Is it Hot in Here? Or is it just me?
Heat moves in three main ways - conduction, convection, and radiation. As Allison Bailes of Energy Vanguard explains, understanding the difference between these three modes of heat transfer is important for building science and creating energy efficient homes.
To understand heat transfer and why it’s important for a healthy, durable, and resilient home, we need to first need to know three concepts…
Must-Know Concepts
Heat is predictable. Regardless of your climate zone, these three concepts apply…
1.Heat always moves from warmer areas to colder areas.
When it’s cold outside, we heat our homes inside. The direction of heat flow is from inside to outside. In summer (or most of the year for us Texans), we are cooling the inside and it’s hot as heck outside. The direction of heat flow is now from outside to inside. This is why your control layers are so important and should be designed to withstand the heat flows most prevalent in your climate zone (or consider the “Perfect Wall”). See the Super Control Layer Freak blog on this point.
2. Heat flows faster where the temperature difference is greater. If it’s 60°F inside and 65°F outside, the difference is not very noticeable to your comfort because there’s not much energy transferring through the enclosure. But, if it’s 70°F inside and 110°F outside, there is a lot of heat flow, and the difference is more obvious. This is why you’re calling your HVAC company more in the summer when you’re noticeably uncomfortable due to the increase in energy transfer through your building enclosure.
3. Air contains moisture.
Warmer air holds more moisture than cold air. Have you been to Texas? You can feel the weight of the moisture in the air as it makes your hair nice and frizzy. If the air is cooled sufficiently to cause the moisture in the air to condense on a surface in the home (See the Just Dew It blog post), it can have a huge impact on building durability.
Heat Transfer
Back to the ways heat moves…
First up - Conduction
Conduction is the transfer of heat between two solid objects that are in direct contact with each other. Think—ironing. It happens at a molecular level - heat energy is transferred as vibrations between neighboring molecules. Metals are very good conductors. This is why metal feels instantly cold or hot when you touch it. The heat quickly conducts from the metal to your hand.
Are you thinking about metal roofs right now? I am.
Next - Convection
Convection is flow of heat within a fluid. Think— boiling water.
As Bailes describes it, convection works like a “conveyor belt for heat.” When a fluid like air or water is heated, it expands, becomes less dense, and rises. Cooler, denser fluid then moves in to take its place. This circulation of fluid transports heat around. Convection currents transfer heat through the atmosphere and oceans. Convection ovens and convection radiators in homes use this process to distribute warm air.
Does this remind you of our stack effect conversation?
If you don’t know. You probably need to get the Actinomycetes Guide (…and clean your drains).
Photo Credit: Schwartz, L. SOURCES OF ACTINOMYCETES ENVIRONMENTAL BACTERIA IN HOMES AND METHODS OF CONTROLLING THEIR PROLIFERATION AND LEVELS; Sept 2021.
As air warms, the molecules move farther apart, and the air becomes more buoyant, floating upwards. As that air rises, cold air is pulled from below to replace it
Finally - Radiation
Radiation involves electromagnetic waves carrying heat directly from its source to whatever solid objects are in its path. No direct contact is needed between the heat source and the warmed object. The heat moves through space, not air, as electromagnetic waves. The sun warming the Earth is an example of heat transfer through radiation.
Radiant floor heating systems work this way too - using electricity as the heat source to generate thermal radiation which travels across open space to people and objects.
When an object, or an assembly is warmed by radiant energy, the energy is actually absorbed into the material. According to the Journal of Light Construction, in order for a surface to be warmed by a radiant heat source, the surface needs to be in the line of sight of that heat source.
This is why shading works. If you put an overhang or an awning between the sun and a window you can reduce radiant heat flow. In that case, the sun warms the overhang or the awning, instead of the window, when the energy is absorbed into those materials.
Source: Journal of Light Construction, “Heat Transfer Through Buildings”
Understanding these heat transfer mechanisms and the three must-know concepts about air are key to designing energy efficient buildings.
Strategies like insulation resist conductive heat flow while technologies like convection fans actively use convective airflow to distribute warmth.
First understanding, then strategically taking advantage of the laws of thermodynamics leads to comfortable yet sustainable living spaces.
WHETHER YOU RENT OR OWN, I’ve organized everything you need here in one place to curate your healthier home.
Module 1 - get to know your home (identifying higher-risk areas, collecting data, organizing emergency contacts, understanding the limits of your builder’s warranty and home/renter’s insurance)
Module 2 - Weekly maintenance (all the stuff you need to do on a weekly basis to maintain the health of your home)
Module 3 - System’s maintenance (HVAC, Electrical, Plumbing, Building Assemblies, Appliances)
Module 4 - Seasonal maintenance (❄️, 🌷, ☀️, 🍂—I cover it all in “zombie prepper” detail)
