re: window u values ? SOLAR.KnowledgePublications.com

re: window u values ?
29 apr 1999
uctraing@ultranet.com (bob) wrote:

>...is there a significant real life difference that could be quantified 
>when comparing a .43 to a .33 ? does anyone provide realistic 
>comparison such as they do with a refrigerator (ie., you'r unit has
>a rating of xx so it'll typically cost $yy to run per year). 

sure. a 3,000 ft^2 house with 300 ft^2 of 0.43 btu/h-f-ft^2 windows in a
5,500 (f) heating degree-day climate will lose about 300x0.43x24hx5500dd
= 17 million btu/year through the windows. with 0.33 btu/h-f-ft^2 windows,
it will lose about 13 million. net savings, about 4 million btu/year,
equivalent to about 40 gallons of oil or 40 therms of gas, ie on the order
of $20 at today's prices... 

>are r-values a straight line ... so the difference between a 
>.33 window (r3) and a .43 window (r2.3) is trivial ?

yes, if you consider 30% (0.43/0.33) trivial.

nick