ᐅ Insulating a pumice block wall with solid wood

I want to try cladding a house wall with double-layer solid wood tongue-and-groove boards.
The wall consists of 12cm (5 inches) of simple pumice stone. In winter, the wall is freezing cold...
1. Two layers of 60mm (2.4 inches) tongue-and-groove boards. The first layer applied vertically directly onto the masonry. (Each piece of wood is drilled and anchored into the masonry).
2. Then a second layer of 60mm (2.4 inches) tongue-and-groove boards screwed horizontally onto the first layer.
3. Battens, 30mm (1.2 inches) thick, applied vertically. This should provide sufficient ventilation behind the cladding.
4. Exterior cladding screwed onto the battens.

Basically, I am optimistic that this should work well.
It would be much easier, however, to first apply the battens and then start building the wall as described above.
With a direct connection of the wood to the masonry, the wood should be able to channel any moisture outwards.
If there is a batten layer between the wood insulation layer and the masonry, I am concerned that water could condense on the wall and, if not properly ventilated, could lead to mold growth there.
Providing ventilation directly at the wall seems questionable to me. Or am I mistaken? What do the experts think?
I would be very happy to receive well-founded advice and thank you in advance.

Hi, if you really only have 12cm (5 inches) of pumice stone, consider adding some proper insulation in front of it if you have the space. A ventilated facade could look like this, for example: Attach about 8-10cm (3-4 inches) thick battens to the pumice stone, fill the space between with mineral wool or rock wool insulation. Then add a breathable membrane, another layer of battens, and finally the cladding installed vertically as tongue-and-groove boards (e.g., larch).

I did something similar, and it worked quite well.

Yes, I understand that, but since I work at a sawmill and the insulation material itself costs me almost nothing (scrap material), and I know several people with solid wood walls who have only positive things to say, I want to try an unconventional approach. I don’t want to use any membranes, vapor barriers, etc. I want to work purely with wood as the building material. I’m just not sure about the assembly and am hoping to get expert advice here.
Thank you very much

So, the solid timber houses of my sister and my parents (both log houses) actually had only about 12-14 cm (5-6 inches) of solid wood. Back then (about 20 years ago), that was enough to meet the energy-saving regulations. But even then, airtightness was important, so insulating strips were placed between the logs to prevent drafts ;-)

If you put a (relatively) tight wooden layer in front of the pumice stone, it will provide some insulation. However, the wood must be protected from the outside (preferably through construction methods rather than chemicals), and the dew point must not be inside the wooden wall (this has to be calculated, but precise values such as the thermal transmittance coefficients of the materials would be required ... are these known for the remaining wood?).

Isn't a certain amount of wood mass required to achieve an insulating effect that eventually forms a structurally self-supporting envelope around the house?

Apparently, the original poster wants to save money by using inexpensive production leftovers. However, I don’t believe this will result in a satisfactory balance between cost, insulation performance, and building physics.

The approach you want to take can be implemented. However, you should be aware that the effect will be only marginal. The U-value of the finished wall will still be quite poor (I just calculated it). You will still have cold walls. Additionally, the dew point shift is unfavorable – everything will become damp (calculated using spruce and oak). Whether ventilation behind the cladding is sufficient when moisture penetrates into the masonry, I cannot assess. I am too much of an amateur for that.

P.S. Calculations were done assuming airtight wood layers, not gaps or spaces. That might result in slightly drier conditions, but with reduced insulation performance.