Types of Thermal Insulation: Comparing Materials, Thicknesses and the Best Fit for the Saudi Climate

Types of thermal insulation at a glance: which material goes where?

Many owners ask for ‘the best thermal insulation’ as if one material suited everything. In reality, types of thermal insulation differ in how they work and in how they tolerate water, fire and loads, so a sound choice starts from where the insulation goes, not from a product name. This guide compares the main families using figures attributed to published datasheets and standards, then shows how a target U-value becomes a thickness.

• Exposed concrete roof: closed-cell spray polyurethane foam with a UV-protective coating, or an inverted roof with XPS boards laid over the waterproofing.
• Roof under tiles or gravel: XPS boards in an inverted roof, because they tolerate compression and long-term wetting better than other boards.
• Metal (shinko) roofs and warehouses: insulated sandwich panels in new buildings, or spray foam, rockwool or glass wool from the inside with a vapour barrier and suitable protection in existing ones.
• External walls: a continuous insulation layer inside a cavity wall or on its outer face, with the concrete columns and beams treated too.
• Where non-combustible materials are required: rock or glass wool, or sandwich panels with a rockwool core.

How does thermal insulation work? k, R and U in two minutes

Heat moves in three ways: conduction through solids, convection through moving air, and radiation between surfaces. Most insulation materials trap air or a low-conductivity gas in tiny cells or fibres that stop it moving, cutting conduction and convection. Radiant barriers deal with radiation alone, which is why they are assessed in an entirely different way.

• Thermal conductivity k (or λ) in W/m·K: a property of the material itself; the lower it is, the better the insulator. It is measured by standard methods such as ASTM C518 with a heat-flow-meter apparatus.
• Thermal resistance R in m²·K/W: a property of a layer of given thickness; it rises as thickness rises.
• Thermal transmittance U in W/m²·K: a property of the whole element (roof or wall) with all its layers, and the figure that code requirements are measured against.

A note on units: R-values in US datasheets are in ft²·°F·h/Btu, which is about 5.7 times the SI R-value of the same layer. This mix-up and its effect on quotations is worked through in our guide to insulation requirements in the Saudi Building Code. For deeper physics (moisture, dew point and advanced materials) see the insulation physics chapter of the encyclopedia.

Why does the roof come first in the Saudi climate?

The roof is the element most exposed to the sun throughout the day. The US Department of Energy’s cool-roof guidelines note that dark roofs can reach 66°C or more in summer sun, while a cool roof under the same conditions can stay more than 28°C cooler. The larger the gap between slab temperature and the air-conditioned room, the more heat flows through every square metre.

That is why insulating the top roof of villas, upper-floor flats and roof annexes is usually the highest-impact step. Walls come next, especially west- and south-facing façades that take strong afternoon radiation. Floors need insulation only where they sit over an unconditioned or open space.

Families of thermal insulation materials: what sets each apart?

Below are the ten families you will meet in quotations. Figures come from specific product datasheets or specification standards and vary between manufacturers; for your project the reference is the datasheet of the product actually supplied. For two of the most common materials in depth, see the polyurethane foam material guide and the rockwool material guide.

1) Extruded polystyrene (XPS)

Rigid closed-cell boards made by extrusion. The US specification ASTM C578 defines XPS types with minimum compressive strengths of 15 to 100 psi (about 100–690 kPa), a minimum R of 5.0 per inch and maximum water absorption of 0.3% by volume, as tabulated in Owens Corning’s bulletin. The Ravatherm XPS X ULTRA 300 SL board declares a conductivity of 0.027 W/m·K, compressive strength of 300 kPa and long-term water absorption by immersion of no more than 0.7% by volume.

These properties make XPS the standard material for the inverted roof, where the insulation sits above the waterproofing and is exposed to water and loads. Its weaknesses: it is a combustible foam plastic (the product above is Euroclass E), and it degrades under UV and some solvents, so it must not be left exposed or bonded with incompatible products.

2) Expanded polystyrene (EPS)

Boards of fused expanded polystyrene beads, usually lighter and cheaper than XPS. In the same ASTM C578 table, EPS types range from 5 to 60 psi in compressive strength and from 3.1 to 4.3 in R per inch, i.e. a maximum conductivity of roughly 0.034–0.047 W/m·K after conversion, with maximum water absorption of 2% to 4% by volume depending on type.

In practice EPS absorbs more water and conducts more heat than XPS, so it needs more thickness for the same resistance. It suits dry, protected locations: inside cavity walls, under floor tiles above a waterproofing layer, and in insulated blocks. It is also a combustible foam plastic subject to the same requirements.

