Understanding insulation R-value is one of the most important parts of choosing roof or ceiling insulation correctly.
Two insulation products may look similar and even have the same thickness, but that does not necessarily mean they provide the same thermal performance. The insulation R-value tells you how effectively the material resists the transfer of heat.
The higher the R-value, the greater the resistance to heat flow.
For South African homes, this matters throughout the year. During summer, effective ceiling insulation helps reduce heat entering the rooms below the roof. During winter, it helps reduce heat escaping through the ceiling and roof.
However, choosing insulation purely by thickness can be misleading. Material type, thermal conductivity, density, installation quality, compression, gaps and the complete roof construction can all influence performance.
This guide explains what insulation R-value means, how R-value is calculated, the current South African roof insulation requirements, and how products such as Aerolite, Knauf, Isotherm and rigid insulation boards compare.
Table of Contents
What Is Insulation R-Value?
An R-value measures thermal resistance.
In simple terms, it tells us how much resistance a material provides against heat flowing through it.
A material with a higher R-value provides greater resistance to heat transfer than one with a lower R-value.
For example:
- R1.25 provides less thermal resistance than R2.50.
- R2.50 provides less thermal resistance than R3.38.
- R3.38 provides substantially more thermal resistance than R1.25.
The unit commonly used for R-value in South Africa is:
m²·K/W
For homeowners, the technical unit is less important than understanding the basic principle:
Higher insulation R-value = greater resistance to heat transfer.
That does not mean that the product with the highest number is automatically the correct product for every application. The insulation must also be suitable for the roof or ceiling construction and installed correctly.
Why Thermal Resistance Matters
The R-value allows insulation products to be compared according to thermal performance rather than simply comparing their thickness.
This is important because different insulation materials conduct heat at different rates.
A 100mm layer of one insulation material may therefore have a different R-value from a 100mm layer of another material.
R-value can help determine:
- how effectively insulation slows heat transfer;
- what thickness may be required;
- whether a roof assembly can achieve the required thermal resistance;
- how different insulation products compare;
- whether adding another insulation layer is worthwhile;
- and whether an existing home may be under-insulated.
When choosing ceiling insulation, the insulation R-value should therefore be considered together with the product type, thickness, fire performance, durability and quality of installation.
How Is Thermal Resistance Calculated?
The thermal resistance of a homogeneous material can be calculated using its thickness and thermal conductivity.
The simplified formula is:
R = thickness ÷ thermal conductivity
Thermal conductivity is often represented by the Greek letter lambda: λ.
A material with lower thermal conductivity transfers heat more slowly.
As a simplified example, if an insulation material is 135mm thick, or 0.135m, and has a thermal conductivity of approximately 0.040 W/m·K:
0.135 ÷ 0.040 = R3.375
This is approximately R3.38.
Manufacturers normally publish the declared insulation R-value for their products, so homeowners do not generally need to calculate it themselves.
When comparing products, use the manufacturer’s declared R-value for the actual product and thickness you intend to install.
Insulation R-Value vs U-Value
R-value and U-value are related, but they describe thermal performance from opposite perspectives.
R-value measures resistance to heat transfer.
A higher R-value is generally better.
U-value measures the rate of heat transfer through an assembly.
A lower U-value is generally better.
For most homeowners comparing ceiling insulation products, insulation R-value is the more useful number.
U-values are more commonly used when evaluating complete building elements such as walls, windows, roofs and other assemblies.
As resistance to heat transfer increases, the corresponding heat-transfer rate decreases.
Product Performance vs Total Roof Assembly Performance
This distinction is extremely important.
The R-value printed on an insulation product is not necessarily the same as the total R-value of the complete roof assembly.
A roof assembly may include:
- roof tiles or metal sheeting;
- air spaces;
- reflective foil or roof underlay;
- bulk insulation;
- ceiling boards;
- and other components.
Each component may contribute some thermal resistance.
Therefore, when SANS requirements refer to the required R-value of the roof assembly, this should not automatically be interpreted as meaning that the insulation product alone must carry the entire figure.
The insulation R-value is one component of the overall thermal resistance of the roof and ceiling system.
