New buildings
As a building material, locally sourced (or possibly newly quarried), reclaimed stone may be a viable option but suffers from the drawback that stone itself has virtually no insulating value.
This can obviously be overcome by including sufficient insulation within the rest of the wall construction but it does tend to lead to a very thick wall with all that that implies for reduced floor areas. This is particularly true if the stones are thick or not evenly dressed. A wall thickness could easily be over 600mm. This has implications for such things as the available floor area the width of foundations and how much light and view is available through windows.
Using a lime mortar reduces the amount of embodied energy compared with a cement lime mortar.
Existing buildings
If you are converting or repairing an existing stone building then there are several options for the insulation
1. apply insulation externally and use the mass of the stone for thermal stability (this is not often an option because the planners will usually want to see the external stone appearance left as it is.
2. Insert a timber frame inside the stone walls and install insulation within its thickness.
3. Spray polyurethane foam on the inside of the stone walls. This not only provides insulation but also seals up the small gaps between stones and mortar which can cause air infiltration. The foam can then be plastered or dry lined. Poyurethane foam has a good insulation U value of 0.022W/m²/ºC. See table
In any case you will need to apply 20 – 30 cm. of insulation to make a serious impact on heat losses.
Which course to take depends on several factors
- with an old building such as a barn where there may be a minimum of internal walls or structure (in other words a large empty space) you will probably need to insert an internal timber frame anyway. This is option 2 (above).
- if external walls are in poor condition they may benefit from extra support from insertion of a timber frame. A structural engineer will advise on this. Option 2 again.
- with a building which has lots of existing solid internal walls such as a house it makes more sense to go for option 1 if planning allows because of the better insulation achieved.
- if the stonework is rough, irregular and prone to air infiltration and possibly driving rain then option 3 will tend to seal and level out the surface. If the internal surface is very irregular, often the case with barns built of cheap undressed local stone, you get a thicker average layer of insulation with foam without the total wall thickness increasing.
Arguably the down side of using petrochemically produced foam is outweighed by its higher insulation value and the increase in space within the building. Furthermore, stone buildings tend to be in areas where the life of buildings is likely to be greater than elsewhere, mainly for aesthetic or conservation reasons, thereby diluting the detrimental effects of a one-off use of the foam.
Insulators such as paper, cork and wool should not be placed in contact with external surfaces such as stone which may be damp.