The next step in insulating the bedroom is the Diathonite skim. All the old plaster has gone and we’re left with a clean brick wall. The room isn’t clean, of course – dust everywhere – but that’s to be expected when plaster is being chipped off and brick/chimney removed.

I made a call to Ecological Building Systems to ask about the recommended way to insulate a wall with wood fibre. I knew how we’d done it two years ago, but change is happening so fast in this area It’s always worth checking. And I was right.

Previously we’d had a carpenter build a wood framework that would hold the insulation. It worked well since wood is itself a good insulator and it meant the wood fibre was held securely in place.

Now I learned that there is a much easier alternative:

  1. Apply a skim of Diathonite – a layer of 10 – 15mm to even up the surface for the other insulation.
  2. Apply the wood fibre insulation – see next blog
  3. Cover with lime plaster

Diathonite is a familiar material to us – we used it on the front wall of the sitting room at a much greater thickness than 15mm. Because of the traditional Victorian bay window it wasn’t so easy to use the wood framework plus insulation as before and keep the shape.

I’d spent hours on the web talking with any technical expert I could find to understand the options. In the end, Claire Nash from Claire Nash Architecture, a group of architects who specialise in eco-retrofit, responded to a rather desperate email with information about Diathonite. And, as ever, once I had the right term it was easy to find out more.

 So what is Diathonite?

Diathonite is a thermal plaster made from lime, cork and clay. It can be sprayed or trowelled onto a wall like any plaster. In the bedroom the aim was to apply a skim in order to create a smooth surface for the insulation to attach to. It could have been any plaster, but why use something that won’t give any thermal benefit when you can add a bit more warmth to the system?

How good an insulator is it?

This is where the brain bending comes in.

Diathonite has a thermal conductivity of .037 W/mK. This is the ease with which heat can travel through the material by conduction (conduction being the main form of heat transfer through insulation). Thermal conductivity is often termed the λ (lambda) value.

So far so good. But how does that relate to the U value? U value measures the amount of heat lost through a given thickness.

So in the case of Diathonite, the U value will change according to how much of the plaster is applied. So the thicker the application, the slower the rate of heat loss and the better the building will be able to retain heat. Which will lead to a lower U value.

U-value = Thermal conductivity / thickness (where the thickness is measured in metres).

If your brain is in a tangle just now, you’re not alone! All you need to hold onto is that for both U value and thermal conductivity, low is good. So a .037 W/mK is great.

What is the advantage of Diathonite?

Diathonite comes into its own when a building is older or there’s a particular structure that won’t take solid insulation so easily – like our bay window. It can vary in thickness according to need and is excellent for following the contours of stonework of curved walls to retain the original features of older buildings.

So if you have a very old house or any difficult areas to insulate, then this might be just what you need. As I’ve learned in my few years of fascination with insulation, there is always an option if you keep looking and questioning. And the more we question, the more manufacturers will look for alternatives – it’s a win win.

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