3.1 Ventilation & Operable Windows
A primary strategy for cooling buildings without mechanical assistance (passive cooling) in hot humid climates is to employ natural ventilation. (The Fan and Landscape sections also address ventilation strategies.) In the Austin area, prevailing summer breezes are from the south and southeast. This matches nicely with the increased glazing on the south side needed for passive heating, making it possible to achieve helpful solar gain and ventilation with the following strategies:
- Place operable windows on the south exposure.
- Casement windows offer the best airflow. Awning (or hopper) windows should be fully opened or air will be directed to ceiling. Awning windows offer the best rain protection and perform better than double hung windows.
- If a room can have windows on only one side, use two widely spaced windows instead of one window.
3.1.1 Wing Walls
Wing walls are vertical solid panels placed alongside of windows perpendicular to the wall on the windward side of the house.
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| Top View of Wing Walls Airflow Pattern |
Wing walls will accelerate the natural wind speed due to pressure differences created by the wing wall.
3.1.2 Thermal Chimney
A thermal chimney employs convective currents to draw air out of a building. By creating a warm or hot zone with an exterior exhaust outlet, air can be drawn into the house ventilating the structure.
Sunrooms can be designed to perform this function. The excessive heat generated in a south facing sunroom during the summer can be vented at the top. With the connecting lower vents to the living space open along with windows on the north side, air is drawn through the living space to be exhausted through the sunroom upper vents. (The upper vents from the sunroom to the living space and any side operable windows must be closed and the thermal mass wall in the sunroom must be shaded.)
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| Summer Venting Room |
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| Summer Venting Thermal Mass Wall |
Thermal mass indirect gain walls can be made to function similarly except that the mass wall should be insulated on the inside when performing this function.
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| Thermal Chimney |
Thermal chimneys can be constructed in a narrow configuration (like a chimney) with an easily heated black metal absorber on the inside behind a glazed front that can reach high temperatures and be insulated from the house. The chimney must terminate above the roof level. A rotating metal scoop at the top which opens opposite the wind will allow heated air to exhaust without being overcome by the prevailing wind.
Thermal chimney effects can be integrated into the house with open stairwells and atria. (This approach can be an aesthetic plus to the home as well.)
3.1.3 Other Ventilation Strategies
Make the outlet openings slightly larger than the inlet openings.
Place the inlets at low to medium heights to provide airflow at occupant levels in the room.
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| Thermal Chimney Effect Built into Home |
Inlets close to a wall result in air “washing” along the wall. Be certain to have centrally located inlets for air movement in the center areas of the room.
Window insect screens decrease the velocity of slow breezes more than stronger breezes (60% decrease at 1.5 mph, 28% decrease at 6 mph). Screening a porch will not reduce air speeds as much as screening the windows.
Night ventilation of a home should be done at a ventilation rate of 30 air changes per hour or greater. Mechanical ventilation will usually be required to achieve this (See Fan Section).
High mass houses can be cooled with night ventilation providing that fabric furnishings are minimized in the house.
Keep a high mass house closed during the day and opened at night.