Fun with the Passivhaus

If you haven't already looked, I recommend you go take a gander at the summary of the Passivhaus performance specifications. It is worth noting that the summary is just that; the Passivhaus approach is far more nuanced and detailed, and seems to be an integrated, carefully balanced energy-flow approach to design. Nonetheless, it does contain some performance requirements, and it might be fun to compare my slightly-improved-from-awful house with a state-of-the-art set of requirements.

For the fun of it, let's compare my energy usage for heating and cooling to the Passivhaus standards. To meet Passivhaus:

• Heating must be less than 15 kWh/square meter per year. Mine is roughly 52.
  
• Cooling must be less than 15 kWh/square meter per year. Mine is roughly 24.
• Overall energy must be less than 120 kWh/square meter per year. Mine is about 86 (this includes all energy, not just heating and cooling).

So interestingly, for overall use, my house seems to meet the Passivhaus standard. This seems like an amazing result, since real Passivhaus houses have:

• 16-inch thick insulated walls
• incredibly airtight building envelopes
• multipane low-e gas-filled windows
• specially designed air exchange systems
• carefully planned and balanced heat flows
  

...none of which I have. But if one looks at the results more closely, one can see that, not shockingly, I won't make Passivhaus certification anytime soon. I'm over budget on the heating use by nearly a factor of 4, and I'm over budget on the cooling by a factor of 1.5 or so. The heating result is particularly poor by comparison since the Central Texas climate is a _whole_ lot warmer than that climate in northern Europe, where the standards were developed. So if my house is spending 4 times more energy per unit of area than a passivhaus, and doing so in a far milder winter, one need not be very impressed with my home's wintertime performance. In fact, if you compare in a way that cancels out the difference in climate by measuring kWh per Heating Degree Day, my house turns out to be more than 6 times worse than the passivhaus per unit of heating per unit of area. That, by the way, is _after_ my attic improvements which increased my wintertime efficiency by 30%. Ouch.

In fact, it seems pretty likely that the overall mildness of the climate here is the main reason that my house falls below the 120 kWh/square meter per year overall energy limit. If there's not all that much heating to be done, and not all that much cooling to be done, it's pretty easy to not spend that much energy even if your house is not all that efficient. That being said, I'm still happy to be under that number (unless my math is wrong... I can post the spreadsheet by popular request).

Another (truly fascinating!) result that I discovered doing this analysis is that I expend far more energy heating during the 3-4 months of relatively mild wintertime than I spend cooling in the 4-5 months of hot, sunny summertime. In fact, despite our long and brutal summers, I spend more than twice as much energy heating the house with the natural gas forced-air heater than I do cooling the house with the air conditioner.

I speculate that this additional expenditure of energy may be due to two factors:

1. A large portion of summertime heat gain is through solar radiation hitting the roof, and the radiant barrier now rejects nearly all of that heat, greatly reducing my cooling load. By contrast, winter heat loss is more evenly spread through windows, walls, and ceiling. In my case, I have not improved either the walls or windows, so their relative inefficiency is costing me more in the wintertime.
   
2. In the wintertime, my heater has to more than fully replace the heat that has left the house. Every 1 kWh of heat energy that leaves the house has to be replaced by slightly more than 1 kWh of energy of burned natural gas (due to imperfect efficiency in the burner system). However, in the summertime, I only have to move heat out of the house, which is far more efficient; my 14 SEER air conditioner can move (ideally) 3.7 kWh of heat out of the house by "burning" only 1 kWh of electricity. (See description of COP and SEER here to understand why).
   

Indeed, the careful reader may observe that if reason #2 is entirely accurate, then I should be expending almost 4 times as much energy in the winter as the summer... and yet, I am only expending just over twice as much. I think that the difference is due to the fact that the internal home heat load (my body heat, the heat generated by lights, televisions, stoves, refrigerators, blogging computers, etc.) works against me in the summer, but works for me in the winter.

In fact, this brings us back to the coolest (warmest?) feature of the Passivhaus: the internal heat load IS their central heat. These houses are so efficient that, even in frosty places like northern Europe, the internal heat of the occupants and their activities is generally enough to keep the house warm. So in effect, the elimination of central heat (and all the ductwork, grilles, filters, etc. associated therewith) helps to pay for all that added insulation and those nice windows, not to mention the fresh air heat exchanger. Brilliant! The houses still tend to cost more (I've seen numbers from 10% more than "standard" construction, down to as little as 3% more as practices and equipment are standardized). However, such price differences can get paid for pretty quickly, particularly in extreme climates.

