Poroton blocks are a special clay block used to build highly insulated walls without
a cavity. The blocks have a honeycomb-like pattern when viewed from above. They have
been used very successfully on the Continent for many years, and are now being used
in Ireland to provide comfortable and healthy living spaces. ‘Poroton’ refers to
the product produced by a group of manufacturers who are members of the Poroton Group
(www.poroton.org). There are other manufacturers who make identical products but
they are not members of the Poroton Group and so market their products under different
names,such as ‘Thermoplan’ and ‘Ecoton’. These products are usually as good as and
sometimes better than the official Poroton products. Here we are using the term ‘Poroton’
to mean this type of block, whether it is an official Poroton product or not.
Clay is naturally a very good material at regulating humidity, absorbing and releasing
moisture readily. It is sometimes used as an internal render in buildings, specifically
for this purpose. The Poroton blocks work by being ‘breathable’, i.e. being able
to transfer moisture from inside to outside, thus eliminating the need for the residual
cavity used in traditional masonry walls.
The insulating properties of Poroton blocks are provided by tiny voids within the
clay. These are formed by mixing sawdust with the clay before it is moulded. When
the blocks are fired at high temperatures to cure them, the sawdust burns and disintegrates,
leaving voids within the clay.
The most efficient way of building with these blocks is using a special thin-bed
mortar with planed blocks. These blocks are cut to exact size, such that only a 1mm
thick joint is required between courses. The blocks are designed with partially interlocking
vertical joints, meaning no mortar is needed in the vertical joints. This all lends
itself to very fast construction. The bottom course of blocks needs to be carefully
bedded in standard mortar to get them perfectly level. Subsequent courses are then
laid by dipping the blocks in the thin-bed mortar and placing them on the wall, or
by using a special applicator to place the mortar on the previous course of blocks.
The finishes used on the external walls are important in order to preserve the breathability
of the wall. This generally means using some type of lime or mineral-based render
that allows moisture to travel through it. Any paints used also need to be breathable.
Beeck mineral paints, supplied by The Traditional Lime Co. (www.traditionallime.com)
are one such make of breathable paints.
As there is no specific insulation layer used in the wall, external walls are usually
365mm or 425mm thick in order to achieve a U-value of 0.27 W/m2K or better. Different
makes of blocks and different block specifications give different U-values, so one
needs to choose the right blocks carefully.
For photos of houses built with these blocks, click here. One supplier of Poroton
blocks used by our clients is: