Energy Efficiency in Historic Buildings
Insulating solid walls
This guidance note provides advice on the principles, risks, materials and methods for insulating solid masonry walls. Traditional solid wall construction is probably the most difficult and in many cases the least cost effective building element to insulate. Whether applied externally or internally work of this nature will have a significant impact on the appearance of the building.
Traditional solid walled buildings are ‘breathing’ structures, in that they exchange moisture readily with the indoor and outdoor environment. In this way they keep dampness within the wall fabric below the levels at which decay can set in. Problems of dampness in a wall occur when it is subject to excessive soakings and/or its ability to evaporate moisture is inhibited by impervious materials, like cement based renders, gypsum plasters and waterproof paints.
It is vital to ensure that insulation is not applied to a damp wall, or to a wall with a history of damp problems that have not been conclusively eradicated. Adding insulation is very likely to make the damp problem worse, and have little or no thermal benefit.
Even on dry walls any insulation must be designed to allow the walls to continue to ‘breathe’, or else damp problems may start to occur. Cement based renders, foil backed insulations, glues and impervious paints can all trap moisture in walls. Fortunately there are good alternatives that are breathable.
One way to insulate a solid wall is to add insulation to the outside but external insulation can radically alter a building’s appearance. Most external insulation systems include an insulation layer fixed to the existing wall and a protective render or cladding. Even if it is already rendered, decorative architectural features such as cornicing, string courses and window surrounds will be affected. Even where the elevations are quite plain, simple alterations such as the deepening of window and door reveals and the alteration of eaves lines can markedly alter a building’s appearance. Planning permission may be required for external insulation whether or not the building is listed; the local planning authority should be consulted before work commences. For listed buildings, consent will be needed and would only be likely to be granted in very special circumstances.
The increased depth of an external render or insulation system will mean adapting existing detailing to the roof and wall junctions, around window and door openings and the repositioning of rainwater down-pipes. These alterations will need scaffolding and possibly a temporary roof to reduce the risk of water penetration during the work. Making these changes to the building will significantly add to the cost of the insulation.
A simple way of insulating flat, even walls internally is to fix insulating boards directly to the internal wall (or plaster) and then apply a finishing skim coat of plaster on the surface. An alternative internal insulation approach for less even walls is to fix timber battens to the wall and to pack the space between the battens tightly with insulation. The inner face can then be finished with plasterboard or more traditional alternatives, such as lath and plaster.
Thought must be given to areas that cannot be directly insulated: where internal walls and intermediate floors butt up against the outside walls. Here there is a risk of creating cold spots which will reduce the effectiveness of the work and could lead to condensation and mould. Insulation must be extended around the corners of party walls and other internal walls to reduce the risk of thermal bridging. Areas around ceiling joists have to be insulated as best as possible.
Internal wall insulation will mean that decorative features such as cornicing, picture rails, skirting boards, door architraves etc will either be concealed or have to be moved. If this is not done carefully it will damage the appearance and character of the rooms. Pipes, radiators, cables, sockets and switches will also have to be repositioned.
Once a solid wall is insulated on the inside, the wall itself will be colder than it was previously. This could slow the rate of evaporation from its surface, making it more prone to damp. In extreme circumstances it could also make the wall subject to damaging frost action. These are both reasons for care and continued monitoring once work it completed.
Another disadvantage of internal insulation is that masonry walls, particularly thicker ones, have useful thermal mass. That means that they need thermal energy to warm them, but once heated they store and slowly release a lot of energy. Under normal circumstances this helps to stabilise the internal environment, but adding internal insulation will lose this benefit.
Internal insulation will reduce the floor area of the rooms which could be significant in rooms that are already small.
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