[Greenbuilding] curious about these assertions

[John Straube]

Alan, you are making a massive (and questionable) assumption and one small error. 
You are assuming PHPP can predict solar overheating.  The basic PHPP is a monthly method and even hourly models have a hard time predicting overheating. Large areas of SHGC 0.5 are almost certain to cause overheating. I just spoke to a PH designer with overheating issues in Connecticut for example. 
The error is comparing the U-value of PHPP windows to US NFRC values. The test values for the former are always higher than the latter by about 10 per cent. 
John 

Sent wirelessly from my BlackBerry device on the Bell network.

-----Original Message-----
From: Alan Abrams <alan at abramsdesignbuild.com>
Sender: smarbawa at gmail.com
Date: Sun, 5 Aug 2012 10:02:37 
To: <jfstraube at gmail.com>; Green Building<greenbuilding at lists.bioenergylists.org>
Subject: Re: [Greenbuilding] curious about these assertions

For some comparison, I have been running a design for a new house through
the Passive House Planning Package, trying to hit the magic value of 4.75
KBtu/SF/year energy for heating.  This mark was elusive, as I thickened
roof, wall, and floor insulation to extraordinary values--to the point
where it was going to make the exercise seem ridiculous.

I also "tried on" various triple glazed N American windows, which were in
the U-0.16 to U-0.20 range.  To hit these values, the windows had low solar
heat gain coefficients--to get below U-0.2, the SHGC had to be below 0.3
(if I recall correctly).  I selected the lowest SHGC glazing for N-E-W
windows, and the highter value (around 0.36, again from memory) in the
south locations.

Still the big Four Seven Five eluded me.

In desperation, I selected a midrange PH certified window from the PHPP
library, with insulated frames and a U approaching 0.1, and instantly the
BTU-ometer needle swung deep into the good zone.

What is notable--and relates to this thread--is that the PH glazing had a
significantly higher SHGC, somewhere in the low 0.5's.  (Despite my fear of
this factor causing overheating or excessive demand on the cooling load,
that did not occur.)  The success of the substitution led me to look at
another module in the PHPP, which analyzes solar radiation from each
compass point.

Where I am going with this, is that there is significant radiation from all
directions, which can be harvested to lower heating loads.  In my mixed
humid climate, even with heavy latent heat loading the cooling side, once a
moderately high level of insulation is reached, the cooling load is less
significant than the heating load.  So higher SHGC's in winter may very
well trump the negative impact in the summer.

I don't have time next week to get back into this project, but when I do, I
will try adjusting the SHGC value while keeping the U value constant, to
try to isolate the effect of solar heat gain from directions other than
south--and will report back to this list accordingly.

None of this is directly transferable to a glass box office building, where
lighting and occupant loads alone may be adequate to heat the building.  In
a building that must run AC in the winter, certainly solar gain should be
controlled.  However, in the case of a home, some counter intuitive effects
may be involved.


Alan Abrams
*Abrams Design Build LLC*
*design for purposeful living*

6411 Orchard Avenue Suite 102
Takoma Park, MD 20912
office  301-270-NET- ZERO (301-270-6380)
fax      301-270-1466
cell     202-437-8583
alan at abramsdesignbuild.com
www.abramsdesignbuild.com

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