This Glossary is a handy reference of terms and definitions used in our Sustainable Architecture 101 series.
Sustainable Architecture 101: Glossary
Brick Veneer Wall
A non-load bearing layer of brick is applied to the exterior of a wall and tied to a load bearing timber or metal frame structure.
In the context of Sustainable Architecture, the Building Envelope is the outer shell of a building that protects the interior from the exterior environments. It includes the roof, walls, doors, windows and foundations – in short every part of the building in contact with the exterior.
This should not be confused with the same term, Building Envelope, used in the context of land use planning. In that context, the Building Envelope refers to the maximum land area on a site that the building footprint can physically and legally occupy and forms the outermost boundary within which the designer/architect must design within.
Where solar access to north-facing windows is obstructed, clerestory windows are a good option to allow solar energy into the building. Clerestory windows are typically a row of windows placed very high on a wall. Internally-operated adjustable louvers or blinds can be used for east and west-facing clearstory windows.
Condensation refers to water that has changed from a gas to a liquid. This occurs when water vapour in the air comes in contact with a colder surface – the vapour changes to tiny liquid drops on that surface. Air always contains a certain amount of water vapour, due to respiration, cooking, indoor plants, bathroom, laundries, heaters etc.
Conduction/Conductive Heat Transfer
Conduction is heat transfer from warm to cool areas within a material or between two materials touching each other. Solids, especially metal, conduct heat rapidly whereas gases, such as air, do not conduct heat very well. Inside a home, heat warms the inner plasterboard layer of a wall which transfers heat to the next layer, for example bricks, then to the next layer and so on. If a building is to be designed for energy efficiency, it is important to stop this heat transfer with the help of insulation.
Convection/Convective Heat Transfer
Convection is heat transferred from one place to another through the movement of gasses or liquids. In a building convection transfers heat using moving air. When air is warmed, it rises and is replaced by cooler air. This creates a cycle or convection current capable of transferring heat. For instance, if a room is draughty, the current can get quite strong, the Relative Air Velocity rises and the apparent temperature felt on exposed skin can be significantly lower than the air temperature.
Two glass panels are assembled into one window unit. The gap between the two glass panels should be a minimum of nine millimetres and is usually filled with still air. The performance of double glazing windows can be increased by approximately 20% with a U-value of under 2.0 W/m²K by filling the space with gas instead of still air, such as argon or SS6 gas. Another way to reduce heat loss through glazing is to use low emmitance (Low-E) glass. The glass has a special coating on the inside of the air space which reflects radiant heat back inside the building, hence the transmission of radiant heat from the warmer glass at the inside from the house to the colder glass at the outside gets reduced.
East-facing windows should be located between 30° east of true north and 40° east of south. The window area should be kept reasonably small. East-facing windows need to be shaded in summer.
Heat moves from warmer areas to cooler areas. On hot days heat tries to get inside a house and on cold days, the heat tries to escape. All materials allow a measure of heat transfer through them. Some materials, such as metal, glass or aluminium allow heat to pass through them more easily, whereas others such as wool, still air or thick clothing, are more resistant against heat flow. These materials are referred to as insulation. Even if walls and ceilings are insulated adequately, heat can still be lost through windows, fixed wall vents and other air-leakages. Heat energy can be transferred in 3 ways; radiation, convection and conduction.
Internal window covers
Internal window covers are the easiest and cheapest way to reduce heat loss in winter. Appropriate coverings include drapes, Holland blinds or Roman blinds combined with thin or lace curtains. This works because the window coverings act as a barrier and trap a layer of still air between the glass surface and the curtain -this reduces the heat flow through the glass. In order to maintain the layer of still air the coverings must be opaque, closely woven, fitted completely over the window and must have a cover at the top, such as a boxed pelmet. An alternative is to recess the coverings into the window reveal, but here again, a snug fit is essential. Vertical blinds, timber or conventional venetians don’t work as a good barrier to trap air and should therefore be avoided.
Mean Radiant Temperature
Mean Radiant Temperature is the weighted average temperature of all exposed surfaces in a room. The greater the difference between air temperature and exposed surfaces, the greater the Relative Air Velocity.
North-facing windows should be located between 30° east of true north and 20° west of true north. North-facing windows should be large but only if the solar access is good. If concrete slab floors are present to act as thermal mass, the glass area can be bigger than if timber floorboards are used.
The overall R-value is the total resistance to heat transfer of a building element. It takes into account the resistance performed by each construction material used in walls, ceilings, internal spaces, insulation materials, air films adjacent to solid materials as well as thermal bridging. The sum of the R-values for each component provides the overall R-value.
Windows can be overshadowed in winter by trees, other buildings, fences or the structure of the building itself which can significantly reduce solar access. For instance east and west facing walls can shade adjacent north-facing windows, or side walls can also overshadow large windows in deep courtyards. Windows in courtyards need to be sized carefully in order not to lose heat through permanently shaded areas. In general, overshadowing will occur more on the lower part of walls than on the upper, hence full-height windows adjacent to side wall should be avoided and sill heights should be raised instead.
A Passive House is not an energy performance standard, but a concept to achieve high thermal comfort conditions on low total costs. A Passive House is a building, for which thermal comfort can be achieved solely by effective use of fresh air mass, sufficient to fulfill indoor air quality conditions, without a need for mechanically aided recirculated air.
Raising the sill heights
If other buildings or structures overshadow windows, raising the sill height can minimise permanent shaded glass areas. How much the sill height should be raised depends on various factors, such as the width of the eave, the floor level (how far the building is above the ground) and the distance to neighbouring buildings – the higher the neighbouring building, the higher the sill height should be.
Radiation/Radiant Heat Transfer
Radiation describes any process in which energy emitted by one body travels through a medium to finally be absorbed by another body. Heat (infra-red) radiation which is emitted from the surface of hot objects travels in straight lines to cooler objects, for example direct heat from the sun’s rays or heat from an open fire can be sensed by skin. As an additional example, a warm plaster board ceiling will emit heat to cooler roof tiles on a cold day.
Relative Air Velocity
Relative Air Velocity (‘wind chill factor’) is the apparent temperature felt on exposed skin due to wind. For example, if cold air is leaking in from a window, the air temperature feels lower than the actual air temperature hence the likelihood of feeling cold is increased, even if a heater is turned on.
Reverse Brick Veneer
Reverse brick veneer puts a layer of non-load bearing brickwork on the inside of a building with the load bearing timber or steel frame located to the exterior.
Solar access refers to the amount of direct and diffuse solar energy a building receives. Solar access is measured by the number of hours that the sun can shine into north-facing windows between 9am and 3pm on the shortest day of the year (22 June).
Direct solar energy is the sunlight that falls upon an object. Diffuse solar energy is the energy reflected from other objects. The sun radiates short-wave (direct) energy into a building and this energy is absorbed into objects and radiated as long-wave (diffuse) energy to cooler elements around us. For example, up to 40% of energy gain from north-facing windows in winter comes from diffuse solar radiation.
South-facing windows should be located between 40° east of south and 40° west of south. Glazing areas facing south should be kept reasonably small and should be placed to enable cool summer breezes to pass easily through the rooms.
The term “stack-effect” is derived from chimneys. The heat source – in the case of a chimney, the fire – heats up the air. The hot air rises and gets discharged through stack. Warm air rises as it has a lower density than cold air. This effect can be used to replace air inside a house. For instance, when it’s colder outside the windows can be opened to let in cooler air. The warmer air inside the room will rise towards the ceiling, the warm air can pass through high openable windows and skylights. The warm air inside will get replaced by fresh and cooler outdoor air.
