Kooltherm K10, K10W and K12: Which Phenolic Board for Which Job

This article compares three of the Kingspan Kooltherm boards we stock. K10 G2 and K10W White G2 go under a concrete slab. K12 goes on a wall, across the frame in all five constructions Kingspan publishes, though its datasheet also allows it between studs. The core is the same in all three, so the facing decides where each board belongs. We also carry K3, K8, K17 and K10 PLUS, which this article does not cover, but not K5 or K15.

What we stock

Kingspan calls the three boards Kooltherm K10 G2 Soffit Board, Kooltherm K10 G2W White Soffit Board and Kooltherm K12 Framing Board. Our white listing reads K10W White G2, transposing Kingspan's letters, so a specification will say G2W. All three are made in Australia and CodeMark certified.

We now also carry Kooltherm K3 Floorboard, K8 Cavity Board, K17 Insulated Plasterboard and the K10 PLUS Soffit Board, the last made to order. This article does not describe them, because no Kingspan document for them was read to write it. K5 and K15 come up in our enquiries and we carry neither.

Whether phenolic beats glasswool, polyester or stone wool is a material question that insulation materials compared works through. Here it is narrower: the specification says Kooltherm, which of these three.

The three boards side by side

SMOGRA-RC is in m2/s2 x 1000. Flame index 0, smoke development 3 or lower, compressive stress over 100 kPa on average at 10% compression and vapour transmission over 35 MN.s/g match on all three. The soffit boards read as twins until the smoke growth line, where the white one is allowed 100 against 30, and K12 has no Group number at all.

The facing decides the job

The core is the same in all three, a fibre free rigid thermoset closed cell phenolic, so the facing does the sorting. K10 G2 carries reflective aluminium foil underneath, emittance E0.05 to ASTM C1371, and that face looks down into the space. On a car park soffit or a plant room ceiling it is usually accepted as the finish. K10 G2W swaps it for white foil, which its datasheet lists as a feature and captions as a visible aesthetic finish. That is the whole reason the board exists. K12 has composite foil both sides, emittance E0.06, because the reflective surface improves the resistance of the unventilated cavities it sits between.

The thickness ladders

Both soffit boards run one declared ladder: 25 mm R1.10, 30 mm R1.30, 40 mm R1.75, 45 mm R2.05, 50 mm R2.30, 60 mm R2.75, 70 mm R3.20, 80 mm R3.65, 90 mm R4.10 and 100 mm R4.60. We carry all ten in both, sold by the full Kingspan pack rather than by the board: 12 boards or 34.56 m² at 25 mm, with fewer boards to a pack as the thickness goes up. Some thicknesses are made to order, about 10 business days.

Kingspan's own sheets contradict themselves at the top of that ladder. The Product Data box on both stops at 90 mm, while the R-value table beside it lists 100 mm at R4.60 and every Total R-value table carries a 100 mm row. We sell 100 mm in both, so it reads as an editing miss, though Kingspan is the one to confirm that.

K12 is shorter. We stock 25, 30, 40 and 50 mm, plus 45 mm at R2.05 and 80 mm at R3.60 made to order, all by the full Kingspan pack. Kingspan offers others on enquiry with minimum order quantities, and notes the 80 mm is meant for residential work, with a separate data sheet that we now host on our K12 residential batts page. Our own page glosses one point: the 0.022 W/m·K figure starts at 45 mm, so only the 45, 50 and 80 mm get it. The rest are declared at 0.023.

Silver against white

Kingspan gives both soffit boards the same declared R-value at every thickness, and not the same Total R-value: on a roof with no ceiling over a 150 mm slab, the white one is lower at every thickness for heat flow in.

Heat flow in then out, over 150 mm concrete, calculated to AS/NZS 4859.2:2018 and NZS 4214:2006 under the NCC 2022 energy provisions. The white 90 mm row is Kingspan's too, and we now stock that board. Sub-floor figures are identical in both datasheets. There is no white ceiling void column because that table does not exist, so specifying K10W above a suspended ceiling needs a figure from Kingspan.

The roof gap is RT0.6 at both ends of the ladder, and heat flow out is close. Kingspan does not explain it and publishes no emittance figure for the white facing, the number most likely to account for it. So: the same declared R-value, a lower Total R-value on a roof, identical underfloor. A ceiling void over 300 mm is worth about RT0.5. Total R-value itself is covered in R-values for ceilings, walls and underfloor.

Our own K10W page says the white board has the same thermal performance as K10 G2. Right about the declared R-value, wrong about the roof tables.

K12 by wall construction

The K12 datasheet says the board can sit between studs or work as sheathing over them. Every one of the five constructions it publishes a Total R-value for fixes the board across the frame, so a between-studs build up has no published figure to quote. What sits either side changes the result more than the thickness does. The five published constructions, heat flow in then out.

At 40 mm the same board gives RT3.3 in brick veneer and RT2.0 in the Type C wall on concrete, a spread of RT1.3 from one product. Timber frame beats steel by RT0.2 throughout, which is the steel stud showing up in the arithmetic. Note the blank: the clip and channel table stops at 40 mm although the 50 mm board is in both catalogues.

