Build Thermally Efficient and Sustainable Structures€¦ · Web viewBuild Thermally Efficient and Sustainable Structures Subject Chapter 9: Ventilation Keywords Energy efficient,

Build Thermally Efficient and Sustainable Structures

Chapter 9 VentilationProduced by Pointsbuild in partnership with the Master Builders

Association of NSW

Supported by the NSW Government as part of the Energy Efficiency Training Program — visit savepower.nsw.gov.au

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Building Thermally Efficient and Sustainable Structures: Chapter 2 - Climate_____________________________________________________________________________________________

Table of contents9 Ventilation..................................................................................................................................................1

9.1 Introduction...............................................................................................................................................1

9.2 Modelling Air Movement..........................................................................................................................1

9.2.1 Infiltration..................................................................................................................................................19.2.2 Ventilation.................................................................................................................................................29.2.3 Ceiling Fans................................................................................................................................................5

9.3 Building Design Considerations.................................................................................................................6

9.3.1 Design Principles........................................................................................................................................69.3.2 Airflow through Windows and Doors.........................................................................................................69.3.3 Impact of Insect Screens............................................................................................................................79.3.4 Stack Effect Ventilation..............................................................................................................................89.3.5 Roof Space Ventilation...............................................................................................................................99.3.6 Floor Space Ventilation..............................................................................................................................9

9.4 Climate Solutions.......................................................................................................................................9

9.4.1 Hot Humid Tropical Climates...................................................................................................................109.4.2 Hot Dry Climates......................................................................................................................................119.4.3 Warm Humid Climates.............................................................................................................................139.4.4 Warm Mild and Cool Temperate Climates...............................................................................................14

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Building Thermally Efficient and Sustainable Structures: Chapter 9 - Ventilation_____________________________________________________________________________________________

9 Ventilation9.1 Introduction Air movement through a building is the main element of passive cooling in many Australian climates as the moving air cools the occupants through increasing evaporation rates. It is the least expensive means of cooling a building and the lowest environmental impact.

Air movement within a building may be uncontrolled or accidental and is referred to as infiltration, or may be deliberate via natural ventilation through doors, windows and other openings. It may also be due to forced ventilation provided by extractor fans or simply ceiling or wall fans moving air within a room.

Air movement provides useful cooling in all climates but may be less effective in tropical climates during periods of high humidity.

9.2 Modelling Air Movement 9.2.1 InfiltrationInfiltration is the uncontrolled airflow that occurs through gaps, cracks, wall and ceiling vents that are unable to be closed and flues without dampers. Infiltration is detrimental to thermal performance in all climates, although performance in winter is much more sensitive to drafts than in summer.

The building’s occupants require a minimum flow rate of half an air change per hour that changes oxygen and expels carbon dioxide and water vapour. Larger infiltration rates may be required where the indoor air quality is poor due to the out gassing of surface finishes, carpets, plastics etc.

Infiltration can be reduced via the following means:

Draught sealing doors and windows and caulking gaps and cracks is low cost, and particularly worthwhile on windy sites.

Enclosing the under floor space of timber houses is effective in reducing drafts. Dampers should be fitted to all flues or chimneys. Strategic planting around the house can reduce the speed of the winter winds, and reduce the

resulting heat loss through infiltration.

The diagram below shows some possible infiltration sites.

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Unit of Competency: CPCCBC4021A 1


Figure 1 Infiltration of air through gaps, cracks, vents and poorly sealed doors and windows

9.2.2 Ventilation Ventilation provides benefits to the occupants of a building when the outside air is similar or close to the temperature that is comfortable when the interior of the building is too hot. This often occurs in the summer months in the late afternoons and evenings when the outside air temperature drops. Unfortunately a drop in the beneficial wind speed often accompanies this fall in air temperature.

Although cool breezes may be available in exposed or open locations, it may be useful to use fans to pull cooler air into the building on more protected sites.

A building with high levels of thermal mass is able to store and release more heat, so it is able to take advantage of additional ventilation to a greater extent than the lightweight buildings.

Moving air can also have a physiological cooling effect by allowing better evaporation of perspiration. This requires much higher air speeds than for simply cooling the internal mass of the building. If the outside air is hotter than inside the house, it is better to use ceiling or pedestal fans to achieve this physiological cooling, than to open the doors and windows and heat up the interior of the house.

