Why Solar Thermal Pipework Needs Specialist Pipe Insulation
Solar thermal systems present unique pipe insulation challenges that standard domestic pipe lagging cannot meet. Collector circuits regularly reach stagnation temperatures above 120°C when the system is not drawing heat — and outdoor pipework is continuously exposed to UV radiation and weathering. Standard elastomeric foam insulation such as Armaflex EVO is rated to +105°C and is not UV resistant — it will degrade rapidly on solar collector circuits and is not the correct specification for this application.
This guide covers the correct insulation products, wall thicknesses and installation requirements for solar thermal pipework in the UK, including flat plate and evacuated tube collector systems.
The Correct Insulation for Solar Thermal Pipework: Armaflex HT
Armaflex HT is Armacell's specialist high-temperature, UV-resistant closed-cell elastomeric foam, engineered specifically for solar thermal and outdoor heat pump applications. Key differences from standard Armaflex EVO:
- Temperature rating: -50°C to +150°C continuous (vs +105°C for EVO) — handles solar stagnation temperatures safely
- UV resistance: formulated to resist UV degradation without additional cladding or jacketing
- Weather resistance: suitable for outdoor exposed installations on rooftops and external walls
- Vapour barrier: closed-cell structure with μ ≥ 7,000 — no separate vapour wrapping required
- Fire classification: Class O (BS 476 Part 6 & 7)
Armaflex HT is available in pre-formed pipe sections and flat sheet format for wrapping cylinders, vessels and irregular shapes.
Recommended Wall Thicknesses for Solar Thermal Pipework
The following wall thicknesses are generally recommended for solar thermal collector circuits in the UK. Always verify against the system design and any applicable standards:
- Indoor pipework (plant room, airing cupboard): 19mm wall — standard thermal performance, protected from UV
- Outdoor exposed pipework (rooftop, external wall): 25mm wall minimum — additional thickness compensates for higher ambient temperature variation and heat loss on longer external runs
- Collector circuit flow and return (high-temperature zone): 25–32mm wall — accounts for stagnation temperatures and reduces heat loss on the primary circuit
For systems with long external pipe runs or in exposed northern locations, 32mm wall thickness is recommended to minimise heat loss and maintain system efficiency.
Solar Thermal Pipe Sizes — What You Need to Know
Most domestic solar thermal systems use 22mm copper tube for the collector circuit flow and return. Larger commercial systems may use 28mm or 35mm. The insulation bore size must match the pipe outside diameter (OD) — for standard UK copper tube:
- 22mm copper tube → 22mm bore Armaflex HT
- 28mm copper tube → 28mm bore Armaflex HT
- 35mm copper tube → 35mm bore Armaflex HT
Insulating Solar Thermal Cylinders and Vessels
Solar thermal cylinders and buffer vessels require insulation on all exposed surfaces, including the cylinder body, pipe connections, valves and flanges. Armaflex HT Sheet Insulation in 19mm or 25mm thickness is the correct product for wrapping cylinders and irregular shapes — cut to size and bonded with Armaflex HT Adhesive.
Installation Requirements for Armaflex HT on Solar Circuits
Correct installation is critical on solar thermal circuits — a poorly sealed seam or joint will allow moisture ingress and heat loss that degrades system performance over time.
- Use Armaflex HT Adhesive only — do not use standard Armaflex 520 on HT grade insulation at elevated temperatures. HT adhesive is formulated for the higher operating temperatures of solar circuits
- Bond all seams fully — apply adhesive to both faces of the longitudinal seam, allow to become touch-dry, then press firmly together. Do not leave any unsealed sections
- Bond all butt joints — apply adhesive to both cut faces at each joint between sections. Stagger joints where possible
- Do not stretch the insulation — stretching reduces effective wall thickness and compromises the vapour barrier
- Protect pipe clamp points — use insulated pipe clamps or ensure the insulation is continuous through fixing points
Common Mistakes to Avoid
- Using standard Armaflex EVO on solar circuits — will degrade at stagnation temperatures and is not UV resistant outdoors
- Using polyethylene foam (Tubolit/Climaflex) on solar circuits — polyethylene foam is not rated for the temperatures involved and will melt or deform
- Leaving pipe connections and valves uninsulated — every uninsulated section is a heat loss point that reduces system efficiency
- Using standard adhesive on HT insulation — standard contact adhesives will fail at elevated temperatures, causing seams to open
Products for Solar Thermal Pipe Insulation
- Armaflex HT Solar Pipe Insulation — pre-formed pipe sections in all standard UK pipe sizes
- Armaflex HT Coils — coil format for longer runs and complex pipework
- Armaflex HT Sheet Insulation — flat sheet for cylinders, vessels and irregular shapes
- Armaflex HT Solar Accessories — HT adhesive and tape for specification-correct installation
Frequently Asked Questions
Can I use standard Armaflex on solar thermal pipework?
No. Standard Armaflex EVO is rated to +105°C and is not UV resistant. Solar collector circuits can reach stagnation temperatures above 120°C and outdoor pipework is exposed to UV. Armaflex HT is the correct and Armacell-specified product for solar thermal applications.
Can I use polyethylene foam (Tubolit or Climaflex) on solar pipework?
No. Polyethylene foam is not rated for the temperatures involved in solar thermal circuits and will deform or melt at stagnation temperatures. It is also not UV resistant. Use Armaflex HT only.
What wall thickness do I need for solar thermal pipework?
25mm wall is the standard recommendation for outdoor solar collector circuits. For longer external runs or exposed northern locations, 32mm wall is recommended. Indoor pipework can use 19mm wall.
Do I need special adhesive for Armaflex HT?
Yes. Armaflex HT must be bonded with Armaflex HT Adhesive. Standard Armaflex 520 adhesive is not suitable for use at the elevated temperatures of solar circuits and will cause seam failure.
Is Armaflex HT suitable for heat pump pipework?
Yes. Armaflex HT is also the correct specification for outdoor air source and ground source heat pump primary circuits, where pipework is exposed to UV and weathering. For indoor heat pump pipework, standard Armaflex EVO is suitable.
For technical advice on solar thermal or heat pump insulation, call our team on 0161 775 1190 (Mon–Fri 8am–4pm) or browse the full Armaflex HT range.
0 comments