Armaflex vs K-Flex: Which Pipe Insulation Should You Choose?

June 7, 2026

Armaflex and K-Flex are the two most widely specified brands of flexible elastomeric pipe insulation in the UK. Both are closed-cell rubber foam products used for thermal insulation and condensation control on HVAC, refrigeration and plumbing pipework. But which one should you choose?

This guide compares Armaflex and K-Flex across the factors that matter most to engineers, contractors and specifiers.

What is Armaflex?

Armaflex is manufactured by Armacell, a German company founded in 1954 that pioneered flexible elastomeric insulation. Armaflex is the world's best-known elastomeric pipe insulation brand and is specified as the default product on many UK building services projects. It is available in multiple grades including Class O, EVO, HT Solar, Ultima, Rail and NH Smart.

Browse Armaflex pipe insulation at Pipelagging.

What is K-Flex?

K-Flex is manufactured by K-Flex (IK Insulation), an Italian company and one of the largest producers of elastomeric insulation in the world. K-Flex ST is their standard pipe insulation grade, with K-Flex ST AD (self-adhesive), K-Flex Solar HT and K-Flex Duct also available. K-Flex is widely used across Europe and is increasingly specified in the UK as an alternative to Armaflex.

Armaflex vs K-Flex: Head-to-Head Comparison

Fire Rating: Are They the Same?

Both Armaflex Class O and K-Flex ST achieve Class O fire rating under BS 476, which is the highest surface spread of flame classification. For most UK commercial and public building applications, both products satisfy the same fire performance requirements.

For projects where fire performance is the primary specification criterion, either product is acceptable. Always check the project specification, as some engineers name Armaflex specifically rather than specifying by performance standard.

Read our guide to BS 476 fire ratings.

Thermal Performance: Is There a Difference?

Both Armaflex and K-Flex ST have a thermal conductivity (lambda value) of 0.033 W/mK at 0°C, which is identical. In practice, there is no meaningful difference in thermal performance between the two products at equivalent wall thicknesses.

For energy efficiency calculations and Part L compliance, both products can be used interchangeably at the same wall thickness.

Condensation Control: Which is Better?

Both Armaflex and K-Flex ST are closed-cell elastomeric foams with a water vapour resistance factor (µ) exceeding 7,000. This gives both products excellent condensation control performance on chilled water and refrigeration pipework.

In practice, both products perform equivalently for condensation control at the same wall thickness, provided joints are properly sealed with the appropriate adhesive.

Read our condensation on pipes guide.

Installation: Any Differences?

Both products are installed in the same way — slit tube slipped over the pipe, seam sealed with contact adhesive, butt joints bonded and sealed. Both are available in self-adhesive versions (Armaflex Self Seal and K-Flex ST AD) for faster installation.

Armacell's Armaflex 520 adhesive is used with Armaflex. K-Flex has its own K-Flex 850 adhesive. The two adhesives are not interchangeable — always use the adhesive specified by the insulation manufacturer.

Specification and Approvals

This is where the two products differ most significantly in the UK market. Armaflex is the default specification on the majority of UK commercial building services projects. Many engineers write Armaflex into specifications by name rather than by performance standard, which means K-Flex may require approval as an equivalent substitute.

If you are working to a specification that names Armaflex, check with the engineer or client before substituting K-Flex, even though the technical performance is equivalent.

Price: Which is Cheaper?

K-Flex is generally priced slightly lower than Armaflex for equivalent products. The price difference varies by product and quantity, but K-Flex can offer a cost saving on projects where the specification allows an approved equivalent.

For domestic and light commercial work where there is no formal specification, K-Flex ST is a cost-effective alternative to Armaflex that delivers equivalent technical performance.

Which Should You Choose?

Summary

Armaflex and K-Flex ST are technically equivalent products. They share the same fire rating, thermal conductivity, vapour resistance and temperature range. The key differences are brand recognition, UK specification prevalence and price.

For most applications, Armaflex is the safer choice where a formal specification exists, because it is the default UK standard and substitution may require approval. For domestic work or projects with a performance-based specification, K-Flex ST is a legitimate and cost-effective alternative.