[Greenbuilding] [BULK] night insulation for windows savings

[Lawrence Lile]

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