Floor underlayment is often treated as a film that may or may not be worth having: it goes under the floor, nobody sees it, and it is a small share of the price. But it is doing three jobs at once, none of which substitutes for the others — levelling, isolating vibration and blocking moisture — and each is decided by a different property. Most wrong purchases are not the wrong material; they are three jobs treated as one number: thickness.
This page takes the three apart, gives measured XPE and IXPE figures grade by grade, and states which numbers we do not supply.
Three jobs, three parameters
| The job | What decides it | What correct looks like | What incorrect looks like on site |
|---|---|---|---|
| Levelling — taking out small undulations in the subfloor | How even the thickness is, not how thick it is | Plank joints close fully, no hollow sound underfoot | Humping at the seams, noise when walked on |
| Vibration isolation — cushioning footfall and furniture point loads | Compression behaviour (expansion ratio and apparent density) | Still recovers after long compression | Dents under furniture legs; the layer thins and hardens |
| Moisture blocking — stopping vapour rising from the subfloor | Whether the layer is continuous (overlaps and terminations) | No path for vapour | Local darkening, floor edges lifting |
The three do not compensate for one another. Thicken the underlayment and the levelling tolerance grows, but isolation and moisture control do not improve. Raise the density and compression resistance improves, but levelling and moisture control still depend on something else.
Thickness: it buys feel and tolerance, not sound insulation
Underlayment is commonly 3–5 mm. One material boundary has to be stated first: XPE foam is made in single layers of 3–15 mm, so below 2 mm XPE cannot be supplied as a single layer — it has to be IXPE, whose single layers run from 0.1 to 15 mm. An enquiry for a “2 mm XPE underlayment” means the supplier either changes the material or changes the thickness.
More thickness buys feel underfoot and tolerance for the subfloor; it costs displacement room at the plank joints. Flooring manufacturers usually set a maximum underlayment thickness, and that maximum overrides the instinct that thicker is more comfortable. Beyond it, the loading on the joint changes.
Thickness and sound insulation are not the same thing. Underlayment affects impact noise, and only as one part of the whole floor build-up; the final performance depends on the mass of the surface layer, the cavity, and perimeter sealing. Using a thicker underlayment to solve an insulation problem is the wrong direction.
Density: how much thickness is left in five years
Thickness is visible at purchase; density is what you learn five years later. The measure is compression set (GB/T 6669, 23±2 °C for 22 hours) — the proportion that does not come back after compression. Lower is better.
| Expansion | 10× | 15× | 20× | 25× | 30× | 35× | 40× |
|---|---|---|---|---|---|---|---|
| Apparent density (kg/m³) | 100±10 | 66.7±8 | 50±6 | 40±4 | 33.3±3 | 28.6±2 | 26.8±3 |
| Compression set (≤%) | 5 | 7 | 8 | 10 | 10 | 11 | 11 |
| Dimensional change, cross / machine (≤%) | 4 / 6 | 4 / 6 | 6 / 8 | 6 / 8 | 6 / 8 | 6 / 8 | 6 / 8 |
| Water absorption (≤%) | 0.2 | 0.3 | 0.3 | 0.4 | 0.4 | 0.4 | 0.4 |
| Thermal conductivity (≤ W/(m·K)) | 0.073 | 0.065 | 0.055 | 0.049 | 0.045 | 0.040 | 0.040 |
Read it left to right: the lower the expansion ratio, the higher the density and the lower the compression set. The 25–40× grades commonly used for underlayment (40 down to 26.8 kg/m³) sit at ≤10% to ≤11%; the 10–15× grades (100 to 66.7 kg/m³) sit at ≤5% to ≤7%. The gap is not marginal: one ceiling is a tenth, the other is under half of that.
So if the underlayment has to still be doing its job in five years, moving to a lower expansion ratio is more effective than adding thickness. Thickness does not change how fast the material packs down; it only extends how far it can before it does.
Dimensional change: low density also moves more
Dimensional change for XPE and IXPE at 70±2 °C for 22 hours is specified in two bands by expansion ratio (GB/T 8811): 3–15× is ≤4% across and ≤6% along; 20–40× is ≤6% and ≤8%.
Floating floors mean the underlayment covers the whole room. Dimensional change is not itself alarming; what is alarming is dimensional change pinned at walls and doorways, which becomes humping. Perimeter gaps and movement joints follow the flooring manufacturer’s instructions and the design — do not leave that to site discretion.
Moisture: continuity decides it, not water absorption
The foam is closed cell, and absorption by GB/T 1034 method 4 is ≤0.2% up to 10×, ≤0.3% at 15–20× and ≤0.4% at 25–40×. But a low absorption rate does not make the layer a moisture barrier. Vapour travels through the seams, not the cells.
- Overlaps. Rolls must overlap, and the overlap must be taped.
- Walls and doorways. Where the underlayment stops at a wall it has to be terminated, not left with an exposed cut edge.
- Penetrations. Pipe collars, floor sockets and fixings need the hole edge made good.
Get those three right and the underlayment is a continuous membrane; miss one and it is a membrane with a hole, and vapour will concentrate through it. A foil facing makes the intent clearer, but it is still “continuous or nothing”; the mechanism is on aluminium foil thermal insulation foam. Where vapour control is strict — walls, for instance — the barrier is usually part of the construction system and the foam supports it rather than carrying it alone.
