SB: Re: Bales in Floor

["Robert W. Tom" <archilogic@yahoo.ca>]

12/17/01 10:33:46 AM, "S. G. Fawthrop" <S.G.Fawthrop@Eklectika.net> wrote:
[snipped & pasted]
>3" concrete on 3" gravel as a base.
>2-string bales separated by 4" in each direction.
>3" concrete on top.
>based on a ground temperature of between 45 and 50 degrees it seems 
>to me that the bales would help reduce heat loss considerably and 
>the extra expense (estimated at about $3/sf) may be worth it.

> I am located in the mountains of south-west
>Virginia at an altitude of about 2800ft.


Steve;

If yours is a predominantly-heating climate, then for comfort's sake
it probably would be "worth it" to decouple the slab from the earth.

As to whether there would be a reasonable payback period, it would 
depend upon some specific variables related to your situation
 (ie HDD/yr, fuel costs (present & future) etc) , information which 
I don't have. 

But using a bale-insulated waffle slab as an insulation strategy 
is IMO, highly questionable at best and potentially disastrous at worst.

First of all, the concrete webs between the bales represent thermal 
bridges which will compromise the effective thermal resistance of the
floor assembly ( thermal conductivity of normal density 2400 kg/m^3
concrete  = 1.4 to 2.9 W/m*degC  which translates to R-0.05 to 0.1 per
inch) .  

A quick number-crunch tells me that the effective R-value
for the slab as per your specs, would be about  R-4.4, 
(optimistically assuming an ideal , in-lab R-value of  R-46 for 
very dry straw and straw  density of ~12 lbs/ft^3,  reasonable
since the bales will be compressed )

Yup, that was  R-four-point- four, average overall R-value for the slab
assembly when one takes the concrete webs into account .
That's a little bit less than what an inch of XPS  or 1.5" of EPS
or Roxul would yield.  Other gearheads may want to check 
the arithmetic but I'm pretty sure that it's correct.

Then there's all that concrete that's consumed in the 4-inch wide
"ribs" between the bales.

In a floor measuring 6 bales by 6 bales, there would be 
2.6 cu yards (ie 70.2 cu ft) of concrete just in the webs 
and that's not counting the concrete that would be at 
the perimeter, which would likely be there anyway, in a stem
wall design.

So the concrete in the largely redundant webs alone consumes 
more concrete than  the entire 3" thick floor.  That doesn't seem 
like a very effective use of a high embodied energy material to me.
If one were to design a concrete beam or pile supported suspended
slab my off-the-cuff guesstimate would be that the amount of 
concrete consumed could be reduced by 75%.


Then there's the issue of the longevity of the straw.

With a waffle slab, one is essentially creating a vapour-impermeable
bell jar to trap moisture in the straw .  A CMHC study a year or two 
ago confirmed what one suspects would happen in such a scenario
... rotten straw.

And what if your design is such that you wanted to have plumbing 
and heating paraphernalia requiring water-carrying pipes 
beneath the floor surface ?  This is the stuff of SB nightmares.

Would I build a SB-insulated waffle slab ?  
Only if I had a bunch of concrete that I wanted to waste and if I didn't
care how poorly-insulated the slab would be and if I wanted to
create an unhealthy environment seething with Fetid Black Goo.

Moral of the story: There are better ways to create an insulated slab.

--- * ---
Rob Tom
Kanata, Ontario, Canada
<ChaffArchiLogic@yahoo.ca>
(winnow the "chaff" spamguard from my edress in your reply)

Please visit http://www.theHungerSite.com daily





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