[Cob] Cob 'outsulation'

[Rhys Dyer]

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> Message: 1
> Date: Tue, 10 May 2011 22:41:12 -0400
> From: Mike <aspiringbodhisattva at gmail.com>
> Subject: [Cob] The matter of insulation. :)
> To: coblist at deatech.com
> Message-ID: <BANLkTi=V0OB_Bi=bOw5te3NaUQwjSg=JRg at mail.gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1
> 
> Hi folks!
> 
> I'm in Chapel Hill, NC, where I've gotten permission to build a small cob
> workshop in my backyard (12x12x12 max, since they're not going to inspect
> the cob - just the electrical and plumbing I'll run to it). Got the
> foundation and drainage trench dug and just about filled.
> 
> I (shh) am hoping to be able to stay in this "workshop." I'm curious what
> options might fit cob Besides straw bales... I'm not a total purist (at
> least this time), and, for instance, lined the foundation trench inside out
> and out to grade with R-5 1" thick solid foam board... more out
> of curiosity than belief it's the right application. :) Has anyone had any
> experience adding a modicum of insulation into cob - I've heard of
> sandwiching foam board slices with wire to hold it together in the cob
> walls, though I worry about anything that might lessen the monolithic
> quality to the walls. And while one cob book mentions you can plaster foam
> board, I just can't see that working with how the boards resist moisture.
> Just curious, any hybrid thoughts?
> 
> Best wishes,
> mike
> 
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For those of us who are prepared to use some hi-tech material, there is another approach which can be considered. There is a material called 'aerogel' (used widely in the space programme) which can be purchased as a roll of flexible woven material in various thicknesses. It has to be mechanically attached to the substrate, is vapour permeable, withstands compression without loss of insulative properties, does not degrade with time, yadda, yadda, yadda. It can be laid under solid floors if you want to.
 
Here in Australia you have to jump through some silly hoops to get a house design 'energy rated' before a permit to build is issued. My house design is a circular 5 metre diameter cottage of cob with all proper attention paid to passive solar considerations , 600mm thick cob walls, double-glazed windows and insulated slab. The rating system gives that a score lower than some McMansion 10 times the size made of industrial material, it seems largely based on the assumed heat transfer through the cob walls!
 
The only practical way to get through was to propose to wrap the whole building with an insulant outer layer. Aerogel fits the bill and by putting a lime render over the outside of that, you'd never know it is there. The proposal is to attach the aerogel to the cob via a series of outside battens, connected to small deadmen at regular intervals in the cob wall, and render over the whole. The effect is not unlike a lath and plaster' layer over the wall.
 
The method has the verbal approval of the insulation manufacturer and adds about 10mm for the insulation and whatever extra it takes for the thicker layer of render. So the appearance of the house is not altered. The same material (augemented by an outer foil reflective layer to limit gain by radiant heat) is to be used in the roof and I am also considering using the same insulation under the (earthen) floor, with solar hot water for in-floor heating.

I think the rating method is invalid and biassed towards 'normal' methods and materials. I wanted to keep away from unnecessary 'hi tech' material in my design: but in effect the choice came down to 'hi tech and cob house' or 'no cob house' and I want the house, so...
 
The disadvantage? Well, the stuff is pretty pricey and supply can be slow. But the small size of the overall design helps to minimise the overall cost.
 
Just a suggestion.
 
Rhys Dyer