Cob Re: Carpet/cement per Pat

[Don Stephens]

On Mon, 29 Jun 1998, Patrick Newberry wrote: You mentioned Earth - not so
> good sandwiched between the layers of cement/carpet, Yet latter on you
> mention the wonderful aspects of a living roof (I too have a high
> opinion of) But then why is a living roof good and earth between two
> layers not good?  I have thought of putting  living roof on top of this
> but at this point I'm still leaning toward the sandwich roof. 
> Also on the shed style roof, it will probably not be "flat" like a 
> sheet of plywood. Last time I used this method, the carpet dipped 
> between by 2x4 supports due to the cement carpet drooping between 
> supports. My brain told me (but it's been wrong before) that  hey 
> this is better because it's more like the corregated roof (right 
> word?) anyway I did't fight the wavy-ness by pulling real tight 
> between the support. 
> I'll will go ahead and start making some "test" sandwiches. 
> One test we did ealier here was using peanut shells and a small 
> amount of cement.. Being in Geogia (not too far from Jimmy's place) I 
> have access to as many peanut shells as I want for free. We mixed 
> them with cement and made inch and a half thick squares. I stood 
> gently on it (about 150 lbs) and it  held. My son stood on it and it 
> broke (about 250 lbs) I have thought this might be an interesting 
> middle layer for the roof as the weight of  the squares as much less 
> than  a cement square would be. ...Pat, Mauk GA
> > 
> > Earth - not so good.  Earth is wonderful stuff and super in cob, rammed
> > earth, stucco, floors, thermal storage, growing stuff on your roof,
> > buffering short-term temerature fluctuations, etc., but as INSULATION,
> > it's heavy and only about R-.2 per inch (dry), eg.: a foot of earth is
> > less insulating than 1" of straw (~R-3).  Weight of a cubic foot of dry
> > earth ~ 120# dry - Weight of a square foot of compressed straw 1" deep " ~
> > 8 ounces!  heavy = more structure = more effort and or expense!
> > PS:  Are you receptive to the idea of a "living Roof" surface on top? 
> > That's one of the techniques of which I'm particularly fond and one I've
> > been experimenting with since the 60s.  It's ecofriendly, cheap, gives an
> > almost eternal life to the waterproofing membrane underneath (no UV, no
> > freeze and thaw cycle, no dry and wet yoyo, no physical trama from foot
> > traffic or falling branches), manages run-off better and it's "purty".

The common confusion with earth is between INSULATION and MODERATION.  as
I indicated above, earth has a very poor ability to insulate - prevent the
transfer (escape/intrusion) of heat.  On the other hand it has a high mass
which very effectively moderates temperatures - it takes a lot of heat to
warm it up, a lot of cold to cool it down.  This means if your day
temperature is ~ 20 F. but it drops to - 10 F. overnight, the soil surface
in the morning will have dropped down almost to the - 10, but even just 3"
down the soil will remain an AVERAGE of all the temperatures of the past
week - say ~ 15.  So the underlying roof won't "see" those short-term
overnight drops.  Likewise, in summer, if you have two or three blazing
days of + 105 F., under even three inches of dirt your roof will never
"see"  it, instead remaining the several week day/night average of say ~
75 F.! 

Also the PLANT LIFE on top will reduce windchill and increase air film
insulation factors in winter and the shading and transpiration cooling it
provides will help in summer.


So plants/dirt on top shield against short-term weather extremes, but to
prevent the slow penitration of outdoors summer heat or the slow escape of
indoors winter warmth, you want INSULATION, and at R-.2/inch, it's just
too expensive to suport the R-40 (almost 17 feet!) you would theoretically
need in some northern climates,

I say "theoretically"  because the soil's thermal flywheel effect is such
that it takes (in dry soil) about a month for heat to travel 18", so in 6
months (9' of soil)  your winter heat would be escaping but it wouldn't
matter because you'd enjoy that cooling 'cause it would be summer!  They
use that principle in Australia (See AUSTRALIAN EARTH-COVERED BUILDING by
Baggs - Rog, have you seen that one?) to let summer sun arrive in winter
and winter cool arrive in summer, but it means holding up hellacious
amounts of dirt with "hell-fer-stout" concrete roofs and walls.  They
should discover subsurface insulation as a way to hugely deminish those
loads. (As I've indicated before, I'm into minimizing concrete for
eco-reasons.) 

Bet that's more than you wanted to know about the thermal properties of
dirt, but because of the chronic insulation/moderation confusion about the
stuff, I thought maybe it needed to be said.  If irrelevant to you, just
hit the delete button....Don