Building codes and green building standards are continuing to raise the bar on energy efficiency and high performance in buildings. This is achieved in wood-framed buildings by addressing both insulation levels and air tightness. While this is a positive trend, there are some notable wall design issues to address. Specifically, determining the best amount and type of insulation to use may be unclear, particularly in light of controlling water vapor or moisture that can become trapped in constructed wall assemblies. This is especially true in the case of providing exterior continuous insulation as part of a framed exterior wall. Codes and best practices suggest different amounts of continuous insulation for different climate zones. There is also concern that the continuous insulation can impact the ability of the wall to “breathe” and release any trapped moisture from within the assembly so, in some cases, it can impact the choice of an interior vapor retarder on the warm, inner side of the building. All of these variables and options have led to some significant confusion concerning the best way to properly address both code-required exterior thermal insulation and vapor management in wall assemblies. This course will help provide clarity on the differences between the varied prescriptive code requirements for continuous insulation in different climate zones, along with principles and choices related to proper moisture management.

All images courtesy of Huber Engineered Woods LLC, except as noted

The energy performance of exterior walls is enhanced by including exterior continuous insulation. With new integrated sheathing, this layer is built in to the back of the nailable sheathing that goes directly against the framing.

Why Continuous Insulation?

Framed wall construction, whether using wood studs or metal studs, has an inherent weakness from a thermal efficiency point of view. Simply put, the framing allows more heat to flow through it than insulation does. This is quite observable and measurable using standard techniques that test different materials for the amount of heat flow or heat transfer through them. Those tests are grounded in the fundamental laws of physics and thermodynamics that, among other things, point out that heat always seeks a balance by flowing from a warm source to a cooler place.

Heat Transfer

The means to measure heat transfer in building products is based on U-factors, which indicate how many British thermal units (BTUs) of energy pass through a defined size of material (i.e., one square foot) over time (specifically one hour) for each degree Fahrenheit in temperature difference. (The greater the difference in temperature between the two sides of the material, the faster or more intensely that heat flows.) In order to determine how much heat is transferred through any specific material, its U-factor is determined by testing that material on a square-foot basis over time, while measuring the temperature difference between the two sides. The resulting number is generally a decimal (e.g., 0.5), with smaller numbers indicating small amounts of heat transfer (think insulation) and higher numbers indicating more heat transfer (think conductive metal). Applying this to a building, the fundamental formula used is (U x A) x dT where U= the tested U-factor for one square foot of material, A= the area in square feet installed in a construction assembly, and dT is the design or actual temperature difference between indoors and outdoors. All thermal energy calculations in building enclosures (i.e., walls, roofs, etc.) are based on this fundamental formula.

It is worth pointing out that while scientists and engineers like to work and think in fractional U-factors, most of the general population prefers whole numbers, which has made R-values the popular means to talk about thermal capabilities of materials. This is still very legitimate since the testing and calculation process is exactly the same. The difference is that instead of indicating the results as heat transfer through a material, they are reported as heat resistance—the direct inverse to heat flow. Since U-factors and R-values are the multiplicative inverse of each other, to convert U-factors to R-values and vice versa, you divide one by the number you are attempting to convert. So, an insulating material with a tested heat flow U-factor of .05 is easily then divided into 1 (1/.05) to indicate a resistance R-value of R-20. Similarly, an insulation product with an R-value of R-20 is converted to a U-factor as ¹/₂₀ = .05. Hence, it has become common for individual materials and products to be promoted and marketed based on their R-values. It is also somewhat easier to think in terms of higher R-values equaling greater resistance to heat flow, which essentially translates into better energy performance in building enclosures. From a calculation point of view, R-values of multiple materials can be added together to determine a total R-value, but U-factors cannot be combined together.

Thermal Bridging

As most design professionals are well aware, construction assemblies are very rarely monolithic. Rather, they require different materials that are assembled to make up the overall construction. In framed exterior walls, the framing members are spaced at 16 or 24 inches on center with upper and lower plates, not to mention additional framing around door or window openings. This framing defines the fundamental thickness of the wall and the spaces between or around the framing are commonly filled with insulation. Then, continuous layers of interior and exterior sheathing, such as gypsum board or wood panel products, cover the framed and insulated areas to create a wall ready for finishing. In order to accurately determine the true thermal performance of this typically constructed wall, at least two calculations are needed: one based on cutting a cross section through the framing and the other based on a cross section through the insulation. Then the resulting numbers need to be applied to the appropriate percentage of the total wall area to produce a weighted average UA for the whole wall.

In typical situations, the framing can account for 20 to 30 percent of the area of any given exterior wall with only about 70 to 80 percent of the wall area actually containing insulation. Since the framed sections will not have the same U-factor/R-value of the insulation, the thermal effectiveness of the wall is directly compromised. It is easy to ask, is this 20 to 30 percent framing area really a big deal? It turns out that the answer is yes. Any building material, including framing or sheathing, that has a capacity to transfer heat more than insulation will follow the laws of physics and do so. In this case, every stud or other solid structural item, like floor band joists, columns, etc., is acting as a breach in the insulated wall, allowing heat to transfer through it. This solid connection between the warm side and the cool side of an assembly acts as a “thermal bridge,” allowing heat to flow freely between the sections where the insulation is present.

To illustrate this, let’s look at U-Factor Example 1 showing 2-by-6 wood stud framing at 16 inches on center with R-20 insulation between the studs. We have identified the section through the studs as A1 and the section through the insulation as A2. Entering the tested and known R-values (from independent sources) of the various materials, we find that the total R-value through the studs is only R-7.95 (U-0.126) compared to an R-21.07 (U-0.048) through the insulated portions. Assuming 22 percent framing and 78 percent insulating areas, the weighted average for the total wall produces an overall effective R-value of R-15.34 (U-0.065). This is a reduction in overall thermal performance of more than 27 percent due to the thermal bridging of the studs, which is quite significant.

Calculating the thermal performance of framed walls with only cavity insulation needs to account for the heat transfer through the studs as well as through the insulation.