Passive Design

Designing the building and the spaces within it to benefit from natural light, ventilation and even temperatures.

Daylighting

For energy efficiency and for the health and comfort of occupants, daylight should be used as much as possible to light a home. The main principles for daylighting in houses are to:

  • use diffused light instead of direct sunlight
  • ensure that daylight is able to penetrate into the building through the form and planning of the building
  • avoid over-glazing, which can create excessive heat gain in summer and major heat loss in winter
  • eliminate glare.

Design requirements for daylighting must be balanced with requirements for passive heating and cooling, views, and privacy. Daylighting must also be integrated with artificial lighting design to achieve appropriate lighting levels and flexibility.

Lighting levels and types of lighting

For detail on recommended lighting levels and on types of lighting for different activities in the home, see artificial lighting.

Even daylighting

Designing to utilise evenly diffused daylighting will provide the most energy savings and the best light quality. Even lighting can be achieved through careful placement and sizing of main windows. Daylight illumination falls off with distance from the windows. There are several ways a designer can allow light to penetrate into the interior:

  • Specify top-lighting using roof-lights. Roof glazing needs to be carefully considered to minimise heat loss in colder conditions – using IGUs (Insulated Glazing Units) for roof glazing is essential.
  • Use open planning to minimise solid walls.
  • Shape the building to allow clerestory windows or south lights.
  • Specify fully glazed internal walls or walls with some glazing (borrowed lights).
  • Specify proprietary tubular rooflights.
  • Plan spaces so that all spaces requiring daylighting have access to an external wall.
  • Design long and narrow (or stepped) rather than square plan forms.
  • Specify glass with spectral coatings that allow light in but reduce glare and UV radiation.
  • Design rooms that have floor-to-ceiling and wall-to-wall glazing.
  • Design to have natural lighting from two or more directions – a single source of natural light is more likely to create glare.
  • Increase the room heights at the external walls.
  • Locate rooms to get maximum daylight when and where it is needed e.g. a breakfasting space on the east side
  • Use reflective surfaces adjacent to the top of the window (inside and outside) – this is said to increase light penetration from 1.5 times the window head height to 2.5 times. The benefit is less if only one side of the window is treated. These light shelves work better when ceiling heights are higher.
Rooflights and clerestory windows improve daylight levels in deep rooms 
Rooflights and clerestory windows improve daylight levels in deep rooms

Because daylight illumination falls off with distance from the windows, adding clerestories, roof-lights or borrowed lights can improve the level and distribution of daylight.

Maximising daylight and ventilation 
Maximising daylight and ventilation

To ensure daylight reaches the back wall of a space, make sure the distance from window to wall is no more than 1.5-2 times the height at the top of the window. Designing spaces that are not too wide also aids cross-ventilation.

  

Selecting reflective surfaces inside the rooms such as light colours, gloss finishes or mirrors can increase the penetration of natural light into a building. Reflectance values suggested to increase daylight penetration are 80% for ceilings, 50 – 70 % for walls and 20 – 40 % for floors. The reflectance values for white paint is approximately 75% when new and 55% when old.

Factors that can affect daylighting levels that need to be considered during design include:

  • the typical amount of cloud cover
  • shadows from adjacent buildings, landforms and vegetation
  • any demands for specific task lighting
  • the use of tinted or reflective glazing.

Avoiding direct sunlight/overglazing

Direct sun may be needed to heat thermal mass such as a concrete floor immediately adjacent to a window. However, direct sunlight is an extremely strong source of light and heat, so it should be controlled to prevent visual discomfort, overheating, glare, and deterioration and fading of carpets, fabrics and equipment. Practical methods of moderating or controlling direct sunlight and heat gain include:

  • correct planning window orientation, location and size to balance heat gain/loss, provide view, allow thermal mass to be utilised
  • shading of direct sun entry, particularly in summer
  • specifying adjustable louvres – horizontal or vertical depending on orientation
  • designing the landscaping to provide shading
  • specifying adjustable interior blinds
  • specifying obscure or patterned glazing
  • specifying solar control glass (tinted or reflective) or films – generally these limit the penetration of light into buildings
  • specifying gas rather than air filling of IGUs – argon is a cheaper, clearer gas that works best with a 10-12 mm thick gap between panes, while krypton is more expensive, but works better with a 6 mm gap
  • specifying glass with a low emittance, a spectrally selective coating, a sputtered or soft coat, or a hard coat.

Spectrally selective glass coatings are designed to admit or exclude defined portions of the visible and infrared spectrum. Sputtered or soft coats can reflect up to 80% of radiant heat (clear glass is 16%) while hard coats which are more durable for exterior use reflect around 40 to 50%.

Glass that is readily available in New Zealand includes clear float, tint, low emissivity, reflective, spectrally selective, combinations, and self-cleaning varieties.

All windows are a potential source of heat gain and heat loss so their size and placement should be controlled by the optimum requirements for daylight and view. They should not be larger than necessary. (See WERS for more information).

Emerging technologies

There are several emerging technologies which, as they become commercially available, will be helpful for controlling sunlight and heat gain. These include:

  • evacuated windows where a vacuum is created between the panes of glass in an IGU
  • so-called superwindows that are triple glazed
  • aerogel – a silica-based, open-cell, foam-like material that is transparent
  • photochromic glass, where transparency changes as the light levels change
  • thermochromic glass that changes in response to temperature changes
  • liquid crystal glazing (currently very expensive) where the addition of an electric current can change its transparency
  • active-particle dispersed glazing – which uses electricity to change the state of liquid particles between two panes of glass to alter transparency
  • active-electrochromic glazing – which uses a small electric current to change the properties of very thin layers of rare metals to change translucency.