Specifying the Fiberglass Scrim in FSK & Foil Facings for Flexible Duct: Mesh Density, Grammage and Roll Format

If you buy FSK facing by the roll, you have probably compared datasheets that all look alike: foil thickness, kraft grammage, basis weight, WVTR, a tensile number in each direction. What those datasheets rarely explain is which layer actually does what when the facing is dragged off a rooftop curb, bent around a large-diameter duct, or stretched across a wind-loaded run — and why two facings with identical foil and kraft can behave completely differently in the field.

The answer is almost always the middle layer: the fiberglass scrim. This guide is written from the scrim manufacturer’s side — upstream of the facing converter — to show you what the scrim carries in an FSK laminate, how to read a facing datasheet back into scrim specifications, and how to put the right scrim line into your next RFQ. For the wider context of how scrim specifications work across our four application lines, start with the specification-level selection guide; this article goes deep on duct and facing.

1. What the scrim actually carries in an FSK facing

The scrim does. In every published facing datasheet we reviewed, the converter places the fiberglass scrim laminated between foil and backing precisely “so the facing carries handling and wind loads on exposed duct runs.” The same source lists the classic field failure among common procurement errors: buying facing without the scrim for a high-wind rooftop run — it tears during installation.

This matches what we see from the scrim side. A laid scrim is engineered with minimal elongation and high modulus, so a duct wrapped in a scrim-reinforced facing keeps its designed cross-section and airflow path instead of deforming under positive or suction pressure.

2. Reading a facing datasheet the scrim way

Here is the part facing datasheets don’t teach: the MD/XD tensile row is, functionally, a scrim specification in disguise. Two published FSK grades make the point cleanly:

Datasheet row	Grade A (FSK-718A type)	Grade B (FSKE-7151A type)
Scrim, MD	2-inch spacing (8 yarns per 100 mm)	3-way scrim reinforcement
Scrim, XD	3-inch spacing (12 yarns per 100 mm)	(same 3-way grid)
Kraft	80 g/m²	50 g/m²
Heat-seal coating	—	PE, 20 µm
Basis weight	115 g/m²	102 g/m²
Tensile MD (ASTM D828)	150 N/25 mm	125 N/25 mm
Tensile XD (ASTM D828)	75 N/25 mm	65 N/25 mm
Burst (ASTM D774)	40 N/cm²	30 N/cm²
WVTR (ASTM E96, Proc. A)	0.1 perm	0.02 perm

Read it the scrim way and three patterns emerge. First, mesh spacing is quoted two ways that mean the same thing — “2-inch spacing” and “8 yarns per 100 mm” describe the same grid (25.4 mm spacing ≈ 8 yarns/100 mm); always confirm which convention your supplier uses before comparing quotes. Second, MD tensile runs higher than XD on both grades, reflecting the closer machine-direction yarn spacing — that asymmetry is a design feature, not noise, and it should match the dominant load direction of your duct installation. Third, burst strength moves with scrim density across grades (40 vs 30 N/cm² here), because burst is precisely where the grid distributes a local load.

One row deserves a caution: WVTR. Published grades range from 0.02 to 0.1 perm, and the spread comes from foil and construction choices, not the scrim — water vapor does not pass through glass filaments. Treat WVTR as a facing-level property to verify per construction, not a scrim selection criterion.

3. Choosing mesh density by duct environment

Facing converters summarize the industry ladder this way: around 4×4 mesh per inch as the mainstream flexible-duct grade, with 5×5 or 6×6 specified where tearing must be avoided — rooftop runs, wind-loaded exposures, reinforced jackets for large diameters.

From the scrim side, the same ladder reads as yarn spacing and grammage. Our fiberglass laid scrim range for composites and facings runs from open, light grids (12.5 × 12.5 mm at 6.2–6.6 g/m²) to fine, heavier ones (5 × 5 mm at 15.2 g/m²), in square or tri-directional constructions, at grid densities from 0.8 to 3 yarns/cm — which spans and overlaps the facing-grade spacing above, leaving headroom to tune the scrim to the laminate rather than the other way around.

The tri-directional option matters more in ductwork than most buyers expect. A flexible duct flexes, bends and gets kicked sideways during installation — loads that arrive diagonally, not along MD or XD. A square grid has no yarns aligned for that; a tri-directional construction does. That is why our HVAC-grade scrim (CFT123030PH) is offered with both square and tri-directional constructions.