This distinction explains why a ceiling insulation product with a declared value around R3.35 to R3.38 may form part of a roof assembly that achieves a total thermal resistance around R3.7.
South African Insulation R-Value Requirements
South African energy-efficiency requirements for buildings are addressed through SANS 10400-XA.
The updated SANS 10400-XA:2021 framework uses South African energy zones rather than the older climate-zone approach found in many outdated insulation articles.
For roof assemblies, the current deemed-to-satisfy framework generally requires a total roof assembly R-value of R3.7 m²·K/W across energy zones 1 to 7.
The high-humidity area classified as Zone 5H has a lower total roof assembly requirement of approximately R2.7 m²·K/W.
This is an important change from older information still found online.
Many older South African articles show different roof R-value requirements such as R2.7, R3.2, R3.5 or R3.7 according to the old climatic zones. That information relates to the earlier version of SANS 10400-XA and should not automatically be applied to new work.
Always confirm the applicable energy zone and current specification when insulation is being installed as part of a new building or formal compliance project.
How Much Thermal Resistance Must the Insulation Provide?
For a conventional roof and ceiling construction, the roof covering, air space and ceiling provide part of the total thermal resistance.
Bulk insulation then provides most of the additional resistance required.
For many conventional South African roof assemblies outside Zone 5H, an added insulation R-value of approximately R3.35 is used as the general deemed-to-satisfy level.
This is one reason why products around R3.35 to R3.38 have become important in the South African ceiling insulation market.
However, the exact requirement can depend on the complete construction.
A flat roof, cathedral ceiling, exposed roof structure, metal building, thermal ceiling or specialist construction should not automatically be treated exactly the same as a conventional tiled pitched roof with a horizontal ceiling.
Aerolite Insulation R-Value
Aerolite Soft Touch is a glass wool ceiling insulation widely used in South African homes.
Current published values include approximately:
| Aerolite Thickness | Product R-Value |
|---|---|
| 50mm Aerolite | R1.14 |
| 100mm Aerolite | R2.50 |
| 135mm Aerolite | R3.38 |
For most conventional residential ceiling applications where the higher South African roof thermal requirement is being targeted, 135mm Aerolite is normally the thickness of greatest interest.
Its insulation R-value of R3.38 provides substantially greater thermal resistance than the thinner 50mm and 100mm products.
A homeowner replacing very thin or ineffective ceiling insulation should therefore not assume that any thickness of Aerolite will provide the same result.
The thickness and corresponding insulation R-value matter.
Knauf Insulation R-Value
Knauf Ceiling Roll is another glass mineral wool product used for ceiling insulation.
Current South African technical information gives approximately:
| Knauf Thickness | Product R-Value |
|---|---|
| 50mm Knauf | R1.25 |
| 75mm Knauf | R1.88 |
| 100mm Knauf | R2.50 |
| 135mm Knauf | R3.38 |
The 135mm Knauf insulation R-value is R3.38, making it suitable for applications requiring substantially higher thermal resistance than thinner ceiling rolls.
Knauf Ceiling Roll is manufactured with ECOSE Technology and is available in formats designed to accommodate common ceiling-joist spacing.
When comparing Aerolite and Knauf, the thermal performance of their main 135mm ceiling products is very similar on published R-value.
Other considerations such as handling, roll dimensions, acoustics, availability, price and installation conditions can therefore become important when choosing between them.
Isotherm Insulation R-Value
Isotherm is a polyester bulk insulation product.
Published product information lists different R-values according to both thickness and product specification.
Current published figures include approximately:
| Isotherm Thickness | Product R-Value |
|---|---|
| 50mm | R0.79 |
| 75mm | R1.18 |
| 100mm – lower-density version | R1.57 |
| 100mm – 11.5kg/m³ version | R2.33 |
| 135mm – 11.5kg/m³ | R3.14 |
| 145mm – 11.5kg/m³ | R3.37 |
This illustrates an important point about insulation R-value.
Thickness alone does not tell the entire story.
Density, thermal conductivity and the exact product specification also matter.
For example, two products both described as 100mm polyester insulation can have quite different declared thermal resistance.
When buying polyester ceiling insulation, check the declared product specification rather than simply buying according to thickness.
