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Cob House
Cob is earth used as a building material. Straw is mixed in with it to
improve its strength. If your soil is too sandy, you add a little clay. If your soil
has too much clay in it, you add a little sand. If you are lucky, your earth may
be just right. This is ‘ready-mix’.
From an environmentally-friendly point of view, it does not get much
better. You source your raw material yourself, right where you want to
build. No tree is cut down, no rock is quarried, no metal is mined, no oil
is extracted. The raw material does not require any melting or heating at
high temperatures, or the addition of any chemicals or massive quantities
of water, to turn it into a building material.
In generations to come, if your home is no longer occupied, cob will
eventually disintegrate back into the earth and it will leave no trace. At the
end of the day, a cob home should not cost any more than a standard-built
house. It is possible to build it cheaper than a standard house. A curved cob
wall is actually stronger than a straight one, as it becomes self-buttressing;
it supports itself. This opens up so many possibilities for a completely
individual building, full of personality and free from the 1.2m x 2.4 (4ft. x 8ft.)
module dictated by so many modern building materials.
Cob buildings are built as monoliths – huge, thick, solid walls. Traditionally
they were built approximately 600mm (2 ft.) wide, up to a storey-and-a-half
or more.
They need ‘a good hat and good boots’, large overhangs and stone or
block plinths to minimise the amount of rain reaching the cob walls. They
need to be finished with compatible materials, which will protect the cob but
also allow it to breathe.
If you maintain your cob building well, such as renewing your external
limewash every few years, it will last for generations.
Cob does not have a ‘good’ u-value. The current building regulations in
Ireland require that the maximum u-value for a wall should be 0.21 W/m²K.
A 600mm wide cob wall will only achieve values from approxiamtely 0.4 to
0.65 W/m²K, pretty far off the mark. This implies that cob walls need to be
heavily insulated to achieve current standards of thermal comfort.
In Ireland, there is a long tradition of earthen buildings, although most
people don’t realise it. If the cob walls are well maintained and the plaster
is still intact, most people assume the building is stone. They have no idea
that they may be living in a ‘mud hut’.
Home & Build Winter 2016
Winter 2016.indd 62
08/01/2016 4:31 p.m.