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Green Building: R value vs. K value, and U value

By Juan Garcia

As Seen In: ICF Builder Magazine

Energy terms are often confusing and sometimes not suited to the situation in which they are used.

Terms like “tested R-value,” “performance R-value”, and “K-value” need to be better understood in order to appreciate the advantages that ICF construction offers. In the past there has been much speculation about the high R-values –often 50% or greater compared to conventional construction— associated with ICF buildings. A short review is then in order for the basic course in energy conservation.

Wikipedia.org, the online encyclopedia, defines R-value as “thermal resistance.” In order to give a working definition of thermal resistance it is necessary to understand that there are many factors influencing R-value and that laboratory tests often do not reflect real world performance. For instance, it most certainly never blows or snows on the lab technicians. These differences have given rise to the notion of “performance R-values,” which may be accurate, but are difficult to verify. The most recent measurement is K-value, which measures how well an object conducts heat.

Convection: Convection is the transfer of heat by the motion within a fluid or gas—like central air heating. Convection is the largest form of heat loss from a structure and can be addressed by airtight construction methods like ICF, which limit the amount of air that can get through the building envelope. “Tested R-values,” however, don’t consider the level of airflow through the wall, which is one reason why many claim R-values don’t accurately represent the insulating properties of an ICF wall.

An ICF structure with R-22 walls will outperform any type of conventional structure with double the R-Value in any side by side comparison.

Incidentally, convection is also the most efficient way to heat a structure. Warm air rises off a heated floor; cool air falls to the floor to be warmed.

Conduction: is the transmission of heat across matter. If you’ve ever burned yourself on the handle of a cast iron skillet, you probably know conduction is the strongest form of energy transfer and the most constant. An un-insulated concrete floor is constantly losing heat to the environment 24 hours a day through conduction, which is why some builders install an underslabthermal barrier.

While convection is the largest form of heat loss, walls, floors and ceilings will also conduct heat to and from the environment unless the conductivity, or K-value, of the materials is very low. K-value, represented in physics by the letter lamda, is the ability of a material to conduct heat. In order to build energy efficient structures, materials of a low thermal conductivity are desirable both above and below grade. The lower the K-value, the better the material is for insulation. Expanded Polystyrene (EPS), which is used to make insulating concrete forms, has a K-value of 0.033. For comparison, fiberglass insulation has a K-value ten times higher (0.33), while wood has a K-Value of 0.76, and marble has a K-Value of 11.0.

Radiation: Convection is the transfer of heat by the motion within a fluid or gas—like central air heating. Convection is the largest form of heat loss from a structure and can be addressed by airtight construction methods like ICF, which limit the amount of air that can get through the building envelope. “Tested R-values,” however, doesn't consider the level of airflow through the wall, which is one reason why many claim R-values do not accurately represent the insulating properties of an ICF wall.

Thermal Mass: Another consideration in calculating energy efficiency is the thermal mass of the structure. A thermal mass in the most general term refers to anything used to absorb and hold heat. Dense materials like stone, concrete, adobe or water work best. Since the ICF concept couples non-conductive foam with an energy-holding thermal mass, any energy that may bridge the foam is absorbed and held by the concrete –perfect redundancy for the energy envelope.

Finally, the term U-Value is the total amount of energy transfer through convection, radiation and conduction. This is an architectural term used to describe the energy efficiency of a structure, calculated using a formula that considers the materials specified for the building envelope—floors, walls and ceilings.

Energy efficient structures keep warm air in, radiant energy out and do not conduct energy in or out of the energy envelope.

Total Effective R-Value with ICFs consists of three factors:

1. R-Value of the EPS will reduce conduction by about half.
2. Thermal mass of the concrete will smooth out large swings in temperature.
3. Air infiltration (convection loss) is reduced to near zero, saving 30% or more on heating/cooling costs.

Insulating Properties of Common Materials

Conclusion: In summary, the R-Value is a number that has been poorly defined and is even more poorly understood in terms of performance. An ICF structure with R-22 walls, SIP (structurally insulated panel) roof, and a foam insulated floor will outperform any other type of conventional structure with double the R-Value (R-40+) in energy efficiency, structural strength and return on investment in most any type of side-by-side comparison. Why? Because ICFs are airtight, non-conductive, have good thermal mass, are very strong and have the advantage of increasing in value purely based on the spiraling cost of energy regardless of location. ICF construction is the way of the future. The future is now. This is the first in our newest series: Green Building. Stories submissions about energy efficiency, sustainability, and other topics are welcome at editor@icfmag.com.

 
Primal Consulting, Established 2005   |   Ph: (602) 460-7111   |  Email: JuanGarcia@primalconsulting.com |   © 2009 All Rights Reserved
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