OK, now I understand. I would use only pressure treated or metal structural support and styrofoam for insulation. Then, green wallboard (moisture resistant). I would install it with screws, but not tape the seams. Cover the seams with a nice piece of wood trim which could easiy be removed accessing the drywall sections. Would only be necessary to locate the screws for the center studs for the section when removing one. As for an entirely removable section, I believe it would be necessary to attach the drywall to plywood or another surface. My concern, based on the conditions you describe, is that the plywood would warp and ruin the wall. Another thought - install a flush door (either hinged or a door panel) where the problem areas need inspection. Hope this helps a bit.

What we in the USA call American sycamore is Platanus occidentalis. It tends to have spiral grain, which makes it harder to dry without twist or warp. It also can have shake when open grown and/or in wet soils. Quartersawing it and/or cutting it thinner (3/4" or 1/2") seems to help with the stability issue. It is slightly softer than longleaf pine and slightly harder than loblolly pine (both are southern yellow pines and might be sold as heart pine). Sycamore from Europe, Ireland, and the UK is Acer platanoides, which is a maple (Acer). We call this same tree Norway Maple in the USA. It is a yard tree/invasive weed tree here and also in Ireland and the UK where it came over from mainland Europe several centuries ago. As mentioned, the wood can have nice figure and is stable. It is relatively soft compared to sugar maple. Finally, the London plane tree (Platanus x acerfolia)is a true sycamore but is a cross between American sycamore and Oriental plane tree (Platanus orientalis). It is common in Europe and in the eastern USA urban areas. I would not suggest having the tree sawn/dried/milled for flooring unless it has sentimental value to you or you can do some of the work yourself to save money. Drying is the big problem for most people with green lumber due to space and stacking requirements. 6-8% MC is what is needed for most flooring applications. This is most easily achieved with a dry kiln. If you are serious about this endeavor, then check out woodweb.com or forestryforum.com. Lots of great information on both sites. For reference, this tree at 26" DBH and 30 feet of usable log (sound & straight) will produce about 490 board feet of lumber on the International 1/4" log scale. Assuming a waster factor of 25% in converting green 1" lumber into dried 3/4" flooring, that leaves around 490 square feet of flooring if everything goes well with the drying. Hope this helps. Joe Dwyer Dwyer Forestry Consulting

Omg calling a professional isn’t always an option! So y’all keeping on saying “call a professional” is just redundant. If you can’t afford to hire someone to come in and do it (probably why your here asking advice) then yes you can do it , your only limited to what you say you can’t do! You have been given good advice by all, follow any one of them! If you can afford it to be hired out then trust me do it! If you can’t then your only option would be skim it with putty and sand (watch YouTube) or to make it even more easy... buy new And rip that out! When filling lines trust me when I say thin thin layers. You might hate it and have to do again but it’s better than what u have at this moment!

It looks like a cinder block foundation that was parged so it could be painted.