Sheet-metal jacket

A sheet-metal cladding is a thin metal sheet (typically 0,5-1,0 mm) that is bent around an insulated pipe, duct, vessel or flange and serves as the outermost, protective layer of a technical insulation. The cladding has three main jobs:

1. To hold the insulation in place. It clamps the pipe section, lamella mat or wired mat against the pipe so that the insulation does not slip, fall down or open up at the joints. The cladding is therefore part of the fixing — it is not just "a nice outer layer". [Fastholdelse af isolering - maj 2020.pdf]
2. To protect against external influences. Weather, UV, mechanical wear, condensation from outside, animals, cleaning water, pedestrians, forklift damage — a sheet-metal cladding stops all of it. Inside technical rooms it protects against people tearing the insulation open as they walk past.
3. To reduce heat loss via radiation. A bright aluminium sheet radiates only about 5-13 % of the heat that a black-painted or matt surface radiates. This means that the same insulation thickness keeps more heat inside with a bright sheet on the outside. More on this under the Emissivity section.

Picture a thermos flask: the inner wool is the insulation itself, but the bright metal shield on the outside means the heat cannot escape via radiation. The sheet-metal cladding is the bright shield.

The cladding is normally supplied as prefabricated thin sheet from a workshop based on measured data (cladding diameter, length, angle) on a fabrication slip. The fitter assembles it on site with pop rivets, self-drilling screws or snap-lock beads. [standard for plade og opmåling.pdf s. 7]

The industry agreement and DS 419/DS 446 form the basis for material selection. [standard for plade og opmåling.pdf bilag 1]

Aluminium

By far the most widely used sheet in Denmark for HVAC and district heating.

• Alloy 3003 and 3103 (3S ½H or 3S ¾H) — the ordinary standard aluminium, soft enough to be bent by hand and hard enough to hold its shape. Suitable for most indoor and outdoor purposes. [Standard prefab. tyndplade s. 01]
• Alloy 5052 (57S ¼H) — aluminium-magnesium, slightly stronger and more corrosion-resistant, chosen where the sheet is exposed to greater mechanical loads or salty air.
• The hardness is given as ½H or ¾H, where ¾H is harder than ½H. Hard sheet holds its shape better in bends; soft sheet is easier to seam and bead. [Forklaringer bilag 1]

Thicknesses per DS 446 / the industry agreement (flat alu sheet): [standard for plade og opmåling.pdf s. 01]

Alu-zinc

Steel sheet surface-treated with an alloy of approx. 55 % aluminium and 43,5 % zinc (the rest silicon). The zinc coating weighs at least 150 g/m². [standard for plade og opmåling.pdf s. 01]

• Better corrosion resistance than plain galvanised steel — alu-zinc combines aluminium's passivating oxide layer with zinc's "sacrificial anode" effect.
• Used where the sheet needs to be stronger than plain alu, e.g. in industrial environments, and where stainless is too expensive.
• Used in the same thicknesses as galvanised sheet.

Galvanised steel sheet

Steel sheet with a total zinc coating of at least 275 g/m². A thicker zinc layer than alu-zinc, but less durable over time in damp or salty environments, since the zinc wears away more slowly than the alu-zinc layer. [standard for plade og opmåling.pdf s. 01]

Thicknesses (flat galvanised sheet):

Stainless steel

Flat sheet in AISI 304 (DIN 1.4301) or AISI 316 (DIN 1.4404). [standard for plade og opmåling.pdf s. 01]

• AISI 304 — the ordinary stainless grade. Good in damp indoor environments, atmospheric outdoor environments, dairies and general process plants.
• AISI 316 — molybdenum-alloyed. Withstands chlorides, salt spray, seawater and chemicals. Used near the coast, in chemical process plants and in food production involving acids.

Stainless sheet thicknesses:

Stainless is considerably more expensive (3-5× alu) and harder to work. It may be necessary to use a band and a band tensioner to make the rivet holes line up, because the sheet is stiffer and does not yield under riveting. [Værktøjslærre ny.pdf]

Material comparison table

The emissivity (ε) is a number between 0 and 1 that tells how much of the theoretically possible thermal radiation a surface emits. It is a material property of the sheet's surface:

The practical consequence — as described in the Isover HVAC handbook (Saint-Gobain): for the same medium temperature and the same surface-maximum requirement (e.g. 25 °C surface to avoid the risk of burns), a matt or PVC-coated pipe section needs approx. 20-25 % MORE insulation thickness than a bright alu cladding. Concrete example: a DN 50 district-heating pipe at 80 °C typically requires 80 mm of mineral-wool pipe section under bright alu to land in DS 452 class 3, but 100 mm under matt PVC or dark canvas. That is pure money saved, both in materials and in heat loss.

Therefore: wherever sheet-metal cladding is at all feasible, it is almost always the energy-efficient choice compared with PVC. That is also why the district-heating network and district cooling in Denmark almost always use sheet-metal cladding on the outside.

Important: the emissivity applies only to the OUTER surface. The radiation loss from the pipe itself is stopped by the insulation regardless — it is only the radiation loss from the OUTSIDE of the sheet to the surroundings that is changed by the choice of sheet. It is still significant, because the outer sheet is the only surface the surroundings see.