[MassHistPres] More window material

[slater at alum.rpi.edu]

In order to present a credible case to homeowners, we must debunk this
worksheet that people use. We must show where the assumptions are wrong,
or where the calculation is flawed. If we can't do that, then the
worksheet is valid and we don't have much to argue on.

As far as I can tell, there is only one highly questionable variable,
which is the infiltration factor. I have no idea why the infiltration
factor of an "old" window is 1 versus a "new" window at 0.14. U-value
seems to be more or less universally measured and accepted, though I
suppose this could be challenged as not being the same for the life of
the replacement, especially if low-E glass breaks down.

However, I have no perspective as to whether certain parts of the
calculation are correct. For example, why should I add the infiltration
factor to the U-value? What does this accomplish? 

The formula is basically:

square feet of all windows x [U-factor + infiltration factor] x unnnamed
factor based on degree days x fuel cost per 1,000,000 BTUs. That gets
you the amount of energy loss via your windows. The only factors that
are variable between old vs. new are U-factor and infiltration factor.
Adding the infiltration factor to the U-factor makes old windows 2-3
times less efficient than new windows to start. The rest of the formula
just calculates how much the fuel will cost per unit of efficiency.

One other cost to factor in is maintenance cost. Even if the payback
period is 7 years, if the windows need to be replaced in 15-20 years
there will be a cost but no additional gain in efficiency. The payback
is not even -- if a homeowner owns the house for 25 years and must
replace the windows twice, the homeowner probably loses out. If someone
buys a house and the windows need to be replaced again because they have
failed, there is no energy savings in replacing them, there is just a
large cost. I'll have to think of that angle more. 

I agree with the other post that there are 3 factors in play here --
preservation/architecture, sustainability/environmental, and cost.
Unfortunately, only cost is tangible. 

Ralph Slate
Springfield, MA

<-----Original Message----->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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