Glasswool, Polyester, Rockwool or Phenolic: Which Insulation Material for Which Job

R-value does not choose the material. Two products at the same R-value do the same thermal job, so the decision comes down to something else: whether it itches, whether the job needs a non-combustible material, how much depth you have, and whether the product works by bulk or by reflection. Get that wrong and the R-value on the pack will not save you.

Everyone starts with the R-value because it is the number printed largest. It is the wrong place to start a material comparison, because R-value is the one thing several very different products can have in common.

Why R-value does not decide it

An R3.5 polyester ceiling batt and an R3.5 glasswool ceiling batt deliver the same thermal resistance. That is what the number means. If R-value were the whole story, one of them would not exist.

What actually separates them is everything the number does not describe:

• Handling. Whether it itches, whether it needs the full covered-up kit, how heavy it is up a manhole.
• Fire. Whether the specification calls for a non-combustible material, a Group number, or a service temperature.
• Depth. How much thickness it takes to reach the number, which decides whether it fits at all.
• Mechanism. Whether it insulates by trapping still air or by reflecting radiant heat, because those two are not comparable on the same scale.

Work through those four and the material usually chooses itself.

Glasswool

Glasswool is the default for ceilings and walls in Australian housing, and it is the default for good reasons: the range is the widest, the price is the lowest per square metre at most R-values, and it goes up to high R-values without difficulty.

It works by bulk. Fine glass fibres hold still air, and still air is the insulator. That means the batt only performs at its recovered thickness, and squashing it into a shallower cavity gives away the performance you paid for.

The other thing glasswool brings to a specification is the fire position. In the Bradford range the glasswool Gold products are the ones quoted as non-combustible, and where a job specifically calls for a non-combustible material that is the reason to choose glasswool over the polyester equivalent at the same R-value.

The downside is the one everyone knows. It itches, and a whole ceiling is a lot of handling.

Polyester

Polyester batts do the same thermal job at the same R-value. Bradford Polymax is 100 per cent thermally bonded polyester, non-irritant, low allergen, and made from up to 50 per cent recycled material with no manufacturing waste.

It gets chosen for three situations, and none of them is thermal performance:

• The owner is installing it. No itch, no full covered-up kit, light to carry.
• Someone reacts to glasswool. A real consideration in an occupied house.
• The space gets accessed. A ceiling used for storage or full of services will be crawled through again, and polyester makes that a much better afternoon.

The trade is the fire position. Polyester is not the material carrying the non-combustibility claim, so where the specification calls for that, use the glasswool equivalent. Polyester products do carry their own fire test data, for example the Martini polyester baffle is fire tested to AS 1530.3 with minimal flame and smoke generation, but a test result and a non-combustibility claim are not the same thing.

Polyester also shows up where the format suits the job. The Martini polyester floor blanket comes as a 430 mm wide roll rather than batts, which runs out along a joist bay instead of being cut piece by piece, and doing underfloor work on your back is exactly where not itching earns its money.

Stone wool

Stone wool, usually called rockwool, is not a cheaper or dearer version of glasswool. It is the material you reach for when heat or fire is the actual requirement.

Two things define it. It is non-combustible, and it holds its form at temperatures that take other materials out of service. That is why the grades are named by service temperature: Bradford Rockwool Fibertex runs to 650 °C in one grade and 820 °C in another, and the choice between them is the plant temperature, not the R-value.

In building work it turns up in specific details rather than as general insulation:

• Party walls. Fireseal party wall batts seal the vertical joints in Class 1 party walls, where a joint packed with combustible material or left as a void does not maintain the separation the wall was designed to provide.
• Bushfire Flame Zone. Fireseal FZ roof batts pack the vulnerable roof edges where the design problem is direct flame and ember entry rather than radiant heat alone, rated for continuous service to 650 °C.
• Industrial lagging. Mesh backed blanket exists because a cut blanket loses its integrity exactly where it is being bent around pipework, which is where lagging usually fails first.