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Figure 2 Methods for achieving air movement in a house

The building should be designed so that the airflow is not blocked and encouraged to flow through the building from the direction of prevailing cooling breezes during the summer months. Good cross ventilation requires uncongested paths through the interior as well as openings on opposite sides of the house. This is easily achieved with living spaces which can be made to span the house but is much more difficult to attain with smaller rooms such as bedrooms.

Figure 3 Designing for cross ventilation

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Windows and other air inlets should be located low enough to create air movement around people.

Figure 4 Designing for ventilation

Attachments to the building, or the shape of the building itself, can be used to create high and low pressure areas that redirect breezes through the house.

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Figure 5 Use of walls to create high and low pressure areas to force air flow through the building

9.2.3 Ceiling FansThe addition of ceiling fans may achieve comfort when the temperature is within acceptable levels.

The impact of a ceiling or wall fan and the air movement generated is indicated below.

Figure 6 Airspeed contours for ceiling and wall fans

Whole of roof fans can be beneficial in cooling applications, particularly where cross ventilation design is inadequate however they do not create sufficient air speed to cool occupants.

Air intakes are usually located in the centre of the house (hallway) and are used to draw cooler outside air into the building through multiple rooms when conditions are suitable. They exhaust the air through

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eave or gable vents via the roof space that cools the roof space. Control systems for whole of roof fans should prevent operation when external air temperatures are higher than internal.

9.3 Building Design Considerations9.3.1 Design PrinciplesThe main elements for passive cooling of a building are :

Orientation for exposure to cooling breezes. Increase natural ventilation by reducing barriers to air paths through the building. Provision of fans to provide ventilation and air movement in the absence of breezes. Provide paths for warm air to exit the building. Floor plan zoning to maximise comfort for daytime activities and sleeping comfort. Appropriate windows and glazing to minimise unwanted heat gains and maximise ventilation. Effective shading (including planting). Adequate levels of appropriate insulation. High thermal mass construction in regions with significant diurnal ranges. Low thermal mass construction in regions with low diurnal range. Use of light coloured roofs and walls to reflect more solar radiation and reduce heat gain.

(Your Home Technical Manual - Fourth Edition)

9.3.2 Airflow through Windows and Doors Air movement through the building depends on the area of wall or window that can be opened. Use windows designed to deflect breezes from varying angles. Locate windows on walls with best exposure to common cooling breezes and design for effective cross flow of air through the building.

This depends on the window type, some of which are illustrated below.

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Figure 7 The type of window determines the amount of air flowLouvres can be almost completely opened, while casement windows give substantial openings and some ability to catch the wind coming from particular directions.

Sliding windows and double hung sash windows have a maximum openable area of around 50%.

Awning or hopper windows are limited as to how far they can be opened unless the top hinge can be locked to keep the pane near horizontal.

9.3.3 Impact of Insect ScreensInsect screens reduce the benefits of cross ventilation significantly and their use should be carefully considered if ventilation and passive cooling principles are applied within a building.

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Figure 8 Reduction of air flow through insect screens

9.3.4 Stack Effect VentilationConvective air movement relies on hot air rising and exiting at the highest point, drawing in cool air from lower parts of the building and is known as stack ventilation.

Convection produces air movement capable of cooling a building but has insufficient air speed to cool the occupants.

The tendency for warm air to rise and to be replaced by cooler air can give rise to stack effect ventilation. Low level air inlets and high level outlets induce air movement upward through the building if the outside air is cooler than the air in the building.

Solar chimneys can also be used to ensure effective convective air movement.

Clerestory windows, highlight windows, roof ventilators, and vented ridges, eaves and ceilings will allow heat to exit the building in nil breeze situations through convection.

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Figure 9 Stack effect ventilation

9.3.5 Roof Space VentilationHouses with higher roof space ventilation rates experience a deterioration of performance in winter, but show an improved summer performance. Heat losses increase in winter if roof cavities are well ventilated, but in summer heated air is able to escape faster.

Houses having vents that can be closed in winter perform best.

9.3.6 Floor Space VentilationHigh floor space ventilation rates in houses with suspended floors can adversely affect both summer and winter performance.

Enclosing the under floor spaces of these houses can improve performance significantly.

9.4 Climate SolutionsThe following design suggestions demonstrate a variety of passive cooling principles that use the principles of good ventilation in combination with other elements of the building design and envelope.

