Mineral Wool Cavity Insulation

What is mineral wool cavity wall insulation?

Mineral wool cavity insulation is a semi-rigid slab or batt that gets built into a masonry cavity wall while the wall is going up. It is spun into fibres either from recycled glass and sand (glass mineral wool) or from volcanic rock (stone wool), and it works by trapping still air in amongst those fibres so heat cannot cross the cavity easily.

It is the most widely used cavity insulation material in the UK, and for good reason: it is non-combustible, it costs less per square metre than rigid board, and because it is soft and flexible it squashes neatly around wall ties instead of needing to be cut around them.

One important distinction. This is the product you build into a new wall. It is not the blown or injected insulation that gets pumped into an existing, already-built cavity by drilling the outer leaf. That is a different job, it needs a PAS 2035 assessment and a TrustMark-registered installer, and it is not something you can buy supply-only. If your walls are already up, this range is not the right product for you.

The benefits of mineral wool cavity insulation

• Non-combustible - Euroclass A1 to BS EN 13501-1, the highest reaction to fire classification there is, and a property of the slab itself rather than of the wall it sits in.
• Lower running costs - a continuous, tightly fitted layer across the cavity cuts the heat escaping through your walls and lets your heating work less hard.
• Fits around wall ties - the soft fibre structure lets the slab close around ties and irregularities, so you get fewer gaps than you would with a rigid board.
• No clips or discs on full fill - the slabs friction fit between the two leaves, which means less kit on site and a quicker build.
• Water repellent and breathable - manufactured with a water-repellent additive and non-hygroscopic, so the slab sheds water rather than soaking it up, while still letting water vapour pass through the wall.
• Useful acoustic performance - the density and fibre structure help absorb sound and dampen vibration, taking the edge off external noise.
• Good value per square metre - generally the most cost-effective way to insulate a masonry cavity, particularly on larger builds.

Glass wool or stone wool?

Both materials do the same job and both are Euroclass A1, water repellent and breathable, so this is a handling, density and price decision rather than a fire or moisture one.

Glass mineral wool (Knauf DriTherm, Superglass Superwall) is lighter and softer to handle, and generally costs less per square metre. It compresses easily around wall ties, which makes for quick work on a long run of wall. Stone wool (Rockwool) is denser and heavier, with higher temperature resistance, and holds its shape well in the cavity.

Where the two differ in a way that affects your order is exposure zone certification, and that varies product by product. The table below sets out the position for each.

Exposure zones: which slab is certified for your wall

Exposure zone is the quickest way to narrow the range down, and it is worth checking before anything else. Zones come off a wind-driven rain map, with site-specific variation by calculation to BS 8104, and the certified position varies by product rather than by material.

If you are building in a very severe exposure area, Knauf DriTherm is the straightforward choice: its BBA certificate covers all UK exposure zones including very severe, across all three lambda grades. Superglass Superwall is also certified across all UK zones, subject to the conditions set out in its certificate. Confirm the zone for your site and check your warranty provider's requirements, and if you want a second opinion before you commit, call 03003 034 578 and we will check the certificate with you.

Choosing a thickness

Thermal conductivity (lambda) and R-value are properties of the slab and are declared on the Declaration of Performance, so you can rely on them. A U-value is not: it depends on your outer leaf, your inner block density, your internal finish, your wall ties and your mortar joints, and it has to be calculated for the actual build-up to BS EN ISO 6946.

The table below is Rockwool's published figures for one specific full fill build-up (facing brick outer leaf, 100mm medium dense block inner leaf, plasterboard on dab). Use it to size your cavity, not as a value for your own wall.

Under Part L, the limiting external wall U-value for a new dwelling is 0.26 W/m²K. New and replacement thermal elements in an existing dwelling, which includes extensions, face a tighter 0.18 W/m²K. That catches people out, so it is worth spelling out what it means in practice: a 100mm cavity fully filled at 0.032 lands exactly on 0.26 with no headroom at all, and a 100mm cavity cannot reach 0.18 in mineral wool whatever grade you use. If you are extending, you need a wider cavity, or you need to look at PIR or phenolic partial fill instead.

