The material, explained
What is
Isotherm insulation?
Isotherm insulation is a flexible polyester blanket made from recycled PET fibres. It sits above the ceiling and slows heat transfer between the roof space and the rooms below. The result can be more stable indoor temperatures through hot summers and cold winters.
Choose the thickness, grade and coverage to suit your project. A brand name or a low pack price alone does not tell you how much thermal resistance you are buying. Our team helps you compare the specification, quantity and delivered cost before you order.
01
Recycled polyester
A glass-fibre-free material for customers who specifically prefer a polyester insulation blanket.
02
Five thickness options
Explore the available sizes below, then check the selected variation’s price and availability.
03
Advice before you order
Compare this material with Aerolite or Knauf and choose around your building, budget and priorities.
Find your thickness
Choose your Isotherm insulation thickness.
Start with the required thermal performance and the space available. Open a thickness guide for practical buying advice, then select that size in the quote options.
145mm Isotherm insulation
The thickest listed option
145mm Isotherm insulation is the thickest option listed on this product page. Consider it when you want to compare the upper end of the polyester range for a new ceiling or a substantial insulation upgrade. Check the supplied density and declared thermal performance before deciding.
The manufacturer reference table below identifies the relevant grade rather than treating every pack with the same thickness as identical. For a quotation, send your ceiling area, delivery location and any specification from your builder or designer. That gives our team the information needed to check the order properly.
Allow enough clear space for the blanket to retain its thickness. Compressing a thicker product into a shallow gap is not a reliable way to obtain its stated performance. Existing insulation, roof leaks, access and services also need consideration before the material is fitted.
Before adding to your quote
Confirm the grade, pack dimensions and coverage. Compare the delivered cost per square metre with an appropriate Aerolite or Knauf alternative if you are still choosing.
135mm Isotherm insulation
Check the product R-value
135mm Isotherm insulation is another option for customers comparing polyester ceiling blankets. It belongs in the same buying conversation as the thicker size, but the two should not be described as delivering identical thermal performance. Match the product to the design rather than choosing on thickness alone.
When comparing quotations, ask whether an R-value refers to the insulation material or to the complete roof and ceiling assembly. Those are different figures. The reference table below separates the product value and density, helping you compare the material on a consistent basis.
This size may be considered for renovation work or a new ceiling where the specified performance and available space suit it. A product recommendation should account for the existing construction, local conditions and installation method. It is not automatically the correct specification for every home.
Make a like-for-like comparison
Compare the stated product R-value, coverage and delivered price. Ask our team to include a glasswool alternative when fire performance is a key part of your decision.
100mm Isotherm insulation
Density makes a difference
100mm Isotherm insulation deserves a careful specification check because published manufacturer literature includes more than one density at this thickness. The same nominal depth does not guarantee the same thermal resistance. Ask which grade is being supplied and compare its declared value.
For an existing ceiling, first establish what insulation is already present and what the upgrade needs to achieve. A thinner blanket should not be substituted for a specified thicker product simply because the initial pack price is lower. The entire roof and ceiling design still matters.
Pack coverage also affects the number you need. Use the dimensions confirmed for the selected variation, divide the required area by that coverage and round up to complete packs. Make a separate allowance for awkward edges or unusable offcuts where the layout calls for it.
Ask for the grade in writing
Keep density, product R-value and pack coverage on the quotation. This makes it easier to compare another supplier’s offer without confusing different specifications.
75mm Isotherm insulation
For a specified thinner layer
75mm Isotherm insulation is worth considering where the project calls for a thinner polyester layer. It may form part of a planned upgrade or another suitable application, but available cavity depth and the desired thermal result need checking together. A restricted space alone is not a complete specification.
Before buying, compare the required material performance with the data for the actual supplied grade. Avoid describing a thinner product as equivalent to the thicker options. If it cannot meet the project’s needs, changing the design or material may be a better use of the budget.
For an upgrade, assess the condition of the existing blanket before adding more material. Wet, badly compressed or contaminated insulation needs attention first. Keep the new layer continuous where the design permits, while observing the required ventilation and clearances around heat-producing equipment.
Plan the upgrade first
Send us the ceiling area and details of the existing insulation. We can help you compare suitable supply options and prepare a quotation for the agreed specification.
50mm Isotherm insulation
The thinnest listed option
50mm Isotherm insulation is the thinnest option in this range. Choose it when the application specifically calls for that thickness and grade. It should not be presented as an automatic replacement for a thicker ceiling blanket or as a universal solution for a new home.
A low pack price can be attractive, but the useful comparison is the cost of meeting the project’s requirements. Check the required thermal performance before comparing how many square metres each pack covers. Buying a thinner material that cannot achieve the intended result can be a false economy.
For a proposed additional layer, consider the existing material, roof layout and available access. Do not assume that adding layers will produce a verified roof performance without checking the assembly. Our team can help you identify the information needed for a sensible product comparison.
Buy for the application
Confirm thickness, grade and coverage before ordering. If you are uncertain about the specification, request advice before committing to a quantity of complete packs.
The figures behind the choice
Isotherm insulation specifications.
These are manufacturer brochure reference values for Isotherm insulation, not confirmation of the grade currently supplied. Match your quotation to the exact product and current technical documentation.
| Thickness | Density | Product R-value | Reference length |
|---|---|---|---|
| 145mm | 11.5 kg/m³ | 3.37 | 5 m |
| 135mm | 11.5 kg/m³ | 3.14 | 5 m |
| 100mm | 11.5 kg/m³ | 2.33 | 6 m |
| 100mm | 8.5 kg/m³ | 1.57 | 6 m |
| 75mm | 8.5 kg/m³ | 1.18 | 8 m |
| 50mm | 8.5 kg/m³ | 0.79 | 10 m |
Source: Brits Nonwoven manufacturer brochure, specification table. The brochure lists more than one width. Confirm actual dimensions and coverage; do not calculate an order from reference lengths alone.
