| The US Department of Energy states that | | | | attic can cause a significant amount of heat to |
| between 50% and 70% of the average home | | | | enter the home during the summer. |
| energy expense goes to heating and cooling. | | | | Thermal exchange between the air flowing within |
| Therefore, homeowners may reduce the amount | | | | an air duct, located within the attic, and the air |
| that they spend on energy by insulating their | | | | within the addict can be minimized by insulating |
| home. | | | | the inside surface of the roof. What this does is |
| Insulation reduces the amount of heat lost by a | | | | to effectively make the combined interior of the |
| home during winter months and the amount of | | | | house and the attic one single insulated volume. |
| heat entering a building during summer months. | | | | Diffusion is the transport of energy that does not |
| The way in which thermal energy can be | | | | involve the movement of matter. For example, |
| transferred between a building and the | | | | heat can be transported from the interior of a |
| surrounding environment is important to choosing | | | | house to the outside environment through the |
| and applying the proper insulation. | | | | walls and roof even if there were no gaps near |
| In general, there are three ways in which thermal | | | | the window sills or doors. The transport of energy |
| transfer occurs: | | | | through the walls of a structure is not |
| 1 - diffusion, | | | | accompanied by the transport of matter, making |
| 2 - convection | | | | it an example of diffusion. |
| 3 - and radiative transfer. | | | | The rate at which energy is transported through |
| Of these three physical mechanisms, diffusion and | | | | the walls and roof depends upon the temperature |
| convection are the most important to the | | | | difference between the interior of the structure |
| understanding of insulation. | | | | and the outside environment as well as a quantity |
| Transfer by convection occurs when the motion | | | | called the thermal resistance or R-value. |
| of matter (e.g., an air current or flowing water) | | | | Therefore, an insulator having a high R-value will |
| carries heat along with it. Gaps along window sills, | | | | help to maintain the temperature within the |
| doorframes and elsewhere may cause heat | | | | interior of the home. Polyurethane spray foam |
| exchange between the interior of a house and | | | | has one of highest thermal resistances of any |
| the outside environment or an uninsulated attic. | | | | insulation with an R-value of between 5.6 and 8 |
| Therefore, losses due to convection can be | | | | per inch of thickness. |
| minimized by choosing an insulator that is able to | | | | As we have seen, polyurethane spray foam |
| fill these small gaps. Board stock is not able to fill | | | | insulation can cope with both of the two primary |
| such small spaces. However, spray foam insulation | | | | physical mechanisms by which a house can lose |
| can. | | | | thermal energy to the outside environment during |
| Thermal energy can also be transported by the | | | | the winter and absorb thermal energy from the |
| water flowing through the pipes of a home as | | | | outside environment during the summer. In |
| well as through air ducts located within an | | | | particular, it is able to reduce the rate at which |
| uninsulated attic. Warm air flowing through the | | | | heat is transported through the walls and roof |
| ducts of a cold uninsulated attic will result in a | | | | while simultaneously preventing air currents from |
| significant loss during the winter. Conversely, cool | | | | transporting heat through small gaps near |
| air flowing through the ducts of a hot uninsulated | | | | windowsills and doorframes. |