Natural ventilation

Objectives

1 Discuss the different types of natural ventilation application in

building design

2. Discern some techniques to reduce solar and internal gains in a

building

Key Words

Stack Effect Positive Air pressure Inlet Openings Solar Gain

Airtight Building Envelope Windward Glazing Exhaust Openings

Introduction

A building is a man-made environment (a machine in Le Corbusier

terms) within the bigger natural surrounding. The whole endeavor of

architecture, apart from space definition, is to moderate the level

of interaction with the environment.

In doing so, openings are provided on the envelope of the

building for ventilation and space cooling purposes.

However, providing the “holes” on the “walls” is not a simple task

of necessity. Rather it is a multi variable function of the immediate

surrounding (wind direction, nearby elements like buildings, trees,

shrubs) and the opening parameters (size, placement and

orientation).

Taking into consideration all above the elements

reduces energy consumption for air conditioning

and mechanical cooling.

Description

Natural ventilation is the use of

outdoor airflow into the building

to provide ventilation and space

cooling.

It can be achieved with operable

windows when the spaces to ventilate

are small and the architecture permits.

In more complex systems warm air in

the building can be allowed to rise and

flow out upper openings to the outside

(stack effect) thus forcing cool outside

air to be drawn into the building

naturally though openings in the lower

areas.

These systems use very little energy but

care must be taken to ensure the

occupants‘ comfort. In warm or humid

months, in many climates, maintaining

thermal comfort via solely natural

ventilation may not be possible so

conventional air conditioning systems are

used as backups

Concepts

Natural ventilation is a whole building design concept. the design

utilizes the stack effect and wind pressure to supply

outdoor air to building interiors for ventilation or space cooling

purposes.

The aim is to have an airtight building envelope while

controlling outdoor air supply to provide the required ventilation.

Features of naturally ventilated buildings include operable

windows, exhaust units located high in the building with

intakes located low in the building, and open building plans

to facilitate air movement. Designs can incorporate lobbies,

internal stairwells, vent chimneys and small fans to move

vent air.

Natural ventilation reduces energy

consumption for fans and mechanical cooling

and in most cases gives occupants control

over their office space. Further benefits

include no fan noise and in some cases

elimination of the mechanical cooling

Benefits

provides ventilation (outdoor air) to ensure safe,

healthy and comfortable conditions for building

occupants without the use of fans, provides free

cooling without the use of mechanical

system, reduces building construction cost

and operation cost,

when carefully designed, reduces energy

consumption for air conditioning and circulating fans

and eliminates fan noise

Limitations

Greater temperature swings than normal with

mechanically cooled spaces will occur and have to

be acceptable to the occupants, flow rate varies

with the outdoor conditions, Cool air drawn

into room, warm air extracted occupants must be

willing to open or close vents and windows

to regulate thermal comfort, not as effective

during warm, humid, summer months, difficult to

retrofit in buildings and fire code may restrict

design options.

Application

1.The long façade of the

building and majority of the

openings should be oriented with

respect to the prevailing

summer breezes,

2. exhaust vents or outlets should

be on the leeward side as high as

possible in the building,

3. vegetation and site objects should

not obstruct inlet openings,

4. rooms should have an inlet and

outlet openings located in the

opposing pressure zones,

e.g. windward and leeward walls,

windward wall and roof

5.inlets should supply air low in the

room, outlets Should Be Located

Possibly Across the room and at

high level,

6. vertical distance between

the inlet and exhaust openings should take

advantage of the stack,

7. all occupied spaces should

have an inlet opening and outlet

opening, one or both of which may be

an operable window,

8. the total area of outlet

openings should be operable and

accessible by the occupants,

9. inlet openings should not be

obstructed by furniture and interior

partitions, to ceiling height

should not be less than 3m

Maximize reflectivity of ground and

building surfaces outside facing winter

sun Provide reflective panels outside

of glazing to increase winter irradiation

Some techniques to reduce solar and internal gains

1.window areas should not be excessive and be

protected with exterior shading devices

Window size > 0.3 A x B

Window sizes for industrial buildings: glass

area-1/20 of room area; window width-

1/10 (M+N-O+P)

2. design for high thermal capacity

and exposed ceilings for night cooling

3. minimize warming of walls by the sun through the use of light

colored exteriors, trees and shrubs to provide shading and

evaporative cooling,, grass and other ground cover to keep ground

temperature low, and ponds and fountains to enhance evaporative

cooling

4. internal loads such as lighting controls, high efficiency

lighting,, high efficiency mechanical equipment and pipe and

duct installation should be low

5. narrow floor plan

Basic Principles

•As the wind approaches the face of the building, the

airflow is slowed, creating positive

pressure. Separated air increases suction (negative

pressure) along wall surfaces.

•If windows are placed in both windward and leeward,

the building would be cross ventilated and eddies will

develop

•Windows placed in sidewalls will enhance ventilation

sue to increased suction and greater

air circulation Ventilation is best for room with windows

on 3 adjacent walls than 2 opposite walls

• If wind is from windowless side, there is poor

ventilation

• A building with one window wall negligible ventilation

will occur, but architectural solutions are available

• As airflow passes through a well-ventilated room it

forms an air-jet

• If openings are near to wall, ceiling, floor, the air

stream attaches itself to the surface

• It is best to offset the inlet and outlet

• The window size for the inlet and outlet should be the

same

Attic and crawl space ventilation

1.when there is attic space under roof, venting of attic will

reduce heat build up, provide escape route for moisture and

in cold climates, help prevent ice dams from forming

2. Even when there is no attic, the venting effect

can still be achieved with at least a 2.54cm air

space above the insulation

3. In some cases, the argument can be made for having no venting

at all. This can be done in dry climates and building types where

vapor is less of a problem or if the “wet” side of

the roof is sealed against vapor migration.