Start from the finished floor level set by thresholds and adjoining rooms, measure down to the highest point of the slab, then subtract the floor finish and the screed: what is left is the space for insulation.
When renovating, the available height almost always dictates the type of insulation. Measuring it correctly before requesting a quote prevents the discovery on-site that you are three centimetres short.
The finished level cannot be chosen freely. It is dictated by fixed elements that cannot be moved: exterior door thresholds, stair treads, the floors of adjacent rooms that are not being renovated, and the clearance under interior doors. The most restrictive of these reference points sets the level.
An old slab is rarely level. Measure the distance between the slab and the finished level at several points, room by room, and use the smallest value: this is what limits the thickness you can install throughout. Also, note the height of any conduits and pipes laid on the slab.
From the total, first subtract the flooring and its adhesive, then the minimum thickness of the planned screed, as detailed in our article on screed thickness according to type and substrate. What remains is the space available for insulation.
With this height, the thermal resistance achievable is determined by the lambda value of each insulation material. When space is limited, an insulation material with a low lambda value, such as sprayed PUR, makes all the difference, as explained in our article on the insulating screed.
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