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Understanding Low E Coatings, U Value, and R Value

Post Source: http://www.replacement-windows.com/windowbb/viewtopic.php?t=1170

Heck of a question...or questions...not sure where to start.

I would say no, the addition of LowE units wouldn't be too much of a good thing. What they will do is moderate the solar energy gain and loss by blocking some solar gain (I know we are talking about High Solar Gain products, but even HSG will stop some solar energy - at least more solar gain than clear glass will allow to enter), and it will keep some of that heat from escaping when the environment wants to remove warmth from the room.

Nature wants a balance. Nature does not like things that are unbalanced. Nature wants to reach equilibrium. If the room is warmer than the outdoors, then nature wants that additional warmth in the room to equalize to the outside temperature. The trick of any sort of insulation is to keep that nature from doing so...keep the warmth where we want it despite nature's idea. Be it wall insulation, LowE on windows, additional window panes, whatever, we are trying to thwart natures attempt to equalize things.

Heat travels in three ways - conduction, radiation, convection.

Imagine an electric heating element. Touch the element with an aluminum rod, and soon the rod will be too hot to hold (and you might be electrocuted - but that is another discussion). A conductor is a conductor be it conducting heat, electricity, light, water, doesn’t matter. So the electrical analogy is valid although the specifics of various conductors would certainly differ.

If you were to hold your hand above that heating element, you would feel the heat radiating from the element. Some of that heat is actually conduction as well heat is being conducted by the air molecules but some of that heat would be radiant as well.

LowE coatings affect radiant heat. The airspace and gas infill in a multi-pane unit affects conduction.

Baseboard heat is a good example of a convection system. So is a whole house forced air system. In simple terms, convection is the movement or displacement of air mass. When warm air is attempting to equalize with cool air, the warm air mass will attempt to the cooler air. This sets up air currents. In the case of a baseboard heating system, warm air rises and cool air falls. When the heater warms the air it is in contact with (by conduction and radiation), that warm air rises and is replaced by cooler air again, air mass movement or a convection current.

And, here is a quick (relatively!) explanation of R value and U value...

R-value measures the resistance to heat flow of a material.

U-value measures heat conduction thru a material.

The formula for computing U-value is: Btu / (hr x degrees F x sqft)

The formula for computing R-value is: (hr x degrees F x sqft) / Btu

To illustrate a simple example, imagine that we have one sqft of fiberglass insulation (thickness not important for this exercise).

We have a temp of 70º on one side of the material and 0 º on the other, we can now find the U-value if we know how much energy it took to keep the 70º side at 70º for one hour.

If it took 3.68 Btu’s to keep the 70º constant for that hour, then we have 3.68 / (1 hr x 70º x 1 sqft) = a U-value of .05257.

The same calculation for R-value would be (1 hr x 70º x 1 sqft) / 3.68 or an R-value of 19.02.

19.02 = 1 / .05257 or .05257 = 1 / 19.02 - Thus the inverse or reciprocal relationship between R-vale and U-value.

So why use U-value for windows and R-value for about everything else?

Walls are built to stop the outside from coming in while windows are added to allow the outside to come in. Walls stop heat and light windows pass heat and light.

Really simplified - when a person builds a wall they want to know how much of the outside is going to stay outside and when a person installs a window, they want to know how much of the outside the window is going to allow inside. Walls resist heat flow and R-value measures resistance to heat flow; U-value measures heat flow and windows pass heat

Makes sense?

Post Source: Who still makes high solar heat gain windows?
http://www.replacement-windows.com/windowbb/viewtopic.php?t=1170
Posted: Wed Jan 25, 2006 8:13 am