Originally Posted by
Jerry Peck
The U factor is how the materials of the structure is constructed, with each material adding to the U factor.
The R-value is the reciprocal of the U factor, thus a U factor of "Walls 0.15" would equal an R value of "Walls R-5.88".
The "sub 0" may be the sub floor area?
Jerry: The U-factors of materials cannot actually be added. Lower U-factors are better at stopping heat flow by conduction. Therefore, if you have a U=.02 material over a U=.05 material, adding them would appear to give you worse performance rather than better.
Pardon me if anybody's head explodes, but here's a definition: the U-factor tells us how many BTUs of heat pass through one square foot of a given material or assembly of materials per square foot of area per degree F difference in temperature. British Thermal Units are the way we quantify heat. A BTU=roughly the amount of energy contained in the head of a kitchen match; 100,000 BTUs equal one therm. Therms are how utility companies measure and charge for natural gas.
R-values represent resistance to heat flow. R-values can be added and then converted to U-factors by dividing them into 1. Since R-values get bigger as performance improves and U-factors get smaller, and since most Americans think that bigger is better, most window salespeople harp on R-values.
If you can find a window salesman who knows what a U-factor is, you might have a shot at a grown-up conversation about his product.
The U-factor can be found on the National Fenestration Rating Council (NFRC) sticker. Window R-values are as stated by the manufacturer, and are taken at the center of the glass. Just as home inspectors are third-party evaluators of real estate, the NFRC is a third party evaluator of windows. Talking about R-values is like taking the seller's word for the condition of the house.
There cannot be a U=0 value, so it's hard to interpret that part of the manufacturer's statement unless they rounded off. It can be very, very small, but it can't be zero because that would violate the Second and Third Laws of Thermodynamics. Heat absolutely, always, and inevitably goes from warm to cold. It might take it a very long time, but it will obey the Laws.
This is also why we cannot get to absolute zero temperature: there is nowhere for the last tiny bit of heat to go.
If you think State Highway Patrolmen are strict, The Laws of Thermodynamics make them look like a charitable organization. No slack. Absolutely no slack.
If you've read this far you may be thinking "All this stuff is beyond the scope, who's Ed trying to kid?" Everything here, except how to read an NFRC label, should be covered in middle school general science or earth science and again in elementary high school physics. It should be common knowledge. You can bet that students in India and China have no trouble understanding basic heat transfer. Let's keep up with them.
