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: 123

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

Featured Forum Discussions

Too many BTU's. Too much horsepower?

Started by Steve in General Forum. Last reply by Eric Kjelshus on Saturday. 4 Replies

Stack/wind pressure and flow networks.

Started by Daniel James Grundy in Training. Last reply by Daniel James Grundy Mar 23. 5 Replies

BDT with vermiculite in hollow CMU walls?

Started by Brad Cook in General Forum. Last reply by John Nicholas Mar 23. 2 Replies

Videos

  • Add Videos
  • View All

Latest Activity

Adrian Sheils liked Home Energy Magazine's blog post The Elephant in the Room
2 hours ago
Adrian Sheils liked Sarah OConnell's blog post Crowdsourcing for Innovation
2 hours ago
Chris Dorsi commented on Home Energy Magazine's blog post The Elephant in the Room
"This was a great session, witness the fact that we had to schedule a second showing to accommodate…"
12 hours ago
Chris Dorsi liked Home Energy Magazine's blog post The Elephant in the Room
12 hours ago
eric anderson added a discussion to the group Energy Auditing Equipment for Sale, Trade or to Purchase
Thumbnail

Equipement still for sail (what is left from what I was selling before)

For some reason I can't reply to my previous thread.  I sold the blower door and duct blaster/flow…See More
19 hours ago
Mark Sackerson replied to angela stanzione's discussion Used Weatherization and auditing equipment for sale in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Angela, Do you have any blower door sets left? mark"
20 hours ago
Mark Sackerson replied to Jeffrey Anthony's discussion Blower Door Kits in the group Energy Auditing Equipment for Sale, Trade or to Purchase
"Jeffrey, do you have any blower door kits left? mark msackerson@franzoso.com"
20 hours ago
Mark Sackerson joined allen p tanner's group
Thumbnail

Energy Auditing Equipment for Sale, Trade or to Purchase

Discuss the pros and cons of the equipment you are interested in prior to purchase. Post equipment…See More
20 hours ago

Home Energy Pros

Welcome to Home Energy Pros – the unique digital community by and for those who work in the home energy performance arena.

Home Energy Pros was founded by the developers of Home Energy Saver Pro (supported by the U.S. Department of Energy) and brought to you in partnership with Home Energy magazine.  Home Energy Pros is sponsored by the Better Buildings Residential Network. Please honor our Guidelines

© 2017   Created by Lawrence Berkeley National Laboratory.   Powered by

Badges  |  Report an Issue  |  Terms of Service