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There is no single correct EPS board thickness. Choose it from the required thermal performance of the complete roof, wall or floor assembly, then verify the board’s declared lambda, compressive strength, moisture properties and fire protection. A grade name such as EPS 70 or EPS 100 is useful shorthand, but it is not a complete specification.
The right selection depends on the building element, climate and jurisdiction, target U-value or R-value, other layers, thermal bridges, loads and the tested construction detail. The workflow below shows how to make that choice without relying on a generic thickness chart.
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What EPS insulation is—and why the label is only the starting point
EPS means expanded polystyrene. Rigid EPS boards are used in roof, floor and wall insulation, as well as other construction applications. They are made in different densities and grades, with different declared thermal and mechanical properties.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallTwo boards with the same nominal thickness can deliver different thermal resistance, and two boards with the same grade label can have different declared values depending on the manufacturer and product formulation. A responsible specification therefore names the exact product, its declaration and the assembly in which it will be installed.
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Building regulations and energy targets normally apply to the whole element, not to the insulation board in isolation. A wall, for example, includes finishes, sheathing, framing, cavities, air layers and thermal bridges. A floor also has to deal with imposed loads, screed or slab construction and ground or external moisture. The board is one part of that build-up.
What “EPS 70” and “EPS 100” mean
Grade numbers commonly indicate a nominal compressive stress at 10% compression, expressed in kilopascals (kPa). They are not a universal performance certificate and they do not, by themselves, tell you the board’s lambda, moisture behavior, fire classification or suitability for a particular detail.
Typical white EPS property examples
The British Plastics Federation Expanded Polystyrene Group publishes the following typical white EPS examples. The page does not state a publication year. Treat the figures as illustrations of the range, not as guaranteed values for every product carrying that grade name.
| Grade | Typical compressive stress at 10% compression | Typical thermal conductivity (lambda) |
|---|---|---|
| EPS 70 | 70 kPa | 0.038 W/mK |
| EPS 100 | 100 kPa | 0.036 W/mK |
| EPS 150 | 150 kPa | 0.035 W/mK |
| EPS 200 | 200 kPa | 0.034 W/mK |
| EPS 250 | 250 kPa | 0.034 W/mK |
These are “Typical white EPS property examples — British Plastics Federation Expanded Polystyrene Group, publication year not stated on the page.” The table shows why a higher grade can provide more load capacity, while the improvement in typical lambda is comparatively modest. It does not justify choosing a higher grade solely for a lower number: the exact declared value and the structural requirement still control.
The same industry group reports that some grey EPS products in the EPS 70 or EPS 100 range can achieve thermal conductivity as low as 0.030 W/mK. That statement applies to some products, not every board in those grades. Confirm the value in the current manufacturer’s declaration at the thickness and conditions relevant to your project.
How thick should EPS insulation be?
Thickness is the result of a calculation, not a universal rule. You need at least four inputs:
- the element being insulated—roof, wall, floor or another detail;
- the target whole-element U-value or R-value set by local regulation, the project brief or an energy model;
- the declared thermal conductivity of the selected board and the thermal resistance of every other layer; and
- the effects of repeating thermal bridges, junctions, fasteners, moisture and construction tolerances.
The U.S. Department of Energy’s educational guidance frames insulation selection around location and the R-values required by code or by the project’s energy goal. It is broad guidance, not an EPS thickness schedule for a particular building. Local rules can differ by country, climate zone, building type and element.
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A useful preliminary relationship
For a first comparison, a board’s resistance is related to its thickness and declared lambda: increasing thickness increases resistance, while a lower lambda gives more resistance at the same thickness. That simplified relationship is not a final design calculation. Surface resistances, other layers, thermal bridges, moisture corrections and the way the regulatory method treats the assembly can change the result.
The 2016 EUMEPS White Book discusses deriving design conductivity from declared values and accounting for thickness and moisture effects. Use that material as technical background, then apply the current standard and calculation method required in your jurisdiction. Do not turn a catalogue lambda into a guaranteed in-service value without checking the declaration and the design conditions.
The practical thickness workflow
- Identify the element and exposure. State whether the board is inside a wall, outside a wall, above a roof deck, below a roof covering, over a floor slab, beneath a screed or in contact with ground. Record whether it will be dry, intermittently damp, persistently moisture-exposed or part of an inverted roof.
- Set the required whole-element performance. Obtain the applicable U-value, R-value or energy target from the local code, approved calculation or project designer. Do not substitute a generic online chart for that requirement.
- List the complete build-up. Include finishes, masonry or framing, sheathing, membranes, cavities, slabs, screeds and any repeating or linear thermal bridges that the calculation requires.
- Compare boards using declared data. Check lambda at the stated thickness or product condition, not only a marketing headline. If a thinner board is being considered, calculate the resistance of the complete build-up again.
