We all know windows are cold in winter and do a poor job of keeping the heat in.  Unfortunately, the calculations that show the potential savings for new windows are dismal, with an extremely long payback period.

We are also aware that all too often an audit will over estimate the potential saving, all be it, for many reasons.  Well, my window question is, are we adding to this problem by over stating the initial heat loss through those windows?

I'm sure whole house heat loss calculations have been verified against actual energy bills, but that is essentially averaging the highs against the lows.  If we look at just the windows and apply the typical heat loss calculation along with the typical HDD, we are assuming an inside temp of 70° (base 65°).  But those windows are actually much colder, thus, the actual heat flowing through them is much less.  To add insult to this over estimate of heat loss, once the new windows are installed the surface temp increases.  Now our over stated savings for new windows shrinks as the new higher surface temp actually increases the delta T.  Despite modestly increasing the r-value of the window we now lose a portion to the increased temperature drive.  That take back actually occurs throughout the house anywhere insulation levels have been increased.

The bottom line is, we over estimate the heat loss from our windows from the start and then the window temps increase to steal a significant portion of our savings.  R-value goes up 50% while the delta T increases (just guessing) 25%, that would be significant.

The numbers are just a guess of course, but they illustrate the race condition between improving the r-value while increasing the temperature drive.  But surface temps of windows during cold spells can easily drop to 40° or lower while our calculations are still using 70°. 

Bud

Tags: Window, heat, loss

Views: 120

Reply to This

Replies to This Discussion

Here are some numbers.  I will post the related math if someone is interested.

18 windows in my 7500 HDD climate, U= 0.5 and total area = 180 ft²

The typical calculated heat loss would be 16.2 mmbtus (million BTU's).

Replace those windows with U= 0.25 windows and the heat loss is cut in half to 8.1 mmbtus with an apparent savings of 8.1 mmbtus.

However, the initial heat loss was never that high.  From this link, https://www.energyguide.com/info/window2.asp , at 0° outside we should expect an inside surface temp of about 44° on a simple double pane unit and 56° on a more high tech unit.

Factoring in the lower surface temp on the simple double pane window it would cut the heat loss approximately in half.  Therefore, the initial heat loss est should have been 8.1 mmbtus.  Now recalculating with the new windows but allowing for the increase in surface temp, the new heat loss becomes about 5.9 mmbtus.  Thus the savings are closer to 2.2 mmbtus and not the 8.1 mmbtus typically estimated.

Ouch! Those windows that were hard to justify on a savings basis just became impossible.  Comfort, yes. but payback will take a very long time.

This take back due to changes in surface temp occurs throughout the house as we improve the insulation.  It will take a sharper pencil than mine, but it does account for part of the shortfall when projected savings do not meet expectations.

Bud

RSS

Videos

  • Add Videos
  • View All

Twitter

Latest Activity

Luis Hernandez replied to Luis Hernandez's discussion Searching for a multifamily study book in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Hey Craig!  Well, let's get serious and keep asking all our friends in the Home…"
58 minutes ago
Luis Hernandez replied to Luis Hernandez's discussion Airsealing an old basement ceiling
"Hey there Ed,      I appreciate you comments... this is an interesting case because…"
1 hour ago
tedkidd replied to Luis Hernandez's discussion Airsealing an old basement ceiling
"bit.ly/ESHPhabitat1 our habitat project -mostly dirt floor..."
2 hours ago
Bryan Gabriel replied to Luis Hernandez's discussion Airsealing an old basement ceiling
"Luis, thanks for being open to sharing your experiences and photos of your project. What is the…"
3 hours ago
Craig McManus replied to Craig McManus's discussion Inventions that save energy, licensing to a US manufacturer?
"Yes happily."
4 hours ago
Michael D'Arcy replied to Michael D'Arcy's discussion Small video we created to promote Home Performance
"Thank you so much. Feel free to share that and anything you may find that we do with your network…"
6 hours ago
Ed Minch replied to Luis Hernandez's discussion Airsealing an old basement ceiling
"I agree that conditioning the basement is by far the best option.  If you are going to…"
6 hours ago
Chris Heenan replied to Craig McManus's discussion Inventions that save energy, licensing to a US manufacturer?
"Craig, as your product progresses to market would you mind coming back and sharing it's info…"
6 hours ago

© 2016   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service