Given the particulars of Central Texas, I doubt we will be without air conditioning in our houses anytime soon, especially since even if _all_ external heat were rejected, we'd have to deal with moving internal heat load outside. However, it's within the realm of possibility that central air conditioning might become less common; I have seen new small efficient houses with no central A/C and no ductwork. These are operated with a single wall unit A/C at a roughly central location, typically near the kitchen, and simple air holes placed over interior doors to allow airflow throughout the home. Combined with a high-efficiency heat-exhausting fan at a convenient point in the ceiling (such as a bathroom where you want moisture reduction anyway), these systems apparently work quite well. A small step towards a Passivhaus-type concept, but one in the right direction, and a very affordable one as well.

Moderate efficiency, radical conservation!

As I wait for the end of September, what I think of as the official end of the air conditioning season here in Texas (although there is little doubt the A/C will run a bit in October as well), I have musings about the way of thinking about energy efficiency.
First, to me, the definition of "efficiency" is the accomplishing of a given task fully and completely but with less use of energy than the way the task was accomplished previously. For example, if the task is to move 2 people from point A to point B, using a smaller vehicle than before to carry said people will typically result in the task being performed more efficiently, in that the task is completely performed, but less fuel was burned to accomplish it.
There is a similar concept called "conservation", which does not stipulate necessarily that the exact same task be performed, but simply that energy not be used unnecessarily. For example, to look at our example from a conservation mindset, one might ask some questions before one decides how to move the people from point A to point B:

1. How far is it from point A to point B, and what mode of transport is appropriate for this distance?
2. What is the goal of moving 2 people from A to B? Is there another way to accomplish this goal that does not involve moving people at all?

Depending on the answers to these questions, the conservation mindset might produce solutions to the problem ranging from "send them via bicycles" to "let them make a phone call from point A, and stay there". In effect, the conservation mindset allows one to question the very reason energy is being used in the first place, and to think differently about how certain needs are met.

In this blog, I have focused much more on the "efficiency" mindset, in that it is the least amount of change from the way things are done today. There should be no great mental hurdle for people to understand that doing the same thing in almost the same way, but using less energy while doing it, is a great thing. Thus, living in a house with air conditioning and heating is not questioned, but the amount of energy those systems use and how they use it is questioned, analyzed, and improved.

However, the conservation mindset allows the possibility of even greater improvements. For example, consider air conditioning from the conservation point of view. This point of view asks the question: "What is the point of conditioning the roughly 20,000 cubic feet of air in my house, day in and day out, for about 5 months every year? What am I trying to accomplish? Is there a less energy-intensive way to accomplish this goal?"
As it turns out, the main point of conditioning 20,000 cubic feet of air every day is to make my roughly 2.5 cubic feet of body more comfortable. Is it possible that there is a way to make that body more comfortable without using all that energy? Certainly, in the winter when the picture is reversed, and I want to warm that 2.5 cubic feet of body, I can put warm clothes over the 2.5 cubic feet part of the problem, and save a lot of energy! As an aside, that works well in the winter, because of course, our bodies are quite active producers of heat, so slowing the flow of that heat from the body produces excellent results.
Writing the above, I am reminded of the fine progress the German people have made on energy efficient houses in the cold. In fact, it looks like we are beginning to adopt some of those strategies here. I particularly enjoy the fact that these houses are so efficient as to recognize that you can even capture the heat from the air that naturally escapes the house by planning to have it escape through a heat exchanger, so that the heat is transferred from the outgoing stale air to the incoming fresh air, saving energy even as the house is ventilated with what would have been cold outside air. That strikes me as the product of a conservation mindset, although a lot of the rest of the Passivhaus construction (super thick insulation, multipane windows, etc.) probably falls more under the rubric of extreme energy efficiency, a sort of "if R-20 insulation is good, R-60 insulation is better" approach.

At any rate, it seems to this humble blogger that the combination of the tame but disciplined approach of energy efficiency, along with the potentially more radical questioning approach of conservation yields the most powerful punch against our nefarious enemy, out-of-control energy use. Food for thought along those lines: if I have tripled the cooling efficiency of my house (and I have) using only "mild" efficiency techniques, what could be accomplished with a full conservation mindset? Out-of-the-box suggestions and discussions welcome!

Looking forward to end-of-summer numbers soon, and perhaps analysis updates to follow. Until then, be efficient and conservation minded!