[Greenbuilding] Passive House Overheating

[Alan Abrams]

the issue of overheating from direct solar gain raises a number of related
issues.

first, the use of exterior shading.  southern exposures in moderate
latitudes only require a 18"-24" overhang to fully protect a 5'-6' window
in early summer, and allow full sun in the dead of winter.  E-W windows are
almost impossible to control with overhangs--but can be inexpensively and
effectively controlled with Interactive Shading Devices (aka shutters).

this raises the subsidiary issue--to preserve cross ventilation while a
(louvered) shutter is closed--precludes conventional outswinging casement
windows.  But double hungs--the traditional solution--have significantly
higher air leakage rates than casements.  for example, aluminum clad
WeatherShield casements are 2 1/2 times tighter than their DH's, per lin.
ft of edge, tested at the same design pressure.   (moreover, DH's have the
extra edge at the lock rail)

this leads one toward the tilt-turn style window, commonly associated with
PH.

second, so-called overheating is subjective--comfort being dependent on RH
as much as temperature.  in an airtight PH with stabilized RH, 80 deg F is
fine.  similarly, 80 deg with a cross breeze is great for sitting around
doing a sudoku.  to a certain extent, PHPP can be "gamed" to take advantage
of this, by manipulating the "summer ventilation" module.

third--is the overheating local within the building, or is it distributed?
in a classic PH house, which relies on low air circulation rates, a given
room on the sunny side might be subject to pronounced overheating, while
the shady side of the building might remain comfortable.  but in a building
that relies on a conventional mechanical system--to lower latent heat
loads--the local heat gain might be satisfactorily distributed and
diluted.  PHPP of course has a provision to evaluate local over-cooling in
winter, but not for overheating.

regardless, it is not entirely unreasonable to organize spaces in a
building so that even if a given space overheats, it is not used during the
critical hours--for example, to place the bedroom of a night owl type on
the west side of a building.  in which case, one may need to toss PH under
the bus.

-aa




On Wed, Aug 15, 2012 at 11:43 AM, Matt Dirksen <dirksengreen at gmail.com>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>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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>
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