9.4.1 Hot Humid Tropical ClimatesIn these climates:

High humidity levels limit the body’s ability to lose heat by evaporation of perspiration. Sleeping comfort is a significant issue – especially during periods of high humidity.

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Design eaves and shading to permanently exclude solar access to rooms. Consider shading the whole building with a fly roof.

Maximise shaded external wall areas and exposure to cooling breezes through the building. Use single room depths where possible with maximum shaded openings to enhance cross

ventilation and heat removal. Design unobstructed cross ventilation paths. Provide hot air ventilation at ceiling level for all rooms with shaded opening clerestory or highlight

windows or ridge vents. Shade outdoor areas around the house with planting and shade structures to lower ground

temperatures. Use insulation solutions that minimise heat gain during the day and maximise heat loss at night. Choose windows with maximum opening areas (louvres or casement) and avoid fixed glass

panels. Include ceiling fans to create air movement during still periods. Consider using whole of house fans with smart switching to draw cooler outside air into the house

at night when there is no breeze. Use low thermal mass construction generally. (Note: High thermal mass construction can be

beneficial in innovative, well considered design solutions). Use planting design to funnel cooling breezes and filter strong winds.

(Selected from Your Home Technical Manual - Fourth Edition)

Figure 10 High humid climate9.4.2 Hot Dry ClimatesIn these climates:

Hot dry climates occur in a wide range of latitudes and geographic locations. This creates a variety of diurnal ranges and winter heating requirements with hot to very hot, dry summers.

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Evaporative cooling from ponds, water features and ‘active’ or mechanical cooling systems is ideal for arid climates where low humidity promotes high evaporation rates.

Evaporative cooling or a ceiling fan should be used if required. Use high mass solutions with passive solar winter heating where winters are cool or cold and

diurnal ranges are significant. Use low mass elevated solutions where winters are mild and diurnal ranges are lower. Consider high mass construction for rooms with passive winter heating and low mass for other

rooms. Shade all windows in summer and east and west windows year round. Well sealed windows and doors with maximum opening area allow maximum exposure to cooling

breezes and exclude hot, dry and dusty winds.


Figure 11 Hot dry climate - courtyard design with evaporative cooling

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Figure 12 Hot dry climate - two storey solution

9.4.3 Warm Humid ClimatesIn these climates:

Passive solar heating is required during winter months. Adjust eave overhangs to suit the particular micro-climate.

Use high mass construction in areas with significant diurnal range (usually inland). Use low mass construction where diurnal ranges are low (usually coastal). Orient to maximise exposure to cooling breezes and use ceiling fans and convective ventilation to

supplement them. Elevated structures can increase exposure to breezes. Include evaporative cooling and water features. Use insulation to prevent heat loss and heat gain.


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Figure 13 Warm Humid Climate - high mass solution

9.4.4 Warm Mild and Cool Temperate ClimatesThese climate types require less cooling. Building should use good orientation, shading, good levels of insulation and design for cross ventilation to provide for comfort.

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Acknowledgements

Figure 1 Dr Holger Willrath - The Thermal Performance of Buildings - Short Course Notes



Figure 4 Your Home Technical Manual - Department of Climate Change and Energy Efficiency - Fourth Edition as

amended - published 2010















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Questions - Ventilation

1. Which window is most effective for ventilation?A. SlidingB. HopperC. AwningD. Double hungE. Louvres

2. Stack ventilation refers toA. A large row of windows providing the equivalent to cross ventilation B. High pressure wind pushing air through windows at the top of the buildingC. Cold air entering down low that pushes hot air up out high D. The extraction of hot air from the top of a building

3. Highly ventilated roof spaces produce better performing buildingsA. At all times B. In summer C. In winterD. Never

4. High infiltration ratesA. Are required to maintain indoor air qualityB. Are caused by using aluminium framed windowsC. Cannot be avoided in high wind areasD. Should always be avoided

5. Insect screensA. Reduce airflow more at high wind speeds than at low wind speedsB. Significantly reduce airflowC. Make only a small difference to airflowD. Make very little difference to airflow

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Build Thermally Efficient and Sustainable Structures€¦ · Web viewBuild Thermally Efficient and Sustainable Structures Subject Chapter 9: Ventilation Keywords Energy efficient,