[Greenbuilding] [BULK] night insulation for windows savings

[Lawrence Lile]

The technique that you are looking for is called "bin data".  In the 1980's this was a commonly used technique, but it is falling out of favor as more powerful computer programs get cheaper. 

Bin data sorts temperature records into little piles, or "bins" that represent maybe a 5F spread, and record how many hours that particular bin occurs within a year.  You could do a lot with a spreadsheet and bin data. 

I don't have a good source for Bin data anymore, although there is no doubt something available through ASHRAE.  

Lawrence Lile, PE, LEED AP
Project Solutions Engineering

-----Original Message-----
From: Eli [mailto:elitalking at hughes.net] 
Sent: Monday, December 17, 2007 10:18 AM
To: Lawrence Lile; Greenbuilding at listserv.repp.org
Subject: Re: [BULK] [Greenbuilding] night insulation for windows savings
Importance: Low

Thanks for your reply.

I understand (Area /R) X temperature difference.  The question is what
number to put in for temperature difference.  I think degree days is a
useful concept for a monthly or seasonal heat demand.  However, in the
winter, the nights are long and the temperature difference is greater at
night.  Therefore, night time heat loses are not proportional to day time
heat losses.

Perhaps there is a rule of thumb about what portion of the degree days is
night time.

However, if the data exist, it would not be that difficult to calculate with
a spread sheet.  Temperature reading each hour of the year averaged with
temperature readings of each hour in previous years gives a average temp for
each hour of the year. Subtract those values from 65F to come up with degree
hours for each hour.  Add up the night time hours degree hours for each hour
of each date.  This could be added up for monthly and heating season totals.
Then back to (Area/R) X Degree hours for both exposed windows and covered
windows.

In my attempts to model thermal performance, infiltration continues to be
another mystery to figure.  Seldom are the edges perfect.  However, even
with some infiltration, it restricts the heated air that would otherwise be
exposed to window.

You are absolutely right that blocking the direct radiant exposure to the
cold glass immediately improves comfort.  Convincing people that they can
turn down there heat as a result of their improved comfort is another thing.
Radiant floors that deliver heat feel warmer at a given temperature than
forced air heat.  In theory, that would allow the occupant to lower the heat
to equal comfort with forced air heat.  However, studies have shown that
home owners keep the same temperature and increase their comfort instead.

Again, I appreciate the comments.  I am looking to develop more objectivity
is assessing the energy savings of various components to justify the
projects cost and keep the projects cost in line with the benefit.

Eli
----- Original Message ----- 
From: "Lawrence Lile" <LLile at projsolco.com>
To: "Eli" <elitalking at hughes.net>; <Greenbuilding at listserv.repp.org>
Sent: Monday, December 17, 2007 8:45 AM
Subject: RE: [BULK] [Greenbuilding] night insulation for windows savings


There are some simplified ways of doing this, and more complicated ones.

I'll start simple.  Nick Pine, I am sure, will chime in directly, since he
seems to really have a knack for this stuff.

First, remember if the insulation allows a lot of air leakage around the
edges, it is possible for it to be very ineffective.  Regular curtains allow
a convection current of air at all the edges.  Really good window treatments
will have a magnetic seal, or some other means to get them good and tight.
There is another benefit that even a leaky curtain will bestow - with a bare
window, you are radiating heat from you body to all outdoors, with a curtain
you are radiating to a warmer object.  At the same temperature, a room with
window treatments will feel warmer.

Let's take a real simple case:  Pretend it is always zero F outside, and
always 72F inside.  How much energy in BTU/hr will your window insulation
save?  Say your window is ten square feet, and it has an R value of 2 (U
value of .5, approximately), and doesn't leak.  Later we will generalize
from this.

Delta T = 72F - 0°F = 72° This is the temperature difference that drives
heat flow

Before insulating:  BTU/Hr = U*A*(Delta T) = .5*10*72 = 360 BTU/hr

If you add a layer of Rmax, with an R value of 2.5, plus an air gap, which
the mfr claims is worth R 2.77, you are adding R5.27 for a total of 7.27.

U is approx.  1/R (this is also a little simplified) 1/7.27 = .138

.138*10*72 = 99 BTU/hr so you are saving 261 BTU/hr, or about 3/4 of the
heat loss.

That ratio, 3/4, will hold true for any outdoor temperature, so you can
generalize and say that adding a foam panel to a ten square foot window will
save about 3/4 of the heating energy.

It gets more complicated to use real weather data and get a precise energy
savings number.  The next closest thing to real weather data is Degree Days.
After making a number of assumptions, a degree day number gives you a
general idea how much energy a building might use.  In my area, there are
about 5000 degree days per year.  Converting to hours, 5000 * 24 = 120,000
degree-hours.  Remember the U*A*Delta T formula above?  Degree hours can
give you energy use over a year.

BTU over a year = (degree-hours)*U*A or (degree-days*24)*U*A

So your insulated window might use approximately 165,600 BTU/year, whereas
your bare window might use 600,000 BTU.  At $30/million BTU, the bare window
is costing you $18 a year to heat, but the insulated window is costing $5.

Keep in mind, degree days is a wildly inaccurate method, there are far more
accurate methods that always involve a computer and reams of data.  But the
degree days method is the best way, IMHO, to put things in perspective on
the back of an envelope, and compare two alternatives.






Lawrence Lile, PE, LEED AP
Project Solutions Engineering