Above the heating pipes is not the same layer as below them
| Position | Main job | Property to look at first | Selection direction |
|---|---|---|---|
| Below the pipe: insulation layer | Reduce heat lost downwards | Thermal conductivity; in a wet system, also carry the weight of the screed | Often foil faced; density chosen to meet the load |
| Above the screed, below the floor: underlayment | Levelling, isolation, cushioning | Compression set, thickness consistency | Thin, toward the lower expansion ratios |
Within one grade table those two requirements pull in opposite directions: higher expansion gives lower conductivity but higher compression set. So a single roll rarely optimises both — especially in a wet system, where the screed’s weight sits on the insulation layer and the density cannot simply be chosen low. See underfloor heating insulation foam for the construction split.
One point that is often missed: above the pipes, the foil facing is usually pressed flat by the screed or the floor, so it no longer reflects. Where foil helps and where it is only a vapour barrier is covered on the foil page.
Choosing by substrate
| Substrate | What to solve first | Direction | Watch |
|---|---|---|---|
| Concrete | Flatness and moisture condition | 3–5 mm, toward the lower expansion ratios | Whether the slab is fit to receive a floor is a construction-code question; underlayment is not a levelling compound |
| Timber | Joints, nail heads and local steps | Around 3 mm, even thickness | Nail heads must not protrude; treat the joints first |
| Renovation over an old floor | Residual adhesive and level differences | Set by the height difference and the flooring maker’s allowance | Do not use underlayment to bridge a large step |
| Underfloor heating | Heat transfer and cushioning | Thin, density on the high side (lower expansion) | Thickness ceiling set by the flooring maker and the heating design |
Four installation rules
- Use a single layer, within the flooring manufacturer’s maximum thickness.
- Overlap and tape the seams to keep the vapour path continuous.
- Leave the perimeter gap the manufacturer specifies; do not jam it tight to the wall.
- Do not stack material or drag loads across the underlayment before the floor goes down, and patch any damage.
The data we give, and the data we do not
What we give are values measured on the foam layer by standard methods, banded by expansion ratio: apparent density (GB/T 6343), compression set (GB/T 6669), dimensional change (GB/T 8811), water absorption (GB/T 1034 method 4) and thermal conductivity (GB/T 10297). Flame-retardant grades are available: oxygen index 27, meeting GB 8410 A0 and UL94 V2, with a low-smoke option.
What we do not give is sound reduction: impact sound insulation (ΔLw and similar indices), airborne sound insulation, or the thermal and acoustic performance of the floor assembly. Those are properties of a construction, tested on the whole assembly to the relevant standard; they cannot be derived from the density or thickness of one layer. Any claim that converts a material figure directly into a sound reduction figure is missing the construction half.
Common questions
What is the difference between 3 mm and 5 mm?
Mainly feel underfoot and tolerance for the subfloor. The difference in compression set comes from density, not thickness — a 5 mm low-density roll can resist long-term compression less well than a 3 mm high-density one.
Is higher density always better?
No. Higher density gives better compression resistance and recovery, but adds weight, hardness and cost. Choose by traffic and load: areas under long-term furniture deserve higher density; areas that only see footsteps do not. XPE covers 3–40×, apparent density 26.8–300 kg/m³.
Can underlayment act as a moisture barrier?
That depends on two things: whether the layer is continuous, and whether it is foil faced. Bare foam has low absorption but is not itself a continuous membrane; once the seams and terminations are done properly it takes on that role.
Can I use thick underlayment over underfloor heating?
Not recommended. A thick layer adds thermal resistance upwards and raises the finished floor level. The thickness ceiling comes from the flooring maker and the heating design, and is usually the thin end of the range.
Will underlayment improve sound insulation?
It affects impact noise, and only as one element of the assembly. How much it improves is an assembly figure, and this page gives no number for it — see above.
Boundaries
What this page gives is material data and a selection method, not an installation acceptance standard, and it does not replace the flooring manufacturer’s requirements. The values in the tables are type-test limits (≤), not measured values for every batch, and different thicknesses at the same ratio also scatter. If the construction has special requirements — load bearing, fire compartmentation, long-term damp — send the construction drawing and the service conditions and we will confirm which combination we can supply.
What Bofeng Foam supplies
Bofeng Foam is a manufacturer of cross-linked polyolefin foam, with a 10,000 m² plant, five production lines and 5,000 tonnes of annual capacity in Zhuhai, Guangdong. XPE runs from 3× to 40×, apparent density 26.8–300 kg/m³, single layer 3–15 mm, heat-laminated multi-layer to 200 mm; IXPE single layer 0.1–15 mm suits thin underlayment. Width is 500–2000 mm. Foil facing and self-adhesive rolls are available, and finished stock can be slit, die-cut and punched. Telling us the substrate, the floor thickness and whether there is underfloor heating will place the specification more reliably than a single “3 mm”.
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See also flooring foam, floor damping foam, underfloor heating insulation foam, aluminium foil thermal insulation foam, high-density XPE foam, laminated foam and the XPE foam product page.