Saturday 8 March 2014

Interior Wall Finishes

With the high thermal mass concrete slab well under way my thoughts were then focused upon the interior finish for the cast in-situ walls.

From the outset of my house design I had a very clear vision of how the concrete finish within the house should look. I was seeking a smooth "plate" finish as illustrated below.

Smooth "plate" finish required for internal walls - Photo courtesy (1)
My aim was that this finish should be achieved as a result of casting in-situ concrete and require little subsequent finishing work.

Over the last couple of years I have observed local building practices and finished results here in Crete. The general practice for building domestic houses is to create a strong reinforced concrete frame, in-fill with roughly laid insulating bricks and cover the whole lot with a thick layer of plaster.

This was not what I was seeking.

A large exposed concrete thermal mass (floor, walls and ceiling) was fundamental to the design and energy strategy for the building (i.e.retaining heat in winter and coolth in summer) together with a large area of South facing glass for winter sun and a large canopy/hood to keep the windows shaded in the heat of the summer. (See previous posts).  For the thermal mass to operate at maximum efficiency the concrete needed to maintain its integrity and remain uncovered by plaster etc.

Having seen some specialist examples locally that appeared to provide the level of finish I was seeking all seemed well.

However over the past six months or so it has become apparent that:

- there was doubt that a near perfect finish could be obtained throughout the building - with it being suggested, by a consultant interior designer, to use sculptural pieces of wood to hide any imperfections!

- it may be unrealistic from a cost viewpoint (since I am on a tight budget) to hire the necessary steel plates, from Athens, to form the internal former for casting the concrete.

- the example I saw of the external plate finish was not, as I understood at the time, obtained by a single fair finish technique but was achieved by using standard sawn timber for the wall former and then using moulds in panels on this rough surface to in effect "plaster" thin panels which are bonded to the rough concrete and then blow holes etc. filled and hand finished as required. Colour was added to this panel mix to darken the panels from the grey/white tone of the locally produced white cement and white sand mix.

- the smooth high quality "industrial look" finish, whilst being a long standing personal favorite and something I have always wanted to do, may not be seen as the most appropriate for our isolated rural environment.

- the desire to strive for a "perfect" finish may impose a level of stress that I do not need or want.

I therefore considered an alternative approach to obtaining a finish that would be acceptable, hide minor imperfections,  be a single fair face finish technique and therefore should prove a little cheaper.

In his book "Architectural Insitu Concrete" David Bennet details a project ("The Jones House") undertaken by Alan Jones Architects for himself and his family. In this project the concrete perimeter walls were formed with cheap Orientated Strand Board (OSB) boarding which is the same board that is used in constructing hoardings. The impression left by the boarding is soft and semi natural linking the interior with the natural surroundings.

OSB boarding - Photo courtesy (1)

As part of my continuing observation of all aspects of the plot from many spots around our plot, i.e. by sitting under the dappled shade of olive or carob trees through the heat of the summer listening, and observing - well someone has to do it!! - I realised that the pattern of the olive tree leaves and the dappled shade created very closely resembled the texture left by OSB boarding.

OSB is a wood based panel consisting of wood strands from forest thinnings combined with liquid resins and wax binder. The sheets used in "The Jones House" project were 2,440 mm x 1,220 mm x 18 mm thick. They were used twice, I assume once each side, and then the sheets were used elsewhere on the site.

Internal walls with OSB finish - Photo courtesy (1) 
The strength of OSB comes mainly from the uninterrupted wood fibre, interweaving of the long strands or wafers, and degree of orientation of strands in the surface layers. Waterproof and boil proof resin binders are combined with the strands to provide internal strength, rigidity and water resistance.
OSB cuts and handles just like ordinary wood.

The mat of surface strands leaves a bamboo lattice finish on the concrete. It is best to seal the face with water repellent to minimise moisture movement and dimensional change during construction.

Bamboo lattice finish - Photo courtesy (1)
In the "Jones House Project" the resulting skeletal shell, the exposed concrete walls and polished concrete floor, created an immense thermal store which gave a stable thermal environment.

Just what I am looking for!!

A Note on Colour

The question of the colour of the concrete finish has also been considered. The original vision was for something that could best be described as mid-grey. However having spent time in Crete it has become apparent that:

- due to the intensity of the light for the long summer season the need to reflect a large proportion around the house is important as stepping out of a dark interior in mid-summer can quite literally hurt/harm ones eyeballs.

- perhaps more importantly the grey skies in the depths of winter need to be offset by something brighter and thus make less demands on the PV system.

Given that we will be using locally sourced cement and sand which are both white and local aggregate which itself is light in colour the resultant concrete finish will be a light grey/grey white in colour. I am assured it will be very similar to the "plate" finish example above.

Whilst the floor slab was being laid a "test panel" using the the same concrete mix was therefore made and over the next couple of weeks was allowed to cure so that I could quickly see exactly what colour would emerge when coated with a clear silicone seal.

"Test panel" to check colour on mix for the interior "fair faced" finish.

Having been assured by the Engineers, in consultation with a concrete specialist, that the smooth "plate" finish was obtainable in a single fair faced finish using plastic moulding sheets that could be ordered from Athens and my satisfaction with the light colour of the local cement/sand mix test panel a straightforward cost effective solution emerged for the interior finish.


(1) "Architectural Insitu Concrete" - David Bennett