Specifying edge insulation and perimeter detailing

The perimeter edge strip is often specified purely as a movement joint, with its thermal contribution overlooked, or specified purely for thermal reasons without recognising it also needs to function as an isolation joint. A properly specified edge detail addresses both jobs explicitly — stating a material and thickness that gives worthwhile thermal resistance while also being compressible enough to allow the screed genuine freedom of movement.

This dual role means the edge strip cannot simply be swapped for whatever offcut material happens to be available on site; it should be a deliberately chosen product referenced in the specification.

Edge insulation needs to run continuously around the full perimeter of the pour, including around columns, pipe penetrations and door openings, since gaps undermine both its thermal and isolation functions. It should be fitted securely before the pour so it cannot be displaced by the liquid screed, and trimmed back neatly afterwards so it sits tidily beneath the skirting or finish.

At door thresholds, where the strip is often trimmed away to nothing, a specific detail — rather than simply omitting the strip — helps limit both heat loss and the risk of a hard, unyielding junction at that point.

A softer edge strip suits a cementitious pour's slightly greater shrinkage, while an anhydrite floor with lower shrinkage can sometimes use a firmer, thinner strip without losing isolation performance.

Whichever product is chosen, it should be named against the actual screed formulation being poured, rather than picked from a generic builders' merchant list.

The damp-proof membrane and the edge insulation strip meet at the same junction, and a gap or overlap error there undermines both the tanking and the isolation detail.

This junction is easy to get wrong on a busy site, so it is worth a specific mention rather than assuming the two trades will coordinate it themselves.