Insulation is several different products solving several different problems

They are all doing the same thing by different means

Insulating materials do not block heat. What they do is trap air, or occasionally a heavier gas, in pockets small enough that the air cannot circulate and carry heat by convection. Still air is an excellent insulator; moving air is not. Everything else about the product is scaffolding to hold that air in place.

The number attached to a material describes how readily it conducts, and lower is better. But a material’s conductivity only becomes a wall’s performance once you multiply by thickness, and thickness is usually the constraint. A very good material at forty millimetres can’t beat an ordinary one at two hundred.

This matters because marketing tends to compare materials and buildings tend to constrain depth. The honest question is never which product performs best in the abstract. It is what will fit in this location, be installed without gaps, and stay dry for decades.

Fibre is cheap, thick and forgiving

Mineral fibre, glass fibre, sheep wool and blown cellulose all work by holding air in a mat of tangled strands. They are inexpensive per unit of performance, they tolerate irregular surfaces, and they can be laid to almost any depth where depth is available. Lofts are their natural habitat for exactly that reason.

Their weakness is compression. Squash a fibre quilt and you expel the air that was doing the work, so performance drops far more than the loss of thickness suggests. This is why boarding a loft directly on top of the insulation is a well-meant mistake, and why raised boarding systems exist.

They also handle moisture in a comparatively benign way, allowing vapour to pass rather than trapping it against a cold surface. That is a real advantage in older buildings that were never designed to be sealed, and it is one reason fibre remains the default in roof spaces despite the alternatives.

Rigid board buys performance per millimetre

Foam boards achieve lower conductivity than fibre, which means more resistance for a given thickness. They cost more, and what you are buying is depth rather than heat. Where depth is genuinely scarce — a floor with a fixed finished level, a wall lined internally, the space between rafters — that is a purchase worth making.

Board is unforgiving in a way fibre is not. It comes in rectangles, buildings are not rectangular, and any gap left at an edge becomes a route that bypasses the insulation entirely. Cutting tightly and sealing the joints is most of the workmanship, and a badly fitted board can underperform a competently laid quilt.

The vapour behaviour is also different, since most foams resist moisture movement. That can be an asset or a liability depending on the construction around it, and getting it wrong is how water ends up sitting where nobody can see it. Insulating a solid wall internally is the case where this genuinely needs specialist design.

Blown and injected fill reaches what nothing else can

Some voids cannot be opened up: the cavity in a masonry wall, the space above a sloped ceiling, an inaccessible corner of a roof. Injected fill and blown fibre exist for these, delivered through small holes and left to settle into whatever shape the void happens to be.

Cavity fill is the common case and it is genuinely effective in a suitable wall. Suitability is the operative word. A cavity that is narrow, obstructed by mortar droppings, already damp, or on an elevation exposed to driven rain may be a poor candidate, and filling it can move water across to the inner leaf.

So this is one where the survey matters more than the product, and where the right answer is sometimes no. Retrofitting a wall that should have been left alone is expensive to reverse, and the symptoms show up on the inside as staining rather than as an energy figure anyone would notice.

Reflective products work on a different mechanism entirely

Foil-faced products address radiation rather than conduction, reflecting heat that would otherwise cross a gap as infrared. The physics is sound and the application is narrow, because a reflective surface only reflects into a space. Pressed against a solid material it conducts like anything else and the effect disappears.

So these products need an air gap on the reflective face, kept clear and kept clean, since dust on a foil surface progressively converts it into an ordinary surface. In an assembly designed around them, with the gap maintained, they contribute usefully. Marketed as thin replacements for two hundred millimetres of quilt, they do not.

Depth beats material, and moisture beats both

For most households the productive order is unglamorous. Get the loft to a decent depth, since it is the cheapest surface in the building to treat and the one where depth is effectively free. Deal with the walls next, by whichever method the construction allows. Floors usually come last, and over a heated space below they may not be worth doing at all.

The commonest failure is not choosing the wrong material; it is leaving gaps. A hatch, a run of pipes, the edge where a quilt stops short of the wall plate — these leave paths no amount of premium product elsewhere compensates for. Continuity is the whole game.

And every insulation decision is also a moisture decision, because adding resistance changes where in the construction the temperature falls low enough for water to condense. In a loft that is usually manageable with ventilation. In a solid wall lined internally it is not a do-it-yourself matter, and it deserves a designer who will model it rather than a rule of thumb.