Felt and canvas

Felt and canvas is the traditional Danish finish for technical pipe and duct insulation. It is a three-layer finish where each layer has its own function:

1. Insulation innermost — pipe sections or lamella mat (mineral wool) tied or fastened onto the pipe.
2. Felt jacket — a layer of insulation felt (approx. 0.7 mm thick) cut to the circumference, with overlap, and temporarily held in place with wire.
3. Canvas on the outside — typically cotton canvas in 1 m wide rolls, wound diagonally in strips with a 20 mm overlap.
4. Paste is brushed onto the canvas — it stiffens as it dries, pulls the canvas taut and fixes the entire construction.

For the beginner: Why a combination, and not just a single material? Because each layer solves its own task:

• The felt provides a smooth, uniform surface that hides small irregularities in the insulation and prevents fibres from the mineral wool pressing out through the canvas.
• The canvas provides the weave-flexible, shock-absorbing exterior that can be painted and repaired in spots.
• The paste stiffens the canvas into a hard, smooth "shell" — when dry, it feels almost like papier-mâché. It is this stiffness that makes the finish durable.

As the guide writes about the fundamental prerequisite: "First, it is important to stress that the substrate is in order. If the insulation is not neat and uniform on the surface, it will be revealed when you finally brush paste onto your canvas" [pap og laerred vejledning.pdf].

Felt and canvas is a craft tradition from the time when central heating really took off in Danish towns at the beginning of the 1900s. When hospitals, schools, factories and apartment buildings got boilers in the basement and pipes throughout the building, there was a need for a finish that was:

• Cheap and easy to obtain (cotton, felt and starch paste were available everywhere).
• Possible to paint in different colours, so pipes could be colour-coded (red = hot, blue = return, etc.).
• Repairable — when a piece got damaged, you could simply patch it with a new piece of canvas.

Through the 1950s and 60s, felt and canvas was standard in industrial installations, heating centres and institutions. Today, in new construction it has largely been replaced by PVC cladding (moulded plastic shells) and sheet-metal cladding (aluminium/galvanised sheet). But in:

• Older basements and boiler rooms the classic felt-and-canvas finish is still in place
• Renovations where the original appearance is to be preserved
• Hospitals and institutional construction with traditions

felt and canvas is still chosen. And not least — it is still part of the curriculum on GF2 and the AMU course 49058 "Technical pipe and duct insulation, felt and canvas (3 days)" [Teknisk roer og kanalisolering, pap og laerred - spoergsmaal.pdf].

The felt is the thin layer that lies innermost, against the insulation, and provides the smooth substrate for the canvas.

• Standard insulation felt (approx. 0.7 mm) — the ordinary type used on heating installations indoors [pap og laerred vejledning.pdf].
• Asphalt-impregnated felt — denser, more water-resistant, can be used where there is a risk of splashing or moisture. Note: with old felt there may be residues of tar products — wear gloves.
• Floor felt — according to the school questions, floor felt is used "at larger diameters" [school test q. 13]. It is heavier and more stable over large surfaces.

The thickness is typically 0.5–1.0 mm. The guide specifies exactly 0.7 mm as the one that "will be visible through the canvas" if the edge is not torn unevenly. That is why you tear the edge of the circumferential overlap — rather than cutting it straight. When the felt is torn, it becomes thinner at the edge, and the line becomes irregular and imperceptible under the canvas [pap og laerred vejledning.pdf].

Calculation of felt piece (example from the guide):

• Pipe diameter: 114 mm
• Insulation: 30 mm (applied on both sides of the pipe) = 60 mm
• Insulated diameter: 114 + 60 = 174 mm
• Circumference: 174 × 3.14 = 546 mm
• Add 50 mm overlap → cutting dimension: 596 mm

[pap og laerred vejledning.pdf]

The canvas is the exterior, the part you see. It holds the felt in place, absorbs small impacts, and provides the surface for paste and paint.

Classic cotton canvas is the ordinary choice on heating pipes indoors. It comes in 1 m wide rolls and is cut into strips with a width corresponding to the insulated diameter + 20 mm overlap [pap og laerred vejledning.pdf].

[Pudesyning.pdf, fabric register no. 1, 5, 17]

When cutting the canvas, the selvedge on the roll must be cut off at each end — it gives a cosmetically poor result if left in place. Use a very sharp knife with a smooth blade [school test B, q. 4].

The paste is the final — and perhaps most important — element. It is the paste that stiffens the canvas into a hard shell and holds the overlaps in place.

Pevicol (Nordcoll A/S) is one of the classic pastes, though it is PVA/PVC-based with N-Methyl-2-pyrrolidone (NMP) as the carrier solvent. The MSDS sheet states:

• Composition: NMP 80–85 %, polyvinyl chloride (PVC) 10–20 %.
• Classification: Xi, R-36/38 — irritates eyes and skin.
• Application: "Gluing of PVC materials" — typically on PVC cladding, but also used on felt-and-canvas installations.
• Personal protective equipment: Safety glasses and gloves (rubber/latex).
• Fire: Solvent vapours can form explosive mixtures with air. Flash point 95 °C.

[PevicolMSDS engelsk sikkerhedsdatabl.pdf]

Arabin Tørklister 203 (Dana Lim) is a more classic and harmless paste — it is the one the guide recommends:

"Before you start, you mix up a paste, for example Arabin. It should have a consistency like a good pea soup. Remember to whisk it together in cold water." [pap og laerred vejledning.pdf]

Datasheet details:

• Based on starch — no solvents.
• Not classified as a hazardous product.
• Code number (1993): 00-1 — lowest safety category.
• pH approx. 4.0, water-soluble, biodegradable.
• Respiratory protection against dust: Class P2, white (only when working with dry powder).

[Sikkerhedsdatablad Danalim Toerklister 203.pdf]

The consistency rule from the school test is clear:

Q. 7: Which consistency gives your paste the best result? Answer: "As thick as you can pick it up with your hand" — that is, like a good pea soup [pap og laerred vejledning.pdf]. Too thin = runs off; too thick = clumps and cracks; with lumps = uneven surface.

Application can be done with:

• Paste brush (brush-like brush — standard)
• Painter's glove
• Paint sprayer (on large surfaces)

But not with your hands — that is the wrong answer in school test q. 7 (set B). Pasting with your hands is impractical, uneven, and does not improve hygiene.