' the Woodlouse: The wonderful world of wood-pulp


Tuesday, 23 August 2011

The wonderful world of wood-pulp

As explained previously (see The Importance of Fitting In) when we wrap our bungalow with strawbales the bedroom that projects forward of the main building will remain as brick outside, in order to visually tie the building with the other brick bungalows in the street and appease the planners (see Elevated Greenery).  This presents some problems.

We want the whole building to be very well insulated and maintain a comfortable temperature without central heating.  The cavity walls of the existing bungalow will be filled (with glass-fibre granules made from recycled glass) but this only gives a U-value of around 0.57.  To achieve a central-heating-free home, we need the external walls to have a U-value of around 0.15 or less (there's some explanation of U-values here).  The strawbale wrap will easily achieve this for most of the building.  But in order to retain the external brick wall of that bedroom some kind of internal insulation will be needed there.  This in turn, presents some problems.

It is always better to insulate externally.  This reduces the chance of condensation (and concurrent mould growth) on your walls and extended moisture build-up within the walls.  Moisture vapour (from breathing, washing, cooking, generally being alive etc.) will condense when it meets a cold surface.  When a wall is insulated externally then the internal surface of that wall is no longer a cold surface, so condensation is far less likely to form.  Then, if the wall structure is also moisture-vapour permeable (for example: strawbales with clay plaster and lime render) the moisture vapour can pass through the wall and be ventilated outside without harming the structure.  This doesn't substitute for the importance of removing most moist air from inside the building by ventilation, but it ensures the longevity of the building structure through reducing the chance for damp-related decay, and in conjunction with ventilation ensures a condensation and mould-free existence (for a much more detailed explanation of the importance of moisture permeable walls see this pdf report from Natural Building Technologies).

We have no choice however, so have to find an effective way of insulating that space without causing condensation build-up where the insulation meets the surface of the existing wall.  Any material we use there must be moisture-permeable but more importantly hygroscopic (will absorb and diffuse moisture) so that any moisture vapour that might condense is drawn either to the surface of the material or outward through the existing wall, where it can evaporate.  We will strip the paint from the walls and probably remove the gypsum plaster and replace it with a clay plaster layer  (one of the good things about this lovely stuff is that it is highly hygroscopic, making it act as a humidity regulating layer on the walls).  This then opens the wall structure up to moisture-permeability throughout as the clay bricks it's built with are also hygroscopic and breathable although not to the same extent as the unfired clay in the plaster.

So what to put on the wall to insulate it?  There are many varieties of lightweight and efficient board out there that could give us the necessary U-value, but generally these are oil-based (polystyrene, polyurethane foam etc) with very high embodied energy (see table of embodied energy and insulation values in Fact Backed Rant).  They are also neither moisture permeable nor hygroscopic; this could be positive in that they act as a moisture barrier, but in reality they remove the helpful condensation and mould prevention that a breathable wall will give.  Also, any moisture that does find its way between these boards and the wall will be trapped.

The best option we've found so far in terms of  performance is Pavadentro, a wood-fibre board.  This wood-fibre board has excellent environmental credentials as it is made the waste (pre-consumer waste) of a material (wood) that is renewable even in it's virgin form.  The board is also both hygroscopic and moisture-vapour permeable, making it the ideal material for the job.

The same company also manufacturer wood-fibre boards that can be used as floor insulation.  We hope to use these to insulate on top if the cold concrete slab of the existing bungalow, ensuring we do not lose heat through the floor.  Here again there is some risk of condensation where any moisture meets the concrete, and use of hygroscopic/moisture permeable floor material minimises the risk of rotting this could otherwise cause.

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