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Renovating a terrace over a room: how much space is left for PIR?

PIR insulation

At a glance

On a terrace above a heated room, the door threshold alone does not determine how much PIR insulation will fit. The available depth must accommodate everything from the substrate to the walking surface, while allowing water to drain. Before comparing quotes, take levels at the doorway, at the outlet and at the highest point of the substrate. A designer needs all three to see whether the proposed build-up will fit. Here is how to draw the section, which figures to take from the board data sheet and what to check before ordering.

PIR insulation board with thin aluminium facings and a smaller sample showing the cream-coloured core

Three levels on one sketch

Use one datum throughout. A threshold measured from the indoor floor and an outlet measured from the old paving cannot be compared directly. Mark which layers will remain after the strip-out.

Account for every layer

List the parts of the specified build-up: falls, insulation, waterproofing, supports and paving. Their order depends on the system. The PIR board accounts for just one line in that schedule.

Draw one section at the door and another through the outlet. If one runs out of space, making the drawings fit by flattening the fall will leave a drainage problem. Resolve the detail before buying boards.

What does the PIR data sheet tell you?

Compare the declared thermal resistance of the thickness that actually fits. The U-value applies to the whole assembly; the foam’s thermal conductivity alone cannot establish whether the refurbished terrace will meet the design brief.

Check the facing on each side and the board’s intended use. Two offers can quote identical dimensions and similar conductivity yet specify different facings; the alternative may not suit the adhesive or membrane.

Check the corner by the door

Trace the path of water at the corner: from the threshold, past the jamb and onto the waterproofing. If the line stops where two trades meet, agree with the door installer and roofer who will complete that junction.

Check access to the outlet after paving. A removable piece above it lets you clear leaves without lifting a large area. Mark it on the paving plan.

What loads reach the insulation beneath the paving?

A paving slab on pedestals transfers its weight at a few points. Give the designer the support layout and bearing areas. Thickness alone is not a basis for choosing insulation under these loads.

List large planters and heavy furniture separately. A load in the middle of a paving slab can call for a different detail from one above a support. The check must include the capacity of the existing structural deck.

Inspect before the insulation is covered

Before covering the work, photograph the threshold, outlet and wall junctions. Record the insulation board reference and waterproofing system. Anyone adjusting the door or repairing the outlet later will know what lies beneath the paving.

Attach the dated final section to the order. If a substitute board is proposed, check it against that drawing and the same waterproofing system. Only then compare the total costs.