Designing for the sun
Designing for the sun will give you a home that keeps you warm and comfortable in a more cost-effective and environmentally friendly way. With the right balance of elements your home can soak up free heat from the sun and keep you warm day and night.
There are some simple rules of thumb for creating a home that's designed to harness the sun's free heat. Along with plenty of insulation, double glazing and good building airtightness, the key principles are:
Positioning a home to make the most of the sun
Windows sized appropriately for the direction they face
Thermal mass to store the sun's heat
To really optimise your home, it is worth getting professional advice from someone experienced in designing for the sun. Not all designers and architects are experienced in this, so it pays to shop around and ask for references.
Positioning a home to make the most of the sun
For optimal positioning to make the most of the sun:
- Choose a sheltered, north-facing site that is protected from prevailing winds
- Look out for objects that can shade the home, like trees, hillsides and other buildings. If you're really keen to see whether an object will shade the house, you can work it out using a sun path diagram from Victoria University's Centre for Building Performance Research.
- Elongate the home along the east-west axis, to maximise north-facing area. The optimal ratio for north to east should be at least 1:1.5.
- Avoid the building shadowing itself on the north side.
- Put living areas and frequently-used rooms on the north-facing side of the house to make the most of the heat and light from the sun.
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Put less-used rooms like the bathroom, laundry and garage on the south-facing side - this acts as buffer against heat loss.
Windows sized for the direction they face
The right balance of windows will allow your home to collect the sun's free heat and daylight while avoiding excessive heat loss or overheating. A lot of heat can be lost through windows if not sized properly for the direction they're facing - even with double-glazing.
With the right combination of insulation and thermal mass, windows should follow the following rules of thumb.
- Sun-collecting windows face north. They should be within 20 degrees plus or minus of due north. Note that due north is west of compass north - calculate due north in your location on the Te Ara website.
- Windows are medium-to-large on the north (sun-collecting) side of the house (provided it is not shaded). Aim for north-facing glazing that is 6-10% of the home's floor area. Houses with large thermal mass areas should aim for 10-20% of the home's floor area but care is needed to get the balance right.
- Windows are moderately small on the east and west sides. Aim for glazing that is 2-3% of the house's floor area on each of these sides, keeping adequate daylight access and ventilation in mind.
- Windows are very small on the south side of the house. Aim for the smallest size necessary to bring in adequate daylight and ventilation. Clerestory windows, which rise above the roofline of the north-facing half of the roof and lead up to the southern half of the roof, can be a good way to bring sun and light into south-facing rooms.
- Skylights can be good for letting in daylight, but since most skylights in New Zealand are placed in sloping roofs, most are not suitable for passive solar heating in winter and can cause overheating in summer. If you do add skylights when building or renovating, make sure you place them carefully where additional daylight is needed.
Daylight simulation software can help with sizing and positioning windows and skylights for natural daylighting - talk to your designer about this.
Thermal mass to store the sun's heat
Thermal mass that is exposed to the sun will soak up heat during the day and release it at night. Whether it is a concrete floor slab or masonry wall, sunlight through north windows should hit the thermal mass for at least 3-6 hours a day for it to work well.
Concrete floor slab
The simplest and most cost-effective way to do use thermal mass is with a concrete floor slab.
- The concrete floor slab should be 100mm thick well insulated both underneath, around the footings and around its perimeter.
- To work effectively the contrete should be uncovered (polished) or tiled in areas exposed to the sun. In rooms that don't collect sun, the concrete floor slab can be covered or carpeted.
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If a concrete slab is not possible, use a suspended concrete floor in rooms with north-facing windows.
Masonry walls
You can also use masonry walls as thermal mass where possible. They can be internal (e.g. as a feature wall) or external, and can be solid or internal masonry veneer.
- The masonry wall must be exposed to sun coming in through north-facing windows to be effective.
- External masonry walls must be well insulated on the outside, with the mass exposed to the inside of the house (that is, without internal lining, but it can be plastered, painted or wall-papered).
- Internal masonry walls don't lose heat to the outside so don't need insulating.
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Internal masonry walls can also be effective in conducting heat to the rooms behind that wall that don't receive direct sun.
Design to prevent overheating
As well as creating a warm home, good design will prevent a home from overheating.
- Properly designed overhangs will limit summer sun, but allow plenty of sun in during winter. Overhangs are good for northerly windows.
- Overhangs don't work well on east and west sides as the sun is too low - deciduous trees and movable shades or louvers are better options here.
- Ventilation is important for preventing overheating, and is most easily achieved with openable windows, particularly in rooms prone to over heating. Have a minimum of 30% of your glazing openable.
- Any skylights should have effective shading to block out the sun on hot summer days. You should also be able to open skylights for effective ventilation and cooling.
Building and renovating tool
Use our online tool to find out areas where you can make energy efficient choices tailored to your building or renovating project.
