Source: Franklin Associates, Ltd.
U.S. Department of Energy (DOE) and U.S. Census Bureau

“There is more to consider than stated R-value, when selecting insulation material today”.

1. Clear Wall vs. Whole Wall R-Value:

 Clear wall: We now understand that insulating the space between studs with fiberglass is not the same as insulating an entire wall. We must consider the entire wall as a system. The lumber itself creates ‘thermal bridging’ and air infiltration within the wall, around windows, doors and at corner connections between the wall, ceiling and floor. So we’ve learned that it is important to evaluate insulation of the entire wall as a system, not just the area between the framing members (studs).

Consider fiberglass insulation, which performs well in controlled laboratory tests. But how does it perform when actually installed in homes? Fiberglass is susceptible to air exfiltration through its open cells – when air enters a wall through a crack in the siding or near a window or door opening. It flows right through the fiberglass, significantly reducing its ability to resist thermal flow. Moisture also causes fiberglass to lose its insulating capacity. Stated R-Value does not account for these shortcomings in real life applications.

According to Builder Magazine, based on ASHRAE studies and others, most R-Value calculations are based on conventional wood frame construction using “clear wall” or “center of cavity” criteria. R-Value only takes into account the insulation and framing members that make up the “clear” section of the wall, not the corners and intersections with windows, roof/ceilings or floors. Furthermore, the center-of-cavity method rates R-Value only at the point where the insulation is the thickest–right between the studs. Ratings are based on lab tests, not actual real life performance. For these reasons, R-Values of traditional bat insulation tend to be greatly overstated.

Whole wall R-Value: On the other hand, this takes into account the interface details of exterior walls, which intersect with other walls, the roof, deck, doors and windows. The framing members (studs) and connections create what are called “thermal shorts” – points where overall R-Value is greatly diminished, due to high conductivity of the materials themselves.

“EPS insulation used in structural panels offer tighter interface details than fiberglass bats, thereby eliminating air infiltration,” said de Campos. “For this reason, the use of whole wall R-Values is more accurate than the respective R-Value of the components alone”.

2. Thermal Drift

Another factor that affects the performance of insulation products after they are installed, is called “thermal drift”. “Depending on the insulation material used, the R-Value can be reduced over time, as the material ages”, said de Campos. “This should be considered when the designer calculates expected performance of insulation being recommended”. Some foam plastic insulation materials use blowing agents that have a high resistance to heat flow causing the insulation to have an abnormally high R-Value at the time of manufacture.

It is now known that these blowing agents diffuse from the cellular structure of the foam until a level of equilibrium is reached years after it is manufactured. As the high R-Value gases diffuse out of the open cell cellular structure, the ability of the insulation to prevent thermal flow is reduced, losing as much as 30% or more of its original insulating ability. Closed cell expanded polystyrene (EPS), which contains approximately 96% air does not use these types of blowing agents. “If you compare EPS with some foam plastic insulation materials right after they are manufactured, the other materials may have a higher R-Value initially” said de Campos, “but EPS is stable and does not experience any thermal drift and does not lose R-Value over its life. In the long run, the thermal performance of EPS insulation is constant over time and when all costs and performance factors are considered, it typically provides the greatest insulation value available”.