Date:         Sat, 8 Mar 2003 15:34:40 -0500
Reply-To:     Marc Perdue <marcperdue@ADELPHIA.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Marc Perdue <marcperdue@ADELPHIA.NET>
Subject:      Re: window tint (LONG)
Content-Type: text/plain; charset=us-ascii

Cary and all,

First, a quid pro quo:  I'm taking a risk of possibly over-simplifying this
process and offending some people's sensibilities or sounding patronizing.  That
is not my intention.  Just trying to shed a little light (yes, I intended that
poke at humor) on a subject about which I have a small amount of knowledge.
That could possibly be a dangerous thing . . .  :^)

While there's generally a correspondence between lightness and darkness of
tinting material and the amount of light transmitted, that is not always the
case.  What you need to find out about from the tinting manufacturers is the
transmittance and reflectance of their materials.  Here are a couple of
definitions that will help understand this process:

Reflectance--The ratio or percentage of the amount of light reflected by a
surface to the amount incident.  The remainder that is not reflected is either
absorbed by the material or transmitted through it.  Good light reflectors are
not necessarily good heat reflectors.*

Transmittance--The ratio of the radiant energy transmitted through a substance
to the total radiant energy incident on its surface.*

*From "The Passive Solar Energy Book, Expanded Professional Edition", by Edward
Mazria, Rodale Press, Emmaus, PA, ISBN #: 0-87857-238-4

The amount of heat in your van's interior is going to be affected by the amount
of sun light that comes into it and by the temperature differential between the
exterior of your van and its interior.  Any time the outside of the van is
warmer than the inside, you will get heat flow into your van through
conduction.  By the same token, if the interior of your van is warmer, heat will
be flowing OUT of your van by the same process.  In addition to the heat
transfer through conduction (and convection, i.e., how much air is moving around
your van and transferring heat in or out . . . less of an impact), you have to
consider how much heat is being added through the input of solar energy.  This
is the main problem with vehicles.  They let in a whole lot more energy through
the windows than they are able to give off through conduction; thus the build up
of heat inside a vehicle.

Now, normally one would think that darker materials would absorb heat faster
than lighter ones and this is true, given materials with otherwise similar
properties and dependent on whether they are opaque, translucent, or
transparent.  With a transparent material, you need to consider the material's
transmittance.  A darker window tinting material will have a lower transmittance
value, i.e., they don't allow a lot of light to be transmitted through them to
be converted into heat within the vehicle.  Somebody else already pointed out
that the light gets converted to heat when it hits objects inside the vehicle
and gets slowed down.  The glass itself, and the other materials that light
passes through, also slow the light down and convert it to radiant energy
somewhat before it even hits anything inside your van.

Lighter materials that are opaque work to prevent heat transfer by having a
higher reflectance than darker ones.  Window tinting films can be given higher
reflectance values by the sputtering of metals onto their surface as somebody
else already mentioned.  You can also engineer the material to allow even less
energy to pass through by making it darker, thus giving it a lower transmittance
value, and by sputtering the metals onto its surface to give it a higher
reflectance value.

So, what happens to the energy once it gets into the van and what effect do the
tinted windows have on the heat buildup in your van?  The light energy hits
things inside your van, slows down, and gets converted to radiant energy
(heat).  It is then either absorbed by the materials inside your van and
re-radiated later (your seats feel hot), it warms the air if the materials can't
absorb the heat fast enough through conduction, or it is conducted through the
van's exterior to the outside if the internal temperature is higher than the
outside temperature.  At this point you need to consider your windows' ability
to CONDUCT heat out of the van.  This is a function of the windows' R-Value, or
resistance to conductivity.  Glass generally has a very low R-Value, but you
change that slightly with the addition of window tinting films.  The key word
here is SLIGHTLY.  BUT, having said that, materials with low transmittance
values will NOT conduct heat as effectively out of the vehicle.  However, the
total effect on heat transfer is relatively minor compared with the R-Value of
the glass.  When you consider the fact that there is very little insulative
materials in vehicles, the overall effect of low-transmittance window tinting
film is pretty small.

Energy loves an equilibrium.  In the absence of any other energy inputs, the
interior of your van will seek to match the exterior temperature.  To reduce the
heat buildup inside your vehicle, you need to reduce the amount of heat coming
into it.  You cannot block out all solar energy coming into your car as long as
the sun is shining on it, whether directly or indirectly.  Therefore, you will
always have heat buildup inside your van regardless (irregardless?) of whether
you have window tinting film on your windows or not.  The object is to reduce
the amount of incoming energy so that your weak air-conditioning has a prayer of
being able to cool you off.  Or, in my case, so that the 4-60 ventilation system
has a chance to cool you off through evaporative cooling.

Regarding UV rays, as others have said, you want to prevent UV rays coming in
because they damage the materials in your van and your skin.  They also get
converted to lower-level energy (heat) once inside your van.

Regarding the experiment, it is pointless.  As I said above, as long as there is
a net energy input into your van, there will always be heat buildup.  The amount
over time (4-6 hours) will come to an equilibrium because the vehicle will be
giving off as much heat as is coming in.  A better experiment would be to see
how quickly each van would cool off with the AC on after having reached that
high temperature.  The tinted van would win, I'm sure.

I hope you all find this helpful, or at least amusing.  :^)

Cheers,
Marc Perdue