Convective Heat Flow

How often have you heard the adage that heat rises? Actually that is not an entirely accurate statement. As we just saw, heat can transfer through conduction in just about any direction from warm to cold (up through roofs, down through floors, sideways through walls, etc). However, when fluids such as air or water are heated, they respond by changing density causing the warmer air or water to move or flow upwards over the cooler portion. Therefore, it is actually heated air or water that rises. If the warmed portion subsequently cools because it has moved away from its heat source, then it will begin to get denser and fall setting up what we commonly refer to as convection currents. As long as there is a warm portion and a cool portion, the natural result will be the ongoing convective heat flow of rising and falling air or water that is seeking to find its equilibrium. 

This convective heat flow process is not necessarily a bad thing since it can be used to an advantage inside buildings to produce natural ventilation or otherwise capitalize on what is commonly referred to as the "stack effect" or "chimney effect." Essentially, heated air is allowed to rise and escape through a designated opening causing lower inside air pressure that draws in cooler outside air through other designated locations. This chimney effect principle is also the centuries old basis for fireplaces and flues to work properly. However convection has been found to be a detriment of building thermal performance when design or construction flaws cause it happen unintentionally. And it doesn't require flaws in the entire building for it to be an issue since wall and roof cavities are among the most common building spaces to generate convection currents. In an un-insulated framing cavity the warm side heats the air closest to it causing it to rise while the air on the cool side drops down. This air motion actually speeds up and facilitates the heat transfer from the warm to cool side of the cavity which may be directly contrary to the design intent.

Completely filling framing cavities with higher density insulation reduces or eliminates convective air flows in the cavities. Photos courtesy Bonded Logic Inc.

 

One of the most common misperceptions about stopping convective heat flows in cavities or voids is that simply stuffing insulation into the space will solve the problem. The reality is that not all insulation products stop air flow. Anyone who has ever felt a cold draft around an electrical outlet or switch on an exterior wall insulated in this manner has experienced the negative effects of convection currents in wall cavities. Fiberglass for example gains its R value by virtue of the airspaces that are created around strands of glass fiber. All by itself, it does not stop the flow of convection currents. In fact, most HVAC air filters are made of the same fiberglass material demonstrating the ability of air to flow through it. Other materials, including products manufactured at a higher density, gain their R values through different material make ups that do resist air flow and can be effective in stopping convection. One example of such a product includes recycled cotton batt insulation. An emerging practice is to use a hybrid insulation system where all openings and edges of all cavities are sprayed or covered with an air stopping material to prevent convection from travelling between cavities. Then the cavity is filled with an air flow resistant insulation.

Radiation

Insulation used as a radiant barrier will include a reflective coating of foil or other materials. Photos courtesy Bonded Logic Inc.

A heat source in its purest form will radiate heat outward in all directions. This is true if that source is a burning ember, a stove top element, or the sun. This characteristic of heat has been used in directional radiant heaters that transfer heat by electromagnetic waves through the air to heat a building surface or people requiring a line of sight between the source heater and the intended contact surface. If a building assembly is subjected to a radiant heat source, such as sunshine or a radiant heater, then the contact surface will warm and the conductive or convective heat flow processes will take over. This may be fine if it is intended or desired, but more commonly, designers prefer to reflect away this radiant heat and control the temperatures on the walls and roofs of the building. Using materials with reflective properties on exterior wall or roof surfaces will keep these surfaces cooler than darker, absorptive materials that turn that sunlight into heat. Lower surface temperatures mean a lower ∆T across the assembly which means the rate of heat transfer is slowed and the U and R values achieved are closer to those anticipated.

Inside the building, radiant heat transfer principles have often been used to reflect heat appropriately within a desired space. This is commonly done in windows that use glass with reflective properties to keep heat outside in warm climates or heat inside in cold climates. Insulation materials that use a reflective coating, either as an integral or added component, can work the same way. When installed in cavities or open spaces they have been shown to boost performance beyond the measured R values of the materials. To do so requires open air space where the radiant heat can truly be reflected. If simply sandwiched in between other materials, the reflector is simply acting as another material that conducts heat through it.