Key points
- Thermal conductivity λ (W/m·K): the lower it is, the better the material insulates per centimetre.
- Thermal resistance R = thickness (m) ÷ λ. To reach the same R you need less thickness of a material with lower λ.
- In a peer-reviewed study (2024), declared values for tested products: PIR about 0.022, EPS about 0.032, XPS about 0.034, mineral wool about 0.035.
- Outdoor storage weakened insulation by about 4% on average; protect materials from sun and moisture before installation.
In a hot climate like Saudi Arabia's, insulating walls and roofs is one of the decisions with the greatest effect on the electricity bill and comfort. But comparing materials needs a common figure: thermal conductivity λ (lambda).
1) What is thermal conductivity?
λ is the amount of heat passing through a metre of material per degree of temperature difference, in W/(m·K). The lower the number, the better the insulation.
From it we calculate the thermal resistance R of an insulation layer:
The higher R, the better the insulation. So a weaker material (higher λ) can give the same performance with more thickness.
2) Comparing common materials
In a study published in the journal Materials (via PubMed Central) on external wall insulation, the declared values of the tested products were1:
| Material | Declared λ (W/m·K) | Measured λ after outdoor storage | Thickness for R = 1 |
|---|---|---|---|
| Polyisocyanurate PIR (polyurethane family) | 0.022 | 0.0230 | ≈ 2.2 cm |
| Expanded polystyrene EPS (white "styrofoam") | 0.032 | 0.0332 | ≈ 3.2 cm |
| Extruded polystyrene XPS | 0.034 | 0.0354 | ≈ 3.4 cm |
| Mineral wool (rock/glass) | 0.035 | 0.0369 | ≈ 3.5 cm |
The thickness column is calculated with the formula above from the declared values. Values vary between products and densities; always use the declared λ in the data sheet of the product itself.
3) How to choose: considerations beyond λ
| Consideration | Why it matters |
|---|---|
| Moisture and water | Closed-cell materials (like XPS) are normally used where there is moisture exposure, as in inverted roofs. Mineral wool needs protection from wetting. See roof insulation. |
| Compressive strength | Insulation on walkable roofs or under tiles must carry loads. |
| Fire behaviour | Varies greatly; rock wool is usually non-combustible, while foams need protection and detailing per the fire code. See sandwich panels. |
| Available space | If thickness is limited, a lower-λ material achieves the target in less thickness. |
| Thermal bridges | Uninsulated concrete columns and beams leak heat; continuous external insulation reduces this. |
4) By location of use: where to read the details
| Location | What drives the choice | Maintenance and risks | Guide |
|---|---|---|---|
| Roof | Water and compressive strength; an inverted roof puts XPS above the membrane2 | Check the membrane and gutters before the rains | roof insulation |
| Walls | Insulated blocks or continuous insulation that deals with thermal bridges | Store materials away from sun and damp before installing1 | blocks, ICF |
| Sprayed on site | Thermal and moisture barrier in one layer | Vacate during spraying and observe re-entry time3 | spray foam |
| Attic and under the roof | A radiant barrier is a complement, not a substitute4 | Dust reduces its effectiveness | radiant barriers |
| Windows | SHGC and U of the actual product5 | Frame sealing | windows |
Any performance value (λ, R or U) belongs to a specific product, system and test; do not carry it over from one product to another.
5) The Saudi energy conservation code
The Saudi Building Code National Committee issued the residential energy conservation code SBC 602, which divides the Kingdom into climate zones and sets maximum heat transfer coefficients (U-values) for building envelope elements in each6. A wall or roof U-value is calculated from the sum of its layers' resistances (including insulation) and must meet your zone's requirements; the designer calculates it when the permit is issued.
The source for this paragraph is a peer-reviewed study that explains SBC 602, not the code text itself6. Before design, ask the engineering office for the current edition of SBC 602 and the U-values required for your climate zone.
6) Practical tips
- Start with the roof: it's the most sun-exposed element in low-rise buildings.
- Keep insulation continuous without gaps between boards, and deal with thermal bridges around columns and beams.
- On roofs: coordinate thermal insulation with waterproofing, falls and drainage in one system. See roof waterproofing.
- Ask the supplier for the data sheet stating λ, density, compressive strength and fire classification.
Sources
The number next to a statement points to its source in this list. Sources back the general information; anything marked “practical advice” is editorial experience, not a quoted source. Anything that affects safety, structural design or electrical installations needs a licensed professional, and in Saudi Arabia the binding reference is the Saudi Building Code.
- 1Analysis of Thermal Properties of Materials Used to Insulate External Walls — Materials (MDPI)
- 25 Reasons to Choose an Inverted Roof (Protected Membrane Roof with XPS) — DuPont Performance Building Solutions
- 3Vacate and Safe Re-Entry Time — Spray Polyurethane Foam Application — U.S. EPA — Safer Choice
- 4Attic Radiant Barriers — Building America Solution Center — PNNL / U.S. DOE
- 5High-Performance (ENERGY STAR) Windows — U-factor, SHGC, VT, low-e — Building America Solution Center — Pacific Northwest National Laboratory (PNNL) / U.S. DOE
- 6Residential Buildings Thermal Performance to Comply With the Energy Conservation Code of Saudi Arabia (SBC 602) — Engineering, Technology & Applied Science Research (ETASR)