Sustainability: Historic Green – Passive Strategies for Heating and Cooling

Written by: Corie Baker, AIA, LEED AP, Pond and Company

The continued use and reuse of historic buildings is, at its core, a sustainable building practice – it reduces demolition waste, it reduces the use of new materials and resources, and it limits new infrastructure needs. In addition, historic buildings often utilized passive heating and cooling strategies, which are still relevant today and can help reduce the loads on modern mechanical equipment.  

                                                                                                                                                                                                                     

A historic building may employ one or more of the following passive strategies for heating and cooling.

Trees

When our older homes and neighborhoods were planned, trees were strategically preserved to protect our buildings from harsh winds and to regulate solar gain. Deciduous trees were often located along southern exposures as they provide shade from the harsh sun in the summer and then loose their leaves in the winter to allow the sun in when the heat is needed. In addition, trees help lower surrounding air temperatures by transpiring water and shading surfaces. Because they lower air temperatures, shade buildings in the summer, and block winter winds, they have proven to reduce building energy use and cooling costs. “USFS estimates the annual effect of well-positioned trees on energy use in conventional houses at savings between 20-25% when compared to a house in a wide-open area. (USFS meteorologist Gordon Heisler). Projections suggest that 100 million additional mature trees in US cities (3 trees for every unshaded single family home) could save over $2 billion in energy costs per year.”

Overhangs, Awnings and Eaves

You will notice that many historic structures incorporated front porches with large overhangs, awnings over windows or large roof eaves. These components have all been carefully sized to allow the winter sun to reach into the home when the sun is low in the sky and the solar heat gain is welcome and prevent the summer sun from directly penetrating into the building when the sun is high in the sky and we are trying to cool our buildings.

Cross Ventilation

Cross ventilation relies on wind to force exterior air into the building and interior air out of the building. Historic buildings were oriented to maximize the cross ventilation based on prevalent wind direction. In addition, most room were designed with more than one window/door, as cross ventilation requires both and inlet and an outlet. Optimally, openings were located opposite each other.

Passive Solar Design - Thermal Mass

The simple definition of thermal mass is a solid or liquid that will absorb and store warmth or coolness until it is needed. Materials such as concrete, masonry and water have the ability to act as a thermal mass and help maintain a consistent indoor temperature, even with large fluctuations in the temperature outside. In older homes, these materials were often used as exterior wall systems or floor slabs but mayhave been  incorporated into interior walls or as a central fireplace. When exposed to direct sunlight in the winter, the thermal mass will absorb heat during the day and then release that heat at night, when it is most needed. In the summer, thermal mass should be shaded so that it can draw the heat from the surrounding inside air and cool the room.


The Stack or Chimney Effect
The Stack effect is caused when warm air rises to the roof of a building and creates a suction at the lower level of the building, pulling fresh air through the windows at ground level. This creates a natural air flow which lets the “used” air out and brings in fresh outside air at the lower levels. Many older homes incorporated large attic fans which helped draw the cool outdoor air in through the windows and the warm air up into the roof and out the roof vents. Additionally, double hung windows can create a localized stack effect. In many historic structures, especially those with tall ceilings and large double hung windows, the bottom sash can be opened to allow the cool air in at the bottom and the top sash can be lowered to let the warm air out at the top.

In summary, historic buildings took passive approaches to heating, cooling and ventilation that are still valid today. Many historic buildings are constructed of brick or masonry and have high thermal mass properties. In addition, windows are typically arranged in a way that maximizes cross ventilation, bringing welcome breezes through the space. Overhangs, awnings and strategically located trees are also used to provide shade to south facing facades where the sun is most intense. These passive strategies can reduce heating and cooling loads, and therefore require a smaller HVAC system than other homes of a similar size. This can result in less material used, lower costs associated with the HVAC system and lower utility costs for the homeowner. Historic buildings bring their own set of unique challenges, but the benefits of continuing to use these buildings from both a preservation and a sustainability perspective, far outweigh any challenges.