[Greenbuilding] curious about these assertions
alan at abramsdesignbuild.com
Sun Aug 5 11:16:46 PDT 2012
John, thanks for the warning. Nevertheless, most of the windows on east and
west will have conventional, old timey wood shutters, louvered so the
windows can be opened even in full sun. others are within a covered porch.
Why not, even if the house would qualify without them?
BTW, the spreadsheet reports slightly varying results when toggled from
annual to monthly loads. will report on this as well.
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)
alan at abramsdesignbuild.com
On Sun, Aug 5, 2012 at 11:56 AM, John Straube <jfstraube at gmail.com> wrote:
> Alan, you are making a massive (and questionable) assumption and one small
> 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.
> Sent wirelessly from my BlackBerry device on the Bell network.
> *From: * Alan Abrams <alan at abramsdesignbuild.com>
> *Sender: * smarbawa at gmail.com
> *Date: *Sun, 5 Aug 2012 10:02:37 -0400
> *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
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