Re: [GBlist] Low E glass

[Stephen Thwaites <stephen@thermotechwindows.com>]

BTW: none of this justification should be necessary - low e glass
justifies itself on comfort issues alone


But, for those who prefer a longer answer i offer the following
perspective based on a Canadian approach - the Energy Rating
(ER):

The ER takes into account:
energy losses
- frame conductivity & glass conductivity (window U-value)
- air leakage (this is a small effect in a new window that has
weatherstripping)
energy gains
- solar (window SHGC)

The gains and the losses are added together (with a pinch of climate
data) and out pops the ER

A negative ER means the bigger the window, the higher the heating bill
A positive ER means the bigger the window, the lower(yes lower) the
heating bill

The British Fenestration Rating Council has developed an ERUK (based on
UK climate data)
I believe Austrialia has a similar approach (based on Aussie climate
data)

The challenge in the US is the range in climates from New England to
California precludes a singular ER approach. (A cynic would say the
bigger roadblock is the habit of the large window companies of
exclusively selling low solar heat gain low e,( it looks bad in an ER
type comparison) - but its too early in the year to be cynical)

But since Upstate NY has somewhat similar weather to Canada the
following generalized double glazed data should be a reasonable guide:

clear glass 
ER=-25

Low solar gain low e
(SHGC (glass only) = 0.4)
ER=-19

High solar gain low e
(SHGC (glass only) = 0.7)
ER=-7

This is for an average orientation, but it turns out the ranking stays
the same when the climate data is fudged for each orientation.

So, if you're still with me, the short answer is; yes low e cuts down on
the solar gain, but only slightly when compared with how much better it
retains the solar gain.



Stephen Thwaites P.Eng.
Thermotech Windows
Ottawa, Ontario



Corwyn wrote:
> 
> On Monday, December 31, 2001, at 10:58 AM, Richard Averett wrote:
> 
> We are building a house in Upstate New York,(42 North latitude).  Does anyone have an opinion on whether Low-E glass on the South side will interfere with solar gain in the winter?  Thanks.
> 
> Maureen,
> 
> I am struggling with the same question myself, and though I don't have many answers, I do have some thoughts (And in addition I am sure my thoughts (if wrong) will be quickly corrected (no way easier to get good information than to post bad)).
> 
> Anyway, the simple answer is yes, it will interfere. The question then becomes, is it still worth it? The two attributes of windows that are relevant are U-value and SHGC (solar heat gain coefficient). You can find these out for some windows by looking in <http://www.nfrc.org/> One tells you how much heat you lose, and the other how much you gain. I think the simplified equations are as follows
> 
> Heat Loss (in BTU/Day) = U-value (whole unit not center of glass) * square feet of glass * temperature difference (in degrees F) * 24 hours/day.
> 
> Heat Gain (in BTU/Day) = SHGC * square feet of glass * Available Solar Energy * % sunshine.
> 
> temperature difference = inside design temperature (probably 68-72) - the outside temperature (you might want to use average daily temperature for the coldest month)
> 
> available solar energy can be found using the PEC solar calculator for your latitude, orientation (degrees off true south) <http://arch.ced.berkeley.edu/vitalsigns/res/res_online.html>
> 
> To take my situation on an average day in February for two (random) windows
> 1) Double Pane, No low-e, Air filled: U-Value = .48, SHGC = 0.60
> 2) Double Pane Low-e #2, Argon filled: U-Value = ..31, SHGC = 0.34
> 
> My house will face 7 degrees east of True South (10 W of Magnetic South).
> At my latitude a (almost) south vertical window receives 1842 BTU/ft^2 on a sunny day.
> The average temperature in the nearest measured city (Portland ME) in February is 24.5 degrees F, the average % sunshine is 59%.
> 
> Window 1 is therefore:
> Heat Loss = .48 * 10 * (68 - 24.5) * 24 = 5011.2
> Heat Gain = .60 * 10 * 1842 * 0.59 = 6520.68
> Difference = 1509.48
> 
> Window 2 is
> Heat Loss = .31 * 10 * (68 - 24.5) * 24 = 3236.4
> Heat Gain = .34 * 10 * 842 * 0.59 = 3695.052
> Difference = 458.652
> 
> So, in this example. the (probably cheaper) non low-e, air filled windows are better (all else being equal) than the argon filled low-e windows (on the south side). Note that this assumes that you are able to benefit from all the solar gain (if you have to open the window to cool the house off...) Thermal mass will help with that.
> 
> Hopefully this will either help, or prod some more knowledgeable into helping,
> 
> Corwyn
> 
> --
> Corwyn
> corwyn@midcoast.com



______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________