## General shape of an energy efficient building (passive house)

The shape of a passive or a low energy house should be kept simple and should follow the rule that the area of the building envelope should be as small as possible. Theoretically, an ideal building would have a shape of a sphere as of all the objects having a given volume, the sphere is the one with the smallest surface area. In reallity, building a sphere shaped house is neither practical nor possible. Nevertheless, the building should still be kept as compact as possible. To check the optimality of a certain house shape we calculate a Area to volume ratio - the rate between house envelope area (A) and house heated volume (V). More energy efficient houses have lower ratios. Interestingly, more thermaly efficient building shapes in most cases also mean lower construction expenses, as there is less wall/roof/foundation for the same space.

Tip: Simple-compact architecture is IMPORTANT! Various unneeded details often also result in additional heat bridges and larger energy consumption. Here is an example of an energy efficient passive house. If you want to see how it was built check out the House blog.
The following table shows various building shapes having the same volume but different area of its outer shell. The volume in all cases is 400 m3.

 Shape Surface area Area to volume ratio(A/V) Drawing Comment Sphere 263 m2 0,66 m-1 Just for comparison. Although sphere has an ideal shape factor it is not a practical shape for a building (at least not for a single family house). Cylinder 301 m2 0,75 m-1 Cylinder is still quite an unpractical shape for a house (imagine square furniture fitting rounded walls). However, houses that are close to the shape of a cylinder are not uncommon (e.g. octogonal house). Cube 326 m2 0,81 m-1 Cube is much more practical shape for a house and still retains an extraordinary area to volume ratio. It is an almost ideal shape for a low energy house. Cuboid(11.1 m x 6 m x 6 m) 339 m2 0,85 m-1 Interestingly, cuboids that are not too flat or too narrow still retain very good area to volume ratio.  This makes them suitable for low energy houses. However, you should avoid too flat or too narrow cuboids. Flat cuboid (16.6 m x 8 m x 3 m) 414 m2 1,04 m-1 Flat cuboid is a shape of a simple singlestorey house. Its area to volume ratio is over 1 and is not an appropriate shape for a low energy house. L - shaped house(13 m × 6 m + 9.2 m x 6 m) x 3 m 435 m2 1,09 m-1 L shaped house has an even worse area to volume ration than flat cuboid. Its surface area is 109 m2 larger than the area of the cube. There are also additional problems, as one part of the building throws shade on another part. U - shaped house(...) 456 m2 1,14 m-1 U shaped object is completely unsuitable for a low energy building. Not only due to a large surface area, but also due to shade related problems.

## Room placement

It is good idea to place rooms that are normally occupied during the day like living room, rooms for children, kitchen, dining room, etc. in the south part of the house. Sleeping room, bath, WC, utility etc. can be placed in the north part of the house. There are at least two important reasons for this:
• more light during the day
• better use of passive solar energy
Tip: Room placement is closely connected to house orientation and properties of the plot. On many plots it is not reasonable to place living room in the south part of the house as for example north part offers better views, there is another house blocking the sun from the south side etc. You should carefully consider this before buying a plot.

## Windows placement

Largest windows areas should be placed in the south as this is the part of the house that gains most of the energy. However, total glazing in the south side of the house should not exceed 40% of the total south front. In the north side there should be as little windows as possible as north side loses more energy by heat transmission than it gains by sun radiation. Windows on the east and the west side of the house in central European climate loose on average as much energy as they gain. However large windows on the west are problematic during summer if not adequately shaded. Nevertheless west and east windows should not be too large.

Roof windows loose large amounts of energy during winter and gain too much heat energy during summer. They should be avoided.

Tip: Windows placement is closely connected to room placement  (e.g. large windows are common in living room, smaller windows in bathroom, etc.) therefore it is important to properly position rooms.

## Balconies, chimneys, roof windows etc.

All these elements typically present large heat bridges that are difficult if not impossible to omit. Therefore, if possible these building elements should not be used. These elements are ofter quite expensive, so this is also one way to cut the costs of the house.

Tip: Before you build a balcony, ask yourself: "How will I use it?". So If you have a terrace in front of the house, you might find out that you don't need additional balconies. This way you can save a lot of money and omit many troublesome heat bridges.

## Bigger is better? Not really!

House owners often complain that they build too big and too expensive. House size is directly related to energy consumption. Smaller house needs less energy as there is less space to heat. Rooms even though not used consume energy for heating.

Tip: Before planning a house ask yourself: "How much space do I really need? How much space will I actually use?" This way you can save a lot of energy and money for construction and for heating unneeded rooms.