For installations targeting an added R-value around R3.35, the published 145mm Isotherm insulation R-value of approximately R3.37 is the relevant comparison.
[Internal link: Isotherm Insulation]
Insulation R-Value Comparison
The following table gives a simple comparison of commonly used ceiling insulation products.
| Product | Thickness | Approx. Product R-Value |
|---|---|---|
| Aerolite | 50mm | R1.14 |
| Aerolite | 100mm | R2.50 |
| Aerolite | 135mm | R3.38 |
| Knauf | 50mm | R1.25 |
| Knauf | 75mm | R1.88 |
| Knauf | 100mm | R2.50 |
| Knauf | 135mm | R3.38 |
| Isotherm | 100mm, 11.5kg/m³ | R2.33 |
| Isotherm | 135mm, 11.5kg/m³ | R3.14 |
| Isotherm | 145mm, 11.5kg/m³ | R3.37 |
These values show why comparing only insulation thickness can lead to the wrong conclusion.
The declared insulation R-value is a much better measure of thermal performance.
Does Thicker Insulation Provide Better Thermal Resistance?
Generally, yes.
If the insulation material and specification remain the same, increasing the thickness normally increases the R-value.
For example, 135mm glass wool provides considerably more thermal resistance than 50mm glass wool.
However, thickness comparisons should ideally be made within the same product family.
A 100mm glass mineral wool product and a 100mm polyester product do not automatically have identical thermal resistance.
This is why the insulation R-value should always be checked rather than assuming all materials perform equally simply because they have the same thickness.
What Happens When Ceiling Insulation Is Compressed?
Compression can reduce the effectiveness of flexible bulk insulation.
Products such as glass wool and polyester insulation depend partly on trapped air within their structure to resist heat transfer.
If a thick insulation blanket is heavily compressed into a much smaller space, it should not be assumed that the product will retain the same published R-value.
The updated South African standard specifically addresses the need to avoid compression of flexible bulk insulation in relevant roof constructions.
Correct ceiling installation should therefore allow the insulation to recover and remain as close as practical to its intended nominal thickness.
This is particularly important where insulation is installed underneath roof sheeting, between structural members or in restricted roof spaces.
A high insulation R-value printed on the packaging cannot compensate for poor installation.
Gaps Can Reduce Insulation Performance
Insulation should create a reasonably continuous thermal barrier.
Large gaps around ceiling joists, roof trusses, access hatches, pipes and other obstructions can allow additional heat transfer.
The same applies when sections of insulation are omitted because an installer does not want to cut around difficult areas.
A ceiling containing R3.38 insulation over only part of its surface will not perform like a correctly insulated ceiling with continuous coverage.
Installation quality is therefore just as important as selecting the correct insulation R-value.
For detailed installation information, see our guide on how to install ceiling insulation correctly.
[Internal link: How to Install Ceiling Insulation Correctly]
Can You Install Two Layers of Insulation?
Yes, multiple layers of insulation can be used in suitable applications.
The thermal resistance of individual layers can generally contribute cumulatively to the overall insulation system.
For example, additional insulation can sometimes be installed over existing ceiling insulation if the existing material remains suitable and the new installation is correctly designed.
Using a second layer can also help reduce thermal bridging through ceiling joists when installed in an appropriate direction.
However, several factors should first be checked:
- condition of the existing insulation;
- electrical wiring;
- downlights and heat-producing fittings;
- ceiling strength;
- roof access;
- ventilation;
- geysers and plumbing;
- available roof-space height;
- and whether the existing material contains moisture or contamination.
Do not simply force additional insulation into a restricted space if doing so causes significant compression.
What Thermal Performance Is Best for a South African Home?
There is no single answer for every building.
For a conventional new roof assembly, the applicable SANS requirement and energy zone should guide the specification.
For an existing home being upgraded voluntarily, it usually makes sense to install a meaningful level of thermal resistance rather than adding a very thin layer simply so that the roof can be described as insulated.
In many South African residential ceiling installations, products around R3.35 to R3.38 are therefore an important benchmark.
Examples include:
- 135mm Aerolite at approximately R3.38;
- 135mm Knauf Ceiling Roll at approximately R3.38;
- 145mm Isotherm at approximately R3.37.
