Cob: Insulation etc

[Mike Holland]

Thought I'd add my two francs' worth to the insulation question...  We are
renovating a 17th-18th century cob (bauge) building in the centre of France
(photos etc still visible at http://perso.wanadoo.fr/mica ).  The walls are
12 - 18 inches thick, no windows on the north wall, relatively small
non-insulated windows on the south facing side.  We heated through the cold
season on an open log fire and consumed about 10 cubic metres of wood in 1m
logs, plus a couple of 13kg bottles of LPG, at a total cost of around
2500-3000 francs.  It was well livable but on the coldest nights (less
than -12 C) we were glad of our thick duvet...  We've therefore done some
research on local solutions for insulation and we're going for a hybrid
solution.  Firstly in the new bathroom area, we'll use plasterboard backed
with 20cm of Rockwool (mineral wool - slightly less hazardous than glass
fibre for handling, but you still want to wear mask & disposable overalls,
and vacuum well afterwards to remove the fibres/dust).  We're using this
primarily because we want it done quickly and these materials are standard -
and local tradesman know how to install them.

But once that's done, in all the other areas we're going to use a material
made from treated hemp fibre.  There's a company near us that have a
patented treatment.  Without going into all the boring detail (I have a 20
page technical dossier on it if you're interested), they make two products.
The first is used loose, for insulation under floorboards, between rafters
and in cavity walls - it's supposed not to heap up, though I don't
understand how.  As a clever (and eco-friendly) touch, the bags in which it
comes are microporous & water resistant, so you staple the bags to the
rafters and pour the product in between - no packaging waste...  The second
is used as a filler in pure lime, allowing you to carry out a number of
tasks.  Firstly, you can apply thick lime render (up to 10cm) to walls
(usually internal, though they have some customers who have used it
externally): the finished result is slightly rough (which suits our type of
vernacular building) and oddly warm to touch.  You can use it for flooring
based on (what the French whimsically call a 'hedgehog') hardcore, gravel
and sand, in that order: you apply it in a 10-15cm layer.  Since the lime &
hemp mixture 'breathes' you don't usually need a DPC.  Thirdly, you can use
it rammed in shuttering in woodframe partition walls, to make lightweight
walls.  The French architectural conservation authority have approved it for
use in repairs to woodframe buildings, and if you're ever dealt with that
lot, you'll know how finicky they are...  It's fire-resistant and, as an
added bonus, rodents are repelled by it, so your cob walls are safe.
Incidentally, when you apply it as a render to cob, you need to put on a
thin coat of pure lime first to improve the cohesion to the surface.

In terms of insulation, according to the figures the company provides
(caveat emptor), the product has a high thermal inertia, which means - as
far as I understand it - that the internal temperature of a given mass rises
(or falls) relatively slowly: so a high (or low) external temperature takes
a relatively long time to travel to the interior, which smoothes out the
peaks & troughs in external temperature.

We went to see their factory and were well impressed with what we saw (and
felt...).  It is in line with our aims to use 'natural' products where
possible, and it looks kind of nice too...  Remains to be seen whether it
works as claimed...

I'll let you know how it works, when we've wintered with it...

Mike