If your job does not have a fire or temperature requirement, stone wool is not the value choice. If it does, nothing else on this page substitutes for it.

Phenolic board

Rigid phenolic board is the answer when depth is the constraint. Kingspan Kooltherm is the range we carry, and the number that matters is thermal conductivity: 0.022 W/m·K at 45 mm and thicker, which is among the lowest of any board on the market.

In practice that means it reaches a given R-value in noticeably less thickness than the alternatives. The K10 soffit range runs from 25 mm at R1.10 up to 100 mm at R4.60, all in 2400 x 1200 mm boards. In a carpark soffit or a wall build up where every millimetre is being argued over, that is the whole argument.

There is a second argument that has nothing to do with thickness. A batt sits between the studs, so the studs stay as a thermal bridge straight through the wall. A board fixed across the frame covers that path. That is how Kooltherm K12 framing board is used, and it is a genuinely different strategy from filling a cavity.

Phenolic also brings certification that commercial specifications ask for: CodeMark certified for NCC compliance, NCC Group 2 to AS 5637.1, spread of flame index 0, smoke development 3 or lower, and FM 4880 approval for thicknesses 25 to 119 mm. The core is fibre free.

Reflective closed cell foam

This is the category most often compared incorrectly, because people put its material R-value next to a batt's and conclude it does nothing.

Reflective products work differently. They reflect radiant heat rather than resisting conducted heat through bulk, and they only do that with an air gap in front of the reflective face. The material R-value is low by design: Kingspan AIR-CELL Permishield XV 70 is R0.20 as a material, Insulwhite is R0.15. Those numbers are not the point.

The number that matters is the system figure, and the difference the installation makes is dramatic. Kingspan quotes AIR-CELL Permifloor in an unenclosed sub-floor at RT 0.8 for heat flowing in and RT 1.6 flowing out. Enclose the same sub-floor with the same product and it becomes RT 1.6 in and RT 3.0 out.

Nearly double, from a decision about the sub-floor rather than a change of product. That is the lesson for the whole category: with reflective products, how it is installed is worth more than which one you bought.

Two practical cautions. Vapour classification varies across the range, from Class 2 vapour barrier to Class 3 vapour permeable, so check which one the build up needs. And some products are restricted by building class: Insulshed 50 is made for Class 10a buildings, meaning sheds and garages, not habitable rooms.

The whole picture in one table

Fire, and why the product alone does not carry it

This is the point most likely to cost someone a rejection on site.

Fire performance belongs to the tested system, not to the product on its own. A Flame Zone rating applies to a roof detail installed to specification. A party wall rating applies to the wall system. Buying a batt that appears in a compliant system does not make your installation compliant if the rest of the system is different.

Commercial specifications add a second layer. They often name a Group number to AS 5637.1, a spread of flame index, or a smoke development index alongside the R-value. A product that meets the thermal number but has no Group classification can be rejected, and that is a substitution problem rather than a performance problem.

The practical rule: read what the specification asks the material to do, not just what R-value it wants. If you send us the specification we will tell you which products in our range actually carry the documentation it names.

What goes wrong

• Swapping polyester for glasswool on a job that names non-combustible. Same R-value, different answer.
• Comparing a reflective product to a batt on material R-value. Different mechanisms, so the comparison is meaningless without the system figure.
• Installing a reflective product with no air gap. It cannot work by reflection against a surface it is pressed into.
• Buying stone wool as general insulation. You pay for heat tolerance you are not using.
• Compressing any bulk product to make it fit. Bulk insulation only delivers its rated R-value at full recovered thickness.
• Assuming a product carries a rating because the system it appears in does. Check the documentation, not the marketing.

Frequently asked questions

Sources

General information. Fire performance, vapour classification and system R-values belong to tested systems and specific build ups, not to a product in isolation, so confirm any figure quoted here against the current manufacturer documentation and your own specification before substituting one material for another.