From: kk881595@longs.LANCE.ColoState.Edu (kevin knappmiller)
Subject: Re: opening of the first
Date: Wed, 18 Nov 1992 18:11:36 GMT
Reply-To: kk881595@longs.LANCE.ColoState.Edu
Organization: Colorado State U. Engineering College

In article <L27SBYL6@cc.swarthmore.edu>, dpeders1@cc.swarthmore.edu (Daniel Pedersen) writes:
|> dietz@cs.rochester.edu (Paul Dietz) writes:
|> > In article <3987.1017.uupcb@spacebbs.com> howard.smith@spacebbs.com (Howard Smith)  writes:
|> > >
|> > >Actually, there are thousands of homes and businesses in the U.S.
|> > >that rely on passive solar heating, usually employing conventional
|> > >triple-paned glass windows. Many of them, mostly in Colorado and
|> > >northern California, also rely on solar for electricity.
|> > 
|> > 
|> > I would dispute your claim.  Triple paned glass windows are really
|> > just conservation -- as long as you have a window, you want to make
|> > the losses through it be low.  If anything, this *reduces* solar
|> > heating, as the triple pane windows are somewhat less transparent that
|> > double pane.
|> 
|> I disagree.  
<Some correct facts with somewhat erroneous conclusions removed>
|> > 
|> > If, instead, you are suggesting that additional triple-paned windows
|> > be installed to provide net heat gain, then I would be dubious as to
|> > their economy as well, at least for consumers who have access to
|> > natural gas or oil (i.e., most.)
|> 
|> Hmm.  Adding more triple pane windows would increase the input of solar
|> energy.  the question is if the walls are better insulators than the windows
|> (my guess is yes).  If this is the case then the increased energy in will be
|> balanced by increased heat out, and there will be a point at which we will
|> lose more than we gain.  Since well insulated walls are (IMO) cheaper than
|> triple pane windows (Andersen windows are wonderful, but $$$$), I would tend
|> to agree that it is not economically feasible.  Although for a small
|> increase in window space in a well designed house, I would expect to see a
|> net heat gain.
|> 

There are some issues that this discussion is missing.

What are windows good for?  Collecting energy?  Yes, but also
lighting, and more importantly in a residence, the view of the outside
surroundings.  It is the VIEW that many homeowners are willing to pay for.
Using multipane windows is a good thing.  If there are large south and/or
west facing windows overheating of the sunspace is a big potential
problem, especially if people want to stay in the space because of a
nice view.  Multipane windows decrease the solar transmissivity, but
they also decrease the convective losses and with a low-E coating also
the radiative losses.  This means you can put in more windows, at an
admittedly higher cost,  without overheating the space, and keep
the space warm on cold cloudy days. (assuming insulated window
coverings, otherwise they would also be an advantage at night.)  

It's true that glass is basically opaque to I.R. radiation, but it
is the conduction and convection through the window, resulting in a higher
outside surface temperature of the outer pane that then radiates heat to
the colder surroundings.  The overall heat transfer of the window is
what is important to the heat loss.  For many of the advanced windows, as
compared to a conventional outside wall, the thermal performance of the
window is often as good or better than that of the wall.  
The thermal performance of the glass and space combination
that is,  the frame is often the weak component.  There are some advanced
frame technologies out there, but since they do not typically contribute
to a better view, only to the thermal cost, they may not be a good deal
financially.

Lastly consider heat transport out of the sunspace to the rest of the
house and the storage of that heat, usually in thermal mass.  Having
high transmissivity high themal loss windows is often worse than
useless.  You end up with a sunspace that is too hot during the day
and too cold at night.  Advanced multi-pane windows help this problem,
but it is still tricky to get the heat out of the sunspace and 
preferably into mass on the other side of the building especially
if one is relying on natural convection to move the flow.

Kevin Knappmiller
Solar Lab
Colorado State University
Fort Collins, CO  80523
(303)491-8215
kk881595@longs.lance.colostate.edu
Newsgroups: sci.energy