Metal roof insulation is not a single product decision. It is a system decision, and the system has to do three separate jobs at once: reduce heat transfer through the roof plane, control condensation on the underside of the steel, and dampen the acoustic impact of rain. Specifying only for one of those jobs while ignoring the others is where most roof insulation problems begin.
This post covers the three main insulation layers used under metal roofing in Australia, how total-system R-value is calculated, and where NCC 2025 climate-zone requirements sit for each.
The Three Jobs Roof Insulation Has to Do
Thermal performance is the most discussed. Heat flows through a metal roof sheet almost instantly. Steel has negligible thermal resistance on its own, so without insulation the roof plane adds almost nothing to the building's thermal envelope. The NCC 2025 sets minimum total R-values for roofs by climate zone, ranging from R2.0 in the warmest northern zones up to R6.3 in alpine and cold-temperate zones.
Condensation control is less visible but equally consequential. When warm, humid interior air contacts a cold steel surface, moisture condenses on the underside of the sheet. In a residential roof with no vapour management, that moisture drips onto ceiling insulation, saturates it, reduces its R-value, and eventually causes corrosion and mould. NCC 2025 Section J and the condensation provisions in Volume 2 both require builders to consider vapour management as part of the roof assembly, not as an afterthought.
Rain noise matters in occupied buildings under metal roofing. An uninsulated corrugated or Trimdek roof over a bedroom or office is acoustically unacceptable to most occupants. Insulation immediately under the sheet absorbs impact energy and reduces the transmitted noise significantly. This is not a code requirement in most cases, but it is a practical one.
Anticon Blanket: The First Layer Under the Sheet
Anticon (short for anti-condensation) blanket is a composite product: a layer of glasswool or polyester insulation bonded directly to a reflective foil facing. It is installed draped over the purlins or battens before the roof sheets go on, with the foil face pointing upward toward the underside of the steel.
The foil face does two things. It reflects radiant heat back toward the sheet rather than letting it pass into the building, and it provides a non-absorbent surface against the steel so that any condensation that forms on the sheet runs down the foil and drains to the eave rather than soaking into the insulation fibre.
Anticon products are available in a range of thicknesses, typically from 50 mm to 100 mm, with R-values from approximately R1.3 up to R2.7 depending on thickness and product specification. The R-value stamped on anticon is the total R-value of the blanket itself, measured to AS/NZS 4859.1. It does not include the air gap below the blanket or the reflective component, which are calculated separately when assessing total system performance.
For metal roofing over habitable spaces, 75 mm anticon (around R1.8) is a common minimum. In climate zones 4 through 7, that alone will not meet NCC minimums, so it needs to be combined with ceiling-level bulk insulation to reach the required total.
Anticon is the standard insulation product for commercial and industrial metal roofing, sheds, patios with enclosed ceilings, and residential roofs where access for ceiling insulation is limited. It installs in one operation alongside the roof sheet, which keeps labour costs down on large areas.
Reflective Sarking: Foil Membrane and Air Gap
Reflective sarking is a different product doing a different job, though it is often confused with anticon because both involve foil. Sarking is a membrane, not a blanket. It has no bulk insulation component. Its thermal contribution comes entirely from the reflective air gap it creates when installed correctly.
Under metal roofing, sarking is typically installed over the battens and under the sheets, or it can be installed on the underside of rafters in a cathedral ceiling assembly. The foil reflects long-wave radiant heat. For the reflection to work, there must be an air gap of at least 25 mm on the reflective side. Without that gap, the foil is in contact with another surface and its reflective R-value drops to near zero.
Sarking also acts as a secondary weather barrier. AS 4200.1 classifies sarking membranes by vapour permeability, and the right classification matters for your climate zone. In humid climates (zones 1 and 2), a vapour-permeable sarking allows moisture to escape from the roof cavity. In cold climates (zones 6 and 7), a low-permeability membrane may be specified to limit vapour entry from inside. Getting this wrong can trap moisture rather than managing it.
