Replies

1. 
   Aedi | Apr 18, 2019 01:21pm | #1

   I'd say the U-0.30 windows are about 25% better :p

   In all seriousness though, it is better to think of thermal resistance in terms of proportional difference than absolute difference. The difference between an R5 and an R10 wall is much more noticeable than the difference between an R50 and an R55 wall. Since a lot of a wall's heat is lost through its weakest points, that extra 25% could make a big difference.

   But it is impossible to answer your question without extra information. If you don't have many windows, then it won't make much of a difference and you should go with whatever is cheaper; conversely, having many windows would would weigh heavily for the higher U-factor. Plus if the windows have different glazings and emissivity values, it changes the whole comparison.

   Calculating the whole-wall R value with the two different windows would be a good start to know how big of a difference it makes.

1. 
   RussMill | Apr 18, 2019 01:58pm | #4

   Thanks very helpful.

2. Expert Member
   Dana Dorsett | Apr 18, 2019 01:37pm | #2

   >"In an r-40 wall i wouldnt worry about an R1 difference but SHOULD WE ON WINDOWS, care about that difference. Climate zone 4 on edge of 5!"

   An R40 (whole wall) wall has U-factor of 1/40= 0.025 BTU/hr per square foot per degree F temperature difference.

   An R41 (whole wall) wall has U factor of 1/41= 0.0244 BTU/hr per square foot per degree F temperature difference.

   So a change of R1 only results in a 0.025- 0.0244= 0.0006 BTU/hr per degree F difference (big whoop, right?)

   The difference between U0.30 - U0.24= 0.06 BTU/hr per degree F difference.

   The effect of an R1 improvement in windows is a 100 x bigger than an R1 improvement in an R40 wall.

   That means an R1 improvement on just ONE ten square foot window has a bigger impact than an R1 improvement on all the walls and attic combined.

   But you shouldn't "...worry about..." it- you should do the math.

   In a zone 4/5 location it's definitely worth upgrading to an R0.25-ish or better window, which isn't a hugely expensive upgrade compared to going from a code-min U0.060 wall assembly upgraded to a U0.025-ish wall assembly. Just about any argon filled double-low E double pane gets you there at a price point well below triple panes.

1. 
   RussMill | Apr 18, 2019 01:56pm | #3

   Thanks Dana, for putting in perspective.

3. 
   Peter Yost | Apr 20, 2019 06:20am | #5

   It's worth noting that higher performance windows most often lead to increased thermal comfort as well as greater energy efficiency.

   A quick and easy way to compare window performance is the Efficient Window Collaborative window selection tool: https://www.efficientwindows.org/new_selection1.php

   Peter