A building envelope is the primary interface with the external environment. It is the physical separator between the conditioned and the unconditioned environment of a building; the design and construction of the exterior of a building including the resistance to air, water, heat, light and noise transfer. It also includes the broader aspects of appearance, structure, safety from fire and security.
A good building envelope involves using exterior wall materials and designs that are climate-appropriate, structurally sound and aesthetically pleasing.The three basic elements of a building envelope are weather, air and thermal barriers. These three elements are the key factors in constructing your building envelope. The building envelope of a house consists of its roof, sub floor, exterior doors, windows and of course the exterior walls.
The external environment is very dynamic. People want the internal environment to be constant. To create a good building with low energy and high comfort for its occupants, the design relies first on the envelope and second on the services.
An efficient building envelop is compromised by
- fabric heat loss (i.e. walls and windows)
- solar radiation
- air leaks
- thermal bridging
Simple ways to minimise heat demand
- HEAT Retain heat
- COLD Avoid overheating
- AIR Supply air naturally
- LIGHT Utilised daylight
- ELECTRICITY Use electricity efficiently
- Air leakage/convection – sealing the building, ventilation
- Radiation – Heat from the sun and thermal mass
- Conduction – Insulation, thermal bridges, R-values, U-values. R-values express a material’s thermal resistance (how well it blocks heat) while U-values express thermal transmittance (how well it passes heat along).
- R-values: Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time, ) through it or . Thickness of material would be a factor on how well it blocks heat. Thermal resistance varies with temperature but it is common practice in construction to treat it as a constant value. The higher the number, the better the building insulation’s theoretical effectiveness. R-values are additive so a material with R-value 12 and another with R-value 3 will have a total R-value of 15
- U-values: Is the overall heat transfer coefficient that describes how well a building element conducts heat or the rate of transfer of heat (in watts) through one square metre of a structure divided by the difference in temperature across the structure. It measures the rate of heat transfer through a building element over a given area under standardised conditions. It is expressed in watts per meter squared kelvin ( W/m²K ). This means that the higher the U value the worse the thermal performance of the building envelope. A low U value usually indicates high levels of insulation. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials. Typically, the internal resistance for a wall is 0.13 W/m2K and the external resistance is 0.04 W/m2K.
- R-value is the reciprocal of U-factor. A material with a high R-value will therefore have a low U-value and vice versa.
- GLAZING: Note that the proportion of envelope heat gain for glass is 87%, 8% walls, 5% roof, 0.6% floor and 0.4% doors. The proportion of heat loss through glazing is 49%, roff 18%, floor 18%, walls 14% and doors 1%.
DESIGN: FORM FOLLOWS FUNCTION
The envelope is a crucial part of the architectural design and determines the formal qualities of the building. It is the primary element exposed to weathering and determines the climate of the interior. Form should consider multifunctional (architectural functions, environmental and human functions)
Types of facades
Punch windows: A single window frame and glass assembly surrounded by cladding as opposed to a number of frames coupled horizontally or vertically (window wall assembly).
Structural walls: A wall that bears a load resting upon it by conducting its weight to a foundation structure. The materials most often used to construct load-bearing walls in large buildings are concrete, block, or brick.
Curtain walls: A wall that provides no significant structural support beyond what is necessary to bear its own materials or conduct such loads to a load-bearing wall. Currently most popular – Design facades independently from the structure using a lot of glass, usually as a panel cladding (curtain wall). However, the curtain wall system more sustainable (multilayered or multifunction layers)
Most advanced facade technologies are design to try and overcome the deficiencies of a glass facade. It becomes a delicate balancing game of natural light, control of solar load, and ventilation. Use of active skins to make buildings able to respond to external conditions, such as operable/ automatic facades, new / advanced materials and technology.
Shading and ventilation
Selection depends on tenants and how involved they will be. Systems are always run by the buildings management system (BMS), building design and control strategy will determine its effectiveness and must also be master control by system (rain, security, etc).
Double skin facades
There is a inner facade layer and an outer facade layer. Integrated sun shading, natural ventilation and thermal insulation devices or strategies are available. For example, sun shading devices can be located between the two skins. The main layer of the glass is the insulator and the air space between the two layers act against temperature extremes, noise, wind, etc. An air cavity can allow total natural, hybrid or totally mechanical ventilation system.
- Ceramic Frit
- Bi Metals
- Built-In Photovoltaics
- Facade Greenery
Ultimately, the future is heading towards:
- Moving away from multi layered construction to multifunctional layers
- Learning to undo the curtain‐wall damage
- Energy and material efficiency
- Structure and form optimisation
- Simplify the form to meet cost restraints
Building Integrated Photovoltaics (BiPV)
Facade costs are often high so photovoltaics can be competitive as their cost has been declining. Surface potential to supply 46% of the load of Australian buildings (Prasad & Snow, 2005).
The University of Melbourne Alan Gilbert building, located on Grattan Street Parkville Victoria, was built in 2002 with the first large-scale BiPV. Cells were laminated into the glass. There is 426 m2 polycrystalline cells giving 46 kWp. 40,000 kWh/a cost $755,000 and saves 36,000 tonnes of CO2. The photovoltaics are placed on a 60 degree slope on the north facing side and cost A$1770/m2 in 2002.
The table shows that photovoltaic facades are comparable with glass walls. It is also much more cost effective compared to both stone and polished stone are
Case Study Saucerbruch Hutton