Soffit Insulation Board: What Goes Under a Slab and What the Code Asks

Soffit insulation board goes on the underside of a concrete slab where the space above is habitable and the space below is not. On a car park under apartments, an undercroft or a podium, that slab is the thermal boundary. The board we supply for it is phenolic, it takes eleven fasteners per sheet, and its own datasheet stops well short of calling it non-combustible.

Why a slab soffit gets insulated

A soffit is the underside of something, and here it is nearly always a suspended concrete slab. It gets a board because the slab separates a heated space from an unheated one: an apartment floor over an open car park, a bedroom over an undercroft, a podium with retail below and dwellings above.

Insulating from below is usually forced by the job, because the slab is poured and the only face left to reach is underneath. A batt sits in a cavity, held by the frame and out of sight. A soffit board goes up overhead, stays up for the life of the building, sits in a space open to the weather, and in most car parks it is the finished ceiling too. It often sits under a fire separating element as well, which is why most people searching for this product are really after the fire answer.

Three soffit situations, and the weakest one

Kingspan publishes Total R-values for Kooltherm K10 G2 in three applications, all on 150 mm concrete, calculated to AS/NZS 4859.2:2018 and NZS 4214:2006 as the NCC 2022 energy provisions require. The three are a roof slab with no ceiling below, a roof slab with a suspended ceiling void deeper than 300 mm, and an unenclosed sub-floor. The last is the car park, the undercroft and the podium, and for heat flow in it is the weakest of the three at every thickness. Take a 50 mm board: RT2.4 as an unenclosed sub-floor, RT3.1 as a roof slab with no ceiling, RT3.6 over a suspended ceiling void. Heat flow out runs closer, and at 70 mm and at 100 mm the sub-floor and the roof slab land on the same number.

Kooltherm K10 G2, unenclosed sub-floor on 150 mm concrete. The white K10 G2W publishes an identical table.

Set those against the board on its own and the point lands. Kingspan declares the 25 mm board at R1.10 and the element at RT1.3, and at the other end the declared R4.60 and the element RT4.6 are the same number to one decimal place. A 150 mm slab and two surface air films add about R0.2 at the thin end and nothing you can see at the thick end, so anyone assuming the concrete carries a meaningful share of the target is assuming something Kingspan's own arithmetic does not support. Our variant titles carry the declared board R-value, because that is what you are buying, but an energy report asks for the element. Which number belongs where is worked through in R-values for ceilings, walls and underfloor, and the declared ladder across all ten thicknesses, with the silver against white comparison and which of the three boards to buy, sits in Kooltherm K10, K10W and K12 boards.

What the datasheets claim about fire

Both boards carry an NCC Group 2 classification, a spread of flame index of 0, smoke development of 3 or lower and an FM 4880 approval, and what those mean is set out in fire performance of insulation. What sits underneath them is the point here, because the search bringing most people to this page is for non-combustible soffit insulation. Neither board makes that claim.

The words non-combustible appear exactly once in the Kooltherm K10 G2 datasheet and exactly once in the K10 G2W datasheet. In both they sit in the same place, a footnote under the FM Approval row, and read: This approval is valid for ceiling / soffit installation only, with non-combustible walls. Please contact us for correct installation instructions.

That is a condition on the approval, not a description of the product. Read quickly it looks like the board is being called non-combustible. It is not. Neither document cites AS 1530.1 or states a non-combustibility result anywhere in eight pages, and neither carries a fire resistance level.

That last absence matters here. Where the slab separates a car park from dwellings it is doing a fire separating job, and no FRL can be read off a board datasheet. The board is a thermal layer, the rating belongs to the slab and its own design, and nothing behind this article addresses what a board fixed underneath does to it. That is a question for the fire engineer, and it is the one most often put to a supplier who cannot answer it.

What happened when a soffit system was burned

Classifications describe a material under a test method. They do not tell you whether a ceiling full of board stays up. The one piece of evidence for that sits in an unlikely place, the data sheet for the fasteners. ICCONS state that their insulation fastener was tested for reaction to fire as part of a ceiling only soffit system built with Kingspan Kooltherm K10 G2, to AS ISO 9705:2003. The fasteners held the insulation for the full duration, because the shafts stayed intact, protected from the heat by the phenolic foam itself. At 600 seconds only minimal debris had fallen, limited to the area directly above the burner. The burner was then stepped up from 100 kW to 300 kW, and a heat release rate of 1 MW, which is flashover, was reached at 648 seconds. The test ran the full 20 minutes anyway, and at the end debris was limited to small charred pieces of foam and foil facing, with most of the foam still attached to the ceiling.

