Insulation R values are key to minimizing energy consumption in buildings and reducing greenhouse gas emissions. But there are several factors to consider when selecting insulation, including climate zone, building type and wall construction techniques.
A key factor is understanding the difference between nominal and effective R-values. The higher the effective R-value, the greater the insulating power.
Nominal R-Value
R-Value is a common term associated with insulation products in the construction industry. R-Value is a measure of the material’s ability to resist heat flow through the product, and the higher the value, the more effective it is.
The R-Value of a material is typically measured at its installed thickness, and the value increases in a linear fashion with increased thickness. However, the R-Value of a material can be impacted by other factors that impact its performance.
One such factor is compression. Because insulators rely on air pockets, when they are compressed the air is no longer trapped and the R-Value of the material decreases.
The R-Value of a building is also impacted by the direction in which heat flows through the building envelope. For instance, if a wall is framed with studs on either side of the insulation, the studs offer a direct path for thermal transfer that is unrestricted by the insulation. This is called thermal bridging and it significantly reduces the R-Value of a wall. This is why the use of continuous insulation in a metal framed building is critical to meet energy codes.
Effective R-Value
Insulation is a vital component to efficient building design, ensuring comfortable indoor temperatures throughout the year while significantly reducing energy costs. However, it is important for architects to understand the difference between R-Values to make sure they are using insulation with the highest possible performance ratings.
The R-Value of insulation depends on a number of factors, including climate zone, building type, and wall construction techniques. For example, wood framed walls require different insulation than steel studs and joists, and a brick facade will need different R-Values than an airtight ICF (Insulated Concrete Form) wall.
Another factor that affects R-Value is the amount of thermal bridging present in the structure. A study conducted by Oak Ridge National Laboratory found that if there are gaps between insulation and framing members, the effective R-Value of the insulation can drop significantly. This is because these gaps provide a path for heat to flow that cannot be mitigated by insulation alone, even with a high R-Value. This is why energy codes typically mandate continuous insulation with a higher R-Value than nominal R-Values.
Climate Zone
The climate zone in which a building is located is another factor that will affect the R-value of an insulation material. Different climate zones have different requirements for the minimum amount of R-value required in a building. This information is available in the International Code Council’s IECC (International Energy Conservation Code) and can be used to determine the exact R-values needed for a specific building site and climate.
For foam insulators, the type of gas used to form the bubbles within the insulation can also have an effect on its R-value. Some foam insulators use gasses that are even worse conductors of heat than air, and these types of insulators will have lower R-values than those made from only air.
Generally, R-value increases proportionally with the thickness of an insulation layer. However, this relationship does not always hold true for all materials. For example, some cellulose batting and cotton wool become less dense when they are compressed, which may have an impact on their thermal resistance. In addition, fasteners and other non-thermally efficient attachment methods can create thermal bridges that will decrease the effective R-value of the insulation.
Building Type
The R-value of a building material reflects its ability to restrict the flow of heat. However, it does not account for the other two mechanisms of thermal transfer – convection and radiation.
For this reason, R-value is not the best indicator of how effective an insulation product will be over a long period of time. However, it does provide a good baseline for comparison.
Generally, thicker materials offer higher R-values than thinner ones. But it’s also important to consider how the insulating materials are constructed. As a result, many manufacturers offer insulation products that provide a range of R-values depending on the construction technique.
For example, a wood-framed wall insulated with fiberglass may have an R-value higher than a wood-framed wall insulated using SIPs. These differences are not accounted for by the R-value metric, but they can still significantly impact a project’s energy performance. For this reason, many energy modeling programs and code calculations require U-factors (or equivalent R-factors) for assemblies like doors and glazing. Consult manufacturer literature for more detailed information.
Wall Construction Techniques
The R-value of a particular insulation material is influenced by the thickness of the material. For example, a thicker fiberglass batt has a higher R-value than a thinner cellulose or foam board. Additionally, the installation methods used will affect the actual R-value of a wall. For example, compressing two layers of a given type of insulation together significantly reduces the installed R-value.
It is also important to consider the parts of a wall that are not insulated when determining a building’s R-value. For example, the wood studs in a wall provide a parallel path of heat conduction that is not covered by the insulation materials used in the cavity. This is known as thermal bridging.
Finally, it is critical to consider the effect of moisture on a building’s performance. Moisture that infiltrates or is trapped within a wall assembly can contribute to in-service deterioration and damage of structural components, and can result in condensation and mold growth that may impact indoor air quality. In many cases, this moisture must be tracked, isolated, and effectively managed to ensure long-term building performance.