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where does the Low E coating go?

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Posted by Doug on September 21, 2003, 9:52 pm
 
Hi All,

We're building a house near Ottawa, Canada.  We've tried to
incorporate passive solar design into the house - concrete slab on
grade over 3" of polystyrene and lots of southern windows.  Our budget
is already blown so we cannot afford really high performance windows.
The ones we've selected have a SHGC of .54 and a U factor of .35 - not
the best but not too bad.
Finally my question...  For solar heating during the winter, how much
more effective would it be to have the low e coating on the 3rd
surface (counting from the outside to the inside) as opposed to the
second surface?  And what will be the effect of this in the summer
time when we want to keep the re-radiated heat out?

It seems to me that the low e will work on either surface but I want
to know in terms of percentage, how much more effective it is to place
it on one surface or the other.

-Doug

Posted by Bill Kreamer on September 22, 2003, 5:57 pm
 


Doug, here's some info I found on Low E windows, fwiw.  Kind of long -
sorry.  - Bill Kreamer



Low E Windows



For most purposes, there are two bands or regions of infra red, called near
infra-red (short-wave), and far infra-red (long-wave).



Far infra-red is the region of wavelengths longer than 2.5 microns.  This is
low temperature radiation.  Almost all energy radiated from low temperature
objects (room and environmental objects) is located there, but almost no
solar radiation is in this region.  Short wave solar radiation absorbed by
room objects is re-radiated as low temperature far infra-red radiation.



A "standard" ("northern") low-e coating has a high reflectance of radiation
in the low temperature far infra-red range so it prevents low temperature
radiation loss through the window from the room.  The standard northern
low-e coating has normal transmittance in the near infra red, so it is
"high-solar-gain".  It is typically produced with pyrolitic or "hard-coat"
technology.  The coating does not stop conduction loss; that requires a low
u-factor (high R-value) type of window construction (gas filled, etc.).



The name low-e was given to the northern coatings because they were
developed first, even though the non-standard "southern" coatings are also
low-e, but in the near infra-red band.



The near infra-red region comprises shorter wavelengths, from around 0.7 to
2.5 microns.  This is also referred to as "solar infra red".  In this band
lies most direct solar energy radiation.  A "southern" or "low-e-squared"
coating is "non-standard".  It consists of two or more coatings (whence the
term "squared") applied in a manner depending on the manufacturer.  Because
it has low transmittance in the near infra-red, it is "low-solar-gain", and
blocks a good part of solar energy from entering through the window.



It is usually made with sputter-coating or "soft coat" technology (but some
hard-coat low-gain products are now available).



So to make a long story short, for northern (i.e., heating dominated)
climates, you need standard low-e, which has high reflectance in the
long-wave, low temperature, far infra-red region.  It prevents low
temperature radiation through windows, but doesn't block high temperature
(solar) radiation.



In northern heating-intensive climates, you should not put "Southern" "low-e
squared" glazing in south facing windows, or you will block a good part of
direct solar radiation in the winter.



Instead, use a "standard" "Northern" high-solar-gain low-e glazing, and one
with low conductivity, for northern climates.  Look for a solar heat gain
coefficient (SHGC) of .5 or higher and an NFRC-rated u-factor of .35 or
less.  In cooling-intensive climates, look for a low-e with a SHGC of .4 or
less, with low conductivity not being quite as important.



However...



Window selection can not be done with an assumption that the winter gain is
more important than the summer cooling load reduction.  Annual energy
balance, heating and cooling fuel, and capital equipment costs should all be
factored in.  As far north as mid-Wisconsin, southern low E can reduce
sizing of cooling equipment with negligible solar gain loss (if it is a very
efficient structure), so that southern glass might be selected.  It can't be
a gut feel, it has to be analysis.



To quickly and easily do analysis, there is a great program from Lawrence
Berkeley National Lab, called RESFEN.  It can be downloaded for free from
http://windows.lbl.gov/software/resfen .  You don't need advanced degree to
run it, and you can do analysis in less than 10 min.



For more information on selecting efficient windows, visit
http://www.efficientwindows.org/



Also...



The following is from Jim Wasley, Assoc. Prof. Architecture, Univ.
Wisconsin-Milwaukee.



Hi Bill,



The short answer is that there are a variety of low-e coatings available-
some that are 'tuned' to optimize heat gain and some to optimize heat
rejection. The specific window will identify as such. All low-e coatings do
reduce to some extent the amount of radiation transmitted.



