Principles of Energy Efficient House Design. The Comparison of Two Dwelling Concepts

Executive Summary

This report compares the thermal performances of the dwellings of type A and type B in calculating the heat loss parameter of each dwelling. The dwelling type B as a semi-detached house performs better. The energy efficiency is considered further in both houses and shows that changes of the heating system (space and water) are necessary. Further decreases in energy demand can be achieved in changing the construction of the dwellings to timber frame as it ensures the same U-value but is less thick. However, the improved constructions of the detached type A do not achieve a similar good performance as the semi-detached dwellings of type B.

Introduction

In times of climate change and scarcity of resources the importance of energy efficient building design has increased significantly and customers demand new solution to get independent of rising fuel prices and to save energy. Therefore, energy efficient design has to comprise more than just insulation of the envelope of dwellings. Whilst the insulation still is one of the most important mean to stop the heat loss of the fabric, the concepts of heating, lighting and internal appliances have gained a great importance in energy efficient design. Therefore this report throws light upon two dwellings of different types. The thermal performances are calculated and compared. Moreover this report provides some suggestions to increase the buildings' efficiency further by considering the heating systems or the construction modes.

Comparison of House Types

As an indicator for the dwellings' thermal performance the heat loss per °C and m2 has been chosen, because it takes into account that the two houses have different floor areas (constructions details in appendices A).

Following assumptions have been made for the calculations (details in appendices B):

- The air-change-rate was set to 0.75 ach-1. This sounds reasonable because the houses are situated in a sheltered urban site and the windows and doors are sealed tightly. However, the installed extractor fan and extractor hood cause a ventilation effect that is considered in this assumption. The air-change-rate has a great influence to the total heat loss (an air-change­rate of 1.00 ach-1 effects an increasing total heat loss by 13%).
- The geometry of the roof of dwelling type B is assumed and illustrated in appendices A. These appendices include all dimensions that were used in the calculations. The influence of the roof area is marginal when the total heat loss is considered. Changing the roof geometry would not cause great differences in the total heat loss of the dwelling type B.
- Other assumptions can be find in the appendices.

For both dwellings the approximate heat loss per °C is about 127 [W/K]. However, as the dwelling of type B has a greater total floor area, the heat loss per °C and m2 (heat loss parameter; HLP) of the type A dwelling is higher than the one of the type B dwelling. Whereas type A consumes 1.64 [W/mCK] the type B dwelling only needs 1.26 [W/mc;]. This is a difference of 23%.

This difference can be explained by looking at the geometry of the two dwellings. The dwelling of type A is built in a detached way. Therefore its ratio of the exposed surface to the total volume is higher (1.10 [m2/m3]). The dwelling of type B shares - as a semi-detached house - one of its walls with the second semi-detached house. This shared wall has approximately the same temperature on both sides and does not contribute to heat transfers. Consequently its exposed area to volume ratio is lower (0.82 [m2/m3]).

Another reason for the better performance of the type B dwelling is the window to floor ratio. As the thermal conductivity of windows is greater than the conductivity of walls, windows cause a greater heat loss. The window to floor ratio of type B is lower (0.12 [m2/m2]) than the one of dwelling A (0.18 [m2/m2]).

Improving Energy Efficiency

In order to save energy in buildings it is important to know where the vast part of energy is consumed within the buildings. According to the Sankey diagram (shown in Appendix C) this part is the energy demand of the space heating (39% of the primary energy). Consequently this is the reasonable start for an improvement of energy efficiency. Other uses such as lighting, water heating or cooking demand up to 30% of the primary energy and therefore are also considerable.

The presently installed heating system reveals some improvable aspects. As the boiler is a critical element of the space heating system great savings of energy can be achieved if the used gas fired low thermal capacity boiler is replaced. It has just an efficiency of 72% (DETR, 1998). An improved efficiency can be gained by installing a condensing boiler. The efficiency of these boilers is close to 85% (DETR, 1998). Another improvement can be achieved when the control-system of the space heating system is modified. The disadvantage of the central timer and thermostats is that these installations do not take into account the different demands of different rooms in the building. Therefore a decentralised control-system with thermostats in each room and thermostatic radiator valves (already installed) at each radiator is more efficient. This system allows a tailored heat supply to each room and therefore avoids an overheating of bedrooms or halls.

Another great part of energy is consumed to heat the water for the taps. In order to reduce this demand the water heating system also has to be changed. The separate hot water circuit has to be replaced by a system that uses the boiler of the space heating system to heat the water. This can be achieved if the recent low capacity boiler is not only replaced by a condensing but by a condensing- combi boiler. This boiler heats the water for space heating and is able to supply hot water on demand if needed.

Having improved the efficiency of the most energy demanding consumers the appliances used in the house can contribute to better efficiency as well. Using gas for hot and oven would reduce energy consumption, because grid-electricity has just an efficiency of 30%. Moreover it is of vital importance to pay attention to the energy efficiency classes of the installed electrical appliances. Energy-saving light bulbs are an easy way to increase efficiency significantly. In order to achieve a better lighting efficiency the concept of passive solar design is considerable. This would also reduce the heating demand as solar gains are available for heating. Another idea might be the connection of the dishwasher and the washing machine to the hot water system of the house to avoid energy wastage when these appliances heat water by themselves.

