[Greenbuilding] Aggressively Passive: Building Homes to thePassive House Standard

[John Straube]

As you may know, there are 3 numerical requirements of PH, with a bunch of recommendations that vary with whom you speak to (of which I dont think any are required) and what day you speak to them.
1. 120 kWh/m2/yr of source energy
2. 15 kWh/m2/yr of annual space heating demand
3. 0.6 ACH at 50 air tightness
and you should have a mechanical ventilation system.

You have to meet 120 kWh/m2/yr site energy regardless of PV. Sort of. In fact, the PHPP spreadsheet uses a factor of 0.7 to when calculating the factor for PV: I assume this means in an all electric house you could use up to 120/0.7= 170 kWh/m2/yr.  I do not know why PV is rated this way, since in a grid interconnected system like PV, the best factor one would think was possible is 1.0.  Anyway, the problem is that the PHPP software basically ignores the PV generated electricity in its verification: you can add PV but it does not reduce the energy accounted for in the 120 number. 

So yes, this limits the size of the PV array and would avoid very large PV arrays on small houses.  That is good.  We can still have largish PV arrays on large houses.  
For a large house in Sonoma California, this results in an absurdly bad house, even if it met the other two requirements of PH. A 300 m2 house (say 4500 sf) could use 300*120=36000 kWh. This is a large amount of energy which could be provided by a 28 kW PV array, and render the house net zero.  This would meet PH's 120 requirement. But it may not meet the 15 requirement for space heating and has nothing to do with the airtightness.  Meeting 15 is easily possible in Sonoma (where heat pumps work very well).  The only hard part is getting 0.6.  Of course, in Sonoma, 0.6 is a pretty stupid measure, since the energy saved going from 1 to 0.6 is truly trivial.  But if you want PH.

Sensible economics would reduce the size of PV arrays on most house, eg if it is cheaper to save energy in a specific project than it is to produce it, a sensible design would choose to save energy. This is one of the requirements that the Building America research program tries to convey: use the lowest cost means of saving energy, and every home reaches a point where PV is more economical than, for example, adding another 2" of foam to a 12" thick layer under your slab (to pick a favourite example).

Now lets look at a small house in a cold climate like Minneapolis.  Say a 1500 sf house plus a basement, which would have a PH area of maybe 130 m2.  Thus we can only use 15600 kWh of energy of all types, which would be 5770 kWh for an all electric house using the PHPP US values.  This would be hard to do all electric! Also total space heating demand would be limited to 1950 kWh (=66 therms) regardless of how this heating demand is met.  This is really hard and kind of crazy to achieve in a cold climate small house.  Meeting this 15 number is what drives many of the extreme and non-obvious decisions, not so much the 120 number.  At least this the case in cold climates and small houses.

For example, I currently live in a medium size house (1500 sf raised ranch with a fully finished basement) in a coldish climate (7500 HDD F). I cannot meet the 15 kWh/m2 demand number for space heating, although I could likely meet the 120 number, and could reduce the blower door number from 1 ACH at 50 to 0,6 ACH at 50 by spending another $5000 or so (I looked at this during the retrofi: the payback for this option was in the order of 100 years, and it was 4 times cheaper to use PV to generate the energy saved than saving it this way).  But I could not get certification if I wanted to because I dont meet the 15 number: I am over twice this with R45 walls, R80 ceiling, R30 basement walls, triple glazed R6.5 windows, etc.  There is zero benefit to me, society or the environment, to meet that 15 target but it is required for PH.  For a townhouse in a 5000 HDD F climate in, say Frankfurt Germany, 15 would be a nice guideline (oh, I guess that is where PH was developed..) but it still would not matter to the goals of good buildings like the 120 number does. Oh, and I heat primarily with wood from my own property, so the fact that I use more energy for heating is pretty irrelevant.

Just some more thoughts.

On 2011-01-09, at 9:08 PM, Rob Dickinson wrote:

> Interesting post, John.
> 
> I'm afraid I need clarification on a few points.
> 
> If I understand correctly, in PH you can't increase the amount of total kWHs available by adding a solar PV array beyond the energy allotment specified by 120kWh/m2/yr.  To me, this makes some sense because it avoids the examples of very inefficient houses compensating by installing enormous solar PV arrays.
> 
> But if one had an all-electric house and the exact amount of electricity allowed by the area formula (i.e. 120 kWh/m2/yr) were produced on-site using PV, wouldn't that that be more of a 1:1 source energy/site energy, and allow the full 120kWh/m2/yr?   That would seem sensible.  Or is PH not that sensible?
> 
> Hopefully I have expressed my question clearly.  I look forward to a response.
> 
> Rob
> 
> 
> On Sun, Jan 9, 2011 at 5:44 PM, John Straube <jfstraube at gmail.com> wrote:
> 
> 
> 
> -- 
> "I still find each day too short for all the thoughts I want to think, all the walks I want to take, all the books I want to read, and all the friends I want to see." — John Burroughs (1837-1921) American naturalist, writer
> 
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Dr John Straube, P.Eng.
Associate Professor
University of Waterloo
Dept of Civil Eng. & School of Architecture
www.buildingscience.com

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