Is it worth insulating under a metal roof or in the walls of an existing house?

Ceiling insulation is easy to retrofit and easy to check. Walls are the opposite: the work is concealed, the result cannot be inspected, and doing it badly can create a moisture problem that takes years to appear.

The methods

• Blown-in loose fill or injected foam through holes drilled in the lining or the external cladding, then patched.
• Removing and replacing internal linings, which is the reliable but disruptive option and only makes sense during a renovation.
• Adding insulation externally with new cladding, which performs best because it addresses thermal bridging, and costs the most.

The risks worth understanding

• Incomplete fill. Voids and settling leave cold spots you cannot see, and cold spots are where condensation and mould form.
• Moisture. Filling a cavity that was designed to drain and ventilate can trap water against the structure, particularly in cavity brick construction.
• Wiring. Insulation packed around old cabling reduces its ability to shed heat, which matters on circuits carrying real load.
• Termite inspection. Filling a wall can obscure the inspection path in a termite-prone area.

The order to do things in

Ceiling first, always, since it is cheaper, more effective and reversible. Draught sealing second, because it is inexpensive and immediately noticeable. Walls third, and only with a contractor who will explain how they handle moisture and how they verify the fill.