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Trombe Wall Variation - Page 3

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Posted by Tim Jackson on April 21, 2009, 9:51 am
 
daestrom wrote:

A hard surface of any sort traps a layer of still air of effectively
around 3mm or so, depending on air speed (wind or convection). Two
sheets of glass well spaced therefore have four layers.  Closing the gap
brings about the interference effect between the inner two layers you
describe which reaches a maximum around 12-15mm spacing.  Filling the
gap with argon increases the insulance of the inner two layers by about
50%, giving a 25% gain overall.

Radiative transmission is not significantly between the panes of glass,
which being transparent to both visible and infra-red do not absorb (or
emit) much IR of themselves; but through them from surfaces in the room
to surfaces outside, or the sky.  This is independent of and in parallel
to conductive/convective losses.  The object of adding an fIR reflective
layer somewhere in the system is to obstruct these losses.

Trying to calculate the actual numbers suggests that the two mechanisms
probably have similar values, so I suppose the statement as applied to
multi-layer glazing is not so unreasonable.  Certainly there would be
little benefit in going to triple glazing unless some IR control were
incorporated.  The actual radiative loss rate depends very much on the
degree of exposure to clear sky so it is difficult to calculate a
typical value.


Tim

Posted by harry on April 21, 2009, 8:33 pm
 

Ah this is a conundrum indeed. I have a solar heated house (active &
passive).   I came to the conclusion that anys sort of glazing was not
enough so I have interior hinged insulated shutters to all my windows.
(4" polyurethene foam inside).  These are closed at night.
However, no-one could tell me if in my case I needed double glazing on
my South facing windows, single glazing or even quad glazing.  These
are how I heat the house.
 Ie is it better to try to let the heat in or prevent it from
escaping?
In the end I compromise & made my own quad glazing by making frames
with two double glazing units.  (All of it E glass).  Dunno if I did
right or not.
 I have two feet of insulation in the walls
I have no central heating & the house remains warm for 99% of the time
in our climate

Posted by Jim Wilkins on April 21, 2009, 11:20 pm
 
I made a small styrofoam box with a thermocouple on a thin black metal
plate inside and tested samples of glazing material before building my
solar water heater. Except for one set of reject double-glazed panels
it was salvaged or common material easy to replace after a hail storm.

There was a difference at 90C but one layer of old window glass
performed about as well as the other materials at 55C where the water
heater operates. The air temp was ~30C. The small box heated to
equilibrium quickly to compare gain and cooled quickly in the shade to
compare loss rate. You could pour in weighed amounts of water to
obtain quantitative data.

In Germany many houses have these to cover the windows at night:
http://www.rolladen.com/
http://www.rolladenlv.com/  (loads faster on dial-up)

Jim Wilkins

Posted by Tim Jackson on April 22, 2009, 8:46 am
 harry wrote:


I'm a great fan of shutters.  I've restored some old Victorian shutters
in my house, and have started experimenting with new, insulated, designs
where they are not fitted.  Unfortunately sunshine is rare for most of
the year here in the north of England, so solar heating is impracticable.

Seems to me that for the extra work of closing shutters when there is no
sunshine and you don't want to look out of the window, you can save a
lot more energy than any sort of glazing.  As you observe you can make a
very much better insulator than any amount of glass, and one sure way to
stop radiation escaping is to be opaque.  Construction, installation and
maintenance is generally cheaper than glass too.

Obviously in your house you want your south windows to maximise solar
gain. It sounds like you did the right sort of thing, it's pretty much a
case of the more layers the better, the reduction in input per layer is
small but largely independent of number, whereas the reduction is loss
is significant but reduces with number.  If cost and appearance were no
object, at a guess you'd probably optimally want about 10 layers.
Practically at 4 you've probably got as good as you'll get.

Whether multiple layers of e-glass offer any significant benefit I am
not sure, I doubt that they justify the extra cost. They only put 2 in
triple-glazed units.

The economic issues with multi-layer glazing are not just about heat
loss though, one has to bear in mind installation and maintenance costs
too. Many double glazing units are discarded because of moisture
ingress, for example.  In designing your own you need to be careful
about frame losses, the manufacturers probably put more effort into
minimising this than into anything else.  It takes a fairly small amount
of relatively conductive frame material to effectively bypass the glazing.


Tim

Posted by Jim Wilkins on April 22, 2009, 12:30 pm
 
Jack Stephenson designed the polyester film window inserts and sold me
the film.
http://www.warmlite.com/
He published it in Popular Science in the early 80's.

IIRC his prototype used 9 layers. It's based on a tent + sleeping bag
idea he had in the 50's but never manufactured.

He had sawed the wood strip spacers thinner than 3/4" and assembled
them unfinished. I didn't have a thickness planer at the time and
decided that sanding and staining all those little strips to match the
window trim was too much work, plus the available parallel space to
mount them in my windows is only 9/16" deep, so I made then out of
wood 3/4" thick and simply covered both sides.

I tried several ways to cut radiative loss. The simple one is sheets
of aluminized bubble insulation resting on the window sill and held up
by the curtains. They can be held tighter against the window by two
strings running across between hooks but that's too much fuss if there
is furniture in front of the window.

Some people around here have made inside shutters with foam or quilted
fabric insulation. I lined some curtains with aluminized cloth. They
have held up well but I don't think they do much good.

I haven't had any problem with moisture between the layers, even in
the bathroom where the inner film has been repaired with tape. All the
condensation goes to the outer storm window.

Jack had borrowed an infrared camera and photographed his house on a
cold day to check insulation effectiveness. You can clearly see the
difference between the thermal conductivity of wood along and across
the grain. The only warm spot in the photo is the protruding end of
the heavy ridge beam.

I used a cheap Radio Shack infrared thermometer to make an effective
if tedious check of my house. To compare areas of different emissivity
I stuck on black tape and measured it instead.

Jim Wilkins

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