To begin with, I need to get up to speed on the basic arithmetic of what it would mean for my household to be carbon neutral or negative. I want to do the math with enough thoroughness that I can be confident that goal is really achieved. And I also want to do the math on the scalability of the particular choices we end up making, so we understand those tradeoffs too. While there's some value to doing niche things that wouldn't scale, there's obviously a lot more value in doing things that do have significant scaling potential.
Since housing is the largest asset most families have, it's obviously the place to start, and strawbale construction seems like one of the most potentially attractive options in terms of carbon capture, as well as ongoing energy efficiency of the building. In googling around, I've found it surprisingly difficult to find good numbers on this, so I'm going to start with some back of the envelope calculations to give a feeling for the scale. (Feel free to provide references to better calculations in comments).
Many readers may wish to gloss over the math in the next few paragraphs - there will be no test!
In this document we find some basic specs for the size and weight of a straw bale as follows:
Straw Bale Size: Each straw bale shall be a minimum of 360 mm (14 in)Let's figure the straw equilibriates at 15% moisture content by weight (it can't go over 18% or it will rot), so the dry weight of a bale is 45lb in round numbers - about 20kg. Next let's assume as a first approximation that the dry straw is basically cellulose with a chemical formula of C6H10O5. Now the atomic weight of carbon is 12, that of hydrogen is 1, and oxygen is 16. So, remembering our high school chemistry, the fraction of the weight in carbon in the above formula is 6x12/(6x12 + 10x1 + 5x16), or 45%. So our 23kg bale would hold 8.9kg of carbon if it was all cellulose. Let's call it 8.5kg to allow for a few percent of ash content in the straw.
wide, 450 mm (18 in) in height, 900 mm (36 in) in length and shall have a
minimum mass of 23 kg (51 lb.)
Ok, so let's translate this into house terms. The median house size in the northeastern US got up to 2600 sq feet in 2009. Let's figure on a two story structure, with 1300 square feet per floor, and to keep it simple, let's suppose that comes from a rectangle 45' x 29' on its inside dimensions. Let's also figure the bales are stacked on the flat side for maximum carbon capture and insulation value. So the bale is 14" high, and thus if we stack them 15 high, we'll get 17.5', which seems enough to allow for two stories, plus some floor thickness and compression. We have two long walls at 48' (16 bales long), and two short walls at 29' (9.67 bales). So the total baleage (is that a word?) is 2x15x(16+9.67) = 770 bales.
That comes to 6.5 metric tonnes of carbon.
Note this ignores that a real house plan would probably not be a rectangle, thus using more bales, neglects carbon capture in the rest of the building (especially wood framing and interior walls), embodied carbon emissions in other components (especially a concrete foundation), transportation emissions associated with getting components to the building site, etc, etc. But let's just play with the 6.5 tonnes number for a minute, now that we've gone to the trouble of getting it. How much is 6.5 tonnes?
Well, let's compare it to average carbon emissions. For the United States, carbon dioxide emissions per capita are about 19 metric tonnes/year, which corresponds to 5.1 tonnes of carbon. Per person. So for a family of four, living in that median sized house, average share of carbon emissions would be about 20.4 tonnes/year.
Thus the carbon in the house's straw-bales offset about 3 1/2 months of emissions.
Not so good. Clearly, to be carbon negative, our hypothetical family is going to have to emit a lot less than the US average.
Of course, it's probably not fair to consider a house as offsetting carbon emissions across the whole economy, versus the family's own activities. With some care in design, the straw bale house is going to consume little energy to heat and cool, so another way of thinking about the scale is relative to driving behavior. Let's suppose the family were to drive about 20,000 miles per year, at 20mpg, thus requiring 1000 gallons of gasoline. That contains 2.450 tonnes of carbon.
So the straw bales would offset about 2 1/2 years of that kind of driving. More or less, depending on how much driving and the fuel economy of the vehicles in question.
That's not that impressive either, given that the life of the building is going to be measured in decades.
One last calculation. How much air travel does this straw bale offset? Well, air travel involves about 0.2kg per passenger mile. So the 6.5 tonnes of straw bales will offset about 32,500 miles of air travel. A round the world trip! One round the world trip.
So, I think the main conclusion is this: the carbon capture in a straw bale building is nowhere near enough to offset the carbon emissions of normal US fossil fuel usage for anywhere near the life of the building. One would have to get fossil fuel usage down to a very small fraction of the typical total, and then the carbon offset in the building might be useful.