Passive Design for Humid Summer Heat
Jul 28, 2024
While there are record high temperatures all over the country this July, we’ve been experiencing this unusual heat wave in the Northeast too – reaching consistently into the 90’s with super high humidity. Now, I am used to the heat as I lived for many years in southern Arizona, but it is the sticky humidity that I have not quite acclimated to. The muggy conditions create a challenge for the body to transpire in order to cool itself, leaving you feeling hotter than you should. There are lots of great passive cooling strategies for home design that I love to write about and implement – but for today, I’ll focus on cooling strategies for hot-humid conditions.
Passive design is when we utilize the natural forces of the climate and the architectural elements (both spatial and material) to mediate the temperature and humidity for human thermal comfort. You can read more details about thermal comfort in my past blog post on bioclimatic design. Passive cooling strategies for hot and humid conditions can be some of the most challenging, since we are dealing with moisture in the air. When moisture interacts with building materials there can often be problematic effects, such as deterioration and rotting or mold and mildew, so it is important to understand the right type of dehumidification strategies for these conditions.
There are two basic ways that moisture can be pulled out of the air passively. One is by creating a moisture sink – this is when we create a surface that is a colder temperature than the dew point of the air temp, resulting in the moisture from the air condensing onto that surface. This is exactly what is happening when you drink a cold glass of iced tea on a hot and humid day and the surface of the glass becomes moist. In a rural, arid region of France, there is an historic and unique structure, known as the Achille Knapen air well, that was designed to pull moisture from the air in this way by creating a fully shaded large surface area of slate tiles that would be cooler than the air temp at night and cause condensate to form and drip to the base of the structure to be collected for potable water.
The other way to pull moisture from the air without mechanical force is to use either a desiccant (hygroscopic) or a hydrophilic – water-loving – material with a large surface area. The material microstructure and chemistry is one that will attract the water molecules from the air by chemical polarization. Desiccant materials are often used in the packaging industry to absorb moisture to protect products during shipping. There are many different forms of desiccants, including solid, liquid, and phase-change types. Within mechanical cooling equipment, the desiccant is most often a silica gel that is applied across the surface area of a fan-like wheel that has a specialized geometric matrix to maximize the surface area while humid air passes through the matrix so that the majority of moisture can be extracted from the air-mix.
Now, the conundrum with any form of dehumidification cooling is that as soon as the moisture is pulled out of the air the dry-bulb temperature of the air will increase! This is known as the latent heat of dehumidification. Of course, we really don’t want hotter temperatures when we are already experiencing a heat wave. But once we have a drier air mix, the surface of the skin has the ability to transpire and help to cool us off. And getting that moisture out of the air before it interacts with building materials in a negative way can be very helpful, to say the least.
The best passive strategies overall for these hot and humid situations are natural ventilation cooling techniques. These strategies engage the layout of the home’s operable windows in relation to positive and negative pressures around the outside of the house to encourage natural airflow throughout the interior spaces. There are a handful of natural ventilation strategies, including cross-ventilation, stack ventilation, and nighttime flush cooling ventilation.
Cross ventilation utilizes windows that are typically on opposite walls or sometimes on adjacent walls of a room to encourage airflow through the space. Stack ventilation makes use of a window on a ground level as an inlet and then utilizes an outlet opening at a higher point, such as at the apex of the roof or a clerestory. Nighttime flush cooling works with any cross or stack ventilation strategy but occurs during the night when the outdoor air is cooler and helps to flush out any heat build-up from the day in the home.
There is also a specialized passive cooling strategy for creating double-skins for the house walls and roof – this means a secondary outer layer is added to the house while creating an air-gap between the primary walls and roof and the outer layer. This outer layer serves two functions: 1) it provides a self-shading environment to protect the house from direct radiation and create a cooler indoor environment, and 2) the air-gap creates a forced ventilation effect by compressing air through the cavity and pulling heat off of the building surfaces. Sometimes houses are also raised off the ground on a type of platform foundation to allow a self-shaded condition under the house for a cooler environment and to encourage airflow all around the building structure. These are strategies that are best planned in advance of new design and construction for the home and are most beneficial in hot-humid climate conditions.
Mechanical fans are not a passive strategy, but they are low-energy and can be quite effective for hot and humid conditions. Having ceiling fans in a room or an outdoor space can facilitate a moderate breeze that helps the evapotranspiration effect across human skin. Combining low-energy fans with some hydrophilic materials in a room can greatly help create an overall cooling effect in hot-humid climates. The hydrophilic materials can be comprised of natural materials such as woven hemp fibers or coconut coir fibers. These can be used as decorative ceiling panels that are suspended to maximize the surface area and airflow around them, allowing more moisture to be pulled out from the air. There can also be tatami mats, a type of woven natural material, used for the flooring of a space to help absorb some of the moisture.
The spatial separation of hydrophilic materials on both the floor and the ceiling is important since humidity will stratify in two ways. Hotter air rises to the ceiling and can hold much more humidity, so a decent amount of moisture can be captured at ceiling height. However, the moisture in the air is heavier and will tend to stratify at low points due to gravity, so having hydrophilic materials incorporated in the flooring is also helpful. The traditional Japanese home designs with tatami mats and lots of sliding panels for the wall systems is the epitome of these concepts for passive cooling in hot and humid climates. Using hydrophilic materials in the architecture allows for moisture form the air to be better regulated for thermal comfort. Creating a panelized wall system allows for different openings to be created for cross ventilation depending on the prevailing wind situation on any given day. It’s important that these materials are properly pre-treated to handle moisture sorption without microbe development.
Hopefully you can make use of some passive cooling for your home and home workspace this summer – especially if you are experiencing these hot humid conditions like we are in the northeast. If you are interested in optimizing the design of your home for passive cooling, reach out to AIDA, LLC today for a consultation. You can always find more information and healthy home resources at Aletheia Ida Design and Architecture, LLC (AIDA, LLC) at www.aletheiaida.com.