Armaflex EVO and Armaflex HT are both closed-cell nitrile rubber elastomeric foam pipe insulation products manufactured by Armacell — but they are engineered for different applications and are not interchangeable. Using the wrong grade on solar thermal or outdoor heat pump pipework is one of the most common specification errors in the UK, and can result in premature insulation failure, heat loss and system performance degradation.
This guide explains the key technical differences between Armaflex HT and Armaflex EVO, and when each product is the correct specification.
Armaflex EVO vs Armaflex HT — Key Differences at a Glance
| Property | Armaflex EVO | Armaflex HT |
|---|---|---|
| Max continuous temperature | +105°C | +150°C |
| Min temperature | -50°C | -50°C |
| UV resistance | No — degrades outdoors | Yes — suitable for outdoor exposed installations |
| Weather resistance | Requires cladding outdoors | Suitable without additional cladding |
| Fire classification | Class O (BS 476 Part 6 & 7) | Class O (BS 476 Part 6 & 7) |
| Thermal conductivity | λ ≤ 0.033 W/mK at 0°C | λ ≤ 0.036 W/mK at 0°C |
| Vapour resistance | μ ≥ 10,000 | μ ≥ 7,000 |
| Microban antimicrobial | Yes | No |
| Adhesive required | Armaflex 520 | Armaflex HT Adhesive |
| Typical applications | HVAC, heating, chilled water, refrigeration | Solar thermal, outdoor heat pump, high-temperature plant rooms |
Armaflex EVO — The Standard Specification for HVAC and Building Services
Armaflex EVO is the world’s most widely specified flexible pipe insulation for commercial and domestic building services. It is the correct product for the vast majority of UK pipe insulation applications, including:
- Commercial and domestic central heating flow and return pipework
- Chilled water flow and return in HVAC systems
- Domestic and commercial hot and cold water supply
- Air conditioning refrigerant and chilled water pipework
- Indoor heat pump primary circuits and buffer vessel connections
- Condensate pipework from boilers and heat pumps
- Refrigeration suction and liquid lines
Armaflex EVO’s key advantages are its very low thermal conductivity (λ ≤ 0.033 W/mK), high vapour resistance (μ ≥ 10,000), Microban antimicrobial protection and Class O fire rating. It is available in solid tube and self-seal formats.
Where Armaflex EVO is not suitable: any application where pipework exceeds +105°C continuously, or where insulation is exposed to UV and weathering without additional cladding.
Armaflex HT — The Correct Specification for Solar Thermal and Outdoor Applications
Armaflex HT is engineered for applications where standard Armaflex EVO would fail — specifically solar thermal collector circuits and outdoor heat pump pipework. The two critical differences are its higher temperature rating (+150°C vs +105°C) and its built-in UV and weather resistance.
Armaflex HT is the correct specification for:
- Solar thermal collector flow and return pipework
- Outdoor air source and ground source heat pump primary circuits
- Exposed external pipework on rooftops and external walls
- High-temperature plant room pipework where ambient temperatures are elevated
- Buffer vessel and cylinder connections in high-temperature environments
Why the temperature rating matters on solar circuits: solar thermal collectors regularly reach stagnation temperatures above 120°C when the system is not drawing heat. Armaflex EVO at +105°C will degrade under these conditions — the foam hardens, cracks and loses its insulating and vapour barrier properties. Armaflex HT at +150°C handles stagnation temperatures safely.
Why UV resistance matters outdoors: standard Armaflex EVO will degrade rapidly when exposed to UV radiation — it becomes brittle, cracks and eventually crumbles. Armaflex HT is formulated to resist UV degradation without additional cladding or jacketing, making it suitable for rooftop and external wall installations.
Can I Use Armaflex EVO Outdoors if I Clad It?
For outdoor pipework that does not exceed +105°C — such as cold water supply pipes on external walls or some heat pump pipework — Armaflex EVO can be used outdoors if it is protected from UV with aluminium cladding or UV-resistant jacketing. However, for solar thermal collector circuits where stagnation temperatures exceed +105°C, cladding alone is not sufficient — the temperature rating of the foam itself is the limiting factor. Armaflex HT is required.
Adhesive — Do Not Mix Products
Armaflex EVO must be bonded with Armaflex 520 contact adhesive. Armaflex HT must be bonded with Armaflex HT Adhesive — standard Armaflex 520 is not suitable for use at the elevated temperatures of solar circuits and will cause seam failure. Always use the correct adhesive for the grade of insulation being installed.
Which Product Do I Need?
- Indoor HVAC, heating, chilled water, refrigeration, heat pump (indoor): Armaflex EVO
- Solar thermal collector circuits: Armaflex HT
- Outdoor heat pump primary circuits (exposed): Armaflex HT
- Outdoor pipework below +105°C with UV cladding: Armaflex EVO with aluminium cladding
- Outdoor pipework below +105°C without cladding: Armaflex HT
If you are unsure which product is correct for your application, call our technical team on 0161 775 1190 (Mon–Fri 8am–4pm). We are authorised UK distributors for Armacell and can advise on the correct specification for any application.
Frequently Asked Questions
Is Armaflex HT more expensive than Armaflex EVO?
Yes — Armaflex HT carries a price premium over standard EVO due to its specialist formulation. However, using EVO on a solar circuit and having it fail within 2–3 years is significantly more costly than specifying HT correctly from the outset.
Can I use Armaflex HT everywhere instead of EVO?
Technically yes, but it is not cost-effective. Armaflex HT has a slightly higher thermal conductivity than EVO (λ ≤ 0.036 vs 0.033 W/mK) and does not include Microban antimicrobial protection. For standard HVAC and building services applications, Armaflex EVO is the better-performing and more cost-effective product.
How do I tell Armaflex HT and EVO apart?
Both products are black closed-cell foam and look very similar. Armaflex HT is typically marked with “HT” on the product or packaging. Always check the product label before installation on solar or high-temperature circuits.
Does Armaflex HT come in self-seal format?
Armaflex HT is primarily available in solid (unslit) tube format and coil format. It is not generally available in self-seal format. All seams must be bonded with Armaflex HT Adhesive.
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