How can I insulate a wall at the plot boundary without losing much space?

Next, identify its construction and moisture. Information is needed on the masonry, plasters and existing insulation. Examine the areas at the floor and under the roof. If water enters the assembly from outside, the internal thermal layer does not remove that cause. Access for later repair is worth allowing for now.

Do not count only the insulation thickness. Add adhesive or the supporting frame, the finishing layer and any installation space. If the total build-up is 80 mm over a 10 m length, it occupies approximately 0.8 m² of floor plan. This is a simplified calculation for a straight strip, without corrections at corners.

In a narrow room, the width of the passage may matter more than the floor area alone. Check the furniture layout, door opening and access to the radiator. A smaller thickness helps only if the solution meets the other conditions. It is not worth saving a few millimetres at the expense of the required protection or continuity of layers.

Before ordering, the future wall face can be marked on the floor and a simple trial placement of the fittings carried out. Allow for skirting boards, the window sill and space to open drawers. This allows the function of the room to be assessed without guessing from the plan alone. If space is lacking, changing the furniture layout may be more sensible than reducing the insulation below the approved option or removing a required finishing layer.

For the same thermal resistance, a material with a lower λ can be thinner. At an assumed R = 3.5 m²·K/W, insulation with λ 0.022 requires a calculated 77 mm. For λ 0.019 this is 66.5 mm. These are values resulting from d = R × λ. They do not constitute a ready selection for a complete wall.

The available thicknesses and the range of the declared value must be checked. The facing, application and required finish also matter. The thinnest foam does not always produce the thinnest finished build-up. The comparison should cover all layers from the old masonry to the new room surface.

Adding insulation from the inside means the wall receives less heat from the room. Its temperature drops, and its ability to dry out may change. A vapour-tight lining affects vapour transport. An analysis of the whole section and the conditions on both sides is needed. A favourable U-value alone is not enough.

If the neighbouring side is difficult to access, repairing the consequences of an error can be costly. It is worth identifying precipitation, rising damp and the condition of the joints before the work. A damp wall should not be sealed with new cladding without establishing how it will dry out. This is a construction problem that the name of the material does not solve.

Insulation ends at many edges. Thermal bridges can remain at these points. The connection to the floor slab, transverse walls and window must be assessed. A narrow reveal can be difficult to insulate, but omitting it may shift the coldest surface to just under the window frame.

Before installation, plan sockets and cable routing. Every penetration through the sealing layer requires a detail. Do not improvise after the finish has been applied. In old buildings, the ends of beams embedded in the wall are also important. Their thermal and moisture conditions must be considered separately.

Compile the full thickness, the thermal parameter achieved, the cost and the required work on installations. Add the moisture assessment and the type of finish. A more expensive variant per square metre of insulation may reduce the need to remodel the room. A thin board, in turn, may require costly details that change the final bill.

Finalise the decision in the design and the material specification. At acceptance, check the connections that will be concealed before they disappear under the lining. Keep photographs and information about the solution used. They will be needed for later drilling, changes to installations or repair of an adjacent section of the building envelope.