Energy efficiency and historic buildings

By - , Build 113

If you live in an historic home, any energy efficiency improvements need to be in keeping with the character of the house.

Figure 1: Insulation of suspended timber floor.
House in Paradise, near Glenorchy, Queenstown, in the process of refurbishment and insulation. Photos courtesy of Jackie Gillies and Associates.

Many New Zealand houses could be considered of historic importance, as each is an architectural expression of its era. Some are considered to be of national importance and are protected by statute. To retain their usability, these buildings need to serve the needs of modern living. Otherwise they are little more than elaborate museum pieces.

With buildings of historic value, care must be taken to balance the conservation of character with the need to address usability and energy efficiency. The health and comfort of our houses is increasingly in the spotlight, so looking at where heat loss is greatest and how easy it is to remedy is a good place to start.


Typically, around 30–35% of a home’s total heat loss is via the roof. Period houses usually have metal-clad roofs with a loft space, which makes retrofitting insulation much easier. Segment-type insulation can be laid between ceiling joists, but in cold regions, there is still a risk of pattern staining to the ceiling plaster along the lines of the joists. Laying a second layer perpendicular to the first, leaving no gaps, should reduce the risk of this happening.


Another 20–30% of total heat is lost via the windows. Single glazed timber windows are common in historic buildings. Modern aluminium replacement windows that may have been installed can be replaced with suitable style timber double glazed ones. In older windows, the frames/glazing bars are often inadequate to support double glazed units, and it is desirable to retain ornate glazing. If they are to be retained in tact, other solutions must be found. Secondary single or double glazing is an option, as long as frame depth is adequate to allow them to be installed. Ideally, these should mimic the original window sections to complement the original windows.

Some older buildings have internal shutters, which, if refurbished, can significantly improve the window’s R-value. If internal shutters can be introduced sympathetically, the option of insulating should be considered. Thick, floor-length thermal drapes are often in keeping with period buildings, and a pelmet assists in retaining a still layer of air between the fabric and glass.


The amount of heat lost through the walls is 18–25%. Walls are much trickier to insulate than roofs or windows, as both internal plasters/panelling and external claddings need to be retained in tact for conservation purposes.

House in Paradise, near Glenorchy, Queenstown, in the process of refurbishment and insulation. Photos courtesy of Jackie Gillies and Associates.
Figure 1: Insulation of suspended timber floor.

If claddings do need to be replaced because of damage or decay, the opportunity should be taken to fit insulation between the timber studs. Insulating interior walls can reduce noise between rooms plus help to reduce heat loss from warmer to cooler parts of the house.


Between 12–14% of total heat loss is via the floor. Period houses mostly have suspended timber floors, which can be easily insulated. If the subfloor space is accessible, insulation should be installed from below, to avoid disturbing historic flooring and skirtings. Either polystyrene or segment-type insulation could be installed to achieve a high R-value. A layer of ply fixed to the underside of the joists helps to protect the insulation as well as reduce air movement and thus heat losses (Figure 1). Once insulated, it is important that subfloor cross ventilation is retained to reduce the risk of moisture build up under the floor.

With a concrete floor, it may be possible to lay polystyrene insulation on top of the floor, between timber battens, and lay a new timber floor on top.


Draughts account for 6–9% of total heat loss of an uninsulated house. Old buildings are inherently draughty and need to breathe, but this can be controlled. Some obvious sources of draughts are windows, doors, floors and open fireplaces. Windows and doors should be draughtproofed by a specialist to ensure that they fit well. Gaps in floors can be stopped with ‘timber slithers’ or caulking. Open fires need to be converted to more efficient double burners, without jeopardising the historic character.


Improving insulation and reducing excess ventilation should always come first, before addressing space heating, but improved heating will be required to maintain a comfortable temperature throughout the house. Even so, heating old buildings can be difficult and expensive. High ceilings mean that space heating is especially challenging. Zoned, spot or whole-house heating should be considered, taking into account lifestyle and homeowner preference.

The main points to consider when improving the energy efficiency of historic buildings are:

  • insulate wherever practicable
  • address uncontrolled ventilation (i.e. draughts) early on
  • heat the whole house as efficiently as possible.

All this must be done within the context and philosophy of retaining historic character and fabric in tact, ensuring all improvements are reversible.

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Figure 1: Insulation of suspended timber floor.
House in Paradise, near Glenorchy, Queenstown, in the process of refurbishment and insulation. Photos courtesy of Jackie Gillies and Associates.