[Greenbuilding] Passive House Overheating

[Bob Waldrop]

I think there is something to be said for designing to meet extremes.

Bob Waldrop, in Oklahoma City, where we are constantly experiencing 
extremes, summer and winter, these days

On 8/15/2012 10:43 AM, Matt Dirksen wrote:
>
> 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.
>
> Thanks,
>
> Matt
>
>
> On Wed, Aug 15, 2012 at 10:39 AM, John Straube 
> <john at buildingscience.com <mailto: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
>     <http://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 <http://www.BuildingScience.com>
>
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>
>
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