[Greenbuilding] Passive House Overheating

Corwyn corwyn at midcoast.com
Wed Aug 15 19:34:42 CDT 2012

On 8/15/2012 1:59 PM, John Straube 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 :)

"Just" using lower SHGC windows, increases energy needed for heating. 
It is a trade-off.  Opening windows at night (and other such measures) 
does NOT increase winter heating requirements.  I would say that those 
should be where we start.

For this house, halving the SHGC on the windows, would lose 5M BTUs or 
so, at a cost of around $120 per year.  I will note that my simulation 
shows that the worst overheating is in the spring, by the time the trees 
have leaved out, the problem is lessened.

Thank You Kindly,


Topher Belknap
Green Fret Consulting
Kermit didn't know the half of it...
topher at greenfret.com
(207) 882-7652

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