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It almost goes without saying that it’s important to insulate the interior walls and attic space in the home, in order to make the house as comfortable and energy-efficient as possible. However, there’s another area in even the best-insulated home that can be the source of significant energy loss—and that’s through the uninsulated spaces that bridge through to the exterior walls.

PROBLEM AREAS FOR STANDARD INSULATION

Examples of areas that are particularly vulnerable to this kind of energy inefficiency are wall cavities around plumbing, air supply ducts, and open air pockets around bay windows and within and around other irregular structural framing features. It is very difficult, if not impossible, to properly insulate these irregular areas with standard batt insulation.

The insulated areas around plumbing and electrical features are often sites of insulation compression. In this case, batt insulation that is rated at, for example R9, is compressed to fit between the plumbing and the exterior sheathing, and the insulating, or “R’” value can fall several points.

Air supply ducts that butt up against an exterior wall can also reduce the amount of cavity insulation and worse, create temperature extremes between interior and exterior surfaces that can create condensation and significantly increase the potential for moisture damage.

Finally, there’s an interesting, and significant effect known as “thermal bridging”, that basically is the transfer of energy through wood framing. Wood framing is not a good insulator, so every stud acts as a potential thermal leak point, or “short”. Thermal bridging is a factor that can contribute to energy loss in both heating and cooling situations.
Since stud edges comprise about a quarter of the total exterior wall space in the average home, when combined with the other problem areas mentioned earlier, the potential for costly energy loss in an otherwise well-insulated house is significant.

INSULATED SHEATHING CAN PREVENT ENERGY LOSS

One solution to the problems mentioned above is to place rigid insulating foam board sheathing on the exterior. Areas clad with exterior foam sheathing have an insulating “blanket” between outside temperatures and the air-conditioned interior spaces.

The addition of this exterior layer of insulation greatly reduces the energy loss due to thermal bridging and the previously mentioned issues caused by plumbing, electrical and air pockets.

Rigid foam board insulation is made from plastic that has millions of tiny bubbles, or cells, within the plastic. These cells contain air that slows down the movement of heat and cold through the board giving the board its insulating qualities.

MOISTURE CONTROL

The problems that stem from the presence of unwanted moisture in the home cause billions of dollars of damage in American homes every year. Two of the primary sources of moisture problems in the home are infiltration (through defects in the building envelope) and condensation.

Condensation occurs when the water in a gas form changes to liquid because the “dew point” ratio of temperature and humidity has been reached. One very common example of this feature of nature is the condensation that occurs on the outside of a chilled can of cola when it is taken from the refrigerated space and placed in room temperature.

For example, in a room whose temperature is 70 degrees and whose relative humidity is 35%, the “dew point” is 40 degrees Fahrenheit. That means that condensation will form on any surface in that room whose temperature is 40 degrees or lower.

Because interior temperatures in a home are often either much cooler or warmer than outdoor temperatures, it’s important to reduce the temperature differential in the areas of the home where outside temperatures come into contact with interior air-conditioned temperatures. This usually happens somewhere between the interior wallboard and the exterior surface of the home.

For this reason, by sheathing with foam board insulation (thus reducing the temperature differential in the areas where air conditioned interior surfaces meet outside temperatures) one reduces the possibility that condensation will form. In this way exterior foam board insulation helps protect against moisture problems in addition to providing a money-saving “thermal blanket” of insulation on the outside of your home.