A thermal bridge is an element or part of a building, which allows heat to travel through it more quickly than through other parts and is therefore responsible for unwanted heat loss or gain. A thermal bridge arises for instance when poor insulative materials touch each other, when gaps occur between insulative materials and structural surfaces, and when materials with different R-values/U-values come in contact with each other. These thermal bridges allow heat transfer from a warmer to a cooler material. The main thermal bridges in a building are located at the junctions of floor to the wall, wall to the roof and the joins between balconies and windows and door frames.
Human Thermal Comfort describes the state of mind that expresses satisfaction with the surrounding environment.
Thermal Comfort is affected by six variable factors:
1. Air Temperature is the most common measure of Thermal Comfort and can easily be influenced with passive and mechanical heating and cooling.
2. Mean Radiant Temperature is the weighted average temperature of all exposed surfaces in a room. The greater the difference between air temperature and exposed surfaces, the greater the Relative Air Velocity.
3. Relative Air Velocity (‘wind chill factor’) is the apparent temperature felt on exposed skin due to wind. For example, if cold air is leaking in from a window, the air temperature feels lower than the actual air temperature, hence the increased likelihood of feeling cold, even when the heater is on.
4. Humidity or relative humidity is the moisture content of the air. If the humidity is above 70% or below 30% it may cause discomfort.
5. Activity Levels can reduce the heating needs, as lower air temperature is acceptable when occupants have higher activity levels.
6. Thermal Resistance of clothing or warm blankets in a bedroom can reduce the need of heating.
Thermal mass or thermal conductivity is the capacity of an object to store heat. It is an effective way to improve thermal comfort in a building since it will absorb heat when the surroundings are hotter than the mass, and give heat back when the surroundings are cooler. When situated well and in combination with passive solar design, thermal mass can play an essential role in saving energy and be used actively for heating and cooling.
Thermal resistance or thermal conductivity is the ability of a material to resist heat flow. The performance of insulation is usually described as the R-value – the higher the score, the better the performance. Other materials or building products are evaluated by their U-value which is the inverse of the R-value – the smaller the U-value, the higher the resistance against heat flow.
U-value / R-value
The U-value measures the heat transfer through a material or a building element, whereas the R-value measures the resistance to the transfer of heat. U-values are commonly used in technical literature, especially to indicate the thermal properties of glass and to calculate heat loss and gain.
The U-value is the reciprocal of the R-value: R=1/U or U=1/R. For example, if a material has an R-value of 4.0, the U-value is 1/4 or 0.25.
The U-value is expressed using the metric units (W/m²K).
– W refers to the amount of heat transmitted across the material in watts
-m² refers to one square metre of the material of a specified thickness
– K or ‘degree Kelvin’ refers to each degree temperature difference across the material
The table below shows the U-value for some materials. The lower the value, the better the resistance to heat flow, the higher the number the better heat can be transferred through it.
|MATERIAL||USED AS||U-VALUE IN W/m²K|
|Wood Fibre Board||Insulation||0.050|
|Plywood Lining||Internal or external lining||0.13|
|Timber||Structural elements, cladding, etc.||0.15|
|Plasterboad||Internal or external lining||0.25|
|Concrete||Floors, structural elements||1.0|
|Reinforced Concrete||Floors, structural elements||2.1|
Ventilation is the process of “changing” or replacing air to regulate temperature and moisture control, amongst other things. By applying the right design features, natural ventilation and cross ventilation can be used to control indoor temperature and therefore reduce the energy bill significantly. For these reasons, controlling the air movement is essential. Draughts and air leakages will increase the need of supplementary mechanical cooling and heating.
West-facing windows should be located between 20° west of true north and 40°west of south. Windows should be as small as possible. It is essential to shade glazing in summer.
If warm room air comes in contact with the surface of cold glass, especially unprotected single glazed windows, the air cools down and falls to the floor as a cold draught. This effect lowers the room temperature and creates a draught near unprotected glass. The air starts to move, the relative air velocity gets high, and the occupants will feel winter discomfort. Furthermore a person close to a window will lose body heat to the cooler surface of the window, which in turn increases the discomfort.