Plan B?

[worms]

I think I need a Plan B…

The property is a rubble construction farm building, converted to a dwelling in the 1940’s, internally plasterboarded with no insulation. Externally it appears as originally intended and in places where I have had to remove small bits of plasterboard in the past, the walls behind the board have been standard rubble. In winter the walls remain pretty cold and condensation in the bathroom leads to mould growth on the external wall. Plan A was to remove the plasterboard from the bathroom wall, and replace with wool insulation and lime plaster to give a well-insulated breathing wall that used the rubble wall as part of the overall heat store/insulation.

On removing the plasterboard, I find that, because of the window opening that must have been inserted in a previously blank wall, the wall is of brick and cement construction, two bricks thick (placed lengthways so 18 inches thick). Good news is that everything is nice and dry, but no way is it going to breathe!

But what is my best move now? Stick with plan A, battens, wool and lime with no air cavity? Or should I do as per the thermafleece advice, leave an air gap then membrane then fleece and new plasterboard? There is no air brick or equivalent ventilation to the outside and the floor is concrete so any air gap will have very little air movement.

 

[biffvernon]

I'd stick to Plan A.

With the insulation on the inside of the wall, the masonry won't act as a useful heat store - that only happens if you insulate the outside of the wall. Nevertheless, Plan A is fine. The diagram on the Second Nature website showing an airspace between Thermafleece and masonry is for a single skin four and a half inch wall. The air-gap prevents water leaking in and saturating the wool, which would then have a lower thermal resistance. In your situation that doesn't sound as though it's going to be a problem hence Plan A works.

Even your 18 inch brick and cement wall will have a degree of vapour permeability that is worth maintaining. Water vapour passing from the room, through the new lime plaster, will be absorbed by the Thermafleece and released back to the room and through the wall to outside as and when gradients in water vapour pressure allow. It will be a 'breathing' wall. You shouldn't get any condensation and mold growth in the room. (But do open the bathroom window after you've had a shower.)

 

[Penners]

From the Thermafleece website:

Wool generates heat when it absorbs moisture from the air. That energy is known as the heat of sorption and accounts for the warmth felt by wearers of woollen clothes in winter conditions. When the air is saturated with water vapour, wool absorbs some 40% of its dry weight in moisture, producing 960 kilojoules of heat energy for every kilogram of dry wool. This warmth is not noticeable inside the building but it acts to prevent condensation in construction cavities by maintaining the temperature above the dew-point in damp conditions.

Sometimes you read something that gives you an inkling of how little you actually know.

 

[worms]

OK, I'll stick with Plan A for the wall, it will be easier than Plan B and save me an inch or two of space or allow a thicker layer of insulation.