Product R-value is not the total roof R-value.
The blanket is one part of the construction. Roofing, ceiling materials, air spaces and installation details affect the complete assembly. Read our R-value guide for a clearer explanation.
Choose around your priorities
Prefer polyester?
Or a non-combustible material?
We supply Isotherm insulation for customers who want polyester. We also recommend comparing Aerolite Soft Touch and Knauf Ceiling Roll, especially when an A1 non-combustible material is important to you.
Start with documented fire performance.
Isover identifies Aerolite Soft Touch as A1 under SANS 53501-1. Knauf declares an A1 reaction-to-fire classification for Ceiling Roll. The cited 2024 TIPSASA register records a B/B1/2 H entry for 145mm Isotherm under SANS 428; that is a different classification system and is not an A1 declaration.
Ask for current documentation for the exact product. None of these labels makes an entire roof fireproof, and installation clearances still matter.
A polyester blanket may be your preference.
Isotherm’s recycled PET construction appeals to customers seeking a glass-fibre-free material. Compare its handling characteristics alongside the required thermal performance, supplied density and documented fire classification. Material preference is one part of the decision.
Compare a complete quotation.
Compare products that meet the same project requirements. Divide each quoted pack price by its confirmed coverage, then include the quantity, VAT and delivery. The cheapest individual pack may not be the cheapest suitable solution for the whole ceiling.
Good fitting matters
Get the installation details right.
Isotherm insulation should be fitted to the manufacturer’s instructions and the roof design. Gaps, compression and unsuitable installation details can undermine the intended result.
01
Inspect and prepare
Repair leaks and check the roof space before fitting a new blanket. Arrange safe access; never stand directly on ceiling boards. Have electrical concerns assessed by a qualified person.
02
Measure and fit
Measure between the roof timbers, cut accurately and fit the blanket without forcing it into a smaller depth. Follow the specified layout to maintain continuity across the intended ceiling area.
03
Respect clearances
Keep ventilation paths open. Follow the relevant product and fitting instructions around downlights, flues and other heat sources. Do not use a general buying guide as an installation specification.
For professional installation advice in Cape Town and surrounding areas, contact Cape Ceilings & Drywalling. Confirm the material they recommend and install for your project before ordering. Material supply and installation are quoted separately.
From selection to delivery
A clearer route
to your quotation.
For an Isotherm insulation quotation, choose an available thickness in the product options, enter the number of packs and add the selection to your quote. Review the quantity and VAT in the quotation before proceeding. Delivery is confirmed for your address and order size.
For help estimating quantity, send us the ceiling area and any drawings or measurements. Use the coverage confirmed for the supplied pack, round up to complete packs and account for the layout. We can also quote an Aerolite or Knauf alternative for comparison.
Roof Insulations brings more than 36 years of practical insulation experience to the conversation. Our aim is a clearly specified order, useful advice and a material choice you understand.
Before you buy
Your questions, answered.
What is Isotherm insulation made from?
It is a thermally bonded polyester insulation made from recycled PET fibres. The material differs from the glasswool used in Aerolite and the glass mineral wool used in Knauf Ceiling Roll.
Which Isotherm insulation thickness should I choose?
Choose around the specified thermal performance, available space and exact supplied grade. The thickest option is not automatically necessary for every application, and a thinner option should not replace a required specification without checking the design.
Is Isotherm insulation non-combustible?
Do not describe the cited B/B1/2 H classification as non-combustible A1. For an A1 material, compare the current Aerolite Soft Touch and Knauf Ceiling Roll documentation. Request the current test classification for the exact supplied product.
Can I fit it above an existing ceiling?
It may be suitable where the roof space, access and project specification allow. Inspect existing materials, leaks and services first. A roof professional can advise on a safe installation method and the appropriate material.
Does it make a room soundproof?
No insulation blanket alone makes a room soundproof. Sound isolation depends on the complete ceiling or partition, including linings, framing, sealing and sound paths around the construction. Compare tested assemblies when noise control is your main objective.
Can the insulation get wet?
Keep the material dry in storage and service. Repair roof leaks and have wet or contaminated insulation assessed before covering it with another layer. A roof leak should not be treated as an acceptable operating condition.
How do I compare the price per square metre?
Divide the selected pack’s price by its confirmed coverage. Use the same VAT basis for every option and include delivery when comparing the final order. Pack size can differ between thicknesses and formats.
The right material. A clear quotation.
Ready to choose your Isotherm insulation?
Select your thickness above, or ask us to compare Isotherm insulation with an appropriate Aerolite or Knauf option for your project.
Technical sources and specification notes
- Brits Nonwoven Isotherm manufacturer brochure, hosted by Pelican. Reference thermal values and grades; confirm current supply.
- TIPSASA fire register, 31 October 2024. Historical classification entries, not confirmation of a current certificate for every variation.
- Isover’s Aerolite Soft Touch fire-classification information.
- Knauf Ceiling Roll / Loft Roll datasheet, September 2025.
Product information can change. Match thickness, density, dimensions and documentation to the product quoted. Building suitability depends on the complete design and installation.
Reviews
There are no reviews yet