Mike, Randy

This has been an interesting thread, here is my 3 cents to add to it.

If you only want to read one sentence: 1) Low-E coatings really do save
energy; 2) the NFRC data on coating properties and the modeling tools to
predict thermophysical properties e.g. U, are well validated and give
accurate, meaningful results, and 3) the annual energy modeling tools do a
decent job of predicting overall performance- assuming you get the details
right.

Some comments on the earlier part of the thread:

Randy- "how can a thin coating have so much effect"-- You are assuming that
the U value is classic 'conduction' where thickness is directly correlated
to heat transfer such as with foam. In windows the U is an overall
conductance that includes 1) environment to outer glass heat transfer (
driven by convection and long wave radiation; 2) glass to glass heat
transfer through the IGU - including coatings, gases, other internal
gadgets ( conduction, convection and long waver radiation) and 3) inner
glass to room ( convection and long wave IR.

There is a simple diagram that shows this (Fig 3.1) in the WINDOW users
manual -download at
http://windows.lbl.gov/software/NFRC/SimMan/NFRCSim6.3-2013-07-Manual.pdf

( you can find similar diagrams in ASHRAE or engineering texts)

The total U of the window is determined by the details of the 3 components
listed above- in series (and to further simplify we are talking about glass
only here, then you have to add edge, frames etc in parallel- the NFRC
number has all this). Depending on specific conditions, e.g wind speed,
different mechanisms may dominate- with a high windspeed outside the long
wave radiative contribution is proportionately small; with still air
outside it's the largest effect. *Inside* an IGU (between the panes) the
largest component of heat transfer is normally long wave radiation
exchange- thats why a very thin low-E coating is so effective in reducing
the overall window U- it reduces the radiative component by over 90% and
since thats about half the total it reduces the overall conductance across
the gap by about 40%; you still have conduction/convection in the gap- and
you can reduce that by another ~20% by adding argon gas in place of air. (
All of this depends on temp, gap width etc but this is the basic picture;
you can also go to an evacuated gap- then no convection and conduction and
the heat transfer is largely radiation but with conduction through the
little spacers that keep the glass from touching?).

The WINDOW software calcs have 30 years of lab and field validation behind
them and they are accurate re U, SHGC etc. The NFRC whole window ratings
are for specific sizes- if you vary the window size you change the ratio of
glass to frame and that will generally change the numbers- WINDOW can tell
you how much if you need that info. You still need to be careful how
window properties are used in the hourly calculations of an annual
simulation model- you should not be using the NFRC rated values for that
directly because they are calculated at a fixed (extreme) external temp and
15 mph wind - not typical of most climates and buildings. Each annual
energy program has its own method for determining a proper inpact of hourly
wind/temp effects if you import a ?U?; some like EnergyPlus do the
calculations themselves internally or import WINDOW files.

Finally you will find some variation in where and how low-E coatings are
used in a window. Most of the time they are used on one side of the air gap
in the IGU- this is because 1) that's where they do the most good
thermally, and 2) they are protected from exposure and degradation. Some of
the more durable versions are now used in the #4 surface- facing the room-
this has some value since the air is normally still in the inside but there
are some other effects to watch for- the coating will lower the interior
surface temperature and you might get condensation in rooms with a lot of
moisture. More importantly the coatings are also tuned to control solar
heat gain- the majority of coatings sold are low SHGC although most
manufacturers offer high SHGC versions also- the ?winter? night time effect
on U is about the same but the impact on solar gain is dramatically
different. The coating position inside the IGU (#2 vs #3 surface) has
some bearing on the overall SHGC but none on the U. The good news is we
have a vast set of products to chose from when you add all the coatings,
substrates and configurations- so solutions can be tuned to meet varying
needs- thats what architects and engineers are hired to do. But it does
require that the proper performance data and tools be available to select
the best match.

There are numerous technical papers on the details of the coatings,
validation of WINDOW, field performance of low E, etc.- if you cant find
what you want email me. Two more general, design oriented sources are
www.efficientwindows.org (residential) and www.commercialwindows.org
(commercial) - both have background info, data and tools to assess impacts.

Steve

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Stephen Selkowitz

Building Technology and Urban Systems Department (510) 486-5064

Bldg.90-3111
fax (510) 486-4089

Lawrence Berkeley National Laboratory
SESelkowitz at lbl.gov

Berkeley, CA 94720
http://buildings.lbl.gov/

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