The example is an office building with an L-shaped plan. The building is five storeys in height and on a suburban site slightly overshadowed from the north. The location is southern Europe and the climate is as Zone 1, i.e. average temperatures for January and July less than 6oC and 24oC respectively.
The figure below shows the building with dimensions and areas of the passive zones as indicated. In designating the 6 m passive zones at the outside corners the zone most influenced by solar take precedence, i.e. south rather than west or east. Note the inside corner is designated a non-passive zone.
The top floor is all passive zone; let us assume that the centre of the plan is rooflit with a sidelit perimeter zone. There is a slight problem here because LT assumes zero net gains and losses through ceilings of sidelit spaces, but this will result in relatively small errors since heat transfer is dominated by the area of glazing. The areas are now summed in the rough working table, and then entered in the LT Worksheet.
First the appropriate curve for climate and building type has to be selected. The next step is to enter the glazing ratios for the facades. Let us start off with 40% all round - maybe we will change it later.
It is possible that the fenestration has already been designed as part of the initial concept. If working from a preconceived glazing pattern, then remember that the glazing ratio refers to the area of structural opening. A 20% obstruction due to framing and glazing bars is already allowed for in drawing the curves. Also, it is the ratio of this area to the total wall area, not just the area of opaque wall, and as seen from the outside of the building.
For the rooflights, the 'horizontal' total curve shows a minimum at about 15% - let us choose that. Note how rapidly the cooling curve (and the total) climbs, indicating how important it is to adopt small areas and to shade them. Note also that we are already using the curves to influence our choice here, whereas for the vertical glazing we are working from an initial proposal which might have originated from an aesthetic considerations.
If the top floor is lit with a rooflight configuration with glazing tilted to more than 45o to horizontal, use the vertical glazing curve of appropriate orientation. This is almost certainly the best bet for southern Europe. But here we will assume that for the initial proposal the rooflights are low-pitched 'shed' type with horizontal roof apertures.
Now, on the appropriate graphs the specific energy consumption for light and heat on the vertical axis is read off for the appropriate values of glazing ratio.
It is possible to estimate the third decimal place, i.e. divide the smallest division in 10 parts. The LT Curve reading aid is a great help here, or a pair of compasses or dividers can be used to measure off the curves and transfer to the vertical axis. The values are entered on the Worksheet in the second box of inputs, specific energy consumption for the appropriate orientations.
For the non-passive zone the value for zero% glazing ratio is entered - any sidelit curve - they are all the same for a given climate zone.
Before reading off the cooling energy, the application of shading has to be considered. Inspection of the cooling curves shows that there is significant saving of cooling energy use of shading on the south and east/west facades. We will choose Type A(1) shading, that is moveable, with the best (lower) transmission coefficient of 35%. Type A(1) has no effect on the lighting or heating energy.
Read off the appropriate curves and enter the values of energy consumption per m2 in the worksheet. Note that this value has three components:
The next step is simply multiplying the areas by the appropriate specific energy consumption values, summing across the page in the third box , at (8), to give totals for lighting, heating and cooling. In the summary box the totals and the specific energy consumption in kWh/m2 are entered. It is also interesting to enter the ratio of the area of passive zone to the total floor area to indicate the 'passivity' of the design.
This gives the first base case. The value of the total primary energy per square metre is shown in the summary box - 107 kWh/m2. This is a very low value, because the test case has adopted a low energy strategy - mimimising non-passive area, maximising use of daylight, shading and natural ventilation. The LT default values assume good insulation standards and efficient lighting.
The real value of the LT Method is to make comparisons. Many more options and conditions could be evaluated. For example we could show the effect of removing the shading, having the building fully air-conditioned, and (due to poor controls)using no daylight. (To evaluate the latter, read off the lighting curve at zero glazing ratio). The value comes out to be 233 kWh/m2, an increase of 118% .
Filling in the worksheets is quite a chore. To reduce the effort a little, it is useful to work on a sheet of tracing paper placed over the base case LT Worksheet, since unaltered data can be read through. Much better, of course is to set up the worksheet on a computer using a spreadsheet application. This computer method is available as an Excel (Microsoft) spreadsheet application which is available for both the PC and the MacIntosh.
Notes
The LT Curves are drawn from data generated from the LT Model as described previously.
Only the LT Curves relevant to this example are provided here. A full set of LT Curves specific to climate zones 1 and 2 are available in the LT Method manual 3.0 .
The Office Curves assume occupancy from 0800 to 1800 hrs. five days per week. The lighting datum of 300 lux is attained by only 12W/m2, which represents a fairly high efficacy lighting system. Internal gains of 15 W/m2 are modest, reflecting the trend in lower heat outputs from business machines.