As engineering starting points, by installation environment:

Installation environment	Mesh starting zone	Why
Interior duct wrap, protected runs	Open grids (≈4×4 per inch class)	Handling loads only; facing stays economical
External runs, vapor-barrier duty	4×4–5×5 class, verify XD	Wind adds tension; XD spacing governs cross-direction tear
Rooftop / wind-loaded exposed runs	5×5–6×6 class	Converter guidance for exposed grades; tear survival is the spec
Large-diameter jackets	Finer grids within the above	Larger circumference magnifies handling tension

Treat these as zones to start a conversation with your scrim supplier, not a substitution table — the facing laminate, adhesive system and installation reality all shift the final answer.

4. Grammage, width and roll format

Grammage. The scrim’s own grammage — roughly 6 to 15 g/m² across our standard fiberglass range — is a small slice of a facing’s 60–130 g/m² basis weight (industry common range from published converter datasheets), but it moves the tensile numbers disproportionately. That is the economic argument for specifying the scrim deliberately: a few grams per square meter in the right places buys more MD/XD tensile than a few grams of anything else in the laminate.

Width. Converter datasheets list standard facing widths of 1000–1300 mm, and one of the documented procurement errors is neglecting roll width versus duct circumference — causing field splicing, which is where facings usually fail first. Our scrim is supplied from 200 to 2,500 mm roll width, so the reinforcement can be matched to your slitting plan instead of forcing the slit plan to follow the scrim.

Roll format. Facing converters ship 50–200 m rolls (longer on request). Those are converting lengths: upstream, scrim parent-roll economics matter — our rolls run 10,000 to 50,000 meters depending on specification and packaging (lighter grids run longer), which is what keeps lamination lines running without splices and keeps converter yields stable. When you brief your facing supplier, asking about their scrim’s parent-roll length is a legitimate quality question, not a detail.

5. Putting it in the RFQ

When you brief a facing converter — or commission your own lamination — the scrim deserves its own line item, not a footnote inside “reinforced facing”:

1.Grid spec in one convention: yarn spacing per 100 mm (and per inch), stated for MD and XD separately
2.Construction: square or tri-directional — say which, and why, in writing
3.Scrim grammage and yarn: fiberglass, target g/m², so the converter’s laminate math is transparent
4.Binder system: our scrims bond with PVOH, PVC or acrylic systems (customized available) — the binder must suit the converter’s adhesive and lamination temperature
5.Width and roll: width to your slitting plan; parent-roll length expectation
6.Verification: converters verify outgoing rolls for adhesion, scrim coverage and width before packing — ask which of those checks are on their QC record, because scrim coverage is exactly the parameter a cheap quote quietly drops
7.Fire compliance framing: foil facings are quoted at ASTM E84 Class A / Class 1 — a rating of the full facing-and-insulation assembly, as the converter’s own fine print states. Never accept or demand it as a scrim property.

The same scrim-selection logic applies across our other application lines — see how it plays out in single-ply membrane carrier selection and PVC/LVT flooring scrim selection.

FAQ

Does every FSK facing need a scrim?

For interior, protected applications, unreinforced foil-kraft facings exist and serve. The scrim earns its line item wherever the facing sees tension: rooftop runs, wind exposure, large diameters, or any installation where the wrap is handled roughly. The documented failure mode — an unreinforced facing tearing during rooftop installation — is why exposed-run specifications call out scrim-reinforced grades.

Is denser mesh always better?

No. Denser grids raise tensile and burst but add grammage and cost, and beyond the load requirement they buy nothing. The published grades cluster around 4×4 per inch for mainstream duct work precisely because it matches typical loads; densify when the environment — wind, diameter, handling — demands it.

How much does the scrim weigh in the total facing?

Typically a single-digit share: about 6–15 g/m² of scrim inside a 60–130 g/m² facing (industry common ranges). Its outsized contribution is tensile and tear performance, not weight — which is why comparing facings on basis weight alone rewards the wrong laminate.

Can we buy the scrim and laminate it ourselves?

Yes — that is exactly the supply position we sit in. Send the facing construction (layers, foil, kraft, adhesive), your lamination process and target MD/XD properties, and we will propose a scrim construction with widths from 200 to 2,500 mm and rolls up to 50,000 m.

Get a scrim recommendation for your facing

Send your facing construction and installation environment — layers, target MD/XD tensile, width and roll format — through our contact page and our engineers will propose a scrim construction with a trial-roll plan.

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