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Concrete blocks

Concrete blocks, though used widely, have the drawback of being very high in embodied energy due to the cement they contain. They may also produce a wall which is too thick . Achieving Passivhaus standard by packing 300mm of insulation into a traditional brick and block wall produces a total thickness of about 550mm which is so thick that windows become less effective in terms of view and daylight and it actually takes up a great deal of footprint on the ground, which is expensive in terms of buildable space.

There are alternatives to blockwork above ground (and below ground if you substitute brickwork, but this is usually not much better in terms of embodied energy). Concrete blockwork is usually specified for the purposes of being loadbearing, unaffected by moisture, thermally insulating, acoustically insulating and fireproof. Usually a mixture of these.

For the purpose of constructing the below ground wall, down to the foundations, there is no real substitute for blockwork because it is not damaged by moisture (providing it is sulphate resisting, along with its mortar). Similarly for party walls there is a strong argument for their use because all the above properties are put to their best use.

However, above DPC level, timber frame construction, along with plywood, plasterboard and insulation can often do a better job than blockwork. In the case of timber post and beam, SIPS or Walter Segal construction there might not even be any need for below ground blockwork. Once again the argument for walls which are virtually fully filled with insulation to achieve PassivHaus standards may mean that blockwork simply takes up too much space.

Aircrete blocks, which have much higher insulation values than standard blocks may be a way of achieving the desired insulation without creating an unacceptably thick wall. However there is a fairly fixed relationship between the weight of concrete blocks, their insulating properties and their compressive strength. Generally the lighter and better insulating, the less strength they have and this means they might not be strong enough to hold up floors and roof, especially if they are the only means of support. See the Aircrete Products Association web site

Shapes and sizes

The Concrete Block Association have information for the specification of blocks including sizes, density, tolerance, strength and configuration.

The standard block size is 440 x 215 which is designed to coordinate with multiples of bricks. (Bricks are 215 x 65 and the joints are taken as 10mm.) So if you include the joints then a block length is the same as two brick lengths and a block height is the height of three bricks. Also this means that a block set on its end is equal to 6 bricks in height and a brick on end is the same height as a block, This means less cutting of blocks and allows wall ties to coordinate properly.

Building regulations


The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

Site preparation and resistance to contaminants

This section,  Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved Document specially for houses.



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