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I think there is something to be said for designing to meet
extremes.<br>
<br>
Bob Waldrop, in Oklahoma City, where we are constantly experiencing
extremes, summer and winter, these days<br>
<br>
<div class="moz-cite-prefix">On 8/15/2012 10:43 AM, Matt Dirksen
wrote:<br>
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<blockquote
cite="mid:CA+3oXxUHGV2Z=T=_kNqNFCxZGteeUJFLROorRkSL75iDH-_3mg@mail.gmail.com"
type="cite">
<div>
<p class="MsoNormal">I've certainly read up on Passivehaus over
the years, but
this article (and other’s I’ve read) make me wonder how one
designs these
homes to properly accommodate for <i>potential</i>
of overheating – especially during peak loads?</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I am curious how it was determined that a
house is
considered "overheating" when it surpasses 77 degrees ten
percent of
the year? Is that during daylight hours or a 24 hour period?
Because the
reality is, if one’s house overheats for up to a third of the
Summer, I’d
expect some serious complaints to arise.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">In other words: does PH actually design for
the anticipation
of a 10+ person summer birthday party which happens to land in
the middle of a
heat/humidity wave?</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">It would seem to me that a “dumb-ole” back
up air
conditioner would still be required for events like this no
matter what. But if
that’s the case, wouldn’t that count against the 4.75 kbtu/h
measurement?</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Obviously I am seeking more enlightenment
on this subject,
but the “data” simply hasn’t eased my skepticism yet. </p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Thanks,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Matt</p>
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<div><br>
<div class="gmail_quote">On Wed, Aug 15, 2012 at 10:39 AM, John
Straube <span dir="ltr"><<a moz-do-not-send="true"
href="mailto:john@buildingscience.com" target="_blank">john@buildingscience.com</a>></span>
wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000"> <font
face="Helvetica, Arial, sans-serif">I have mentioned the
challenge of high SHGC windows in super insulated homes
(eg Passiv Haus) </font>previously<br>
<br>
A recent article from Maine -you know, that hot humid
place in summer :) - is yet another story I have heard on
this issue.<br>
<br>
A really good builder with some experience in PH renovated
his own home. The article is at <a moz-do-not-send="true"
href="http://coastalcontractor.net" target="_blank">coastalcontractor.net</a><br>
<br>
To quote the relevant parts:<br>
"“We're very close to the Passive House new construction
standard from an annual heating load standpoint," says
Corson. “Instead of 4.75 kBTUs/sqft, we've got this house
down to about 6 kBTUs/sqft. And the peak load in this
house is down to about 10,000 BTUs an hour — which for
Maine, is pretty low." Oddly enough for a house in the
frozen North, however, it's the summer issues that are now
the most vexing. “I need to address shading of the
windows, the cooling load, and summer dehumidification,"
says Corson.<br>
<br>
“Getting this house so close to Passive House, and living
here in the summer, has really given me a sense of the
propensity for overheating," says Corson. “The windows I
used on this house have a solar heat gain coefficient of
.494, the same as the windows we used on the Knox job. For
the new house in Montville that we're doing now, we used a
.62 solar heat gain unit — much higher. If I had used the
.62 solar heat gain window on this house, we would be well
below passive house standards for our annual heating
demand (though not necessarily for our shell
airtightness). But in the summer — we'd be baking in here
right now."<br>
<br>
.....<br>
<br>
The Passive House standard says a house isn't overheated
unless the indoor temperature exceeds 77°F for more than
10% of the year, Corson explains. “Theoretically, on
paper, in the PHPP, the house that we're living in now is
not overheating," he says. "<br>
<br>
Short summary: in super insulated homes high SHGC may look
good on paper for the solar gain they give in the middle
of winter, but hourly spring/fall peaks of gain can cause
real discomfort. Most programs cant reliably predict
this. High SHGC makes sense with really low window areas,
but not with the window area people usually want for view
and daylight.<span class="HOEnZb"><font color="#888888"><br>
<br>
<br>
<div>-- <br>
Dr John Straube, P.Eng.<br>
Building Science Corporation<br>
Westford MA Waterloo ON<br>
<br>
<a moz-do-not-send="true"
href="http://www.BuildingScience.com"
target="_blank">www.BuildingScience.com</a></div>
</font></span></div>
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