How much energy does it take to keep your conditioned air actually conditioned the way you want it. In the summer time, your home is trying to keep heat from flowing in. Or is it to prevent cool air from flowing out? In the winter time you are trying to prevent warm air from leaving. Or is it to prevent cool air from coming in? Learn how this impacts your power bill and your indoor air quality.
To understand this we need to understand how heat transfers and air moves in and out of a building. This first blog will discuss Heat transfer. The second blog will look at air movement (infiltration/ exfiltration) and the third blog will look at pressure changes.
Heat transfers or flows in one of three ways: Conduction, Convection, and Radiation.
Conduction is heat flowing through a solid material. For example, let’s say you have an outer wall that does not have any insulation in it. (This happens more than you would expect.) On a hot summer day the inside of that wall will be warm to the touch. The heat from the outside passed through the solid wall, unimpeded, because heat is attracted to cold. Insulation will slow the conduction of heat transfer.
Convection is the transfer of heat caused by the movement of fluid like air or water. For example, let’s say your friendly DISH satellite technician drilled a hole in your outer wall to pass wires but then left the hole the way he made it. Conduction is when the heat passes from the hot summer day into your home through the hole. Air barriers will slow convection heat flow. This type of hole is easy to appreciate but there are many other that are not so intuitive. Look for these in future blogs.
Your home’s seal or building envelope is composed of continuously touching Pressure and Thermal boundaries. The Pressure boundary is your continuous air barrier that slows convection of heat. The Thermal boundary is your continuous insulation coverage that slows conduction of heat. The two boundaries need to be touching throughout the entire envelope of your house. If they are not touching then your home’s ability to slow the transfer of heat is diminished. A regional Earthcents program pays particular attention to this as it is a key component to energy savings.
Radiation is the movement of heat energy from a hot to a cold surface through air with nothing solid or opaque in between. For example, imagine sitting on your roof or deck on a summer morning. As the sun rises you will feel the heat from radiation transfer. Intuitively, most home owners understand that the hotter it is outside the harder their AC unit will work. This speaks to the temperature difference between your conditioned and outdoor air. The larger the gradient the greater the two are attracted to each other.
Think about your Pressure and Thermal Boundaries and how they slow the convection and conduction of heat. Their ability to slow transfer is diminished the larger the temperature gradient. Shaded houses do not have to work as hard to condition their homes all other things being equal because the temperature difference is not as great in a shaded area.
Hopefully, this gives you an appreciation of how heat flows and transfers. I realize that some more technical folks might feel that this does not fully explain all the aspects heat transfer. For now, I believe it sufficiently explains why we as homeowners need to have a continuously touching Pressure and Thermal Boundary. Look for other blogs on specifics around where these boundaries often break down. There could be some easy DIY projects in store for you that will directly impact your power bill.
My next blog in this series will talk more about air infiltration and exfiltration. Not only does this have a significant impact on your power bill but it can also directly affect your Indoor Air Quality. How natural is your home’s natural ventilation? Are you living with the elements?
Let me know if this was helpful.