PIR/PUR pipe sections

PUR (polyurethane) is a thermosetting plastic formed by a chemical reaction between two liquid components:

1. A polyol (alcohol with several -OH groups)
2. An isocyanate (typically MDI — methylene diphenyl diisocyanate)

When the two components are mixed, they form urethane bonds and foam up into a rigid foam with small closed cells. The reaction is exothermic (releases heat).

PIR (polyisocyanurate) is a chemical relative. The difference is that a large excess of isocyanate relative to polyol is used, together with a catalyst that makes the isocyanates react with each other rather than only with the polyol. The result is a dense ring-bonded (trimeric) network of isocyanurate rings, which is thermally more stable than PUR.

In practice, the difference is:

• PUR = softer, cheaper, ignitable
• PIR = harder, more expensive, far more fire-resistant and temperature-stable

Both types are closed-cell foams — that is, each individual cell (small air bubble) is physically sealed by a thin plastic film. The cells are filled with a blowing agent (driving gas/blowing agent) with low thermal conductivity, and it is this gas — not the polymer itself — that gives the low λ value.

Comparison: PIR has approx. 30 % more fire resistance than PUR and can withstand higher continuous temperature (120 °C vs. 90 °C) [products.pcc.eu].

• 1937: PUR is invented by the German Otto Bayer at IG Farben (later Bayer AG)
• 1950s: Industrial production of PUR foam begins
• 1960s-70s: PUR becomes widespread as insulation, including for refrigerators and district heating pipes
• 1970s-80s: PIR is developed as a fire-safe variant
• Until approx. 1990: CFC gas (chlorofluorocarbons, e.g. R-11) is used as blowing agent — very low λ, but ozone-depleting
• 1990-2000: Switch to HCFC (hydrochlorofluorocarbons) — less ozone-depleting, but still greenhouse effect
• 2000-2010: Switch to HFC (hydrofluorocarbons) — no ozone effect, but high GWP (Global Warming Potential)
• Today: Pentane (cyclopentane or iso-pentane) or CO₂ is used as blowing agent. Pentane has λ ~0.012-0.015 W/(m·K) and is the dominant blowing agent in modern district heating pipes [provak.no]
• 2030+: Research into HFO (hydrofluoro-olefins) as blowing agent with even lower GWP

Ageing: The blowing agent slowly diffuses out through the cell walls, and atmospheric air diffuses in. Since air has a higher λ (~0.026) than pentane (~0.015), the total lambda value increases over time. This is the reason why you must always use the aged (declared) lambda value, not the fresh one.

Glossary: lambda (λ) is the thermal conductivity, measured in W/(m·K) — watts per metre per degree of temperature difference. The lower the λ, the better the insulation.

For PUR/PIR foam, the following applies:

In design calculations you must always use the aged (declared) value, not the initial value. This is specified in:

• EN ISO 8497 — Measurement of thermal transmittance for pipe sections
• EN 253 — Specific to district heating pipes, defines the ageing method

Many manufacturers, however, only state the attractive initial value in marketing material. As an installer you must be critical and ask for the declared λ or the declared CE value after ageing.

Comparison with mineral wool: Mineral wool has λ around 0.033-0.040 W/(m·K). PIR/PUR is thus 20-35 % better, which means you can achieve the same degree of insulation with 1/3 less thickness [dan-iso.dk]. This is decisive in technical rooms where space is scarce.

Key point: On a district heating pipe in the ground, where fire is not a risk, PUR is used (cheaper, and the temperature is typically below 90 °C). In visible technical installations, fire penetrations or high-temperature plants, PIR is chosen.

PUR/PIR pipe sections come in several forms:

• Moulded half sections (two halves laid around the pipe and closed with tape/adhesive)
• One-piece pipe sections with a slit on one side (snap-on, mostly for smaller dimensions)
• Lamella sections for large dimensions (segmented pieces laid around the pipe)

Standard dimensions: Follow the DN system (Diameter Nominal) for media pipes. Inner diameter matches, e.g., steel pipe DN15 (21.3 mm) up to DN500 (508 mm).

Thicknesses: Typically 20, 25, 30, 40, 50, 60, 80, 100 mm.

Surfaces:

• Uncladded (raw foam)
• With alu foil (reflective radiation protection + vapour barrier)
• With PVC jacketing (UV protection for outdoor use)
• With glass-fibre fabric (mechanical protection)

District heating pre-insulated pipes are a special form in which the pipe section is factory-moulded directly onto the media pipe in one long piece (see next section).

The absolutely dominant application of PUR in Denmark is pre-insulated district heating pipes. Approximately 66 % of Danish households get their heat from district heating — it is an infrastructure of many thousands of kilometres of pipes in the ground [mariestage.dk].

Construction (called the "bonded pipe system"):

1. Media pipe — carbon steel, more rarely copper or PEX plastic
2. PUR foam — typically 30-80 mm thick, blown with cyclopentane
3. PE jacketing — outer protective jacket of HD polyethylene, impact- and fracture-resistant
4. Leak monitoring — two copper wires embedded in the PUR foam (Nordic / BroDan system). If moisture enters the foam, the resistance between the wires changes, and the location of the leak can be measured.

Standards:

• DS/EN 253 — Bonded single pipes (single pipes in the ground) [iteh.ai]
• DS/EN 448 — Fittings (bends, T-pieces)
• DS/EN 488 — Valves
• DS/EN 489 — Joints (couplings)
• DS/EN 13941 — Design and installation
• DS/EN 14419 — Leak monitoring systems

Dimensions: Media pipes from DN15 (21.3 mm) up to DN1200 (1219 mm). The typical domestic installations are in the DN20-DN65 range [steelpipeline.net].

Insulation classes (per EN 253):

• Series 1 — Standard insulation thickness
• Series 2 — Thicker insulation (lower heat loss)
• Series 3 — Thickest insulation (lowest heat loss, for low-temperature district heating)

Operation: Logstor states a continuous operating temperature of 120 °C, short-term peak 140 °C, outer surface temperature ~50 °C [logstor.com]. Pressure capacity typically up to 25 bar.

Laying:

• Excavation of a trench, laying on a sand bed
• The pipes are welded together (the media pipe)
• Joints are protected with a heat-shrink sleeve — a PE coupling is placed around the welded joint, heated with a gas torch so it shrinks and seals, after which it is filled with 2-component PUR foam through a hole in the top
• Backfilling with sand and soil

IMPORTANT: District heating pipes are not subject to DS 452 (building insulation). They have their own set of rules under the EN 253 series. The DS 452 tables therefore cannot be applied to district heating.

Brands in Denmark:

• Kingspan LOGSTOR (Danish-based, dominant in the Danish market) [logstor.com]
• isoplus (German) [isoplus.group]
• Brugg Pipesystems (Swiss; flexible CASAFLEX/CALPEX pipes) [bruggpipes.com]
• Uponor (Finnish; mostly flexible PEX-based)
• Thermaflex Flexalen (Dutch)