Fixing, tape and ancillaries

A soffit board fully restrained to concrete takes at least eleven fasteners with a 35 mm minimum head diameter, which is 3.81 fixings per square metre on a 2400 x 1200 board. Four run along each long edge, 50 to 150 mm in from it, and three down the middle.

The reasons are practical: board and slab move at different rates, formwork joints leave ridges of up to 3 mm, and a horizontal sheet bows under its own weight and pulls tape off the joints. In a fire a fixed facer cannot peel away and foul sprinklers. Fewer than eleven, Kingspan says, would not be approved by FM Approvals.

Joints are staggered or squared and taped, silver on K10 G2 and white on K10W. Exposed edges take tape lapped at least 48 mm onto the face, or a metal c-section. Fit the board tight to a fire collar, but never over its face, which has to stay exposed to activate.

K12 has five published methods: clip and channel over concrete, a Type C wall with plasterboard adhered using Kooltherm K12 Adhesive, brick veneer, steel frame and timber frame. Studs sit at no more than 600 mm centres, and brick veneer keeps a clear cavity between 25 and 75 mm. The Type C build up is the most prescriptive of the five: eight or more fixings per board into the concrete, then sixteen daubs of K12 Adhesive about 10 ml each, set 50 to 70 mm in from the edge.

The tape instructions differ in kind, not just in width. The soffit sheets call for taped joints on every fixing method. The K12 sheet names 48 mm reinforced aluminium foil tape inside the clip and channel instructions only, so read it as a system instruction rather than the tape for the product.

The Kingspan Insulation Tape these datasheets name and Kooltherm K12 Adhesive are not in our catalogue and come through Kingspan or a distributor. The fixings are the exception. The ICCONS ballistic point insulation fastener we stock has a 60 mm head in every size, well clear of Kingspan's 35 mm minimum, and we hold silver foil tape at the 96 mm width the soffit sheets name. Neither is a stated substitute for the Kingspan product. Which fastener size goes with which board thickness is set out in soffit insulation board.

Where the fire data differs

Parity across the range is the easy assumption and the datasheets do not support it. The AS 1530.3:1999 results are the same on all three, and fire performance of insulation covers what those indices mean. Past that the three part company.

• K10 G2: Group 2 to AS 5637.1:2015 and AS ISO 9705, SMOGRA-RC of 30 or lower, FM 4880 with UBC 26-3 Class 1 for 25 to 119 mm.
• K10 G2W: Group 2 as well and the same FM approval, but SMOGRA-RC of 100 or lower.
• K12: no Group number and no SMOGRA-RC. The only extra figure is a smoke obscuration of 100 m2/kg or lower, in its specification guide.

Both FM approvals cover ceiling and soffit installation only, with non-combustible walls. K12's datasheet warns twice that its constructions suit NCC Class 1 and 10a housing and Type C construction in Class 2 to 9 buildings, and that Type A and B walls need a Performance Solution.

What Kingspan does not publish

A comparison is only as honest as its holes. None of the datasheets gives these.

• Weight and density. No kg/m3 and no board weight.
• Pack quantity. The datasheets say packs are made up according to quantity, with no boards per pack or metres per pallet. We sell by the full Kingspan pack, and each of our listings gives the boards and square metres in it.
• Acoustics. Not one Rw, NRC or absorption coefficient.
• FRL. No fire resistance level for any board.
• Service temperature. No upper or lower limit for the core.
• Load capacity. Compressive stress is an average only, with no per thickness figure and no creep data.
• Vapour class. Vapour transmission is given, but no vapour barrier class.

Where a tender asks for one, the answer is Kingspan Technical Services, not a number reasoned out from elsewhere. Specifying insulation for a tender covers what a certifier wants on file.

All three sheets do agree on the warranty: ten years, and a specification guide asking for a Project Specific Warranty from Kingspan to be submitted on the job.

Which board for which job

1. Slab soffit to be lined, or where looks do not matter: K10 G2. The lower SMOGRA-RC figure and a ceiling void table.
2. Slab soffit that stays in view: K10W, taped white, no separate lining.
3. Framed wall behind cladding: K12 on the outside face of the frame, which is the build up both frame tables are calculated on.
4. Concrete or block wall being lined out: K12 on clip and channel or as the Type C build up, but check the table before choosing 50 mm.
5. Brick veneer: K12 outside the frame, the highest published figures of the five.
6. Roof slab with no ceiling and a tight energy report: compare both boards on their own tables first.
7. The specification names K3, K8, K17 or K10 PLUS: we stock it, though this article does not cover it.
8. The specification names K5 or K15: we cannot supply it.

Where a drawing names the board, a substitution belongs with the designer, not the supplier. See commercial versus residential specification.

Frequently asked questions

Sources

General product information from the manufacturer's current datasheets, read on 23 September 2026. It does not decide a compliance pathway, a Performance Solution or a fixing design, which belong to the designer or certifier. Kingspan changes specifications without notice, so check the current datasheet before ordering.