Available thicknesses and brands

Mineral wool cavity slabs and batts are stocked from 50mm to 150mm, in a range of lambda grades so you can hit your target without changing cavity width. The range covers Knauf DriTherm 32 and DriTherm 37, Rockwool Full Fill Cavity Batt and Superglass Superwall 36. All slabs are 1200mm x 455mm.

For reference at 100mm: DriTherm 32 gives an R-value of 3.10 m²K/W, DriTherm 34 gives 2.90 m²K/W and DriTherm 37 gives 2.70 m²K/W.

Where mineral wool cavity slabs are used

Anywhere you have a masonry cavity wall with two leaves and standard wall ties, whatever the building is for:

• New build masonry housing - full fill is the default in volume housebuilding.
• Extensions, renovations and self-builds - bearing in mind the tighter 0.18 W/m²K target for new thermal elements.
• Commercial and public buildings - offices, retail units and schools built in masonry cavity.
• Agricultural and rural buildings - see below.
• Party and separating walls - fully filling the cavity together with effective edge sealing is the standard route to a 0.00 W/m²K party wall in SAP. You need both: edge sealing on its own with an unfilled cavity gives 0.20 W/m²K, and with no edge sealing at all it is 0.50 W/m²K.
• Buildings over 18m in England, 11m in Scotland - where Euroclass A1 is the only compliant option.

Agricultural and rural buildings

Cavity slabs and batts are widely used on farms and rural holdings wherever the wall is masonry cavity: farmhouses and farmhouse extensions, farm offices and staff accommodation, workshops, grain and produce stores, livestock housing, and equestrian buildings such as stable blocks and tack rooms. The slab behaves no differently on a farm building than on a housing plot, and the 455mm width sits between standard wall ties in exactly the same way.

Two things are worth thinking about on a rural site. Exposure tends to be higher, because farm buildings sit on open ground with little surrounding development to shelter them, which pushes a lot of holdings into severe or very severe territory. Start with the exposure table above and check the position for the actual site. And Euroclass A1 means the insulation adds nothing to the fire load, which is a genuine consideration in a building storing hay, straw, bedding, fuel or fertiliser.

What these slabs will not do is insulate a portal frame or steel-clad agricultural building. There is no cavity to fill, and that is a different product family.

How mineral wool cavity slabs are fitted

Cavity slabs are 455mm wide right across the category, and that is not arbitrary: it suits standard vertical wall tie spacing, so the slab friction fits between the ties with a closed joint. Fitting is a job most self-builders and competent DIYers take on, and the insulation goes in as the wall is raised.

Full fill slabs fill the cavity edge to edge, tight against both leaves, and hold themselves in place between the wall ties with no clips or discs needed. Partial fill slabs are fixed against the inner leaf, leaving a clear residual cavity to the outer leaf: 50mm minimum, or 75mm in very severe exposure with a fair-faced outer leaf. Our full fill cavity insulation and partial fill cavity insulation pages cover both approaches in more detail, including the PIR and phenolic options.

A few things to get right:

• Butt the slabs tightly together with no gaps, and keep joints closed as you go up.
• Brush mortar droppings off the tops of the slabs and off the wall ties, so nothing bridges the cavity.
• Fit cavity closers at door and window reveals to seal the cavity at the edges.
• Keep packs dry and stored under cover. Mineral wool is water repellent, but it should not be left exposed on site.

Why buy your mineral wool cavity insulation from Insulation UK?

• Great prices - competitive pricing on cavity slabs and batts from the leading mineral wool manufacturers, with no minimum order value.
• The certification stated up front - lambda, R-value, BBA certificate number and exposure zone position listed on every product page, taken from the current manufacturer datasheet.
• All the leading brands - Knauf DriTherm, Rockwool and Superglass Superwall in glass and stone wool, full fill and partial fill.
• Fast UK-wide delivery - delivered nationwide from the Rotherham distribution centre, with free delivery on orders over £500 (ex VAT).
• Expert help when you need it - over 50 years in the insulation industry. If you are working to a U-value target, checking an exposure zone or not sure which thickness you need, call 03003 034 578 and we will point you in the right direction.