If you want to get serious, spray foam which has generally relatively high R-values ranges from 5.5.- 7 per inch and residential construction requirements for walls average between R13-19 depending on where you are located.
Adding a layer of wood to the inside over sleepers would effectively increase the r value of the assembly a very small amount. The foam rubber shown has maybe an r-value of 1? You really must consider 1/2” or 1” to get anything of value. Simply can’t negotiate the physics if heat transfer.
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I'll add a bit to this - with spray foam you would want closed cell spray foam, they make both open cell and closed cell. Closed cell spray foam tends to be more expensive. There are two factors in insulation, the R value which is a measurement of how fast or slow heat moves through the material being used, in this case the R value is like the resistance for heat to flow so the larger the number the better the insulation.
The second factor is how leaky the wall, ceiling, whatever, is. In residential construction it's called the vapor barrier. Think about a plastic bag filled with water. If you have holes in the bag, the water leaks out even though the plastic bag contains the water well. Similar with insulation, of you have holes in your insulation, the heat just leaks out (or in, think tropics).
The advantage of closed cell foam is it has a high R value, and it adheres to the surfaces it touches making a good vapor barrier at the same time.
Whatever material you decide to use, when doing the application keep in mind those two aspects of putting in insulation. You want both good insulation and a good seal.
dj