Your home encloses you from the elements. Not only does it keep you dry when it rains, it keeps you warm when it snows, and it keeps you safe in a storm. The inside of your house is your own personal eco-system. Before we can understand how to maintain this perfectly comfortable eco-system, we ought to discuss what protects us from Mother Nature – the building envelope.
The building envelope is a fancy way to describe the parts of your home that face the outdoors: walls, windows, foundations, doors, and roofs. Each item on this list helps encloses you from the whims of weather. These items all have specific properties and variances that affect how well they keep the heat in our out (depending on the season).
The building envelope physical property most people are familiar with is the “R-Value”. R-value is a unit of measurement that describes how well a physical material prevents the transmission of heat, a property known as thermal resistance. For our Canadian purposes, this generally refers to the ability of a wall to prevent heat from moving from our comfortable interior to the frozen exterior.
Heat will always flow from hot to cold (this forms the basis of the Second Law of Thermodynamics). Without delving too deeply into the physics of thermodynamics, the R-value tells us how many watts (or BTUs for those who are so inclined) of heat will flow through a wall of a given size with a given temperature difference on either side. More heat will escape when the temperature difference is higher (in winter, the difference across an outside wall can be as high as 50 degrees Celsius if it’s -30°C outside) or if the wall has a larger area (a 10’ x 10’ wall will transfer more heat than a 6’ by 8’ wall with a similar R-value). Our obvious goal is to reduce the amount of heat lost to the exterior; to quote my grandfather: “We’re not paying to heat the outdoors!”
We shouldn’t limit our discussion to walls; every part of the building envelope has an R-value. Part of the job of a HVAC consultant is to determine exactly how much heat a building will lose given the R-values of all the walls, doors, windows, foundation walls, and roof. It goes without saying that higher R-value building envelopes require less energy to heat than a building with lower R-values. However, there is another factor that affects indoor comfort, and that is infiltration.
Infiltration is air from outside your home that passes through the building envelope into the conditioned space. Older homes are sometimes called “drafty”, as they are less airtight than newer homes. Older homes were so drafty that there was no need for indoor ventilation given the amounts of outside air passing through the building envelope. This outside air must be heated to keep temperature constant in the home. As infiltration increases, so does your heating bill. In some buildings I review, infiltration can account for more of the heating load than windows, walls and roofs combined. Modern buildings have become much more airtight than their predecessors. Buildings became so airtight in the energy crises of the 1970s and 1980s that “sick building syndrome” became an issue. Buildings built today are provided with dedicated ventilation systems to ensure fresh air is circulated through the building without the cold drafts of yesteryear.
With this brief primer on building envelopes behind us, we can begin to discuss the real meat of this blog – how to control the temperature inside our palace regardless of the season. My next post will describe the various ways we heat and cool our homes.
