Quick answer: For most lofts that are only used for occasional storage, mineral wool laid to 270mm across the joists is the simplest and best-value option. SuperFOIL multifoil, fitted to the rafters, makes more sense when you want to use the loft space, the pitch is too low to lay thick rolls properly, or the roof has awkward areas. Some homes are best served by a combination of the two.
We install both, so we have no reason to push one over the other. Our mineral wool installs start from £600 and our SuperFOIL installs start from £800. What matters is which one suits the loft in front of us. This guide sets out the honest differences.
The basic difference: where the insulation goes
The most important difference isn't the material, it's the position.
- Mineral wool (we use Knauf Earthwool loft roll) is usually laid on the loft floor, between and across the ceiling joists. It keeps the heat in the rooms below and leaves the loft itself as a cold, ventilated space. This is called a cold loft.
- SuperFOIL is a multifoil insulation: layers of reflective foil and wadding in a single roll. We fit it to the underside of the rafters, following the slope of the roof. That brings the loft space inside the insulated envelope, so it stays warmer than a standard cold loft.
So the question is really: do you want to insulate the loft floor, the roof slope, or both?
SuperFOIL vs mineral wool at a glance
| Mineral wool (Knauf Earthwool) | SuperFOIL multifoil | |
|---|---|---|
| Where it is fitted | Loft floor, between and over the joists | Underside of the rafters |
| Our installed price | From £600 | From £800 |
| Thickness | Typically built up to 270mm in total | Thin rolls; SF19+ is around 45mm nominal |
| Effect on loft space | Loft stays cold; floor needs raised boarding for storage | Floor stays free; loft space is warmer |
| Best suited to | Standard lofts with reasonable pitch and access | Lofts used for storage or as rooms, low pitches, awkward spaces |
| Ventilation and moisture | Eaves ventilation must be kept clear | Build-up must suit the roof's underlay and ventilation |
| Reaction to fire | Euroclass A1, non-combustible (Knauf) | SF19+ is Euroclass F; Superfoil sells separate fire-rated products |
| DIY friendly? | Simple lofts, yes, with care | Better left to an installer: detailing matters |
When mineral wool is the better choice
For a typical Cardiff semi or terrace with a reasonable roof pitch and a loft that is used for a few suitcases and the Christmas decorations, mineral wool is hard to beat.
- Cost. It is the cheapest way to get a well-insulated loft, and if there is already a dry layer between the joists we can often top it up rather than start again.
- Proven method. The Energy Saving Trust recommends a minimum depth of 270mm for mineral wool and describes laying a first layer between the joists and a second at right angles across them. It is a well-understood system.
- Fire performance. Knauf rates its Earthwool loft roll as Euroclass A1, non-combustible.
- Quick to fit. Most standard jobs are finished in a day.
The downside is space. 270mm of insulation fills well above the joists, so you can't walk on it or store things on it without squashing it. If you want storage, you need boards raised on loft legs, which adds cost. Our guide to how much loft insulation you need covers the depth question in more detail.
When SuperFOIL is the better choice
You want to use the loft
If the loft is a working storage area, a hobby space, or somewhere you go up to regularly, insulating the roof slope keeps the floor clear and makes the space less cold than a cold loft.
The roof pitch is low
Shallow-pitched roofs, common on some post-war and later homes, leave very little height at the eaves. Getting 270mm of mineral wool right into those edges without blocking ventilation can be difficult. Working on the rafters avoids crawling into that tight space.
The roof space is awkward
Dormers, room-in-roof spaces, sloping ceilings and odd-shaped lofts behind knee walls are often easier to insulate at the rafters.
Things to be honest about with SuperFOIL
- It costs more. Our installs start from £800 against £600 for mineral wool.
- Installation detail matters. Superfoil's published performance figures for SF19+ include two air spaces alongside the core product, so the way it is fixed and battened affects how well it performs. Badly fitted multifoil with gaps and torn joints won't perform as intended.
- Fire rating. The standard SF19+ product is listed as Euroclass F. Superfoil makes separate fire-rated products, and the right product depends on where it is going and how it will be finished.
- It is not a fix for a bad roof. If the roof leaks or the timbers are damp, that must be dealt with first. Covering rafters hides them from view.
Ventilation and vapour: the part that gets missed
Both approaches need the moisture side handled properly. Warm, moist air from the house finds its way into the roof, and if it meets a cold surface it condenses.
With mineral wool on the floor, the loft above is cold, so it relies on ventilation at the eaves or ridge to carry moisture away. Insulation stuffed into the eaves can block that airflow and lead to condensation and mould. Keeping the ventilation path open is part of a proper install.
With SuperFOIL on the rafters, the build-up has to suit the roof. Superfoil describes SF19+ as a 3-in-1 product with a built-in vapour control layer, while SF19BB is a breathable version with perforated layers designed to let water vapour pass. Which product is right, and whether a ventilated gap is needed behind it, depends on what is above the rafters: a modern breathable membrane, old bitumen felt, or in some older Cardiff terraces no underlay at all. We check this on the survey before recommending anything.
If you are already seeing condensation or mould in the loft, read our guide to loft condensation and mould before deciding.
The combination approach
You don't always have to choose one. Superfoil itself says its products can be used on their own or in hybrid build-ups with other insulation. In practice, combinations we consider include:
- SuperFOIL on the rafters over a boarded storage area near the hatch, with mineral wool on the floor in the rest of the loft
- Mineral wool on the main loft floor and SuperFOIL in a low-pitched or awkward section where rolls can't be fitted properly
- Multifoil working alongside other insulation in a room in the roof
The aim is to avoid gaps where one area is insulated at the floor and the next at the rafters with nothing joining them up. That is something we plan on the survey.
Our recommendation
If your loft is a standard shape, you don't need the space and the roof is sound, choose mineral wool. It is cheaper and does the job well. If you want to use the loft, the pitch is low, or the space is awkward, SuperFOIL insulation or a combination is worth the extra cost. If you aren't sure, we will look and tell you straight. Surveys are free and quotes are usually back within 24 hours: request your free quote.