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U Value: A Better Energy Calculation

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Great discussion!

Wayside, I have a feeling from reading your posts (and I could certainly be wrong) that you are asking why the window manufacturers introduced U-factor (or value) when R-value already existed? Why mess with a good thing?

You make a very good point concerning the intuitiveness of using an R-value versus U-value for comparison...going "up" as an improvement just "feels better". In fact, I have used that in discussions myself when people have asked why to use one over the other.

What most people don't realize, I think, is that U-value was around as a calculation long before R-value made its appearance. I am not certain when U-value first was used to calculate energy losses (I believe it was in the 1930's - but don't quote me), but R-value was actually developed in the late 1960's / early 70's by the fiberglass insulation folks. So the question of "which came first the chicken or the egg", at least in the case of R vs U, turns out to be the U-value. This surprises many people since they have grown up seeing R-value measurements everywhere when dealing with home energy products.

Really the only advantage of using R-value over U-value is that R-value is easier to visualize. But, that can be a very important consideration when dealing with folks who live in a world of "sound-bites" and fast food.

When a window company salesman starts bragging that the units he is selling have this "great R-value" he is often (always?) simply giving the center-of-glass reading of the unit. The warmest spot on any window is the center of the glass. This is a very real number, but it doesn't say a thing about overall performance of the unit and is thus misleading.

Ultimately, as generally used, R-value is really just a theoretical number...unless it is derived directly from U-value measurements of a specific circumstance. U-value, on the other hand, is an actual physical measurement. A very common assumption is that since R and U are inverse they are simply two different ways to say the same thing, but I think that FenEx did a great job of explaining why that that isn't necessarily true, in his post.

R-value is based on the assumption that the materials used in an application will actually achieve the results that the materials are capable of achieving. In other words, if a certain thickness of material has an R-value of 19 (remembering that this value was developed using U-value measurements on a limited size sample), then filling an entire wall cavity with that thickness of the material will result in a wall with an R-19 insulating value.

Again, I would point out that FenEx's post easily proved that that idea isn't correct and even worse while a homeowner may believe that they have a certain level of insulation value in their wall, they really don't. In fact, they can be significantly below that level of value without realizing it...despite what they believe or what they were told they would have.

A given R-value only takes into account insulating against conduction...neither radiation nor convection is included when dealing with R-values. So, while filling a wall cavity with 3 feet of fiberglass insulation might give that wall an enormous R-value reading, if that wall is full of holes that allow air movement within that wall cavity, the R-value number is meaningless.

In the case of a window, and as we have been discussing throughout this thread, radiation is a very significant part of a window systems insulating equation. We can know the overall R-value of a window by knowing the actual U-value, and calculating the R-value inverse, but we cannot calculate the R-value by using only R-value numbers.

Wayside, all this verbiage does nothing to discredit your central idea that U-value is simply not as easy to intuitively picture as is R-value. But, the point being that R-value is not necessarily the best way to compare apples to oranges and that there really are, at times, very valid reasons for using one system over another.

ColoJohn, good morning!

Although insulating shades and blinds sound like a good idea, I would suggest checking out this and other sites for questions concerning condensation on window surfaces. One of the biggest contributors to window condensation issues is using shades or blinds that block interior warmth from window surfaces. One of the quickest and easiest fixes for window condensation problems is to open said blinds and shades and allowing room air to circulate over the window.

The folks that sell insulating shades and blinds never seem to mention that possible shortfall.

Your analogy of an electrical circuit really is a good one. Ultimately, a conductor is a conductor and the laws of physics tend to be very consistent when dealing with fluid motion. Nature wants balance. Doesn’t matter if the unbalance is warm and cold, wet and dry, air and argon, high voltage and low voltage nature wants them equal. Consider a tub of water as potential and the hose from the tub as the conductor or a room full of warm air as potential and the window (path of least resistance is the perfect analogy as well) as the conductor.

In both of those cases you can calculate resistance to flow just as you can with a circuit.

P = I * E and R = E / I is true even when dealing with the insulating value of windows... Idea

Source Post: Who still makes high solar heat gain windows?
Posted: Thu Jan 26, 2006 8:24 am