SW-03 · Carpet & Soft FlooringMilliken says a carpet tile's backing keeps it dimensionally stable and flat on the floor 1. Milliken argues that breathable cushion backings handle subfloor vapor better than impermeable hardbacks 2, yet its own installation guide still sets a 75% relative humidity hygrometer limit 3. CRI 104 makes the owner or general contractor responsible for an acceptable substrate 4, while Milliken's guide puts all floor preparation on the installer 3.
Carpet tile is usually chosen by its face: color, pattern and yarn. The backing gets less attention, yet it decides how the tile behaves on the slab. Milliken describes the backing as what keeps a tile dimensionally stable and flat on the floor.1
This piece compares what Milliken and Interface publish about their backings with CRI 104, the Carpet and Rug Institute's standard for installation of commercial carpet.4 The sources disagree in places. Those disagreements tend to land on whoever priced the prep.
What sits under the face
Milliken lists three main backing types: hardback, cushion and textile felt.2 It says hardbackings are typically made of filled bitumen compounds or filled polyolefins.2 Milliken's own hardback, Function Plus, combines hardbacking and latex to lock the tufts, with a felt layer as the foundation.1 Its cushion is recycled polyurethane, which Milliken puts at 90% recycled and 10% virgin polyurethane as a binder.2 Interface names bio-based backings, CQuest Bio and CQuest BioX, and integrated acoustic backings, CQuest Bio SONE and SONE+.5
For a general comparison of the types, see carpet tile backings explained. Much of the performance comparison in circulation comes from the makers themselves. Milliken's blog says felt compresses over time and loses thickness, acoustic performance and wear protection.2 Milliken also sells cushion-backed tiles.1 Treat that comparison as a manufacturer's position.
Moisture: the breathability argument and the limits
Milliken argues that open cell cushion promotes moisture wicking rather than water retention.2 Its claim is that vapor from the subfloor migrates through the backing and evaporates at the seams.2 It says hard backings are impermeable, so moisture from damp subfloors can become trapped as liquid and can promote mold and mildew.2 On that basis, Milliken calls breathable cushion or felt preferable where a damp subfloor, such as newly laid concrete, can be a concern.2
Breathability does not waive the limit. Milliken's guide covers its WellBac Comfort and Comfort Plus backings. It still says not to lay coverings until all hygrometer readings show 75% relative humidity or less.3 On new wet-laid bases, it asks for drying aids to be off for 4 or more days before testing.3 Interface states that humidity in new concrete must not exceed 75% by hygrometer test.5 It also calls for an efficient damp proof membrane in solid floors at or below ground level.5
CRI 104 sends the reader to each manufacturer's written moisture and pH limits.4 It requires RH testing to ASTM F2170, MVER testing to ASTM F1869 and surface pH testing to ASTM F710, with none exceeding manufacturer requirements.4 Before glue-down work or free floating carpet tile systems, the owner or general contractor, or their testing agent, is required to give the flooring contractor a written moisture and alkalinity report.4
Both manufacturer limits above are stated as hygrometer readings. The Milliken guide quoted here gives a plant address in China.3 Ask the manufacturer which test its limit refers to before treating a report as a pass. The test methods are compared in concrete moisture testing.
Flatness, and who owns the prep
Milliken asks for a subfloor that is smooth, hard, dry and level to within 6mm across 3.5m, with no abrupt changes.3 It says cracks and holes in excess of 3.2mm should be filled, and gypsum based compounds are not recommended.3 Interface asks for a base that is firm, smooth and dry, prepared to BS 5325 or a corresponding national standard.5 CRI 104 requires cracks, holes and irregularities to be repaired, to prevent accelerated wear and telegraphing.4
The lighter the hold, the more flatness matters. Milliken's TractionBack is designed to be installed with no adhesive.3 Milliken says the system needs stringent floor preparation to be fully functional, and calls flatness crucial for it.3 It also requires a thorough mopping of the floor before TractionBack goes down.3
The standard and the maker disagree on who owns this work. CRI 104 says the owner or general contractor is responsible for providing an acceptable substrate.4 Milliken's guide says all floor preparation is the responsibility of the installer.3 If the bid excludes leveling and the floor misses the tolerance, settle it before tiles go down. Put it in a written change order. A slab specification does not settle the question either; see slab F-numbers vs flooring tolerances.
Spacing checks also differ. Milliken measures 11 full modules (10 joints), allowing no more than 3.2mm over the calculated distance.3 CRI 104 measures 10 tiles and expects 10 times the tile size +/- 1/4 inch, unless the manufacturer states otherwise.4 Agree on the method before anyone measures. Seam symptoms are covered in carpet tile seams showing.
The published numbers side by side
| Item | Milliken guide | Interface guide | CRI standard |
|---|---|---|---|
| Moisture | 75% relative humidity or less by hygrometer3 | New concrete must not exceed 75% (hygrometer test)5 | Manufacturer limits; F1869, F2170 and F710 testing4 |
| Flatness | Within 6mm across 3.5m, no abrupt changes3 | Firm, smooth and dry; prepare to BS 53255 | Flat, smooth substrate; irregularities repaired4 |
| Heated floors | Floor must not exceed 28°C; heat off 48 hours before laying3 | Surface must not exceed 27°C (80°F); heat off at least 48 hours prior5 | Refers to the Radiant Professionals Alliance4 |
| Conditioning | 15-32°C for at least 24 hours3 | At least 24 hours, minimum 18°C5 | Recommended minimum 24 hours at 65-95ºF (18-35ºC)4 |
| Old adhesive | Smooth until trowel notches are less than 1mm3 | All traces must be removed5 | Must be removed and encapsulated4 |
The backing chooses the fixing method
CRI 104 describes the usual system as a thin film of pressure-sensitive adhesive allowed to dry completely.4 It is designed to prevent lateral movement and allow easy removal and replacement.4 CRI 104 notes that attached-cushion backings with factory-applied adhesive have special floor preparation requirements.4
Interface designs CQuest backings for spray adhesive, or free-lay inside a 2 meter grid of tackifier in a 100 mm band.5 For CQuest Bio SONE and SONE+, it says fixing should be tackifier release adhesive only, applied overall, not TacTiles.5 Interface does not recommend TacTiles on stairs, on ramps, under heavy rolling loads or over existing carpet.5 Milliken says heavy or rolling traffic and ramps may require a stronger tack to prevent lateral movement.3 Where heavy wheeled equipment runs, it recommends adhering TractionBack with release adhesive.3
Cushion adds height at the edges. Milliken says Comfort Plus thickness requires a transition that accepts the carpet without modifying the edge.3 Interface's loose-lay Interlay CQuest Bio SONE underlay adds 4.1 mm to floor height.5 Where no transition molding exists, CRI 104 says the hard surface edge should not exceed 1/16” above the total carpet thickness.4 Check door clearances and route edges against ADA changes in level.
Re-carpet: old adhesive, old backing, swapped tiles
Milliken says its Comfort Plus, Function Plus and TractionBack backings have no chemical incompatibility with existing floor covering adhesive.3 For TractionBack, it asks for trowel notches reduced to 0.8mm or less.3 It says an old pressure-sensitive film will not interfere, but asks for several tiles to be pre-tested for release first.3 Interface and CRI 104 want old adhesive residue removed.5 4 Smoothing and full removal are different labor lines. Price to the guide for the product actually specified.
Black adhesive changes the job. Milliken says never to scrape, sand or mechanically abrade exposed black adhesive or existing resilient floors, which may contain asbestos.3 CRI 104 warns that cut-back asphalt-based adhesive may contain asbestos and rules out powered removal that creates dust.4 It also does not recommend chemical adhesive removers on a substrate receiving new flooring.4 If old cutback was reactivated by PVC-backed carpet, Milliken asks the installer to call its Technical Services.3
Felt raises its own replacement issue. Milliken says felt collapses faster in high-traffic areas, leaving uneven tile thickness.2 Tiles lifted for underfloor access and not returned to their original spots can then highlight seams.2 For dye lots and fit on single swaps, see replacing a carpet tile; layout choices are in installation patterns.
Before a re-carpet: what to ask and put in writing
- Identify the existing backing and adhesive, and whether any adhesive is black. Milliken treats black adhesive as possible asbestos.3
- Name the governing manufacturer guide and the test its moisture limit uses. CRI 104 defers to manufacturer limits.4
- Get the written moisture and alkalinity report CRI 104 expects from the owner or general contractor.4
- Measure flatness against the product tolerance, and state in writing who pays for leveling.
- Confirm whether old adhesive is smoothed or removed. Milliken, Interface and the CRI standard differ.3 5 4
- Confirm the fixing method for ramps, stairs and rolling-load areas for this backing.5 3
- Check transition heights for cushion or underlay products before ordering moldings.
- Agree on the spacing check: 10 tiles under CRI 104, 11 modules under Milliken.4 3
- Inspect the benchmark area. Interface sets a maximum of 100m2 for inspection by all parties who sign off.5
- Keep attic stock, and record tile locations on felt-backed floors.
Sources
- Milliken, “Product Backings (commercial carpet backing options)”. milliken.com/en-gb/businesses/floor-covering/technical/product-backings
- Milliken, “What are the different types of carpet tile backing?”. europeblog.milliken.com/4-myths-of-open-cell-cushion-backing
- Milliken, “Modular Carpet Installation Guidelines”. contenthub.milliken.com/api/public/content/212b9643130e469aa098eefd05147
- Carpet and Rug Institute, “CRI 104 Standard for Installation of Commercial Carpet”. carpet-rug.org/wp-content/uploads/2019/03/CRI-104-STANDARD-For-INSTALLAT
- Interface, “Carpet Tile Installation Instructions”. interface.com/content/dam/interfaceinc/interface/technical/emea/installa
General information for the flooring trade. This article was drafted with AI assistance and checked against the sources listed above; the manufacturer's current instructions and the project specification govern any real job. If something here is wrong or out of date, tell us.
