[Greenbuilding] Straw Bale Flash Demo

Tue Sep 11 20:24:01 EDT 2007

to further complicate the issue, think about property taxes.  depending on 
the methods of the local assessor, the thick walls can be thought of as 
adding 137 sq.ft of floor area (they usually measure the outside ) at 
$200/sq.ft or $27000 to the house value.  in high property tax areas like 
nh, this can add $30/thousand to annual taxes or $810 negating any energy 
savings.  one way around this is to have the walls overhang the foundation 
which is the part usually measured by assessors.
----- Original Message ----- 
From: "LarenCorie" <LarenCorie at axilar.net>
To: <greenbuilding at listserv.repp.org>
Sent: Sunday, September 09, 2007 11:22 AM
Subject: Re: [Greenbuilding] Straw Bale Flash Demo

From: Chris Green <pojeros at telus.net>

> A quote from one of the expansions:..studies by the United
> States Department Of Energy have shown that the cost of
> owning a strawbale home is over 50% less than the cost
> of owning a conventional wood-frame home over the same
> period, due largely to the reduced energy consumption.

 Since strawbale houses are usually one, or one and a half
stories, and walls count for significantly less than half of the
heat loss/gain in a typical single story house, much less half
of its total energy consumption, that claim is extremely hard
to believe.  The only kind of wall that I know of, which can
actually offset heat losses from other area, is a south facing
Solar wall of some sort.

  I have lately been very aware of the cost of super-insulation,
in terms of lost square footage to the living space. I have been
calculating the energy savings, versus not only the construction
costs, but also the loss of interior floor area, at the value of the
rest of the interior area.  Even with cellulose filled walls, which
are insulated about twice as well as strawbale walls, per floor
area that they take up, I find myself thinning my walls, to max-
-imize cost effectiveness.  In a 30ftx40ft house, 20" strawbale
walls take up, about 131sqft more interior floor area than
cellulose walls with the same Rvalue. That is a loss of around
12% of the floor area  At even $50/sqft, that is $6550, which
should be enough to heat a super insulated house for over
twenty years, not even figuring the interest (or lost investment)
cost of having to pay it all in advance. Since the strawbale walls
generally require much greater overhangs, the amount of wood
used and carpentry labor are often not reduced at all.    For the
house above, if the overhangs have soffits, the amount of lumber
in 4ft overhangs will just about equal the wood in the wood walls
with 1ft overhangs, when built with optimized framing techniques.

 Another overlooked factor for all thick walls, is that deep
wall openings significantly increase the amount of wall area.
Calculating heat loss, based on exterior wall areas, minus
window exterior face, ignores the insides of deep window
and door holes, that are very popular on strawbale houses.
For example the house above, with around 978ft², might
have twelve doors and windows, averaging about 17 linear
feet of 16" deep opening perimeter, for a windows area,
equal to 10% of the floor area.  Those interior surfaces will
not have the high Rvalue that the full thickness walls gives.
They will only have an average of between that and the
Rvalue of the 4" thickness where the window and doors
set. That average is about R18, and of course, the framing
around the windows and doors will have significantly less.
That gives an additional UA of over 15.1, which in a 6000
HDD climate represents about an additional 2.2 million
BTU/year, just through the straw. If the walls set on a slab
that has less insulation value than the walls, then there is
more thermal bridging, that never seems to get counted in
the claims.    This bridging is just one more drawback to
thick walls.  It effects, thick cellulose walls and other types
of insulations too, but only about a fourth (cellulose) as much
as with strawbales, with an equivalent Rvalue. Large one story
structures reduce the proportions of walls to roof and floor,
but small houses and two story structures have a higher pro-
-portion of  wall surface, so these factors hit them harder.

-Laren Corie-
Natural Solar Building Designs, Since 1975
www.LarenCorie.com

-LittleHouses- http://groups.yahoo.com/group/LittleHouses

-Energy Self-Sufficiency Newsletter- Free at  www.rebelwolf.com

-WoodGas- Power from Wood. http://groups.yahoo.com/group/WoodGas

-Refrigerator Alternatives-
http://groups.yahoo.com/group/RefrigeratorAlternatives

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