[GSBN] use of ashes in clay plasters and/or floors

RT ArchiLogic at yahoo.ca
Thu Feb 5 12:57:56 CST 2009

On Wed, 04 Feb 2009 08:14:57 -0500, Rikki Nitzkin  
<rikkinitzkin at earthlink.net> wrote:

> Hi all in a debate on the Spanish list serve, talking about
> "insulating" clay floors, the following observation was made:
> "one more question: I don't know if in the insulation layer (of the
> clay floor) it would be possible to add ashes? I know that ashes are
> frequently used as good insulation in clay ovens-both in the floor of
> the oven and in the clay. Would it be possible to re-use a residue
> (ashes) that many people produce in important cuantities?
> note: here in spain a lot of people still use wood-burning stoves and
> ovens, as well as fire-places...
> Any thoughts or comments?

The ASHRAE Handbook of Fundamentals lists the thermal conductivity of wood  
ashes as being

     0.041 Btu/hr*ft*degree (Fahrenheit) @ mass density of 40 lbs/ft^3

The units are peculiar to me because typically, the units for thermal  
conductivity of building materials are listed as Btu*inch/hr*ft^2*degF (or  
in SIU, W/mK) .

My *guess* is that one would multiply the above value by 12 (ie  
inches/foot) to get it into the more familiar units for thermal  
conductivity, but that is just a guess.

For comparison, the same table lists the thermal conductivities of:

dry packed earth (@ 90 lbs/ft^3 density)  = 0.037
white pine (a softwood, @27 pcf density)  = 0.063  ... in units of  


Oficial-looking numbers aside, my gut tells me that ashes wouldn't do much  
towards providing any insulation value to a clay floor, either if mixed  
with the clay or configured as a separate layer underneath the clay.

Why ? Because my guess is that any insulating value that ashes my have is  
probably due to trapped air in the interstices between the ash particles  
and when compacted (as would be the case in a floor construction, most of  
those air pockets would likley be gone.

The other thing is that ashes seem to suck up water quite readily and when  
a materialis water-logged, it becomes a better thermal conductor than  

But all this is just speculation on my part. I've copied Norbert Senf on  
this message and my guess is that he'd know more factual info.

=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  ChaffY a h o o  dot  C a >
(manually winnow the chaff from my edress in your reply)
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