The heat-gain-optimized types come out ahead by mainly reflecting far
infrared low temperature radiation (in heating application this blocks the
radiant loss of heat from surfaces within the house).  Cooling-optimized
types reflect both long wave and short wave infra-red (this prevents most of
the sun's radiation, that is not in the visible range, from ever reaching
the interior). I don't think that the gas fill has much influence on radiant
energy transfer but it does help reduce conductive transfer, making the
windows better insulating. This is good for either heating or cooling, but
makes a bigger difference for heating since the temperature differences
between inside and out are greater.



The portions of the electromagnetic spectrum that we are concerned with from
high energy (shorter wavelengths) to low energy (longer wavelengths) goes:
UV (8%-9% of sun's energy) Visible (46%-47% of sun's energy) Near Infrared
(remaining 45% of sun's energy) and Far Infrared or 'terrestrial' radiation
which is way down the spectrum and represents the wavelengths of things like
the interior surface of a room warmed by the sun. Low-e coatings are
'selective' in that they act to narrow the range of wavelengths allowed to
pass through and reflect the rest. Coatings tuned to capture heat for
passive heating reflect far infrared. Coatings tuned for cooling narrow that
range of transmitted wavelengths and reflect both far and near. Thus they
strip out much of the sun's heat without cutting down on the visible light
too much. This oversimplifies but is the basic idea.



Generally the coatings are on the inside surfaces of insulated glazing units
to protect them. Some processes are more fragile than others- as you would
expect the more fragile 'soft coats' perform better. Placement does have a
small but notable impact on performance- the rule is to have the coating on
the warm side of the air gap- in heating applications on the inside pane so
that energy is reflected back into the room before warming the air gap,
which would increase convective losses. In cooling the reverse.



If you want even more information I could point you to the VITAL SIGNS
project  http://www.arch.ced.berkeley.edu/vitalsigns



This is a curriculum materials project for teaching energy issues in
Architecture. Under resources/ resource packages/ there is a pdf file called
the Glazing Package that I coauthored that attempts to explain all of this
and more in depth. Of course, all of this is now four or five years old,
which in this field may be totally out of date!



All the best,



Jim

------

Again, kind of long -  sorry.  - Bill Kreamer



Posted by Doug on September 24, 2003, 3:44 pm
 Thanks, Bill.

I phoned a window manufacturer and also a scientist at the National
Research Council of Canada.  They both stated that for northern
regions, the low e coating is on the 3rd surface counting surfaces
from the outside towards the inside.  Makes perfect sense.  It's just
that I've had lots of conflicting info on this.

-Doug

Posted by Scott Willing on September 26, 2003, 6:22 pm
 On 21 Sep 2003 14:52:53 -0700, dpreece@storm.ca (Doug) wrote:


I tend to choke on my lunch when I see phrases like "lots of southern
windows."

I just hope you aren't over-glazing. Even in our climate, a good
balance is achieved for passive solar gains with surprisingly modest
amounts of glazing - something like 12% of equator-facing surface
area. (CAVEAT: that's from memory - and this is highly site and
design-specific anyway.)

With too much glass - low-e or not - you can wind up with a negative
benefit. In winter, daytime gains can be more than overtaken by
night-time heat loss (especially if thermal shutters/curtains are not
used) and in summer, unless overhangs are adequate, you can have
yourself a solar furnace.

I know of what I speak. I'm living in a house of well-intentioned but
severely over-glazed design at the moment, and I have a nearby friend
with the same problem. In my case, the original owners went for a
"solar house" concept to try to reduce heating needs, whereas in my
friend's case, she just wanted a swell view. Similar unfortunate
results in both cases.


Maybe you don't need -- or really want -- as many as you think.

Pardon the intrusion if you already know about the importance of doing
the calculations up front. Others may not.

There are professionals, formulas and software available for figuring
this all out. (E.g. see http://www.susdesign.com/design-tools.html  )

This is another one of those things that's a lot cheaper and less
stressful to deal with at the outset than to live with the
consequences and/or try to fix them after the fact.

-=s



Posted by Doug on September 27, 2003, 8:39 pm
 
I've applied some rules of thumb to see if we're overglazed or not.
It may be an exact science to Nick Pine or David Delaney, but not to
me.  The floor we have chosen is a cement slab on grade with no
covering - (acid etched or perhaps ceramic tiles, nothing to impede
radiation).  Substantial thermal mass. We also have three foot
overhangs and a "brieze solae (sp?)" to shade from summer sun.  The
site isn't perfectly clear (some trees will provide shading).  Even
with this, I expect that we are overglazed to a certain extent.  But
the view of the beaver pond will make up for that ;-).

-Doug

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