The final choice should still consider the complete roof construction and installation conditions.
What About 50mm or 100mm Ceiling Insulation?
Thinner insulation still provides thermal resistance.
However, it provides less resistance than the thicker products in the same range.
A 50mm product may be suitable for certain specialist applications, additional layers or restricted cavities, but it should not automatically be treated as equivalent to a 135mm product.
Likewise, a 100mm glass wool product around R2.50 provides useful insulation but considerably less thermal resistance than a 135mm product around R3.38.
This is why comparing price per roll without comparing the insulation R-value can be misleading.
A cheaper roll may simply contain less insulation or provide lower thermal resistance.
XPS Insulation R-Value
Extruded polystyrene, commonly known as XPS, is a rigid closed-cell insulation material.
Unlike flexible ceiling blankets, XPS is supplied as rigid boards.
Products such as Isoboard and Summit XPS can be used in thermal ceiling systems and other building applications where rigid insulation is appropriate.
XPS generally has relatively low thermal conductivity, meaning useful thermal resistance can be achieved with less thickness than some fibrous materials.
However, the correct insulation R-value must be taken from the technical data for the exact board being specified.
Do not assume that every XPS board has the same thermal conductivity simply because it is described as XPS.
Board thickness, product formulation, density and test conditions should all be considered.
[Internal link: Isoboard]
[Internal link: Summit XPS]
Rigid Insulation vs Bulk Ceiling Insulation
Rigid insulation and flexible bulk insulation can both provide excellent thermal resistance, but they are used differently.
Flexible products such as:
are commonly installed above horizontal ceilings in accessible roof spaces.
Rigid products such as:
- Isoboard;
- Summit XPS;
- and other rigid thermal boards
may form part of the ceiling itself or be incorporated into specialised roof and building systems.
The correct product should therefore be chosen according to the construction method rather than simply selecting whichever product shows the highest insulation R-value.
Thermal Performance of Reflective Foil
Reflective roof insulation works differently from conventional bulk insulation.
Products such as Sisalation, Radenshield and reflective foil systems reduce radiant heat transfer when correctly installed in conjunction with suitable air spaces.
Their thermal contribution depends on factors including:
- the emissivity of the reflective surface;
- direction of heat flow;
- roof configuration;
- ventilation;
- dust accumulation;
- and the size and position of adjacent air spaces.
For this reason, a reflective foil should not simply be compared with 135mm glass wool using one standalone R-value without considering the complete installation.
Reflective insulation can be a useful component of a roof system, but it must be installed in the configuration for which its thermal values are specified.
[Internal link: Sisalation Foils]
Why Installation Quality Matters
Buying insulation with good thermal resistance is only the first part of the job.
The product must also be installed correctly.
Common problems include:
- leaving large gaps;
- excessive compression;
- missing narrow areas around roof trusses;
- poorly insulating around roof access openings;
- failing to cover the perimeter of the ceiling properly;
- incorrect installation around electrical fittings;
- pushing insulation away from difficult sections;
- and damaging insulation while repeatedly walking through the roof.
A properly installed lower-performing product can sometimes outperform a theoretically better product that has been installed badly.
The published insulation thermal resistance assumes the product is being used and installed appropriately.
Common Insulation Performance Mistakes
Comparing insulation only by thickness
Two 100mm products may not have the same R-value.
Always compare the declared thermal resistance.
Confusing product R-value with roof R-value
The insulation product is only one component of the complete roof assembly.
Using outdated South African climate-zone information
Many websites still reproduce tables based on the earlier SANS 10400-XA requirements.
The 2021 edition introduced revised energy-zone requirements.
Compressing insulation
Compressing flexible bulk insulation can reduce its intended thermal performance.
Ignoring gaps
Heat can pass through areas where insulation coverage is missing.
Buying insulation purely by price
The lowest price per square metre does not necessarily provide the best value if the insulation R-value is significantly lower.
Assuming reflective foil and bulk insulation work identically
They control heat transfer in different ways and must be evaluated according to their installed system.
Thermal Resistance and Energy Savings
Good ceiling insulation can reduce the amount of heat moving through the roof and ceiling.
During hot weather, this can reduce the rate at which heat enters living spaces.