3) Closed-cell spray polyurethane foam (SPF)

Two liquid components mixed at the spray gun expand into a rigid foam that bonds to the substrate and fills cracks and gaps without joints. The datasheet for BASF SKYTITE C3-2.8, for example, declares a density of 2.7–2.9 pcf (about 43–46 kg/m³), an aged R of 6.2 per inch (conductivity about 0.023 W/m·K), closed-cell content above 90% and compressive strength of about 45 psi (roughly 310 kPa).

Its big advantage is continuity: no joints and no fasteners, and it combines thermal and waterproofing functions when protected by a UV-resistant coating. Its drawbacks are that quality depends on the applicator, weather and substrate moisture; it degrades in sunlight without a coating; and the datasheet itself states that it is a combustible material. Densities and inspection methods are in the polyurethane foam material guide, and field application in our foam insulation service.

4) Rigid PIR/PUR boards

Polyisocyanurate or polyurethane boards made in the factory between aluminium-foil or similar facings. The declaration of performance for Kingspan Thermaroof TR26 states a conductivity of 0.022 W/m·K for all thicknesses from 25 to 160 mm and a compressive stress class of CS(10\Y)150, i.e. about 150 kPa.

These boards give high resistance per centimetre, so they suit locations with limited depth such as annex roofs, internal linings and under metal roofs. They are not suited to prolonged wetting or inverted roofs (the declaration above gives no water-absorption performance), and their reaction-to-fire class as a stand-alone product is F, so they need protection and fire-aware detailing.

5) Rockwool (stone wool)

Fibres spun from molten volcanic rock and formed into slabs or rolls of various densities. The datasheet for ROCKWOOL Thermalrock S declares densities from 40 to 140 kg/m³, conductivity between 0.034 and 0.037 W/m·K at a 23°C mean temperature, Euroclass A1 non-combustible classification and a melting point of 1000°C.

It is therefore chosen when fire is the governing constraint: separating walls, warehouses and facilities that require non-combustible materials, with an added acoustic benefit. But its lighter products are not load-bearing, and it loses much of its effectiveness when wet, so it needs a vapour barrier and protection from rain. Densities and installation are covered in the rockwool material guide.

6) Glass wool

Light glass fibres in rolls or boards, most often used above suspended ceilings and around air-conditioning ducts. The datasheet for Owens Corning’s industrial Fiberglas TIW declares conductivity of 0.22–0.24 Btu·in/h·ft²·°F at 75°F (about 0.032–0.035 W/m·K), densities of 16 and 40 kg/m³, a non-combustible result under ASTM E136, flame-spread and smoke-developed indices of zero for the unfaced product, and vapour sorption below 5% by weight.

It is usually lighter and cheaper than rockwool but less robust, so it should not go under loads or in damp locations. Handling it calls for gloves, a mask and goggles because the fibres irritate skin and airways, as our health and safety guidance explains.

7) Insulated sandwich panels

Two painted galvanised steel skins around an insulating core (PIR, PUR, rockwool or EPS); the whole panel is fixed to the purlins, giving roof and insulation in one step. In the Kingspan QuadCore KS1000RW datasheet, for example, the core conductivity is 0.018 W/m·K and the U-value runs from 0.47 W/m²·K for a 40 mm core to 0.12 for a 150 mm core, with a B-s1,d0 reaction-to-fire classification for the internal face.

Panel performance is set by the core: rockwool cores are chosen where non-combustibility is required, while foam cores give a lower U at smaller thickness. Joints and fixings remain the weak points that need careful sealing.

8) Radiant barriers and cool-roof coatings

Both technologies deal with radiation, not conduction. A radiant barrier is a low-emittance aluminium foil; the Building America guide from PNNL defines it as a product with long-wave infrared emittance below 0.1 per ASTM C1371, working across an air space that faces its reflective side. A cool coating (white acrylic and similar) reflects much of the sunlight: the US Department of Energy guidelines note that light-coloured materials reflect 55–90% of solar radiation versus 5–20% for dark ones.

Both reduce solar heat gain, but neither replaces insulation; the same guidelines state plainly that cool roofs are ‘never a substitute for using sufficient thermal insulation’. Coating properties and application are covered in our acrylic reflective roof coating guide.

