[mikeyouse]

In a sense that's true - it's more natural to consider in the same way that MPG is less obvious compared to gallons per hundred miles. But they're representing the same quality, U-Factor is just the reciprocal of R-Value.

So R-Value represents the thermal resistance and U-Value the transmittance but you can easily take the reciprocal of one to get the other (e.g. my R-2 window is just a U-0.5).

[bialpio]

First off, thanks for the explanation in your other post! The main point I had is that there are gotchas when talking about improvements in R-value in terms of percentages that we the uninitiated won't be aware of. U-value doesn't seem to have that problem if it's just a simple reciprocal of R-value (IIUC, halving U-value halves heat loss, doubling it doubles the heat loss - that's a way easier relationship to understand to me).

[mikeyouse]

Yep, that's a good point and a good way to think about it.

My MPG reference was an eye-opening moment for me -- the US uses MPG which is really backwards from what you care about (in the same way that R-value and U-value are). So increasing your mileage from 10mpg to 12mpg saves exactly as much gasoline as increasing your mileage from 30mpg to 60mpg. This is plainly obvious when you consider gallons per 100 miles, [10->8.3] & [3.3->1.7].