Energy efficiency in historic buildings

As pressure grows to reduce CO2 emissions, so does the need for owners, designers, users and managers of traditionally built structures to improve energy efficiency and reduce fuel consumption. While the advice in this leaflet is intended for domestic properties, it applies to any traditional building. If a building is listed or is in a conservation area any work that may affect its character should be discussed with the local authority Development Control or Conservation Officer.

The embodied carbon
Most energy assessments of buildings are based on the consumption measured by your fuel tank, or gas or electricity meter. However, the embodied energy represented in the existing housing stock, in terms of carbon, has to be considered as well. This is the carbon that has been expended in putting the building up; it includes labour, construction materials, transport and other supply chain issues. The existing built environment contains significant amounts of such embodied energy and has been summarized in one report as “The embodied energy in the construction of a tenement is the equivalent of 1000 years of the energy requirement of its power and heating” (Edinburgh Standards for Sustainable Building). Keeping older properties in use is as important as some of the newer carbon reduction programmes. With appropriate modification, properly maintained traditionally built structures will last for hundreds of years and can play a full part in managing our carbon expenditure now and in the future.

Thermal performance of traditionally built houses
Older structures perform differently from a thermal point of view to modern timber frame houses. They are described as being thermally heavy, or having thermal mass. This is the ability of a building to absorb heat over a period of higher temperatures, and release it when ambient temperatures fall, and has affected building design and orientation since the earliest times. This property is not considered in current energy assessment tools which rely on assessing the amount of insulation, or the insulation property of the building components. This is measured by what is called “U-Value”, a number that expresses how quickly a certain amount of heat will pass through the material. Traditional buildings, due to their thermal mass, can stay cooler during periods of high temperatures. Infrared photography is often used to assess energy performance. It can determine where heat is being lost in a building and can show very small differences in temperature. These images need to be interpreted with care, but when used correctly can show very clearly where heat is being lost.

Heating regimes and equipment
Older structures need to be kept modestly warm over longer periods to prevent moisture absorbing materials from becoming damp. Where possible, keep the house at 10 – 12 °C if unoccupied, and 17 °C or so when in use. There are many heating options available, and the age of your boiler will have a significant bearing on fuel costs. Electric heating, while non-invasive to the fabric, can be expensive. Underfloor heating from a heat pump is an economic way of heating an older structure, if the intervention in the floors is appropriate.

Roofs and attics
25% of heat is lost through a typical roof, so suitable levels of loft insulation must be the basic starting point; at least 270 mm of insulation is required to be effective. There are many types available, from natural materials such as hemp fibers or wool to recycled products made from newsprint, and others made from glass and more modern materials. In most circumstances natural materials are preferable in traditional buildings, as they are better able to disperse moisture and prevent condensation. Insulation of attics and the underside of roof slopes and dormers is often problematic but, with care, and the right material, can be addressed.

Floors
Where old floor boards are in good condition, there is no reason from an economic or aesthetic point of view to lift them. Insulation of timber floors is probably only worth it on the ground floor if there is a reasonable crawl space from which to work. Laying insulation board on top of a timber floor will inhibit water vapour movement, and may give rise to rot and other forms of timber decay. Flagged floors should be left as they are, but there are considerable benefits in overlaying modern proprietary insulation sheets on a concrete floor. In tests carried out by Historic Scotland, the insulation value of the concrete floor was increased by a factor of 6.

External doors
External doors are generally thermally effective, as long as the door framework is around 45mm or so, but the boards in the panels are often thin, and their thermal performance can be improved. This can done by adding a layer of appropriate insulation material on the rear or indoor side. This maintains the character of the door from the outside, permitting modest intervention on the inside. Where possible, keep the finished insulation level flush with the door framework. Draught or weather stripping around the edge or the door and the letter box can also help. In the 19th century heavy curtains were mounted on rising rails to control draughts. Internal doors, unless there are significant heat differentials between rooms, should be left alone.

Windows
The use of thermally efficient glazing is probably the greatest area of conflict and opinion, yet there are many options for improvement. It has to be accepted that a single pane of glass has a low insulation value (the thickness of the glass makes little difference), with a U- value for most plate glass of about 5.2. Draft stripping the sashes can reduce air leakage by 80%, as well as getting full movement back into the window sashes, although it will not improve the U-value. Many companies provide this service, which combines the upgrading work with a general overhaul of the window and the sash cords. The table below, compiled from lab tests, gives a basic sequence of actions, and the consequent reduction in U-Value (from HS tests). While most traditional windows are of seasoned timber that responds well to repair, it sometimes happens that new sashes are needed. Should sashes require replacement, new ones can be made, to matching dimensions, but fitted with thin double-glazed panes within the existing astragal (glazing bar) patterns. While these will perform well from a thermal point of view, with a U value of 2.1 (from tests), consideration must be given to cost and payback times, given the cheaper benefits of the options in the table above.

Walls
An estimated 25% of heating energy is lost through walls. Performance of mass walls has been underestimated and past advice was to clad and insulate within where possible. Recent measurements by Historic Scotland have shown that their thermal performance can be reasonable, and a dry lime bonded wall can have a U value as low as 0.9, whereas most modern assessments assume a value of 2.5 for a mass wall. These measurements can better inform the amount of linings required in refurbishment projects, reducing loss of old material and detailing, and reducing the amount of insulation required. To ensure optimum performance, walls must be dry. Leaking downpipes, cement renders, incorrect paints inside and out can make a wall damp – and a damp wall will lose heat quicker than a dry one. Lath and plaster was generally used in the 18th-19th centuries, in preference to plastering direct onto the stone, showing that our predecessors understood the benefits given by wall linings. Lath and plaster, in good condition, will assist thermal performance. It may not fully reach modern standards, but its removal should be resisted, as cornice and other detailing can never really be replicated economically, and the replacement framing insulation and board itself has an embodied energy cost. Where no lath and plaster or cornice detailing remains, hemp board and other natural insulants may be used, as they transfer moisture better. Vapour barriers and waterproofing measures are not recommended as they will impair the performance of the walls, and act as condensation surfaces. Insulation may be applied externally to walls but the combined thickness of the render layer and the insulation on it usually means that existing detail and surface texture are lost. It is not normally appropriate except in unusual situations such as once-mutual gables or on buildings with very flat facades and generous roof eaves.

Building standards
Some types of work to existing buildings require compliance with Building Standards. However, traditionally-built structures, accepted as working in a different way to modern buildings, are given a degree of latitude in the guidance documents. Compliance where possible is recommended. For more details see the publication Conversion of Traditional Buildings: application of the Scottish Building Standards, which is a guidance document for purpose of the building regulations published by Historic Scotland and Scottish Building Standards Division.