Both halves matter. The assembly stayed up past flashover, which is what a firefighter entering a car park and a resident leaving one both depend on. The foam also charred and the room reached flashover, which is what a combustible lining in a room fire does. It is a system test result, not a material classification, and presented as a non-combustibility claim it would misrepresent an unusually candid document.

If the specification insists on non-combustible

Sometimes the phenolic board is simply not the product. Where a specification requires non-combustible bulk insulation, the item in our catalogue carrying that claim is Bradford Rockwool Fibertex 350. Its product technical statement calls it a medium density, non-combustible stone wool bulk insulation product meeting the non-combustible requirements of NCC 2022 Volume One Amendment 2 clause C2D10(1) when tested or assessed to AS 1530.1. The stone wool is 60 kg/m³, melts from above 1200 degrees C, and the plain board runs R1.40 at 50 mm, R2.0 at 75 mm and R2.75 at 100 mm in 1200 x 600 mm pieces. A 50 mm Kooltherm board is declared at R2.30 against that R1.40, so the swap costs headroom under the slab.

The same document then takes most of it back for this application. Under limitations of use it says the product is not suitable as an exposed internal wall or ceiling lining where a Group Number to AS ISO 9705 and AS 5637.1 is required. It also states the product is not suitable for FRL applications, not suitable in BAL-FZ, and suits applications where it is protected from direct UV light, water and wind pressure.

The trade-off is not a matter of opinion. The product in our range that is non-combustible is barred by its own datasheet from lining a car park ceiling, and wants protecting from the water and wind an undercroft will hand it. The boards made to be that lining make no non-combustibility claim. A soffit required to be both is a design problem, not a product choice, and it belongs in the tender documents early.

How the board stays up

The fixing schedule is the same for the K10 G2 and K10W soffit boards, a minimum of eleven insulation fasteners with a minimum head diameter of 35 mm, and it is set out in the Kooltherm K10, K10W and K12 comparison. Worth adding here is Kingspan's aside on the lighter patterns going around: firms promoting as few as six or seven fixings, it says, have not been in the market long enough for their warranty periods to expire. Where the board sees external wind pressure, extra fixings are assessed against the Australian wind load standards.

The fastener we stock is the ICCONS ballistic point insulation fastener, a drive pin with the washer already assembled, fired from a gas tool instead of drilled and plugged. Its head diameter is 60 mm in every size, clearing the 35 mm minimum with room to spare. In 15 to 50 MPa concrete ICCONS give a minimum spacing of 100 mm, a minimum edge distance of 100 mm and a recommended tensile resistance of 0.15 kN, which is 15 kg, calculated from the characteristic ultimate load with a safety factor of at least three. One footnote changes the arithmetic on site: for cut or modified panels they recommend at least 4 fixings per square metre, so an offcut wants a tighter pattern than a full board. Kingspan publishes no weight and no density for the board, so the dead load hanging off eleven of those fasteners has to come from Kingspan rather than off the datasheet. What the Kooltherm datasheets do not publish covers the rest of those gaps.

Two rows need saying out loud. There is no 70 mm fastener, in the ICCONS document or our listing, and the size in the part number is the board thickness: one up leaves the washer proud, one down crushes the board. A 70 mm board cannot be matched from our range. The 45 mm is the mirror image, listed from the RM Industries price list with no such part on the ICCONS sheet.

Kingspan also allows the board onto a proprietary grid system of metal furring bars or timber battens rather than straight to the concrete, and publishes no specification for it, so that detail goes to its Technical Service Department.

Fire collars are the detail missed most often. Fit the board tight to one to limit thermal bridging and never across its face, because a collar has to stay exposed to activate. Cast in collars are the dangerous ones, flush with the concrete so a board run past buries them unnoticed.

Joints get taped, and Kingspan names 96 mm Kingspan Insulation Tape, silver on K10 G2 and white on K10 G2W. We do not stock the branded product. We do hold silver foil tape at the 96 mm width Kingspan specifies, both the plain 400F aluminium tape and the 493 reinforced aluminium foil tape, and a 100 mm aluminium glass tape whose own datasheet names soffit board joints and seams. None of those is a stated substitute, and we hold nothing in white, so a white soffit cannot be finished from our shelf.

Separately, ICCONS list No License among the GTIXF-160C features while our own product page says the operator should be trained and licensed as the state requires. Those disagree, and the regulator is the authority.

Frequently asked questions

Sources

General product information from the manufacturers' current datasheets, read on 23 September 2026. It does not decide a fire strategy, a fire resistance level, a fixing design or a compliance pathway, all of which belong to the project's designer, engineer or certifier. Where our product pages and a manufacturer document disagree, rely on the manufacturer. Specifications change without notice, so check the current datasheet before ordering.