Building Insulation for Energy Conservation

Energy use reductions and improved thermal comfort in buildings have been the driving factors behind most of the latest advances in insulation products. Most of these advances have come from a better understanding of how heat energy moves or flows into or out of a building under various climate and temperature conditions. Fundamentally, it is the laws of physics that govern the four fundamental ways that heat energy moves - conduction, convection, radiation, and heat transfer through a change of state. In order for insulation to be effective, then, it must appropriately address one or more of these four heat flow processes. Designers need to understand these heat flow characteristics and how insulation affects them in order to accurately calculate or model the energy efficiency of a building design and determine the predicted energy use or overall building performance. Next we will elaborate upon these four heat flow methods.

Conductive Heat Flow

This is the thermal heat flow process that most people think about in buildings and often plays strongest in specifying insulating materials. Any material has the capability of conducting heat energy through it but some simply do it better than others. This heat energy always flows from the warmer side (higher heat energy) to the cooler side (less heat energy) in a natural effort to achieve a balance or equilibrium regardless of the direction of the flow. The rate or speed in which this conductive heat flow occurs in any given material or assembly is measured in "U" values which, defined in simplest terms, is the number of British Thermal Units (BTU's) that flow through one square foot of a material in one hour (per degree Fahrenheit of temperature difference).  The more common "R" values measure the same heat flow in a material just in reverse - in terms of its resistance to that heat flow. Hence, it is simply the opposite or reciprocal of a given U value meaning that mathematically, an R-value is equal to 1 divided by the U value or 1/U. If conductive heat flow moves quickly through a given material then the U value is higher while it's reciprocal resistance or R- value is lower. Brick and gypsum board, for example, have a high U value of 2.5 and a correspondingly low R value of 0.4 meaning heat will conduct readily through these materials with low resistance. Conversely, in materials like insulation that conduct heat more slowly, the U value will be much lower and the corresponding R value will be much higher. The most common way to determine accurate U and R values of material is through ASTM testing that has some inherent limitations since it seeks to confine at least 3 real world variables to determine the test results:

• Test temperatures.The rate of heat flow is directly impacted by the actual temperatures and the relative temperature difference between the warmer side of a material when compared to the cooler side. Typically, the bigger the difference in temperature, (often expressed as ∆T) the quicker the heat wants to flow. In order to produce a relevant test, the ASTM standards call for materials to be tested with warmer and cooler temperatures that are within 10 degrees of the anticipated average or common applications of that material. For most building products, that means that a warm side temperature of 75 degrees Fahrenheit and a cold side temperature below freezing. If your typical installation is different than the test temperatures, then the R values will likely be somewhat different when the building is up and running.
• Material thickness. Most R values are expressed in terms of one inch of material for purposes of comparing different materials. However, materials like insulation that are typically installed in thicknesses greater than an inch can be tested in that full thickness with potentially different results from testing a single inch. If a manufacturer is showing R values for a product that are higher than simply multiplying the per inch R value times its thickness, then it is appropriate to ask for the test on the material at the full thickness that demonstrates this total product R value.
• Average density. Often, the density of an insulating material directly affects the rate of conductive heat flow through it, sometimes performing worse or better if the material is denser. Manufactured products, however, may be produced with various density levels either by design or by virtue of normal tolerances in the manufacturing process. Products that are prone to less control of density are tested based on samples of product over the typical density range. This suggests that there will likely be some additional variation in the actual performance of the product when installed particularly if the density is controlled by the installer as in the case of some blown in or field sprayed applications. Specifying products that have a predictable and tested density range will help with predicting actual building thermal performance.

Of course, all of these conductive heat energy characteristics will work on all of the materials in a building assembly, not just the ones tested. So, while testing can determine how an individual material will perform, there is a trend to move toward thermal testing of a full construction assembly such as a typical total wall or roof section. ASTM test C 1363 for example is a test for a typical wood framed, insulated wall that yields an R value for the entire wall assembly. Currently, it is still more common to calculate the heat flow through the different parts of an assembly and average out the total result. So, in a typical wood framed cavity wall or roof assembly with insulation between the framing for example, the R value of the insulated section (center of the cavity) can be calculated separately from the framing section (through the studs). By determining the percentage of the wall that is framing (typically 20 to 25%) compared to the percentage that is insulated and allocating the respective R values accordingly, a more accurate overall R value of the wall can be determined. 

This same concept of differing conductive heat flows through different materials can apply in unwanted ways if the wall assembly is not detailed or specified to avoid "thermal bridging". In this condition, the insulation is compromised by other, more conductive materials, such that a thermal bridge is created between the warm and cold side of an assembly and allows heat to siphon across it. This is commonly found where structural fasteners, braces, or mounting elements are incorporated into the assembly or where mechanical, plumbing, and electrical components pierce an assembly. Obviously such conditions illustrate the negative consequences of conductive heat transfer in that they compromise the overall heat resistance of the assembly. The result is lower overall R values and lower than expected building performance.