To mesh or not to mesh? That is the question

To mesh or not to mesh, that is the question.

Should underfloor piping be installed onto the mesh, or stapled directly to the polystyrene?

Underfloor heating functions by installing plastic pipes into the slab before it is poured.

Warm water is then circulated through these pipes which results in a warm concrete floor, the floor in turn heats the whole house.

We install our pipes by stapling them directly to the polystyrene insulation. There are a number of reasons for this, one of the main ones being that the pipes are out of the way, not just of the mesh but also any saw cuts in the concrete.

Why is mesh installed?

The mesh installed into a concrete slab protects the surface from cracking. Concrete is very strong when compressed, but weak when you try and pull it apart (in tension) – mesh about a third of the way down the slab will help to stop surface cracking if for example the foundations round the edge of the house sink slightly, as steel is very difficult to pull apart.

Steel embedded in concrete provides an extremely strong structure, and this combination is frequently used in large building projects.

However, for the steel to do its job correctly, it must be fully encased in concrete (the recommendation in a slab is 30mm of concrete round any piece of steel).

 

A foundation model of MaxRaft slab with pipes onto polystyrene.

When pipes are tied to the mesh there are two main issues.

Some builders lower the mesh (to prevent pipes coming up to the surface) – this means the mesh is not protecting against surface cracking correctly (in extreme cases it will protect the bottom of the slab from cracking).

The main problem is that pipe installers invariably follow the mesh lines to install pipe, which means much of the mesh does not have 30mm of concrete round it, instead having 16mm of air and plastic.

The overall result is the integrity of the mesh is compromised and is not doing the job it is designed to do, either because it is set too low or it is not fully encased in concrete (or both).

If pipes are attached to the mesh, it must be at frequent intervals (to stop floating) and the lines of steel of mesh should not be used, the pipe should only cross any piece of mesh (at right angles) – at least 30mm away from any other parts of the mesh. Following the mesh lines (as it convenient and looks neat), defeats the whole purpose of the mesh

There is an argument that the pipes at the bottom of a 100mm slab (where we install them) does lead to a very small loss of efficiency when using a heat pump (which we do). However, taking everything into account, the advantage of having the pipes largely out of the way with the mesh in its optimum position providing maximum strength, is a superior solution.