Regarding the insulation tube:
There are many brands available, such as Thermaflex, Armaflex, K-Flex, Aeroflex, and others.

The list could be quite long. Each brand offers various part numbers due to differences in thickness, density, and fire rating category. Generally, thicker insulation is better, especially for home insulation where thicker and less aesthetically pleasing trunking is used. The most recommended thickness is usually 1/2". Some brands also indicate a fire rating classification, such as Class 0 (Fire Propagation) or Class 1 (Spread of Flame), which adhere to the British Standard 476 Fire Tests. However, in the case of a small unit like an HDB or condo, the difference between Class 0 and Class 1 might not matter much unless you live in a multi-story or large unit.
Typically, installers will provide the cheapest option of 1/2" thickness from a brand that meets either Class 0 or Class 1 fire rating.
Each insulation tube also requires a factory-standard method of jointing. Unfortunately, many unprofessional installers use only black electrical insulation tape, which is considered a bad practice. In reality, there are various adhesives available specifically designed for joining insulation tubes/foams. Some installers may use a single can of PVC adhesive for both PVC piping and insulation tubes.