[Greenbuilding] Passive House Overheating

Matt Dirksen dirksengreen at gmail.com
Wed Aug 15 10:43:33 CDT 2012

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 *potential* of overheating – especially during peak loads?

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.

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?

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?

Obviously I am seeking more enlightenment on this subject, but the “data”
simply hasn’t eased my skepticism yet.



On Wed, Aug 15, 2012 at 10:39 AM, John Straube <john at buildingscience.com>wrote:

>  I have mentioned the challenge of high SHGC windows in super insulated
> homes (eg Passiv Haus) previously
> A recent article from Maine -you know, that hot humid place in summer :) -
> is yet another story I have heard on this issue.
> A really good builder with some experience in PH renovated his own home.
> The article is at coastalcontractor.net
> To quote the relevant parts:
> "“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.
> “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."
> .....
> 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. "
> 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.
> --
> Dr John Straube, P.Eng.
> Building Science Corporation
> Westford MA Waterloo ON
> www.BuildingScience.com
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