Multifoil vs mineral wool: which loft insulation is the better long-term buy?

Mineral wool is what most installers quote by default, and for a loft nobody ever sets foot in, it does a job. But when homeowners ring our advice line having lived with both, the story we hear again and again is that the long-term question is the one that matters. So here it is, asked properly.

What you are actually comparing

Mineral wool is spun rock or glass fibre, laid between and over the joists. It insulates by trapping still air. It is cheap to buy, familiar, and fine in ideal conditions, and that italic is doing a lot of work, as we will see.

Multifoil is a thin multi-layer quilt: reflective foil faces around insulating wadding, typically fixed at the rafters or joists with an air gap to the foil. It does three jobs in one layer: conventional insulation, reflection of radiant heat, and a built-in vapour-control layer. The UK's best-known certified range is SuperFOIL, whose products carry independent BBA certification, which is the figure to trust with any insulation, ours or anyone's.

The part day-one pricing hides: what happens over time

Mineral wool's performance depends entirely on staying thick, dry and fluffy. Real lofts are unkind to all three:

• Compression kills it. The moment boards, boxes or Christmas decorations squash that 270mm down, its insulation value collapses: a compressed roll can lose the majority of what you paid for. Most families use their loft; most wool lofts are part-crushed within a few years.
• Moisture degrades it. Damp wool insulates dramatically worse, and it can slump and gap over the years, leaving cold spots you will never see.
• It does little against summer heat. Wool slows heat conduction, but radiant heat through a hot roof is what cooks bedrooms in July, and reflecting radiant heat is precisely what wool does not do.

Multifoil's strengths are the mirror image. Installed correctly at the rafters with its air gaps, it does not slump, does not compress, and does not absorb moisture the way fibre does: the performance you buy in year one is the performance you have in year twenty. Its foil faces work both directions: heat kept in through winter, radiant heat reflected back out in summer, which is why room-in-roof spaces insulated with multifoil are noticeably more liveable in a heatwave. And because it insulates at the rafters, your loft floor stays free: you can board and use the loft without crushing your insulation, something the wool approach cannot offer without sacrificing performance or paying for raised-floor systems.

That is the honest meaning of "cost-effective": not the price of the roll, but the cost per year of performance you actually keep, plus a usable loft thrown in.

Where wool still has a case

We would be selling you something, not advising you, if we pretended wool had no place. If your loft is purely a cold void, never boarded, never stored in, ventilation generous, then full-depth mineral wool on the floor is a serviceable budget answer, and the 270mm convention exists for a reason. If that is genuinely your loft, wool will do.

But that describes fewer real homes than the default quotes assume. If you store anything up there, plan to board it, have a room-in-roof or conversion, struggle with summer heat, or simply want the job done once and done properly, the case tilts firmly towards a certified multifoil system.

Doing multifoil right (this part is non-negotiable)

Multifoil earns its performance only when installed as its certification describes: air gaps on the foil faces, joints taped, detailing done properly. Squashed flat against a surface, it loses the reflective benefit you paid for. Two rules keep you safe:

1. Buy on the independent certificate, not the brochure. Ask for the BBA (or equivalent) certified figures for the exact product and build-up being quoted; certified ranges like SuperFOIL publish these. Then ask the installer to show how those figures make your roof comply.

2. Ventilation stays sacred. Whatever the material, the loft must breathe. Blocked eaves cause the condensation and mould problems that fill our loft condensation report. A multifoil install designed with its vapour-control layer in the right place actually helps manage moisture; designed wrong, any insulation causes trouble.

The installer matters as much as the product. Our guide to choosing a loft insulation company gives you the six questions to ask before you sign, and if you want to understand the certified numbers you will be shown, our plain-English U-values guide, with its free calculator, translates them.

The verdict

For the way real families use real lofts: boarded, stored-in, lived-under, and hot every July, a certified multifoil system such as SuperFOIL is the better long-term buy. Performance that does not degrade, winter and summer comfort, moisture handled by design, and a loft floor you keep. Mineral wool remains the budget answer for the untouched cold loft, and we will happily tell you if that is what you have.

Not sure which your loft is? That is a five-minute call. We will tell you straight, including if the cheap option is genuinely all you need. And if multifoil is right for you, we will make sure what you are quoted is the certified system installed the certified way.