Knowledge Centre – Passive Strategies

Natural Ventilation
Introduction
Recommendations
Resources & Tools
Energy & Cost Savings

Introduction

Fresh air in a building brings health benefits and increased comfort level to its occupants. Fresh air provision is considered as an efficient and a healthy solution as it reduces the need for mechanical means to ventilate a building.

For NZEB design goal, passive design measures can be judiciously used to influence movement of outside air into a built space by bringing in fresh air. These interventions can reduce and in some cases (in certain climates) completely eliminate reliance on mechanical means to ventilate a building. Thus largely affecting air-conditioning loads.

Various forms such as appropriate orientation and form, openings in building envelope (windows, doors and ventilators), operable windows, internal space planning, etc. are various natural ventilation strategies that can be adopted. Other advanced ventilation techniques are courtyard effect, stack effect, wind tower, or air earth tunnels.

Recommendations

cross ventilation_1

Cross ventilation is dependent on the size and position of openings. Inlets should be in the windward direction and aided with suitably placed outlets that allow egress of wind from the space.

  • For good natural ventilation, building openings should be in opposite pressure zone ( since natural ventilation rely on pressure to move fresh air through buildings).
  • The building can be oriented 0° to 30° with respect to the prevailing wind direction (wind rose diagram) / most preferably orientating longer facades of the building towards predominant wind direction.
  • Maximum air movement is achieved by keeping the sill height at 85% of the critical height.
  • Greatest flow per unit area of the opening is achieved by keeping the inlet and the outlet of nearly same sizes at nearly same levels.
  • Windows should be staggered rather than aligned( see fig)
  • If the space has only one façade exposed to the exterior, it is preferred to provide at least 2 windows on the façade.
  • Total area of openings should be a minimum of 30% of floor area.
  • Window-Wall-Ratio (WWR) should not be more than 60%.
  • Along with orientation to breeze, design of windows to collect direct breezes is important. Use casement windows to catch and deflect wind from varying angles.

cross ventilation_2

Opening controls like louvers, sashes, canopies and screens can be used to control the direction and velocity of air stream flowing into a space. Comparatively permanent controls like canopies can alter the pressure build up at the face of fenestrations and must be designed keeping this factor in mind.

Climate specific recommendations

  • In hot and dry regions, the windows should generally be kept closed during the day to minimize solar heat gains to interior spaces. For regions experiencing high diurnal temperature differences (of the order of 12°C to 15°C cooler in night), Interior spaces should be opened to night time ventilation.
  • In warm humid regions, openings should be designed to capture wind as ventilation is the key comfort defining criteria throughout the day.
  • In cold regions the openings should be kept shut with controlled ventilation.
cross ventilation

Horizontal placement of openings and internal partitions can alter the direction and spread of air stream. Ideally, openings must be placed in opposite walls, and diagonally but not directly opposite to each other. When placed in walls perpendicular to each other, the inlets and outlets should be at the farthest corners of the walls.

Resources & Tools

Online Resources

  1. Kamal, M. (2012). An Overview of Passive Cooling Techniques in Buildings: Design concepts and architectural interventions. Acta Technical Napocensis: Civil Engineering & Architecture, Vol 55 (84-97).
  2. http://passivesolar.sustainablesources.com/#heat
  3. Passive solar design

Publications

  1. MNRE – Architects Guidebook
  2. Geetha, N., & Velraj, R. (2012). Passive cooling methods for energy efficient buildings with and without thermal energy storage – A review. Energy Education Science and Technology Part A: Energy Science and Research, Vol 29- 913 – 946.
  3. TERI – Guidelines for Solar passive design for new buildings
  4. Integrated green design for Urban and Rural buildings in Hot- Dry climate zone – CPWD

Tools

Envitrans
Provides information on wind direction in selected locations.

Autodesk Ecotect Analysis
Autodesk Ecotect Analysis is an environmental analysis tool that allows designers to simulate building performance from the earliest stages of conceptual design. It combines analysis functions with an interactive display that presents analytical results directly within the context of the building model.

Climate Consultant
A graphic-based computer program that displays climate data in dozens of ways useful to architects, builders, contractors, and homeowners, including temperatures, humidity, wind velocity, sky cover, and solar radiation in both 2-D and 3-D graphics for every hour of the year in either Metric or Imperial units.

Energy & Cost Savings

Natural Ventilation is particularly effective in moderate and cold climates. In such climates it can partially offset or even totally eliminate the requirement of a cooling system.

In composite climate,  winter, spring, and autumn months provide an opportunity to run the building on natural ventilation rather than mechanical cooling.  A well designed building can respond to the outdoor conditions and reduce mechanical cooling when the conditions warrant.

In warm and humid climate, especially near the coast, natural ventilation can provide the much needed air movement to provide comfort during humid periods.