[Greenbuilding] Passive House Overheating

Corwyn corwyn at midcoast.com
Thu Aug 16 17:25:54 CDT 2012

On 8/16/2012 3:17 PM, John Straube wrote:
> I don't understand what you are suggesting then Corwyn if not 1-4.
> So you say: 1. don't lower the window SHGC, 2. don't use operable shades
> and 3. don't change comfort expectation.
> Sounds like that is exactly what Corson has, and he reports, to no
> surprise to me, that there is overheating.

My suggestion to him was nighttime ventilation as a start.  This summer 
there have been only 2-3 nights here which were insufficiently cool for 
reducing daytime heat to comfortable levels.  Further efforts might well 
be required (such as shades).   But that wasn't the point I was making 
with you.

> I am arguing against the dogma of "high SHGC windows are generally a
> good solution for low energy homes".  I believe they are generally a bad
> solution.

Well, I certainly wasn't advocating for Dogma.  I was saying that 
according to my model of his house, lower SHGC windows would save him 
less in the summer (even if he used AC to cool the house) than it would 
cost him in the winter.  Is there some reason I should favor windows 
which decrease summer heating at the expense of winter heating if the 
winter heating is much greater than the summer?  In other words, if the 
windows cost 1.5 MBTUs of cooling in summer, but gain 5 MBTUs of heat in 
the winter, isn't that the right choice?

> Also I am arguing that current models don't properly predict overheating
> (even the much lauded PHPP) and that fixating on a specific space
> heating target rather than a total primary energy target can lead one to
> poor designs than otherwise.

As I said, my model did predict the overheating issue, it was still 
(IMHO) the better choice from a total energy standpoint.  No argument 
from me, about fixating on one thing being a poor approach to design.

> Your 1. You are making a major assumption that a house with a SHGC of
> 0.23 (pretty low, probably too low to get decent daylight) is a bad
> house.  What tools do you use to support that and what metric do you use
> to measure this?

I am not sure where I could have possibly implied anything about a 'bad 
house'.  What I was saying is that below a certain point of SHGC, a 
window is a liability (from a heating/cooling energy standpoint).  The 
SHGC value of 0.23 (all else being equal) is just that tipping point for 
the house and windows in question.

Here is another try at this point:  When do you think it makes sense to 
use a lower SHGC window as opposed to using a smaller window.  Corson 
certainly could have used smaller or fewer windows.  This would have the 
benefit of reducing winter heat losses as well as summer solar 
overheating.  Especially as he would be substituting an R-43 wall for 
that window.

> Your 2. I was proposing 3-5 time reduction in primary energy use of
> homes. Who said anything about status quo?  Where did I ever suggest
> that this was good enough or the right direction?

This was a response to your comment "In many houses, people don't know 
or want to bother opening windows at the right time. Based on the my 
walks through suburbia across the US in the early evening when it is 
cool outside and the AC units on house after house are running, I would 
venture to say this is the vast majority of people."  That status quo 
(of those people, not *you*) was what I was referring to.

> Your 3.  This is MORE variation than one would
> see for houses built to the same total energy use.
> It is not my experience that a heating target gets more attention from a
> builder than a total energy target.  Having a target is what seems to
> matter.

Ok, we are apparently encountering different builders then.  Most of the 
ones I encounter, wouldn't understand that they had anything to do with 
total energy.  Clearly, I have more educating to do.

> And of course the environment cares only about total energy use and
> energy type, not whether that energy is used for heating, lighting
> cooking or TV.

True enough.

Thank You Kindly,

Topher Belknap

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

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