What is a U-Factor for Energy Efficient Homes?
As we use more science in building our homes, homeowners may want to understand what is being discussed. U-factor has been a term used in the past, but now is seen more frequently when energy efficient standards are being mentioned.
You may have heard the term U-factor when shopping for energy efficient windows. The lower U-factor is the best option. What does that mean though. If you are interested at all in energy efficient buildings you may be seeing this value mentioned more often. We are applying this value to more than windows. Am I suggesting that homeowners be able to calculate the U-value of their building envelope? No. I do think that you should understand the concept as it is being applied more to home construction.
I realized that building professionals throw out terms that homeowners do not understand when I began as a home inspector. I try to avoid confusing clients, but sometimes you have to use the proper term. Sometimes I am lulled into believing that a term is so common that homeowners understand it. I was giving a lecture when I mentioned what the R-value for insulation in the attic should be. That is when someone stopped me. They knew that the R-value was associated with insulation, but what was it, so I had to explain what the significance of an R-value meant. I did not mention U-value at that speech, but I realized that could be more confusing. I also realized that attaching a meaning to R-value would make the importance of what your R-value is.
The U-factor and R-value are related in a simple way, and they are connected to another value, the K-value. The K-value is a measure the rate of heat conduction through a piece of material that is one inch thick and is one square foot. We refer to the K-value as conductance. If we are speaking of the conductance value of a specific material, we call it the R-value. We refer to this as thermal conductivity, which we refine by taking the material with the one inch thick and one square foot dimension, but we measure the flow of heat when there is a one degree Fahrenheit difference in temperature on the two sides of the material over a period of one hour. We can also call this the thermal resistance of the material, so that is where we get the “R” of R-value. If we want to calculate the U-factor, all we have to do is find the inverse of R-value (U= 1/R).
If the R-value and U-factor are so closely related, why do we need both figures? The U-factor tells us something different, and it functions differently. I can add R-values together to find the total R-value. I add an R-26 insulation over an R-30 insulation, I obtain an R-56 (R30+R26=R56). This does not work with the U-factor. Also, an R-value is associated with a specific material. A U-factor can be calculated for a combination of materials. With K or R, we find out the rate of heat conducting through a material. With U, we find out the amount of heat that is transferring through the material. Knowing the amount of heat amount transferring helps us to calculate the power and energy needed for heating. This is done by looking at the heating load (q) which is calculated by multiplying the U-factor by the area of the structure by the difference in the design temperature and the desired indoor temperature. Design temperature for heating is looking at a temperature that is equaled or exceed most of the time(97.5%) in the winter months. This is great if you live in a cold climate, but in warm climates, like Houston, we have to look at the power and energy used during a summer months, since we will use more energy to cool our homes. Again, knowing the U-factor of our walls, ceilings and floors will help us calculate our energy requirements. In either case (concern of heating or cooling), a lower U-factor means less energy being used.
Since the R-value does not help us calculate the energy needed for the home, we will see more people in the building industry refer to U-factors. Building scientists are already referring to U-factors more often when discussing energy efficient homes. The popular new energy efficient building standard, PassivHaus or Passive House, relies on achieving a certain U-factor rather than stating install a certain R-value with the insulation. One thing to note about this standard is that it was designed for a colder climate than what I have in Houston, so this standard will not work out of the box for my home. However, there are individuals looking at how these standards can be converted to apply to these warmer climates. What I do like about discussions of the U-factor is we are getting away from simply discussing insulation. We are realizing that good design with a comprehension of how materials work together can build a better house.
Lastly, I wanted to mention a simple test that I did on my own home which may help you with some of your own energy efficiency plans. This test was loosely connected with my studies of how I would apply U-factor calculations in my home conversion. I tested the surface temperatures of walls, ceilings, and floors on the exterior and interior of the home during the course of a day. I found that my walls were slightly effected with small changes in surface temperature as we moved into the hottest part of the day. The floor was also little effected (I have a slab on grade foundation). Ceilings were another matter. My roof has a low pitch, which makes movement in the attic difficult. Little insulation is over the edges of the home. I found temperature differences of more than 10F in surface temperatures in interior ceiling surfaces within a distance of three feet. Exterior surface temperatures were consistent. The hotter spots had roof surfaces closer to the interior ceiling surface and less insulation due to my not getting into the edges of the attic area. What does this do for my energy efficiency? Well, these hot surfaces help deliver the exterior heat into the home, which makes the cooling system work harder, reducing energy efficiency. I am continuing to look at these temperature differences. Of course, wall penetration areas, like doors and windows, have greater problems. This is why U-factor is important in discussion about windows, and why the average homeowner will encounter the U-factor when window shopping.
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Kristina Says:
