Jim again,
Why does it feel so hot in Miami (where the average summertime high temperatures are in the high 80s) when a day of the same temperature can feel tolerable in Phoenix (where the average summertime temperatures are in the high 90s)?
It is an example of how the combination of heat and humidity can impact our comfort levels, in both our exterior and interior environments. Want to know more?
Read on.
During my time in school, I became fascinated with the concepts of “Earth Sheltered Design” and “Passive Solar Design”. I was excited by the idea that earth sheltering could moderate the impact of the climatic conditions of the surrounding exterior environment on the interior space. As you go deeper in the ground (at least at the surface), the impact of the heat of the summer day and the coolness of the winter nights could be moderated. The earth could provide a thermal mass that would absorb the heat of the day and give back that heat during the cooler evenings and nights.
Passive solar design takes advantage of the most powerful heat source available to us here on earth, the sun. It is a naturally occurring, regular source of heat that is only impacted by clouds. Let enough sun into your space and you have a natural heat source. Combine that with the thermal mass of the earth and you can create a naturally heated space that stays warm throughout the nights. [Passive Solar Design is also discussed in a previous post entitled “Aquarius - Let the Sunshine In” dated March 17, 2023 in the ROOTS section of this website]
So with the sun providing our heat, let’s talk about humidity. As a part of our architectural education relative to building systems, HVAC (Heating, Ventilation, and Air Conditioning) systems specifically, we learned about the “Psychometric Chart”. This complex chart shows temperature graphed with relative humidity.
There is a “comfort zone” (shown in the area outlined in red within the image above) of relative humidity and temperature that one tries to achieve in buildings using both passive and active systems. People can be comfortable in a set temperature with a normal relative humidity but can also be comfortable in higher temperatures if there is lower relative humidity.
This is the answer to the earlier question about why we feel different in the hot humid days in the US southeast versus the hot dry days in the US southwest. People can typically handle the hotter days in the southwest better than they can the sticky days in the southeast. The same basic principles apply to interior spaces.
In the early days of implementing passive solar designs, in an effort to reduce energy consumption, many designers of these new building forms increased the amount of glass area in their structures. They thought that this would increase the amount of natural light into the spaces and reduce the need for the power required for artificial lighting. This admirable intent led to the undesirable consequence of serious overheating in these spaces. They hadn’t considered that the sun coming into these spaces in the daytime summer months would make them unbearably hot. This is where many of the principles raised in Victor Olgyay’s book, “Design with Climate” come in, including shading and ventilation.
You can keep the glass and natural lighting but you need to provide shading and make the glass operable.
Because the angle of the sun, in relation to the earth, changes over the course of a year, you can take advantage of shading devices to control the heat from the sun. The angle of sunlight is lower in the winter and higher in the summer, so shading devices of particular depth employed over windows can allow sunlight to enter the space in the winter months for heating but keep it out during the hot summer months.
Summertime cooling is another important issue and that is where ventilation comes in. Taking advantage of prevailing breezes and operable window locations can allow air to flow through a building or space. Warmer temperatures of the summer usually mean some level of perspiration occurring on people’s skin. Air movement, or breezes, allow for evaporation of that perspiration from the surface of the skin creating a natural cooling feeling. Unfortunately, when you have higher humidity levels, the air is limited in its ability to take in more moisture and your ability to cool through evaporation is also limited. Again, this illustrates why we say,
“It’s not the heat, it’s the humidity.”
I have been following the developments of all of these concepts for the last 40 years as they have become more sophisticated, more mainstream and have been put into practice and tested in the field. Unfortunately, I had limited professional opportunities to implement them. Our cabin at Runaway Ridge has given me the chance to plan, design and execute these concepts in an application that allows us to be energy efficient, and relatively energy independent. It feels good to give life to the concepts that excited and inspired me so many years ago as a young aspiring architect.