Open Those Windows: Natural Ventilation in Historic Buildings Can Reduce Mechanical System Use, Save Energy and Create Healthier Indoor Environments

Today indoor temperatures are generally controlled by devices – whether they are smart, portable, or wedged in a window. Most heating, ventilation and air-conditioning systems can be set to maintain the temperature of an indoor space within a few degrees.  But has modern technology, like air conditioning, erased a time-tested body of knowledge about how best to design a well-ventilated, temperate building?

At Drexel University, researchers are investigating natural ventilation strategies in historic homes in hopes of understanding how their inherent energy-efficient features can inform modern energy conservation and historic preservation efforts, particularly in hot and humid climates.

“Historic homes were designed for times when there was no HVAC,” said Antonio Martinez-Molina, PhD, an associate professor in Drexel’s Antionette Westphal College of Media Arts & Design, and College of Engineering, who is leading the research. “Taking into consideration modern-day expectations, most are not comfortable buildings - but they perform way better than we expect.”

Using energy and computational fluid dynamics simulations, the research team analyzed several passive cooling and natural ventilation strategies for cooling historic residential structures in the hot and humid climate of San Antonio, Texas. Their results, published in Energy and Buildings, suggest that historic preservation and thermal comfort goals can be achieved simultaneously and can be replicated in multiple historic structures in similar climate regions around the globe.

The team considered feasibility, cooling potential and cultural preservation of six scenarios through the X and Y seasons in San Antonio:

• No natural ventilation
• Ventilation with openings at full capacity
• Ventilation with openings at half capacity
• Cross ventilation, opening windows on opposite sides of the building to improve airflow and regulate indoor temperature
• Stack ventilation, using temperature differences to drive airflow, with warm air rising and exiting through high openings while cooler air enters through lower openings
• Night flush ventilation, using cooler nighttime air to ventilate and remove heat from a building, reducing the need for mechanical cooling during the day

“Our analysis revealed that all the considered scenarios can contribute to energy savings in both seasons, especially in spring, with cross ventilation being the most efficient strategy,” said Martinez-Molina. “Mechanical cooling and mixed-mode ventilation are well researched topics, this is not the case with naturally ventilated residential heritage structures, which house approximately 15% of U.S. population. This work is increasingly important as climate changes create increasingly hot and humid conditions.”

Martinez-Molina cautions, that while opening windows in a historic building is a good practice – it’s not always feasible or sufficient to achieve a tolerable, livable environment.

In addition to assessing natural ventilation strategies, researchers turned their gaze to the inherent energy-efficient features of historic buildings and analyzed how energy retrofits can ensure a balance between energy optimization and historic preservation. Their research on this topic, which was also published in Energy and Buildings, developed a methodology for planning successful retrofits of historic buildings, especially those in hot and humid conditions.

“To enhance the energy performance of historic structures while preserving their cultural and physical values, it is essential to assess the inherent energy-efficient features and evaluate the benefits of integrating them into the energy retrofit plan,” said Martinez-Molina. “This study seeks to address this gap by quantitatively evaluating the effectiveness of various inherent energy-efficient features in a historic residential building located in a hot and humid climate, where the original energy concept aimed to cool the building in the absence of mechanical systems.”

The research team identified and assessed features, like existing shading devices (porch, roof overhangs, and awnings), large windows, high ceilings; and various envelope materials, such as wood, brick and limestone. Using a validated numerical model, researchers were able to present outcomes of their energy simulations and examine the impact of these features on indoor environmental conditions, cooling loads and occupant thermal comfort.

Their analysis provided several useful insights:

• Existing shading devices can effectively reduce indoor temperatures and cooling loads while minimally increasing relative humidity
• Large windows lead to higher temperatures but improve air circulation, resulting in lower relative humidity
• High ceilings generally lower indoor temperatures, but their impact varies with outdoor temperatures
• Replacing wood with high thermal mass materials, like limestone or brick reduces temperature and enhances thermal comfort

Researchers believe this body of work demonstrates that a balance can exist between improving the energy efficiency of historic buildings and preserving existing physical attributes, especially in regions with hot and humid conditions.

In addition to Martinez-Molina, Layla Iskandar and Saadet Toker Beeson, from the University of San Antonio; and Ezgi Bay-Sahin, from Lancaster University, contributed to the research on passive cooling and natural ventilation.

Read the full paper here: https://www.sciencedirect.com/science/article/abs/pii/S037877882400121X?via=ihub

In addition to Martinez-Molina, Carlos Faubel from Drexel, Layla Iskandar and Saadet Toker Beeso from the University of San Antonio, contributed to the research on quantification of energy efficient features.

Read the full paper here: https://www.sciencedirect.com/science/article/pii/S0378778824006625?via=ihub