Although these house types do not possess a mechanical ventilation system, it is considerable to think about a system with heat recovery, when installed, to avoid vast amounts of cold air flowing into the buildings! Cold air infiltration can furthermore be avoided when the porch of type B is closed with a second door so that air infiltration is minimized when persons are entering or leaving the building.

Alternative Construction Modes

The envelope of building type A is constructed as a cavity wall. Its thermal performance meets the requirements of the buildings regulations (U = 0.35 [W/m2K]). However, thermal performance can be enhanced with alternative constructions with better U-values. Therefore two alternative ways of construction are shown in this report.

An enhanced U-Value of 0.25 [W/mCK] is demonstrated by two constructions in appendix D. Whereas the masonry construction has a width of 510 [mm], the timber frame wall just needs 300 [mm]. As both constructions have the same U-value the fabric heat loss (Tabl.E1_1) is the same. However, as the timber frame comprises less volume and therefore creates more inner volume of the building, the ventilation heat loss increases and the HLP - as the overall heat loss is divided by a greater area - decreases.

A comparing calculation to evaluate the benefit of this U-value improvement is shown in appendices E. Although these constructions are improved and timber frame decrease heat demand by over 10% they do not reach the performance of the semi-detached dwelling type B.

Moreover timber frame can be pre-constructed in factories and joint at the site. As an effect the air­tightness of timber frame constructions is better than the one of on-site constructed timber free walls and this reduces the heat loss due to infiltration.

In consideration of these facts timber free constructions do not perform at best when used as external walls. But a house might be constructed with a mixture of timber free and timber frame constructions. Timber frame constructions perform well when used at external walls, as shown above. But they do not provide any thermal mass which is needed in summer to keep the interior cold (saves energy because air condition energy can be reduced). Therefore timber free constructions have advantages in the internal structure of buildings.

Besides the walls, the windows contribute a lot to the fabric heat loss of a building. Therefore an improved U-value needs to be accompanied by a better thermal performance of the installed windows. To improve this performance windows can include up to three single leaves of glass. B etween the glasses heavy gas is captured to avoid convection heat transfer within the construction. Moreover a thin coating of metals can reduce the emission heat loss. These windows achieve U-values of under 2.0 [W/m2K] (Anderson, 1999).

The improvement of walls and windows seem to be most reasonable as they contribute the vast part to the overall fabric heat loss. Up to 80% of the fabric heat loss is due to walls and windows as the calculation in appendices A shows (walls contribute 50%, windows 30%). In comparison to this the heat loss through the roof or the final ceiling is marginal (<10%).

Observations

A more general idea of reducing the energy demand of dwellings is the concept of passive solar design. In orientating the house to the south and increasing the window area on the south side, solar energy can be captured to reduce heating demands in winter. Moreover the daylight entering through the greater window area reduces the energy demand of lighting. The recent window to floor ratio of the dwellings is far to low to capture enough solar gains (0.18 [m2/m2] for type A and 0.12 [m2/m2] for type B). The recommended window to floor ratio is about 0.23 [mc/mc] with 75% of the window facing the south.

In achieving higher efficiency it is also considerable to replace the grid electricity used in the house by electricity produced by a green tariff, own solar panels or geothermal sources.

Overall energy efficient houses can contribute to the company’s image and advertisement can attract a great number of new costumers. And higher investments can be argued by less running costs during the occupancy.

Moreover the dependence on increasingly high-priced energies will be reduced.

Conclusion and Recommendations

As the report shows the timber frame construction helps to decrease heat demand and therefore energy usage. However, it does not contribute that much that the disadvantages of type As shape are compensated. Consequently the best way to improve dwellings A and B is to build timber frame semi­detached houses.

Besides the structure of the houses the heating system is of vital importance if energy efficiency is considered. The present system including the boiler and control system has to be replaced. Another focus must be taken on the appliances with in the building. Lighting and cooking for example demand also a lot of energy. But at this point it has to be acknowledged that the behaviour of the occupants also contributes to the energy efficiency.

Although the implementation of the recommendations increases the building costs, the running costs will decrease and so will the dependence on resource prices.

These aspects are great advantages in attracting new customers and improving the developer’s image and therefore a worth considering.

References

- Anderson, B.R. (1999) Review of Part L of the Building Regulations. Glasgow: Scottish Laboratory (online) available: http://www.odpm.gov.uk/stellnt/groups/odpm_buildreg/documents/page/odpm_breg_600361 .p df [Accessed 17/10/2005]
- Building Regulations (online) available: http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/documents/sectionhomepage/odpm_b uildreg_page.hcsp [Accessed 17/10/2005]
- DETR, 1998
- McMullan, R. (2002) Environmental Science in Building. Hampshire: Palgrave
- Yannas S. (1994) Solar Energy and House Desig. London: E.G. B ond Ltd

Appendix A1 dimensions of dwelling type A (assumptions for the calculation)

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