Solar Passive – Leaf Versus Rectilinear Planform – Which Works Best?

At Solarity we generate solar passive dwelling plans. These are almost always leaf-shaped. Why?

To  examine the differences between curved and rectangular planning we took the plans of our Leaf no 2 design and re planned it into its rectangular equivalent. Here is what we found:

The plans relate to a small dwelling of just two bedrooms. The accommodation of both follows solar passive standard practice of being wide, with the long side containing most glazing facing within 30 degrees of the solar direction. There are other aspects tailored to solar passive design – check other blogs on this website to read about these.

Both plans are single aspect and front entrance, and both have a reflecting pool drawn up against the building on its solar side. The pool has a couple of functions. It acts as a light shelf or reflector at ground level reflecting light into the building. However on hot summer days it acts to cool the house because breezes that pass over water ate slightly cooled. So, both plans are working to the same brief with the same accommodation schedule. After that, however, they are both very different. 

1. We weren’t able to get exactly the same accommodation in both of the plans because the geometries would not allow this – if they did they would be identical plans!

First is the rectilinear plan:

And below it the leaf-shaped plan:

2. Overall the leaf plan is more compact. We couldn’t get the same accommodation into a rectangular shape in the 61 sq. m. gross internal area of the leaf-shaped plan. The rectangular plan is 65 sq. m in gross internal area.

3. The elastic effect of bending the rectangular plan into a leaf shape has the effect of expanding the accommodation behind slightly – the bedrooms and shower room – and compressing the accommodation in front – the Lounge and Kitchen/Dining areas. So, the rectangular plan has larger Kitchen/Dining areas than the leaf, but somewhat smaller bedrooms.

The rectangular plan wins on this count because it is the living areas of a house that are the most used and therefore most useful.

4. There is no doubt that the rectangular form has a better Lounge/Kitchen layout, with more space generally and the useful returns down the  side walls giving more storage there. Moreover, the Kitchen itself being of rectangular geometry is bound to be cheaper to construct than the bespoke curved Kitchen units of the leaf.

5. The rectangular plan is crying out for East and West facing windows, and it has space for these. But we have omitted them as they are net heat loss windows unless triple glazed. 

6. The solar collecting wall fronting the main living areas is of greater area in the leaf-shaped plan. This is because it is stretched out in a bay window form relative to the flat face of the rectilinear plan. The rectangular plan is two 900mm floor-to ceiling panels narrower. This could be significant in terms of solar gain in some circumstances.

7. The curved geometry of the leaf shape bends the two bedrooms effectively to the front of the house where they form part of the front elevation. With the rectangular plan the bedrooms are to the rear, and their glazed doors would be overshadowed for much of the day by shadows from the main front accommodation which projects forward.

8. The ‘feeling’ of the plans are very different. The rectangular plan is logical, hierarchically clear and directs views out in a straight line, whereas the leaf-shaped plan appears to focus views in a wide arc, with the pool in front acting to unify the accommodation. The accommodation of the leaf is not hierarchical in the sense that the bedroom glazed doors are beside the Lounge/Kitchen doors, not behind them as in the rectangular plan.

9. The cooling pond: The pond doesn’t really work all that well in the rectangular version, as it has small offshoots going back to the bedroom ‘wing’ behind. These small offshoots are not big enough to give the scale and scope of reflections into the house that the unified pool of the leaf plan gives. For most people (who wouldn’t have a pond ) this may be irrelevant. But to us the pond is an important part of the natural  non-mechanical environmental controls built in to this design.

10. Characterful or Willful? There is a large difference in ‘excitement’ between the two plans. The rectangular version is clear, hierarchical and disciplined almost to a ‘Roman’ degree, whereas the leaf plan has something of the excitement of a boat about it, with its view in many directions, apparently undirected by straight walls. The question is whether a household values excitement above Kitchen and Living space!

11. Roof. Our solution to roofing the leaf shape is to go in with a nominally flat roof of glass-reinforced resin. If this is done for the rectilinear version too then they come out equal on this. But the rectangular plan has the advantage that it can also have a conventional pitched roof. This can be a deciding factor in some circumstances.

12. Design flexibility. The rectangular design wins clearly here, as each part of its plan can be manipulated separately from the rest. With the leaf plan there is an egg-like aspect that doesn’t allow one to alter one part without it affecting all the other parts. 

A useful exercise in learning the pluses and minuses of each plan type. We would be happy to deliver either. On balance our view is that the leaf shape narrowly gets our vote over the rectangular version. This is admittedly largely because of its excitement aspect, plus its presentation of all of the major rooms to the sun in a fairly democratic manner without the overshadowing aspect of the rectangular plan and its larger solar wall area. However, if larger living areas are preferred, or extension or size flexibility of individual elements are important then the rectangular plan is the best option. 

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