- Check mechanical design. Where the board is loaded, verify declared compressive strength against the design load, area, safety factors, creep assumptions and supporting layers.
- Check moisture and fire details. Confirm that the declaration and the tested or approved assembly cover the exposure. Specify the required protective layers rather than assuming the board can remain exposed.
- Have the final build-up confirmed. Ask the project designer or supplier to confirm the calculated assembly, product identity and installation detail before ordering.
Choosing EPS for floors
Floor applications can be lightly loaded or heavily loaded. A residential floor beneath a screed, a suspended floor, a slab-on-ground and an industrial floor do not impose the same stresses. The grade must therefore be matched to the design load and the complete floor system.
Questions for a floor specification
- What permanent and imposed loads reach the insulation?
- Is the load distributed by a slab or screed, or concentrated through legs, partitions, racks or point supports?
- What thickness and tolerances are acceptable below the finished floor level?
- Will the board be above a slab, below a slab, beside an edge detail or in contact with ground?
- Which membrane, damp-proof layer and drainage arrangement control moisture?
- What cementitious screed, slab or other layer provides the required fire protection?
Never select EPS 100 simply because “100” sounds suitable for a floor. The grade label does not describe every load case, and compressive stress at 10% compression is not the same thing as an allowable long-term design stress. Use the manufacturer’s declaration and the structural or flooring designer’s calculation.
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EPS can be used in several wall arrangements, including cavity, external and internal insulation systems where the complete system is designed for it. The wall’s thermal bridges, fixings, render or cladding, moisture control and fire detailing matter as much as the board thickness.
Wall selection checks
- Confirm whether the board is intended for the specific wall system and whether its dimensions, edges, facings and tolerances suit that system.
- Use the declared lambda in the wall calculation, including the effect of joints, fixings and repeating framing or masonry bridges where required.
- Check how wind, impact, attachment loads and the wall’s support are handled; compressive grade alone does not describe every wall property.
- Confirm the weather layer, vapour control strategy and drying direction for the actual climate and construction.
- Specify the render, board, cavity barrier and lining details that provide the required fire performance.
A lower-lambda grey board may reduce the thickness needed for a target resistance, but only if the exact product is approved for the wall system and its declared value is used in the calculation. Do not assume that a grey version with a familiar grade label is interchangeable with a white board.
Choosing EPS for roofs
Roof insulation may be installed above or below a deck, between or over rafters, or as part of an inverted roof. Each position changes the thermal, structural and moisture questions.
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Warm and conventional roof positions
For insulation above a deck or within a roof build-up, check the continuity of the thermal layer, the membrane arrangement, wind exposure, attachment method and the way the roof covering is supported. The target U-value applies to the roof assembly, including the deck and surface resistances, not to the board alone.
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The 2016 EUMEPS White Book specifically notes that certain applications, including inverted roofs, can affect the moisture factor used for design conductivity. This is why a dry-laboratory lambda should not automatically be treated as the in-service value in every roof position. Use the product declaration and the roof system design for the expected moisture exposure, drainage and covering.
Compressive strength: when it matters and how to verify it
Compressive strength matters whenever the insulation carries construction or service loads. Typical published compressive stresses at 10% compression rise from 70 kPa for EPS 70 to 250 kPa for EPS 250 in the BPF example table. Those figures help explain the grade scale, but they are not a project-specific allowable load.
Ask for the exact declared property and its test basis. The BPF material also notes that compressive and tensile characteristics can be declared at different levels independently. A board with a suitable compressive declaration may still require separate checks for tensile, pull-through, wind or handling performance.
Use the full load path
Identify where the load enters and leaves the insulation. A screed or slab can spread a load, while a narrow support, partition, wheel or pedestal can create a concentrated stress. Check the support beneath the board, joints between boards, long-term effects and the tolerance of adjacent layers. If the calculation is not within the board’s declared scope, obtain a design statement from the manufacturer or engineer rather than extrapolating from the grade number.
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Moisture: EPS is not a blanket answer for wet conditions
Moisture performance depends on the product, the position and the assembly. Do not describe EPS as universally waterproof or assume that every board is suitable for permanent wet exposure.
For a ground-contact floor, confirm the damp-proofing, drainage, capillary break and the product’s declared moisture-related properties. For an inverted roof, account for the moisture factor and water movement addressed by the roof design. For walls, check rain penetration, vapour control, drying and the compatibility of membranes and finishes.
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The EUMEPS White Book (2016) explains that its described method treats moisture correction as negligible for normal building applications but flags specific applications such as inverted roofs as potentially significant. That qualification is important: “normal” is not a licence to ignore the exposure in an unusual or persistently wet detail.
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EPS is combustible. The BPF Expanded Polystyrene Group’s fire-properties page states: "EPS is a combustible material which by the inclusion of a flame retardant can achieve a Reaction to Fire performance of Euroclass E." The same page continues: "However in construction EPS should always be protected from source of fire by such as brickwork or plasterboard in walls or cementitious screeds in the case of floor applications."
