Saturday, March 1, 2008

Hating straw bale

So I've been chatting with Redbeard and Mr. Universe about straw bale construction.  They're like, "we don't think we like it any more."  I'm like, "why not?"  Composite reply:
  • If it gets wet, even once, you're screwed.
  • Very difficult to air-seal any butt joint, such as to a post or window buck.  We're kind of stymied here.
  • Dimensional instability - sagging and settling opening gaps.
  • Labor-intensive construction method.*

Redbeard's quite concerned about the air sealing of this cottage.  I have one half-assessed data point to back him up.  Thursday-Friday the inside temperature held pretty steady at 70 F with the 17000 Btu backup heater running flat out.  The outside temp was (I think) pretty steady around 20 F.  In steady state:

Q = U*A*(Tin-Tout)

17000 Btu/h = U*A*(70F-20F)

UA = 17000/50 Btu/hF

A=Awall+Aroof=2*(18*12+43*12) + 18*43=1464+774=2238 ft^2

U = 17000/(50*2238) = 0.152 Btu/hft2

R = 1/U = 6.6 hft2/Btu

R 6.6 seems pretty bad for a cottage with nominally R 100 walls and ceiling.  I think it is an indication of the awesome power of air infiltration to undermine efforts at super-insulation.  This is admittedly a crude calculation on a shallow database.  If you assume for example that half the heater power was going into the ground, you could justify R 13 for the above-grade structure.  La-de-da.

That R 6.6 is however, about three times better than the long term average R-value which Kimpossible and I came up with for Powerpole's more conventional type of house, based on several years of propane bills.  His nominally R 13-19 sort of house was effectively about an R 2.2.

*Postscript:  Your correspondent has struggled to understand why some people in the sustainability movement are so positive on labor-intensity.  To me it seemed like an obvious step backward.  My current understanding is that labor-intensive approaches make sense if your economy is bad and there is a lot of unemployment, such as in a lot of the third world and potentially in the first world, post-gridcrash.

No comments: