[MassHistPres] More window material

[slater at alum.rpi.edu]

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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