[MassHistPres] More window material

Fri Sep 25 12:23:49 EDT 2009

i'm just now heading out of the house and will respond more later....

my initial response was 'how silly, where did they (not you) get those figures?'
after a bit of thought, my response was 'how silly, where did they get those figures?'

for a VERY thorough study on energy performance of old windows, check this out...i have a hard copy and it's over 150 pages long...do a search for a briefed version...

the payback time for replacement windows is typically 25 to 125 years.....

...jade......ever the cynic

heartwood window restoration
hawley, ma
413-339-4298
  ----- Original Message ----- 
  From: slater at alum.rpi.edu 
  To: masshistpres at cs.umb.edu 
  Sent: Friday, September 25, 2009 12:04 PM
  Subject: Re: [MassHistPres] More window material

  Maybe someone can help me out here. I just did this worksheet for window replacement:

  http://www.dnr.mo.gov/forms/780-1363.pdf

  Based on my calculations, it told me that a house with 30 single pane windows with storms will consume 582 gallons of oil -- just for the windows -- and that if you install low-E replacements, it would consume just 233 gallons of oil just for the windows. Replacing would save 439 gallons of oil annually.

  Let me tell you my assumptions and maybe someone else can try this.

  First, I assumed 30 windows at 32" x 62" -- each being 13.9 square feet. That's about the size of my second floor windows. Multiplied by 30, that's 417 square feet of windows. 

  Second, I used 0.5 U-value for single pane with storms, 0.38 for double-pane with low-E glass. That's an agreed upon number.

  I used the "degree days" method in line 6, since this form is for Missouri and we are in Massachusetts. The calculation says "degree days x 24 / 10^6". I used 6250 degree days, a number I got from an energy website. That changed the factor to 0.15 versus the 0.1 for Missouri.

  I used an energy cost of $19.73 per million BTUs. That was based on oil being $2.75 per gallon. It came from this website:

  http://energy.cas.psu.edu/costcomparator.html

  I treated the fuel efficiency as 100% in the calculation because the energy cost from the previous step already factored that in.

  I assumed that a vinyl replacement window costs $300 installed.

  Using the formula on the spreadsheet, it said that if I keep the original 30 windows, I will spend $1,849 in heating costs, and if I replace them, I will pay $641 in heating costs.

  If that's true, that seriously weakens the argument for keeping the originals pretty significantly -- if a replacement window costs $300, then payback period is just 7 years. If oil goes up by 45% to $4/gallon, then the payback period is just 5 years.

  One big factor is that the calculation uses something relatively new (it wasn't on an earlier version of the form) called an "infiltration factor". It is set to 1.00 for old windows, 0.14 for new windows. That seems a little arbitrary, and it completely changes the results of the calculation. Without this factor, a replacement window without low-E glass is actually less efficient than an original window with storms. 

  I did some research on "low-E" -- there is some thought that it does not last for the life of the window, and may only last for 5-6 years before breaking down. 

  Can someone else validate my numbers? Honestly, they are just not that believable. I have 49 windows in my house -- some of them are quite leaky, such as leaded glass casement windows -- with storms, though the leaded glass transoms above them don't have storms. I also have several windows without storms -- two 2nd floor bathroom windows and a casement window over the kitchen sink, plus a fixed window with multiple lights, plus a couple of 3rd floor casements that open inward. Many of my windows are larger than the example I used too.

  I use about 1,200 gallons of oil per year to heat a 4,000 square foot house to around 65 degrees. If I plug 50 windows into to this worksheet, it says 1,100 gallons of that is due to the windows, and I would save 718 gallons per year by replacing them.

  I just can't believe that on its face. Could these calculations be designed to justify replacement?

  Ralph Slate
  Springfield, MA 

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