Cob: RE: Re: Thermal Mass and R-values

[Michael Saunby]

On 03 August 1999 17:06, Raduazo at aol.com [SMTP:Raduazo at aol.com] wrote:
> Mike,
>     If you have the money to buy sheets of polystyrene you might try 
making
> the wall by standing up the sheet on edge with the 4' dimension vertical 
and
> the 8' dimension horizontal and then start building on either side then 
every
> so often poke a stick through the foam to bond the two independent walls
> together.  The sticks extending through the foam will allow the two walls 
to
> act as a single unit and give you the equivalent strength of a single 
thick
> wall while the insulation will prevent heat transfer through the wall.
> Ed

I guess you meant this to go to the list, or another Mike.  Personally, and 
I expect others on the list would have similar reservations -

1. It's an untried technique so you'd need to figure out a decent 
experimental method.  How to simulate many years of thermal changes (daily 
and seasonal). Physical impacts,  my own cob house suffered a serious shift 
in the roof, I don't know when, maybe a century or more ago. etc.

2. How would you calculate if this is worth doing?  Over how many years do 
you expect to recover the extra costs of materials, and labour since your 
technique isn't without a degree of fiddling?

3. Polystyrene is really dangerous in a fire.

4. Polystyrene also acts as a moisture barrier.  Is this good or bad?

5. Polystyrene carries an environmental cost that many will not tolerate.

Don't get me wrong, I'm not anti-experimentation, it's the basis of all 
worthwhile learning but you need to have some idea of what you're hoping to 
achieve.  If it's only going to save a few quid a year, why bother? If it 
could save hundreds for little extra cost then it might be better than some 
of the alternatives.

BTW in the US do you have tables of heating costs vs. R values of 
materials?  How do you decide what amount of insulation is worth installing 
if you want to get a payback in 10 or 15 years?

Right now I'm working on a (mathematical) model of the dynamic thermal 
behaviour of cob walls.  I've got other stuff to do first though so it'll 
be a while before I post it to the list.  I'm hoping it will start to 
answers some of the questions of whether to insulate or not, and if the 
answer is yes whether to place insulation between walls, on the inside or 
outside.  It might help answer some other questions too.  Anyone else done 
this?  Or even done the calculations for the steady state?

Michael Saunby