SB: Insulated slab was Re: SB: Re: Bales in Floor

["j. gann" <jmygann@yahoo.com>]

What would be a better way to create an insulated slab
?


--- "Robert W. Tom" <archilogic@yahoo.ca> wrote:
> 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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