Your Home's Thermal Pulse

Do you sense that your home is telling you when the temperature is changing outside? Our houses can be a real-time thermometer of sorts – they drop in temperature overnight and heat up by the late afternoon. This is especially true if you don’t have any mechanical air-conditioning or heating actively running. This is a natural state for thermal fluctuations to occur within our buildings in sync with what is happening outside. There are a few concepts and terms that help further define and explain the thermal phenomena within our homes.

First of all, we may experience diurnal swings! No, these are not swings that children play on that move in circles, but rather the change in temperature from day to night or through a 24-hour period. Diurnal swings are most extreme in hot-arid climates that have clear sky conditions, which allow for heat to rapidly escape the earth surface at night causing a large temperature drop. Diurnal temperature swings are least apparent in humid climates due to cloud cover preventing major radiant loss at night (the sky is sort of serving as a thermal blanket or insulator). In mixed climates, there may be some days that have large diurnal swings and other days that have minimal change in temperature from day to night.

When diurnal temperature change occurs, there is a large release of heat energy from all materials and surfaces exposed to the outdoor air and sky at night. When the sun is no longer active overhead, all materials and surfaces stop absorbing radiant heat. The absorbed and stored heat energy begins to emit back out over time, and this occurs more rapidly when there is a clear sky and cooler surrounding air temperature. Thermal energy naturally moves from hotter to colder states, so if the inside of our house was warm during the day it will tend to release that warmth to the cold outdoors at night. Heat transfer can occur by conduction, convection, and radiation. So, if there is also a breeze or wind blowing at night, the convective flow will further any effect of heat removal from building surfaces.

The building materials on both the exterior and interior of our houses will impact the way our home’s thermal pulse behaves. Different materials facilitate heat transfer in varying ways. Heat can be absorbed and stored for longer periods of time in materials that have a high capacity to do this, such as concrete and dense masonry. These materials, when used in building applications, are often referred to as thermal mass. When thermal mass is applied on the exterior walls of our house, the radiant heat from the day will often be stored in the walls rather than transferred to the inside of our home. Then, at night, the stored thermal heat will be released (or emitted) to the outside sky. This strategy aids in keeping our homes at a more stable temperature between day and night. Because of the heat energy being stored inside the walls, the release of the heat is delayed for a longer period of time compared to low heat capacitance materials. This results in a thermal lag effect.

The same concept can be integrated on the interior of houses as well but will have a different result. For example, when we have an exposed concrete floor or an exposed brick wall on the inside of a space, it has the potential to absorb radiant heat from the sun during the day as long as there is an adjacent window or skylight that is transmitting the radiation. The high heat capacitance materials on the interior of our spaces will also store heat energy for longer periods of time but at some point, will begin to re-release the heat into the room. This usually occurs at night when there is no longer a source of heat from the sun and as outdoor temperatures begin to drop. The result in this case is a natural warming effect of the interior spaces at night due to the release of heat from the high heat capacitance materials. This is a great strategy for cold climate conditions!

However, the tricky thing about an interior thermal mass strategy is that the heat released into the home at night can quickly disappear if the exterior walls are not properly insulated. Insulation materials are great in resisting heat transfer, often because they are entrained with air pockets. Air is the least efficient in transferring heat energy because the molecules in a gas are far apart and are not in contact with each other to transfer heat. Heat transfer by conduction occurs when molecules are in direct contact with each other. So dense materials like metals are great conductors of heat since the transfer of heat energy can occur very rapidly from one molecule to the next. Heat resistance is simply the inverse of heat conductance. Materials that are resistant to conduction are often called thermal insulators. Once we have heat inside our spaces accumulating, a well-insulated wall system will help to retain the heat indoors on a cold night.

Let’s consider a house located in a mixed climate where there is a wide range of seasonal temperature changes. In these scenarios, the challenge is to design the house to work well for different types of temperature conditions. There might be hot humid summers and cold arid winters, with temperate shoulder seasons of spring and fall. Just like we might have a wardrobe change-out with lighter clothing in summer and heavier sweaters and coats in winter, our homes also need to be designed for these seasonal change-outs. In summer, the exterior walls and roof will help prevent radiant and sensible heat from transferring to the indoors as long as there is good insulation or thermal mass. During summer nights, natural ventilation can be helpful to open up the house and allow for any heat to drain to the outside to maintain cooler indoor temperatures. Ventilation allows the house to breathe. There are different types of ventilation strategies that can assist the exhaust of heat including cross-ventilation (window-to-window) and stack-ventilation (window to roof vent or clerestory).

The same house design needs to accommodate the cold winter season too! In this scenario, the same exterior wall and roof insulation will assist in retaining heat energy on the inside of the house. While insulation is great for both summer and winter seasons, exterior thermal mass does not help in cold winters – only interior thermal mass will assist our indoor space temperatures to stay warm from day to night as long as radiant heat was absorbed during the day. Ventilation may be necessary for keeping fresh air moving through the indoor spaces, but it is only helpful as a cooling technique. Solar access is one of the best design strategies for winter seasons. When there is alignment between the sun’s position in the sky on winter days and the windows and fenestration of our house, we have the opportunity to allow the sun’s radiant energy into our home. This a wonderful gift from nature on a cold day! To adjust for summer, we just have to be aware of the sun’s changing position during each season and design accordingly. In summer, we can provide shading devices, roof overhangs, or specific window sizes and geometries to prevent direct radiation from entering the home.

Now, aside from the radiant energy of the sun and the general temperature conditions, we also need to be aware of air movement between the interior and exterior. Air movement will occur any time there is a pressure or temperature differential. While heat transfer through air is very inefficient, it can also be a source of rapid heat loss or heat gain when the pressure or temperature differentials are extreme (causing higher velocity flow). It is common knowledge that any small gaps in our house’s components will enable air leaks. Air can leak in both directions depending on the season. In summer, hot air from outdoors will slowly begin to seep inside filling cooled spaces with warmth that is counteracting thermal comfort. In winter, hot air from inside our homes will rapidly leak to the cold outdoors, causing a loss of any internal heat generation. In both scenarios we are losing energy very quickly and working against any thermal strategy that is being attempted. Utility bills for cooling and heating systems can be quite high when there are air leaks in a house due to the fact that the mechanical systems have to work twice as hard to maintain the desired temperature.

Air leaks are often due to small discrepancies of components during construction. It can be quite challenging to have all building materials and components perfectly match and align. In older homes, gaps between components may have developed due to differential settlement. This occurs when the earth under the foundation compresses over time, or settles, in an uneven pattern across the footprint of the structure. With differential settlement, there may be one side of the house that is slightly lower than another side. While this may not be noticeable to the visible eye because it can be extremely slight, it can cause some of the small gaps that begin to occur between building components. Doors and windows that are placed into frames within exterior wall systems will want to hang in a vertical direction with the force of gravity. The exterior wall system, however, may be resting upon a part of the foundation that is settling deeper into the earth. This can result in discrepancies between the door or window component and the wall system, causing small gaps to occur. Air leaks can be identified and treated with relative ease using sills and seals, but it can be challenging to make these treatments aesthetically pleasing.

The thermal pulse of a house really does synchronize with the climate, so the better we can understand the climate, then the better equipped we will be to design accordingly. At their core, thermal design strategies just require a basic understanding of heat transfer, materials, and solar geometry. While there can be many nuances to thermal design, we can apply some basic principles in our own houses by understanding the fundamentals. Proper thermal design provides comfort to inhabitants and reduces dependency on active mechanical systems, which in turn reduces utility bills and maintenance costs. If you are interested in more information as you prepare your house for the forthcoming winter season,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.