Thursday, September 2, 2010

Straw Bale Model House


Today, I continue yesterday's project of exploring the carbon implications of various housing technologies.  In particular, I consider the exact same home, except with infill straw bale walls.  The exterior size of the home has been increased slightly in order to keep the interior sizes of the rooms constant.  The wall detail is assumed to be stick framing with bales notched to infill between, and with 1" of lime plaster on both sides of the wall.

ICE doesn't have values for the density or embodied carbon emissions in straw bales.  I found the density of straw bales as 7.6lb/cu. ft here.  I take the embodied energy of straw from here as .24 MJ/kg, which corresponds to 6.59 ml diesel/kg straw, which, with a density of diesel as 0.832 kg/l, and an average formula of C12H23 corresponds to an embodied carbon emissions content of 0.0047 (extraordinarily low).  Meanwhile, I take the sequestered carbon content to be the same as my estimate of softwood, ie 34% by weight (at 15% moisture content).

That gives this for the overall embodied carbon emissions and sequestered carbon in the home:



That can be compared to yesterday's numbers for a conventional technology home (here shown on the same y-axis scale as the straw bale one):



The embodied carbon emissions barely changed: although straw is a very low embodied energy material, lime plaster is not, and we needed to increase the size of roof and foundation to allow for the thicker walls (remember the model 1.0 house has a full poured concrete basement).  Thus the big gain, and it is big, is in the sequestered carbon in the bales: the overall sequestered carbon is now substantially more than the embodied carbon emissions in the house.

I'll make one controversial point here about house size: if you believe sequestering carbon from agricultural wastes in buildings is a good thing, then smaller may not be better!

Of course, the environmental benefit will come more from the lowered heat usage of the home than the embodied energy upfront.  But straw-bale is not the only way of achieving that, so the next post in this series will look at using foam-insulation based super structural-insulated-panels (SIPs).

Finally, if you'd like to look for my errors, here are the details (units as per yesterday).  Again, remember that embodied carbon emissions are uncertain to a few tens of percent, so these calculations can never be highly precise.

Note: an early version of this post, which was only up for a few minutes, had a units error in the density of straw bale which caused the carbon sequestration to be overstated. It has been fixed.

3 comments:

JCamasto said...

Are you including insulation under the slab & foundation to decouple thermally from the earth?

SIPs = Structural Insulated Panels

-Jim

Stuart Staniford said...

Thanks on the typo - fixed. I added 2" of rigid insulation under the slab and outside the basement walls for future versions of the model (the versions in this post are unchanged). It adds about half a ton of embodied carbon emissions.

Max Vittrup Jensen said...

Hi Stuart,

I appreciate your new accounting approach; including embodied and sequestered energy; better than simply counting the carbon input.
However I found the location is essential to pinpoint. When we made our study ( http://permalot.org/files/u2/BBB1228Final.pdf ) it helped us greatly to know the location in regards to transport of materials, and significantly as we added the part which you -so far- seem to ignore; the 'end of life' cycle. As we knew how far it is to drive to the dump for toxic waste vs. normal landfill or compost (straw), it helped greatly in gaining more precise data.
-It was clearly best to use local milled wood boards, as opposed to imported OSB's from 1000 km's away.

It also helped me to learn to distrust LCA's. [Life Cycle Analysis]. Fact is that we had to use data for lumber which were derived from studies of extremely large automated mills in Finland, though in reality our local lumber is made by 2 mobile bandsaw mills, which is all what our local saw mill consists of.

Your accounting should also illustrate the fundamental mistake of the foundation; The basement. Building on piers saves an enormous amount of high embodied energy concrete+ eliminates flood and radon issues. To ensure floor insulation you simply add a layer of strawbales under the floor.
Building a good ol' stone (or earth-bag) root cellar in the garden provides the storage without the concrete.

Keep it coming; looking forward to see the SIP home, however the real environmental comparison there is the end-of-life issue; SIP's are toxic waste, the straw is simply mulch or bio-mass fuel...

Cheers,
Max Vittrup Jensen
PermaLot Centre of Natural Building,
Czech Republic - www.permalot.org