Those statements are industry guidance, not a substitute for the fire rules in your jurisdiction or the tested classification of a particular system. Fire performance belongs to the assembly: board, adhesives, fixings, cavities, barriers, membranes, facings, render, plasterboard, screed and finish can all affect the result.
Fire checks before installation
- Obtain the current reaction-to-fire classification for the exact product, not a grade-wide assumption.
- Confirm the required protective layers and their minimum thickness or configuration in the approved system detail.
- Check cavity-barrier, opening and junction requirements where the board is in a wall or roof.
- Prevent exposure to ignition sources during storage, cutting and construction.
- Do not leave boards exposed merely because they will eventually be covered; follow the construction sequence and temporary-protection requirements.
How to compare two EPS boards
When a supplier offers alternatives, compare the following in one schedule. A nominal grade or a lower price is not enough to establish equivalence.
| Comparison axis | What to verify | Why it matters |
|---|---|---|
| Application | Roof, floor, wall or other use; system compatibility and approved detail | A board suitable in one position may not be suitable in another. |
| Thermal data | Declared lambda and declared resistance at the actual thickness and conditions | Determines the board’s contribution to the whole-element target. |
| Mechanical data | Declared compressive strength and any separate tensile or pull-through declarations | Loads and attachment forces vary by assembly. |
| Moisture data | Declared properties, exposure limits and design assumptions | Wet or moisture-sensitive positions can change in-service conductivity and durability. |
| Fire data | Reaction classification and required protective layers in the applicable assembly | EPS must be protected from fire sources as required by the system and local rules. |
| Physical specification | Dimensions, edge profile, tolerances, facings and current documentation | Fit, jointing, continuity and installation quality depend on these details. |
Request the current declaration, installation instructions and system approval for each candidate. Keep the product code in the specification so a visually similar replacement cannot be substituted without review.
Cutting, fitting and site handling
The BPF EPS Group notes that EPS can be cut or moulded on site. Cutting does not remove the need for a designed layout. Plan board sizes to maintain continuity, limit unnecessary joints and avoid gaps at corners, openings and junctions.
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- Keep boards dry and protected from ignition sources and incompatible chemicals.
- Use the cutting method permitted by the product and system instructions; collect offcuts so they do not block drainage or cavities.
- Butt boards tightly where the detail requires it, and treat unavoidable gaps with the material and method specified for the system.
- Protect the installed board promptly with the specified screed, plasterboard, masonry, render or roof layer.
- Inspect around penetrations, corners, supports and fixings, where discontinuities can create thermal, moisture or fire problems.
A cutting saw may be useful where extensive on-site shaping is required, but no particular tool can be recommended from the available product evidence. The board manufacturer’s instructions and the system designer’s detail take priority.
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Common selection mistakes and their fixes
Choosing from thickness alone
Mistake: ordering a familiar thickness without a whole-element target. Fix: start with the required U-value or R-value, then calculate the complete build-up using the selected board’s declared lambda.
Treating the grade as a universal specification
Mistake: assuming every EPS 100 board has identical lambda, strength or moisture behavior. Fix: identify the manufacturer and product code and check the current declaration.
Using compressive stress as an allowable design load
Mistake: dividing a grade number by an area and calling the result safe. Fix: have the complete load path, long-term behavior, load distribution and safety factors checked by the responsible designer.
Ignoring moisture because EPS is closed-cell
Mistake: assuming every location is dry enough for the same design lambda. Fix: review ground, roof and wall exposure, membranes, drainage and the moisture assumptions in the product and assembly documentation.
Leaving boards exposed to fire
Mistake: treating a flame-retardant formulation or an eventual finish as permission for temporary or permanent exposure. Fix: follow the required protective layers, sequence and local fire provisions.
Substituting a “similar” board
Mistake: accepting a replacement because the thickness and grade look the same. Fix: compare declared lambda, strength, moisture, fire classification, dimensions and system approvals before substitution.
Project checklist
- Element and exact position identified.
- Local U-value, R-value or energy target documented.
- Complete build-up and thermal bridges included in the calculation.
- Exact board product, thickness and declared lambda recorded.
- Compressive and any required tensile or attachment properties checked.
- Ground, rain, vapour and roof moisture exposure reviewed.
- Fire classification, protective layers and cavity or junction details confirmed.
- Dimensions, tolerances, facings and installation instructions available on site.
- Any substitution approved by the designer or system supplier.
Final decision rule
Choose the EPS board that meets the assembly’s thermal target and the position’s mechanical, moisture and fire requirements—not the board with the most familiar grade or the greatest thickness. Use typical grade tables to understand the market, but base the order on the exact product declaration and a calculation for the complete, locally compliant construction.
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