LIMITATIONS AND APPLICABILITY

The LT method is a tool for strategic energy design and it is very important that its limitations and applicability are understood. The main points to bear in mind are as follows:

• The LT Method should not be regarded as a precision energy model, but should be used to test the relative performance of a number of design options. The precision of the actual number produced is far less important than the trends shown by comparison with other LT calculations.
• As a manual method, only a very limited number of design parameters are required for input. A much larger number of parameters have already been given assumed values, which in some cases already represent good practice in low energy design.
• The method predicts the potential performance of the building, assuming that both systems and occupants function optimally. This is in order to concentrate on the building design parameters.
• It follows that direct comparison of the LT calculated energy performance with a real building is not likely to be relevant, unless it is known that the assumptions made by LT accurately describe conditions in the real building.
• The output of the method is annual primary energy per square metre. Primary energy relates well to both cost and environmental impact. Building energy data and energy targets are often given in delivered energy units. This must be born in mind if compared to LT outputs.
• In using LT, certain best bets will emerge. For example, given a free hand, `shallow (depth-wise) plans' with about 35% glazing and no overshadowing will generally use the least energy. However, LT is useful to ascertain the performance of non-optimal designs where other constraints and design objectives apply.
• The LT method has not been rigorously validated in the strict quantitative sense, where predictions are compared with measured data. This is largely due to the factors outlined above. However, the use as an early design tool has already engendered confidence, in that it has not led to designs which informed opinion would regard as flawed in energy terms. Nevertheless, caution should be applied to the results. An LT analysis is not enough on its own to ensure good energy design. It forms only a part of a spectrum of analytical methods and prescriptive advice to which the designer should respond.