This guide will help you understand R-value and thickness. Selecting the right product and depth depends on where the insulation is being installed (loft, wall, floor, or roof), the space, thermal efficiency, soundproofing, or breathability.
What Is R-Value in Insulation?
R-value is a measure of how well insulation resists heat flow. The higher the R-value, the better the material performs at slowing heat transfer, keeping warm air inside in winter and outside in summer. It’s a crucial number to consider when comparing different insulation types and choosing materials for your project.
How R-Value Is Calculated
R-value is calculated by dividing the thickness of the material (in metres) by its thermal conductivity (measured in W/m·K).
Formula: R-value = Thickness ÷ Thermal Conductivity
So, if a material is very resistant to heat flow (low thermal conductivity), even a thin layer can have a high R-value. That’s why denser or more advanced materials (like PIR foam) can offer better performance in thinner profiles.
Choosing Between Thickness and Material Efficiency
Your insulation choice shouldn’t just come down to R-value or price. It must also reflect the physical space available and the specific performance goals of your project. Here’s how that plays out in different areas of a building:
Lofts and Attics
Lofts typically offer plenty of vertical space, so thicker insulation with a recommended minimum insulation of 270mm, like mineral wool or natural fibres (sheep’s wool, cellulose) can be used without compromising room function. These materials are also breathable and excellent for preventing condensation in older homes.
Cavity Walls
Cavity walls have limited space (usually 100mm or less), so you’ll need a material that offers a high R-value per mm. This is where PIR boards or spray foam excel. They're thinner, denser, and more efficient, ideal for retrofits or narrow wall systems.
Internal Stud Walls
Internal partitions are often sound-sensitive. Mineral wool is the go-to for its acoustic dampening properties, even though it requires a bit more thickness than foam normally between 70mm to 100mm. In some cases, combining thin rigid boards with acoustic membranes can balance both space and performance.
Floors
Space is limited in floors and flat roofs due to structural and head height constraints. Rigid foam boards or PIR boards are best, as they achieve strong thermal values without needing a lot of depth, with thicknesses between 25mm - 100mm.
Flat Roofs
Flat roofs typically require robust insulation to meet both thermal and structural performance requirements. PIR or rigid foam board insulation is usually installed as part of a warm roof system with thicknesses ranging from 150mm - 180mm.
Pitched Roofs
In pitched roofs (especially loft conversions or habitable attic spaces), Mineral Wool Rolls (cold roof - above ceilings) have a recommended thickness of 270mm. Sheep's wool (warm roof - traditional and low carbon builds) thickness ranges from 100mm - 200mm, and PIR boards (thinner, between the rafters) insulation can be installed between and/or over rafters with thicknesses of 25mm - 200mm.
Heritage or Breathable Builds
Older properties or buildings that need breathability (to prevent damp) benefit from natural fibre insulation like wood fibre, hemp, or sheep’s wool. These materials are bulkier and more expensive but ideal for breathable construction systems.
How Thickness Affects R-Value And Why It’s Not Always Linear
While more insulation usually means better performance, the relationship between R-value and thickness is not always perfectly linear. Why?
- Diminishing returns: After a certain point, each additional mm adds less benefit than the one before.
- Installation quality: Gaps or compression can reduce real-world effectiveness.
- Material behaviour: Some materials don’t scale evenly due to settling (e.g. cellulose) or thermal bridging.
So while thickness is important, it must be matched with material quality, application technique, and building type for optimal results.
Tips for Maximising Thermal Performance
- Eliminate gaps - even small voids reduce effectiveness
- Combine layers where needed (e.g., PIR + mineral wool)
- Install reflective vapour barriers in cold roof setups
- Always follow manufacturer's data for true R-values
- Account for thermal bridging in steel or timber frame systems
Recommended Insulation Thickness by Material
When planning insulation for your project, the thickness varies by both material type and application (wall, floor, or roof). The table below outlines typical recommendations to help you meet thermal performance targets effectively.
| Product | Material | Application | Recommended Thickness |
|---|---|---|---|
| Loft Roll 44 | Mineral Wool | Lofts | 270mm |
| Thermafleece CosyWool | Sheep’s Wool | Lofts | 200–250mm |
| Celotex CW4000 | PIR Boards | Cavity Walls | 50–100mm |
| RWA45 | Acoustic Mineral Wool | Internal Stud Walls | 25–100mm |
| Knauf RS45 | Acoustic Mineral Wool | Internal Stud Walls | 25–100mm |
| XPS Insulation Board | Extruded Polystyrene | Floors (suspended or solid) | 100–150mm |
| Celotex PIR Board | PIR | Floors (solid, suspended, under screed) | 25–150mm |
| Thermafleece NatraHemp | Hemp (Natural Fibre) | Heritage / Breathable Builds | Up-to 300mm in roofs, walls and floors |
There’s no one-size-fits-all solution. If you have space constraints, go for high-efficiency materials like PIR. But if you prioritise sustainability, sound control, or breathability, and have space to spare, mineral wool or natural fibres may be your best choice.
Looking to build greener?
Check out our guide on The Environmental Impact of Insulation Materials for insight into sustainable insulation choices.