Floor area in Navigator different from block width x length

Question: I draw a block with dimensions 8 m x 6 m and a height 2.7 m. So the ground floor should have the area 8 x 6 = 48 m2. But 40.345 m2 is on the menu. The wall thickness is 0.087 m so the ground floor area calculated from inner dimensions is 45.6 m2. What does cause so a huge difference?

Answer: When you draw a block in DesignBuilder you specify the external dimensions. The thickness of the walls is taken from the value in the 'Wall thickness' Drawing option in the panel in the bottom left of the screen, which takes its default from the current external wall construction set on the Constructions Model data tab at the building level. The internal dimensions are defined by the external dimensions and the element thicknesses. The area data displayed in the navigator is the internal area - the data supplied to calculation engines.

If you use construction lines to measure the internal dimensions of the block, you will see that the dimensions are 7.43m x 5.43m, giving a floor area of 40.35m2 which is the zone floor area displayed in the navigator. The external dimensions of the block are 8m x 6m which means that the external perimeter wall is actually 0.285m thick.

The most likely explanation is that you created the block using an external wall construction having a thickness of 0.285m but then later changed the external wall construction to a much thinner one (0.087m).

In DesignBuilder version 1 you cannot change the thickness of block walls once they have been drawn. If you have a block you would like to draw that has very different thicknesses in one part relative to another part you should draw two blocks adjacent to each other, setting the appropriate wall thickness before drawing each block.

Article based on support request:

http://www.designbuilder.co.uk/component/option,com_forum/Itemid,1/page,viewtopic/p,899/#899

Note: This article refers to DesignBuilder V3 and previous versions. A new mechanism has been introduced in recent versions to deal with the construction thicknesses. For more information see Program Help Geometry Areas And Volumes chapter.