Polyurethane Spray Foam Insulation and Thermal Diffusion/Conduction
The US Department of Energy states that between 50% and 70% of the average home energy expense goes to heating and cooling. Therefore, homeowners may reduce the amount that they spend on energy by insulating their home.
Insulation reduces the amount of heat lost by a home during winter months and the amount of heat entering a building during summer months. The way in which thermal energy can be transferred between a building and the surrounding environment is important to choosing and applying the proper insulation.
In general, there are three ways in which thermal transfer occurs:
1 - diffusion,
2 - convection3 - and radiative transfer.
Of these three physical mechanisms, diffusion and convection are the most important to the understanding of insulation.
Transfer by convection occurs when the motion of matter (e.g., an air current or flowing water) carries heat along with it. Gaps along window sills, doorframes and elsewhere may cause heat exchange between the interior of a house and the outside environment or an uninsulated attic.
Therefore, losses due to convection can be minimized by choosing an insulator that is able to fill these small gaps. Board stock is not able to fill such small spaces. However, spray foam insulation can.
Thermal energy can also be transported by the water flowing through the pipes of a home as well as through air ducts located within an uninsulated attic. Warm air flowing through the ducts of a cold uninsulated attic will result in a significant loss during the winter. Conversely, cool air flowing through the ducts of a hot uninsulated attic can cause a significant amount of heat to enter the home during the summer.
Thermal exchange between the air flowing within an air duct, located within the attic, and the air within the addict can be minimized by insulating the inside surface of the roof. What this does is to effectively make the combined interior of the house and the attic one single insulated volume.
Diffusion is the transport of energy that does not involve the movement of matter. For example, heat can be transported from the interior of a house to the outside environment through the walls and roof even if there were no gaps near the window sills or doors. The transport of energy through the walls of a structure is not accompanied by the transport of matter, making it an example of diffusion.
The rate at which energy is transported through the walls and roof depends upon the temperature difference between the interior of the structure and the outside environment as well as a quantity called the thermal resistance or R-value.
Therefore, an insulator having a high R-value will help to maintain the temperature within the interior of the home. Polyurethane spray foam has one of highest thermal resistances of any insulation with an R-value of between 5.6 and 8 per inch of thickness.
As we have seen, polyurethane spray foam insulation can cope with both of the two primary physical mechanisms by which a house can lose thermal energy to the outside environment during the winter and absorb thermal energy from the outside environment during the summer. In particular, it is able to reduce the rate at which heat is transported through the walls and roof while simultaneously preventing air currents from transporting heat through small gaps near windowsills and doorframes
By: Andrew Stratton