[Greenbuilding] Passive House Overheating

Alan Abrams alan at abramsdesignbuild.com
Wed Aug 15 14:47:49 PDT 2012


points well taken--passive solar optimization is never perfect (unless
there are deciduous trees in play), because the solar cycle is always out
of phase with seasonal thermal intensity--the hottest months trail the
summer solstice, etc.

so if you can't control solar gain otherwise, you have to make a commitment
whether to favor heat gain or solar shading.  And it may include deciding
whether to sacrifice some notion of absolute comfort for the sake of
reducing total annual energy consumption.

as far as I'm concerned, the risk of overheating is counterbalanced by the
availability of a cold beer.  'course my wife might differ.

AA



On Wed, Aug 15, 2012 at 1:59 PM, John Straube <john at buildingscience.com>wrote:

>  Exterior shading of 24" overhanging a 8-9' high wall works, maybe, May
> June July but does not help in April or September.  These are the bigger
> problems, as these months one can often have outdoor temperatures of 65 to
> 75 and full 200 Btu/hr/ft2 sun hitting windows.
> Exterior shades are absolutely the answer. Unless you have a working
> couple in the home who are not in the house during sunny hours.  I guess
> that is unlikely, eh?  This can be solved with automatic controls on
> automated shades. If you want to spend that kind of money.
>
> Orrr, you could just use lower SHGC windows.  But if you want to hit a
> magic space heating (rather than total energy use) target using an
> imprecise program (like PHPP) then I suppose that simple and low cost
> solution wont work :)
>
> This all reminds me of the old debate in the first round of low-energy
> houses.  One crowd wanted to use more and more glass and hence mroe energy
> collection.  The other said just insulate and reduce energy need.  Now we
> have a similar scenario.  Following the dogma of PH, everyone wants to use
> a high (0.6 or more)  SHGC to push to a rather meaningless space-heating
> target, but in many cases lower (0.5 or less) SHGC will avoid the risks of
> discomfort due to overheating and provide higher R-values (almost always)
> at the expense of potentially slightly increased winter space heating
> energy.  But if you are willing to risk comfort, you can save energy in
> other ways (e.g. operating the house in winter at a colder temperature).
> There are lots of examples of comfort problems already, leading to the
> recommendations of lower SHGC (eg between 0.3 and 0.5 depending on climate)
> and these will only increases as the houses get better insulated.
>
>  Dr John Straube, P.Eng.
> Building Science Corporation
> Westford MA Waterloo ON
>
> www.BuildingScience.com
> On 12-08-15 1:14 PM, Alan Abrams wrote:
>
> 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).
>
>
>
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