During cold weather, insulation slows the loss of heat from the rooms below.
The actual energy saving will vary according to:
- climate;
- building construction;
- insulation level;
- air leakage;
- windows;
- heating and cooling equipment;
- occupant behaviour;
- and the condition of the existing building.
For this reason, it is better to view insulation R-value as a measure of thermal resistance rather than as a guaranteed percentage energy saving.
The higher the effective thermal resistance of the roof system, the more slowly heat can pass through it.
Choosing Ceiling Insulation by Thermal Performance
When choosing ceiling insulation, consider the following:
- Determine the type of roof and ceiling construction.
- Identify the applicable thermal requirement where compliance is relevant.
- Compare the declared insulation R-value of the products.
- Compare the thickness required to achieve the desired performance.
- Check whether the product can be installed without excessive compression.
- Consider fire, acoustic and handling characteristics.
- Make sure the complete ceiling area can be insulated properly.
- Use competent installation where roof access or electrical conditions make DIY work unsafe.
For a broader comparison of the available materials, see our guide to the different types of insulation in South Africa.
[Internal link: Different Types of Insulation]
Frequently Asked Questions About Insulation R-Value
What does R-value mean in insulation?
R-value measures resistance to heat transfer. A higher insulation R-value means greater resistance to heat flowing through the material.
Is a higher R-value better?
For thermal resistance, yes. A higher R-value provides greater resistance to heat transfer, provided the insulation is suitable for the application and correctly installed.
What is the ideal declared thermal resistance?
Under the SANS 10400-XA:2021 deemed-to-satisfy framework, roof assemblies generally target a total R-value of approximately R3.7 m²·K/W across energy zones 1–7, with approximately R2.7 for Zone 5H. The complete roof assembly must be considered.
What is the thermal resistance of 135mm Aerolite?
Current published information lists 135mm Aerolite Soft Touch at approximately R3.38 m²·K/W.
What is the R-value of 135mm Knauf?
Current Knauf South African technical information lists 135mm Ceiling Roll at approximately R3.38 m²·K/W.
What is the R-value of 145mm Isotherm?
Published Isotherm information lists the 145mm 11.5kg/m³ product at approximately R3.37 m²·K/W.
Is 100mm insulation enough?
That depends on the product and application. For example, 100mm glass wool around R2.50 provides less thermal resistance than 135mm glass wool around R3.38. A complete roof specification should be considered where regulatory compliance is required.
Does compressed insulation lose R-value?
Significant compression can reduce the intended performance of flexible bulk insulation. The insulation should generally be allowed to maintain its specified thickness.
Can I put new insulation over old insulation?
In many cases, additional insulation can be installed over suitable existing insulation, but the condition of the old material, electrical services, ceiling structure, roof access and available space should first be inspected.
Does R-value stop heat completely?
No. Insulation slows heat transfer; it does not completely stop it.
Is Thermal Resistance important in summer as well as winter?
Yes. Thermal resistance works in both directions. Ceiling insulation helps resist heat entering from the roof during hot conditions and helps reduce heat loss during colder conditions.
Are all 135mm insulation products the same?
No. Thickness alone does not determine performance. Compare the declared insulation R-value, material specification and intended application.
Does reflective foil have an R-value?
A reflective system can contribute thermal resistance, but its performance depends heavily on the adjacent air spaces, emissivity, roof configuration and direction of heat flow. It should be evaluated as part of the installed roof system.
Final Thoughts on Insulation and Thermal Resistance
Choosing ceiling insulation should never be based on thickness or price alone.
The insulation R-value gives homeowners, builders and installers a meaningful way to compare thermal performance.
For conventional residential ceilings, products around R3.35 to R3.38 have become particularly important because they can provide the bulk of the thermal resistance needed in a roof assembly targeting the current South African R3.7 requirement.
Current examples include 135mm Aerolite at approximately R3.38, 135mm Knauf at approximately R3.38 and 145mm Isotherm at approximately R3.37.
The final thermal performance of the building still depends on the complete roof assembly and, critically, the quality of installation.
A correctly selected insulation R-value, combined with continuous coverage and proper installation, is one of the most effective ways to improve the thermal performance of a South African roof and ceiling.
