[Bldg-sim] How can Low-e glass have dramatically lower U-Value?

Stephen Selkowitz seselkowitz at lbl.gov
Mon Aug 4 10:12:08 PDT 2014

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

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

( 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.



Stephen Selkowitz

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

       fax (510) 486-4089

Lawrence Berkeley National Laboratory
SESelkowitz at lbl.gov

Berkeley, CA 94720
http://buildings.lbl.gov/ <http://BTECH.lbl.gov/>


On Mon, Aug 4, 2014 at 6:49 AM, Michael J Witte <mjwitte at gard.com> wrote:

> Lots of good information in this thread, but I didn't see any reply that
> explains why low-e coatings have a direct impact on the overall u-value of
> the window.  And I saw some incorrect discussion of the radiant heat
> transfer impact of low-e coatings.  Here's my understanding.
> Glass is opaque to infrared radiation (look at the detailed glass layer
> properties in most simulation software and the infrared transmittance will
> be zero).
> The low-e coating is always on an inner face of the window unit, so it
> only impacts the radiant exchange between the glass layers (or in the case
> of some windows, between the glass and an inner plastic film layer).  Low-e
> coatings do not participate in the interior room radiant exchange or the
> exterior radiant exchange with the surroundings.  So you don't need to be
> concerned about any double-counting of radiant effects.
> U-value is the overall heat transfer through the entire window unit (with
> no sun present) divided by the temperature difference.  The rated u-value
> includes the impact of the inside and outside convection coefficients.  As
> was said before, the heat transfer through the window includes conduction
> through the glass and gas layers, but it also includes the radiant heat
> transfer from pane to pane across the gas layer(s).  For a double-pane
> window, there's conduction through a pane of glass, then conduction *and
> radiation* through the gas layer, then conduction through the second pane
> of glass.  This is why the low-e coating has a direct impact on the
> u-value, regardless of how interior and exterior radiant exchange is
> modeled.
> This FSEC page has a nice diagram and explanation of window coatings.
> http://www.fsec.ucf.edu/en/consumer/buildings/basics/
> windows/how/solar_gain.htm
> Mike
> On 7/30/2014 9:42 AM, Blake, Jeff wrote:
>> Randy,
>> Some of the commonly used simulation engines do account for the effects
>> of low-e glass on window surfaces (for example, Energy Plus and ESP-r).
>> That is to say that these engines estimate the long wave radiation from
>> interior surfaces (and devices) to the interior surface of the glass and
>> radiated through the various glass panes and interstitial gases to the
>> sky/surroundings.
>> Low-e coatings effect the overall U-value of windows because of their
>> effect on reflected long wave radiation. Which low-e coating is most
>> appropriate for the North is important since the coatings will affect both
>> the SHGC and the U-value, depending on their type, thickness and the
>> surface they're applied to.
>> The results of a recent study of a Northern Canadian city (in NWT)
>> concluded that hard coat windows, despite their higher cost and relatively
>> higher U-value make sense in terms of both energy savings and
>> cost-effectiveness.
>> Jeff Blake, P.Eng.
>> Sustainable Buildings & Communities | Bâtiments et des collectivités
>> durables
>> CanmetENERGY | CanmetÉNERGIE
>> www.canmetENERGY.nrcan.gc.ca
>> -----Original Message-----
>> From: Randy Wilkinson [mailto:randallcwilkinson at gmail.com]
>> Sent: July 28, 2014 16:35
>> To:bldg-sim at lists.onebuilding.org
>> Subject: Re: [Bldg-sim] How can Low-e glass have dramatically lower
>> U-Value?
>> I went into this thinking that the Low-e coating would simply change the
>> SHGC and not change the U-Values much.  I suspected that Low-e coatings
>> would NOT save energy in the far North since they would block the wanted
>> solar heat gain.  Since the manufacturers are claiming a U-value
>> improvement from the Low-e coating, It looks like I can't calculate the
>> savings using our normal energy modeling software. If the U-value
>> improvement they give is in lieu of a more direct calculation method, and
>> is accepted by the community (you all), then I will accept and move on to
>> the next problem.
>> Thanks to all for your comments.
>> Randy
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