9) Insulated blocks and autoclaved aerated concrete (AAC)

Concrete blocks with polystyrene inserts and autoclaved aerated concrete (AAC) blocks raise wall resistance compared with ordinary hollow blocks. But each product’s conductivity varies with density, so take it from the manufacturer’s datasheet. And a wall is not only blocks: columns, beams and mortar joints break the continuity of insulation, which is why insulated blocks alone do not guarantee the wall U-value limit, as our building code requirements guide shows.

10) Aerogel (specialist use)

Fibre blankets impregnated with nanoporous silica aerogel. The Spaceloft datasheet declares a conductivity of 0.015 W/m·K at 5 and 10 mm thicknesses, a C-s1,d0 fire classification, and that it repels liquid water while letting vapour through. Its high price confines it to tight spots: treating a thermal bridge at a window reveal, or internal insulation in a room that cannot spare extra thickness.

Comparison table of thermal insulation types

The table sums up the above against seven criteria. The conductivity column gives typical values from specific datasheets or specifications; where no reliable general figure exists we wrote ‘per datasheet’. Relative cost is a general estimate for the material alone, excluding installation, protection and finishes.

Bottom line: the differences in k between XPS, spray foam and PIR are small, so the choice among them is decided by moisture and fire. Non-combustible fibres need more thickness and protection from wetting, and reflective coatings cannot be compared centimetre for centimetre with the other materials.

Which type of thermal insulation suits each part of the building?

Exposed concrete roofs

On residential roofs that will not be tiled, closed-cell foam is sprayed directly onto the slab after cleaning and drying, then covered with a UV-protective coating. The advantage is a continuous layer around drains and AC unit bases with no joints. The alternative is XPS boards over the waterproofing, protected by a ballast layer. Both options and the application steps are covered in our roof insulation service.

Inverted roofs under tiles

The typical build-up: slab, screed to falls, continuous waterproofing, XPS boards, a separation fabric, then gravel or tiles. XPS is chosen because it stays exposed to water below and loads above for the life of the roof. The UK guide BR 443 refers to a correction applied to inverted-roof U-values for rainwater effects under Annex D of BS EN ISO 6946, which the designer accounts for.

Metal (shinko) roofs and warehouses

In new buildings, sandwich panels are the simplest solution with a declared U-value. Existing metal roofs are insulated from inside with spray foam or rock or glass wool boards with a vapour barrier, or from outside with foam and a coating. If polyurethane foam stays exposed inside an occupied space, IBC Section 2603.4 requires it to be separated by an approved thermal barrier such as 12.7 mm gypsum board, or an equivalent material that limits the temperature rise of the unexposed face to no more than 120°C after 15 minutes of fire exposure.

External walls: cavity or external insulation?

The cavity wall (block + XPS or EPS boards + block) is the most common approach in new villas; the insulation is protected inside the wall, but columns and beams interrupt the layer unless treated with extra boards. External insulation (boards fixed to the outer face, then mesh and a special render) wraps the whole concrete frame and reduces thermal bridges, but needs a complete system from one supplier and careful workmanship. Internal insulation is a practical option for existing buildings, but it takes floor space and leaves slab edges untreated. The options in detail are in our wall insulation service.

Internal walls and ceilings

Partitions between conditioned and unconditioned spaces, such as stairwells, garages and ceiling voids, are usually insulated with rock or glass wool inside drywall or above suspended ceilings. These locations are dry and unloaded, so fibres suit them, with an acoustic bonus.

Floors over unconditioned spaces

Rooms built over open parking or a cantilever lose cooling through the floor. The fix is either XPS boards under the screed and tiles, since they carry compression, or insulation from below with spray foam or wool boards behind a cladding that protects them from sun and damage.

AC ducts and pipes

The goal here is twofold: reduce heat gain and prevent condensation on the cold duct surface. Glass wool with an aluminium facing that acts as a vapour barrier is used on ducts, and flexible elastomeric foam on chilled-water pipes. Thickness and conductivity follow the datasheet and the fluid temperature; the key requirement is that the vapour barrier stays continuous at joints and hangers, because any gap becomes a drip point.

Cold rooms and freezers

These are usually built from panels with PUR or PIR cores, thicker than roof panels, with vapour- and air-tight joints and insulated floors detailed by the designer for the operating temperature.

From target U-value to insulation thickness: the method step by step

The code does not ask for ‘5 cm’ or ‘10 cm’; it sets a maximum U-value for the whole element, and thickness is derived from the conductivity of the chosen material. Our building code requirements guide solves this for a typical concrete roof in all three climate zones; here we apply the same logic to a metal-sheet annex roof to see how thickness changes between materials.