Reflective sarking on its own contributes approximately R0.4 to R0.7 to total system R-value, depending on air gap size and emissivity of the foil surface. That is not enough to meet NCC requirements alone, but combined with ceiling-level bulk insulation it is a cost-effective way to add reflective performance without the bulk of anticon.
Sarking is also required under metal roofing in bushfire attack level (BAL) zones under AS 3959. In BAL-12.5 and above, the sarking must comply with AS 1530.1 for non-combustibility or meet specific ember-resistance requirements. Always check the BAL rating for the site before specifying sarking products.
Ceiling-Level Bulk Insulation: Where Most R-Value Comes From
For residential buildings with a ceiling below the roof, the majority of the required R-value is typically achieved with bulk insulation laid on the ceiling. Glasswool batts or polyester batts at ceiling level are the most common approach. R4.0 to R6.0 batts are standard in temperate and cold zones.
Ceiling-level insulation works by trapping still air in fibrous material, slowing conductive and convective heat transfer. It does not address condensation on the underside of the roof sheet, which is why anticon or sarking is still needed even when ceiling batts are installed. The two layers address different problems at different points in the roof assembly.
In a typical residential metal roof assembly in climate zone 6, you might see: 75 mm anticon under the sheet (R1.8), a ventilated roof cavity, and R4.0 ceiling batts. The total system R-value is not simply the sum of those numbers. Air gaps, framing, and the thermal bridging effect of steel battens all affect the calculated total.
Reading Total System R-Value Correctly
The NCC minimum R-values are total system values, not product values. This distinction matters when you are trying to meet a compliance target.
Total system R-value for a roof assembly is calculated by adding:
- The R-value of each insulation layer (anticon, bulk batts)
- The R-value of reflective air gaps (where the gap is at least 25 mm and the foil has a low emissivity surface)
- The surface resistances of the internal and external surfaces (typically 0.11 and 0.03 m²K/W respectively)
Thermal bridging through steel purlins, battens and rafters reduces the effective R-value of the assembly below the nominal sum of its components. For steel-framed roofs, the zone method or the proportional area method in NCC Section J is used to account for this. A roof assembly with R1.8 anticon and R4.0 ceiling batts may have a total system R-value of around R4.5 to R5.0 after bridging corrections, depending on framing geometry.
The NCC 2025 climate zone map divides Australia into eight zones. Zone 1 (Darwin, far north Queensland) requires a minimum roof R-value of R2.7 for residential buildings. Zone 7 (alpine areas, parts of Tasmania) requires R6.3. Most of coastal New South Wales, Victoria and South Australia sits in zones 5 and 6, requiring R4.6 to R5.1. Always check the specific zone for the project address, not just the state.
Putting the System Together
For a habitable building under a metal roof, the practical approach is:
- Lay anticon blanket over battens before sheeting. Choose a thickness appropriate for the expected condensation risk and acoustic requirements, not just the R-value contribution.
- Install reflective sarking if the design calls for a ventilated cavity or if BAL requirements apply.
- Achieve the balance of required R-value with ceiling-level bulk insulation.
- Calculate total system R-value using the NCC method, accounting for framing and air gaps.
- Verify the sarking vapour permeability class suits the climate zone.
For unheated sheds, carports and industrial buildings, anticon alone is usually the specification. For commercial buildings with suspended ceilings, the assembly may include anticon, sarking and additional insulation within the ceiling void.
The products involved are not complex, but the interactions between them are. A foil that cannot drain, an air gap that is compressed to zero, or batts installed over a ceiling with no vapour management under the sheet will all underperform against their rated values.
ACS stocks anticon blanket, reflective sarking and associated accessories for metal roofing projects of any scale, with cut-to-length and bulk supply options available. For product specifications or a quote on insulation for your next job, visit acsupplies.com.au.