Concrete is everywhere
Cement-based building products, for construction of the domestic home, have been the go-to material for nearly a hundred years and understandably so. They’re readily available, easy to manufacture and (monetarily) relatively cheap. However, there is an increasing political, environmental and vocal turn-around happening. Some lobbyists are demanding a full stop to cement production and some would like to see more environmentally friendly alternatives. Everyday we read about the effects of global warming and the many reasons why our planet is at breaking point. A slow change is evident and looming on the horizon is the prospect of a more responsible approach to our building materials. Eradicating the use of concrete and its close cousins is never going to happen – but we can reduce reliance and demand.
Growing demand for alternative building materials
Here at Earth Blocks Ltd we’re seeing a higher enquiry rate into our products. This year has been the busiest by far with architects, self-builders and academic research being the primary areas interested……but not the home builders or developers. Convincing the policy-makers that alternative, sustainable building materials are a way forward is not easy. The idea is well received but that’s as far as it goes….perhaps until now.

Hybridization
Historically, all cob buildings were built off of the ground. Stone or brick plinths were/are constructed on the sub-ground material and thereafter cob (and cob blocks or cob bricks) are subsequently constructed atop of the plinth. This tried and trusted method ensures that the cob is not in direct contact with the ground, thereby keeping it dry and remote from inherent and/or accumulated water…..and this is where hybridization (the use of both concrete products and earth products) fulfils this criteria.


Wall base
The concrete blocks used here are standard 125mm that are widely available either direct from the manufacturers or any building material stockist. Damp proof course (DPC) is as equally obtainable. These 2 double courses have a 4:1 mortar ratio of sand and cement, again, standard practice.


Superstructure
Our 125mm cob blocks have been specifically designed to be laid on their edge (rather than on their flat). Laying cob blocks on their flat has always been the preferred method (generally because there are too many stockists/manufacturers that cannot achieve consistent uniformity to their cob blocks whereby they have to be laid flat)! Rant over and let’s get back to our wall. Constructing with cob blocks [on their edge] has many benefits – less cob blocks required per square metre, less mortar used and overall wall thickness reduction.

Building with cob blocks
There are no specialist skills required to construct cob block walls. General builders, bricklayers and, of course block layers, will notice that the only real difference is a little more care is required when handling these cob block products. Whilst compressive strengths are reliable and effective, tensile strengths are not so forgiving…..basically, if you drop a cob block it’ll break.
Binding mortar used for this wall was a non-hydraulic lime/coarse sand mix of 2.5 parts sand to 1 part lime. Natural hydraulic mortars can also be used (NHL2 or NHL3.5) again 2.5 parts sand to 1 part lime. Here’s a tip: we found that the non-hydraulic mortar to be more favourable because a) it has a slower set time b) it can be reconstituted and reused and c) when the bag/tub is sealed, remaining mortar can be used at a later date.

External wall corners
Perhaps an extravagance to round-off the corners? Far from it, our 125mm, single-rounded cob blocks have been developed especially for this purpose. The role these cob blocks play is an important one. 90 degree (external) corners, particularly to cob, are vulnerable to getting knocked and/or damaged. In addition, effective anchoring of angle-bead to a cob substrate would be difficult. If 90 degree corners are required then slips of timber can be introduced to the cob block joints at given intervals during build.
Cavity components
We’ve addressed the use of conventional cavity wall ties within this concept but what about the insulation and cavity closers?
Insulation for this cob block cavity wall system is off-the-shelf, cheap-as-chips 50mm Celotex. A more desirable, planet-friendly alternative can of course be used depending on budget, design and/or preference.
Cavity wall closers are once again off-the-shelf, conventional products and easily acquired. We fixed our closers [to the blockwork] with an adhesive. These should be introduced to all window and door openings.

External finishes
There are various claddings that are compatible with the cob blocks. The go-to’s being hydraulic or non-hydraulic mortars. Here we’ve used an NHL2/coarse sand render for the base-coat at 2.5 parts coarse sand to 1 part NHL2 with a stop-bead introduced just above the DPC.

External top-coat and decoration
Once again, we’ve used an NHL2 mortar for the finish-coat render, only this time the sand is a fine grit. Coarse sand mortars can be equally as effective depending on the type of ‘finish’ desired or specified. Ratio’s stay the same – 2.5 parts sand to 1 part lime.
Painted surfaces is must really, particularly for external lime render applications. Mineral paint systems for example, offer greater wall protection. Here we’ve used a well-known brand of mineral paint that is/should be readily available from building conservation material outlets.
The ‘plinth’ on our cob block cavity wall (below the DPC/stop-bead) has been rendered over with a sand/cement mortar and painted with an off the shelf water-proofer.
