Author Geoffrey C Pitts B.Arch (Hons) M.Phil and advisory editor TRADA Technology's Robin Lancashire offer an extract from TRADA Technology's new best practice guide, Low energy timber frame buildings: designing for high performance.
Energy efficiency in buildings comes at a cost. Energy is another word for fuel - most commonly electricity, gas, oil or solid (coal) fuel. All fuel costs money in some form and the more fuel that can be saved with energy efficient buildings, the lower the running costs for building users and owners.
It is the building envelope that utilises and/or controls the natural elements to assist in providing economic thermal comfort. For cool temperate climates some level of additional space heating will be required but, in most cases mechanical cooling will not be necessary.
The building envelope decisions which affect the 'economic comfort' requirement are the thermal resistance, thermal mass and thermal response of the roof, walls and floor and the thermal performance, natural lighting and ventilation characteristics of the windows and doors. These decisions are the major influences on the overall level of building heat loss, on the amount of solar energy collected and on the usefulness of this energy, i.e. on the space heating requirement. They also influence the level of direct power consumption for lighting and appliances.
And it is in the building envelope where the particular energy saving advantages of timber frame construction can be found. Frequently, the decision to build in timber frame is taken for reasons other than energy efficiency; for example, speed of erection, reduced site labour, dry construction, precision and accuracy of finished building or even cost. However, as energy efficiency continually becomes more and more important to clients and designers, this will become one of the major reasons for the choice of timber frame simply because the system has the ability to incorporate very high levels of thermal insulation.
Adding a convenient 50mm service zone to a conventional 140mm stud wall increases the insulation depth to 190mm
Thermal insulation is not the only energy criterion, however, and a view which is often expressed is that the low thermal mass of a timber frame fabric is such that solar gain cannot be effectively utilised. However, high thermal mass is not necessarily beneficial; but rather, the important factor is the provision of the right level of thermal mass in the right place to utilise solar gain. This can be achieved in timber frame without compromising the structural integrity of the system.
Timber frame houses can be easily adapted to ensure thermal mass is in the right place to utilize solar gain
The climate type under consideration can be broadly described as cool temperate, where winter heat loss is the major energy efficiency design criterion. Within the general cool temperate climate, a number of variations occur. In terms of macro-climate there are the cold dry winters and hot dry summers typical of inland continental areas (north central USA, central Canada and inland northern and central Europe) and cool wet winters with moderate, humid summers typical of maritime regions (UK, Ireland, Denmark, Netherlands, Belgium and Atlantic coastal France). In terms of micro-climate, there are variations caused by local topography and vegetation or existing buildings. All these variations must be considered in the design to reduce energy consumption.
The external climatic conditions which influence energy consumption are the daily and average seasonal temperature (winter and/or summer), the daily and seasonal solar radiation, the winter and summer wind (speed and prevailing direction) and, to a lesser extent, the precipitation.
Internally, air temperature is the major internal environmental factor which affects energy consumption as the building heat loss is directly proportional to the inside/outside temperature difference. While the internal comfort air temperature may vary between 15°C and 30°C, this will normally be between 18°C to 22°C during the heating season. There are four key indices which determine the comfort of occupants in buildings. These are described in the table below.
|Description and Criteria
|Occupants require a feeling of warmth for comfort. The usual index for describing this is the dry resultant temperature (tres) which is a combination of the air temperature (ti) and the average radiant temperature (tr) of the room surfaces. tres approximates to ½ x ti + ½ x tr. In most buildings the average dry resultant temperature is normally between 18°C and 22°C.
|Air movement can cause draughts and if excessive, may require an increase in tres to maintain comfort. In general the air movement in winter should not exceed 0. 1m/s.
|Occupants require the introduction of fresh air into rooms primarily for healthy breathing. Ventilation is also required to reduce odour levels, to minimize condensation risk and in some instances to provide combustion air. Ventilation should not be excessive as this will increase heat loss and so increase energy consumption. In most buildings ventilation which provides 0.5 air changes per hour for the whole building provides a healthy environment without excessive heat loss.
|Occupants can normally remain comfortable over a relative humidity range from 40 to 70 per cent at temperatures between 18°C and 22°C.
Site planning decisions are taken early in the design process and have a significant effect on overall energy efficiency. Orientation is closely linked to all other site planning decisions and is therefore considered in each of the four site planning sections.
One of the particular benefits of energy-efficient planning is that there is little or no cost penalty associated with the decisions, while the potential energy benefits can be high. For a given residential building density there are no additional costs for efficient layout and southerly orientation, and landscaping costs are typically low in relation to overall development costs.
The cost of positioning buildings to maximize solar gain is usually less then the long-term benefit
The energy considerations for site layout are orientation, minimum over-shading and wind protection. Other site considerations are view, privacy, minimum roadworks (length, width and specification) and minimal service runs and the relationship of landscaping and gardens to the buildings. While these considerations can affect the energy decisions, they would not normally be expected to compromise them. The target site density however might be expected to affect energy efficiency decisions adversely, but at even high density (say up to 200 persons per hectare) good orientation with minimum over-shading for passive solar gain can be achieved with careful layout and grouping. There is another issue associated with site density: high density implies a greater number of shared party elements which reduces fabric heat loss which might offset reduced passive solar gains.
Designers need to consider all of these potentially conflicting issues for optimum energy efficiency, but with buildings responsible for up to half the total national energy consumption in industrialised countries, surely that extra work at the drawing board is more than justified.