Installs Membranes and Sealants and Installs Insulating Materials
Chapter A8 – Installs Membranes and Sealants and Installs Insulating Materials
Prerequisite Knowledge:
Topic 1 – Building Science: Heat, Air, and Moisture
Temperature
Temperature is the term we use to describe whether an object or environment is “hot” or “cold”. While there is a tendency to think of these two terms as opposites, it is worth looking at the meaning behind those words. While heat is a product of placing thermal energy into a system, there is no mechanism in which we can place “cold” into a system. In simple terms, the concept of “cold” is strictly relative, and is the result of a lack or absence of heat or thermal energy. Understanding this concept is helpful in the design of buildings when considering heat flow.
Heat Flow and Transfer
In general, heat is transferred from one medium to the next by three means:
Conduction – Which is the flow of thermal energy through solid objects. Much like electricity conducts through electrical wires, heat is able to conduct through solid objects. An example would include a hot pan on the stove or frost appearing on the warm side of a metal door.
Convection – Convection refers to the transfer of heat through air currents. An example would be the HVAC system in a house or a cold wind blowing in on a storm front.
Radiation – This term describes the transfer of heat by electromagnetic waves (typically infrared) through a cold, intervening space such as a window. The most prominent example is how heat is transferred from sunlight onto a solid surface in a building.
It is important to understand how these mechanisms work in order to produce a building envelope capable of efficiently maintaining a specified environment.
*** Try to name the method of heat transfer for the following examples:
- Heat loss through the exterior wall structure
- A window is left open to allow fresh air to circulate through the building
- Heat is lost through a triple pane window filled with argon.
- A house is warmed by a fireplace
R and RSI Values
The primary method through which heat is lost through a building is through conduction. While this process cannot be stopped, the effects can be reduced and slowed to reduce the rate at which heat is lost through exterior walls. Materials used to slow this transfer of heat are called insulators and are designated by their effectiveness. To measure this effectiveness, two types of units are used: R-value, and RSI value, which measure thermal resistance. R-value is the imperial equivalent of RSI values, which are the internationally recognized metric unit.
The two units can be converted, where 1 R value = 0.176 RSI whereas 1 RSI = 5.678 R-values. As a result, R-values are larger than RSI values. For example, a typical batted insulation while fills the void in a 2×6 wall (140 mm) has an R-value of 20, and an RSI value of 3.52.
Relative Humidity
The term relative humidity is not a constant measurement of the amount of moisture in the air, rather it is a value assigned in relation to the maximum amount of moisture air can hold at a specific temperature and pressure. It is expressed as a percentage in relation to this maximum capacity. When the airs capacity to hold moisture is reduced beyond the moisture present, the excess moisture condenses on solid surfaces as liquid water. Conversely, if the airs capacity for moisture is increased with no moisture added, it will feel dry and stuffy.
Generally, as the temperature in a room increases, its capacity to hold moisture increases with it, and as it drops, so does the airs capacity to hold moisture. When it comes to relative humidity, there is a balance that must be maintained. If the relative humidity in a room is too low, it becomes uncomfortable for the inhabitants. If the relative humidity in a room becomes too high, it can have severe consequences on the buildings structure, resulting in mould and rot.
Review Questions
Use the textbook CHBA Builders’ Manual to help answer the following:
- Name three ways heat enters a building.
- Name and give examples of the three mechanisms through which heat is transferred.
- Heat flows from a ______ space to a _______ space.
- Explain the “Stack Effect” in a building.
- What does HVAC stand for?
- What problems can arise if relative humidity is too low? Too high?