What is plasterboard joint tape made of?

Fibreglass

Fibreglass is a type of plastic that has been reinforced with very small fibres of glass. 

These fibres are woven together to create a strong, but lightweight material. 

Fibreglass tape is waterproof and, unlike paper tape, is not prone to developing bubbles.

This is because, as mentioned earlier, the mesh allows the jointing compound to bleed through, and effectively become part of, the tape. 

Despite its strength, fibreglass mesh is elastic and tends to stretch under load. 

When plasterboard settles into position, it sometimes moves very slightly. Unless it is used correctly, the pliability of fibreglass tape means that joints are more likely to develop cracks than those reinforced with paper tape.

Paper

Paper tape is not quite as strong as fibreglass tape. This is partly because it is non-elastic so will tear, rather than warp or stretch, under load.

However, the smooth surface of paper is an advantage as it can be more easily concealed with jointing compound to make plasterboard joints invisible and provide a better finish.

It is important to note that, when using paper tape, there can be a risk of air bubbles forming if an inadequate amount of jointing compound is applied between the joint and the tape.

An even layer of mud is critical to prevent this from happening.