[Greenbuilding] Passive House Overheating

Corwyn corwyn at midcoast.com
Wed Aug 15 16:30:34 CDT 2012


On 8/15/2012 10:39 AM, John Straube 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.
>

I am familiar with this house, to the point of having run an energy 
model on it, both in its original condition, and in its fixed condition 
(before the work was done).  And the overheating was one of the things, 
that I noted at the time.  This was based on an estimate of hourly 
temperatures.

The Climate of Maine is not what I would call *really* hot and humid. 
The temperature here (South of the Corson house) hasn't been over 90°F 
this summer.  My house has not been uncomfortable this summer.  Corson 
is a smart guy, but he may need time to become more familiar with his 
new circumstances, to achieve comfort.

The trouble with my model is that the difference between comfortable and 
too warm, in this climate, depends a lot on night time cooling, using 
open windows (if you aren't willing to use energy on AC).  My software 
is not very smart about when opening windows is desired.  It isn't good 
at estimating the degree of cooling which can be obtained.  I would 
*love* any thoughts on the proper way to do either of those.

Even high levels of SHGC, may have cheap, simple solutions for reducing 
overheating, in some circumstances.  I would maintain that if the goal 
is to reduce energy consumption, that cooling energy NOT be just summed 
with heating energy on a one-to-one basis.  The better our modeling of 
house energy becomes, and the more finely tuned we make our houses, the 
more each factor affects the balance of the whole.

Thank You Kindly,

Corwyn

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




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