1. 1

   Set the target U

   For this example we use 0.202 W/m²·K, the limit for a conditioned roof in Zone 1 as presented in our code guide. Required total resistance = 1 ÷ 0.202 = 4.95 m²·K/W.

2. 2

   Add up the other layers

   On a metal roof the sheet’s own resistance is negligible, leaving Rsi = 0.10 and Rse = 0.04 per BR 443, a total of 0.14 m²·K/W.

3. 3

   Find the resistance the insulation must provide

   4.95 − 0.14 = 4.81 m²·K/W must come from the insulation layer alone.

4. 4

   Convert resistance into thickness

   Thickness = 4.81 × k. We use the conductivities of the specific products whose datasheets are cited above; the values are illustrative, not a design.

5. 5

   Round up, then correct

   Round up to the next commercial thickness, then add corrections for fasteners, air gaps and thermal bridges; BR 443 requires them and allows them to be ignored only if they total less than 3% of the U-value.

Look at the second-to-last row: the simple calculation gives about 87 mm for a 0.018 core, while the panel’s own datasheet states that a 91 mm core gives U = 0.20. The difference is normal, because a declared panel U-value covers the whole product with its details, not the core layer alone. The practical rule: for factory-made panels use the U-value table in the manufacturer’s datasheet; for materials installed on site, calculate with the formula and add a margin for details.

Once the material and thickness are set, you can estimate an approximate cost by area and insulation type with our insulation cost calculator; the final price is set after a site visit because it depends on substrate condition, details and quantity.

Common mistakes and thermal bridges

• Comparing products in different units: a US R-value per inch is not the SI R-value of a layer; convert to k in W/m·K before any comparison.
• Taking thickness from a quotation instead of a calculation: a common thickness does not prove the required U-value is met.
• Leaving spray foam exposed to the sun: foam degrades under UV, and the protective coating is part of the system, not an optional extra.
• Putting a moisture-sensitive material in a wet location: PIR boards and wool in an inverted roof or over failed waterproofing lose their performance.
• Forgetting or breaking the vapour barrier: on AC ducts and metal roofs any break turns into condensation and dripping.
• Ignoring parapets, columns and window reveals: the subject of the next section.

Thermal bridges: where do your savings leak out?

A thermal bridge is a path where heat bypasses the insulation through a more conductive material. The best-known examples in Saudi buildings: a parapet wall tied into the slab without insulation, concrete columns and beams within an insulated block wall, window reveals and lintels, metal fasteners piercing the insulation, and slab edges with internal insulation. The code treats insulation as a continuous envelope layer, so actual performance is set by the weakest points.

Treatment starts at design: carry insulation up the inner face of the parapet to the coping, cover columns and beams with extra boards or continuous external insulation, and use aerogel where space is tight. Thermal imaging after the building is in use shows these points clearly.

Reflective insulation and cool coatings: myths and facts

‘Insulating paint’ and ‘reflective insulation’ are widely marketed as a cheap substitute for real insulation. The table below sets the most common claims against what the references say.

The practical conclusion: a reflective coating is excellent on top of an insulation layer or on a metal roof to lower its temperature, and it deserves regular maintenance and recoating, but it is not counted as a substitute for the calculated thickness. In cooler mountain cities, its effect on winter heating may deserve the designer’s review, since the US guidelines note that a cool roof can increase heating needs during heating seasons.

How to choose the type of thermal insulation for your project

1. 1

   Define the location and its conditions

   Is the insulation exposed to sun or water, or will it carry tiles? These questions alone rule out half the options.

2. 2

   Identify the governing constraints

   Fire requirements, available depth, permissible roof load, and whether work must happen in an occupied building.

3. 3

   Calculate thickness from U

   Set the target U from the code or the owner’s goal, and calculate thickness with the actual product’s conductivity as in the example above.

4. 4

   Compare datasheets on one basis

   Compare k in W/m·K and the material’s water and fire behaviour as the datasheet states, not the advertisement.

5. 5

   Plan protection and details

   A protective coating for foam, ballast for inverted roofs, a vapour barrier for fibres, and treatment of thermal bridges.

6. 6

   Inspect and document

   Thickness measurements, installation photos, datasheets, a written warranty and a post-commissioning check.

If you want a technical opinion on your building, our thermal insulation service team starts with a site inspection that settles location, material and thickness before any quotation, and you can request an inspection directly. For practical articles on specific cases, browse the thermal insulation blog category.

▶Energy conservation and thermal insulation requirements in design under the Saudi Residential Building Code