulation will hardly cool as a result of where it is - little or no likelihood of condensation.

If you have a free cavity between the stone and a plasterboard skin separated from the stone wall then there is a possibility that condensation will form on the stone and, at the contact points, wet the studs. In this case use treated wood in the studs and hope that the dampness will not significantly travel to the plasterboard via the studs. The more warm air that flows through the cavity the higher the risk of condensation, and if that warm air comes from within the heated dwelling then the heat loss is all the greater. If the air is cold air from outside the dwelling (central cooling) then the heat loss will be truly significant but caused slightly differently because the plasterboard skin will be continually cooled down and is itself likely to give rise to risk of condensation (on the wall surface inside the house, paint, wallpaper or tiles)

Thus, if you create a cavity behind a plasterboard skin set away from the stone wall AND fill that cavity with insulation then there is technically a diminishing likelihood of condensation because there will be very little air movement within the cavity and therefore no significant migration of moisture into it in order for its moisture to condense. This depends on the insulating material itself not collecting/conducting moisture to any significant degree and the void being thoroughly filled with insulation and no significant air pockets/"holidays" present. The whole thing will be most challenged when temperature differences inside and outside the dwelling are greatest (freezing outside, heating working correctly).

[The Builder]

Good post from Karl, as always.

With a clear cavity behind the timber studwork.......... tack a thin polythene vapour check onto the cavity side of the studwork to prevent any possibility of moisture entering the stud.

Filling the cavity with foam ......... fine .... a good vapour check in itself. Also, extra insulation.