Water Underfloor Heating between Joists

Joist and suspended floor installations are a lesser known type of water underfloor heating system, but an incredibly versatile and effective one. Since they have no increase at all to the floor height they can be incorporated into existing properties as well as new builds with relative ease. Joist systems involve laying the pipes between the joists using one of the three methods shown below. The most popular choice is the aluminium spreader plate, but the aluminium insulation board and dry screed mix both offer equally effective but different suitabilities.  This type of heating system works by drawing the heat into a heated element and transferring it quickly into the room above, making the response time very quick when turned on. The joist system is less efficient then the screed system, but many time more effective than radiator heating.

There are 3 different fixing methods for water underfloor heating joist systems to choose from…

1x over joist underfloor heating with aluminium heat plate

Aluminium spreader plates

over_joist_underfloor_heating_with_foiled__grooved_insulation the underfloor heating company

Aluminium insulation boards

screed_underfloor_heating_with_dry_mix_system the underfloor heating company

Dry screed mix 

  • A 50mm insulation board is laid between the
    joists 20mm short from the top.
  • Aluminium spreader plates are nailed to the
    top of the joists.
  • 15 or 16mm pipes are pushed into the omega shaped grooves for a firm hold.
  • Aluminium spreader plates are effective and the
    most common choice for joist systems and best
    suited for upper floors.
  • PRO TIP! Put rock wool between the insulation
    and heat plate to slightly convex the aluminium
    plate. When the flooring is laid the plate pushes
    firmly against creating a solid contact with no
    air gap!


  • A 50mm pre-grooved insulation board with an aluminium layer bonded to the surface is laid
    between the joists.
  • 15 or 16mm pipes are pushed into the omega shaped grooves for a firm hold.
  • This fixing method combines two materials into one product and eliminates the separate costs
    for insulation.
  • PRO TIP! If the budget is stretched, this option
    can work out to be the cheapest.





  • A foil faced insulation board is laid between the
    joist leaving a 20mm gap at the top.
  • The pipe is laid onto the insulation and held in
    place with pipe staples.
  • A dry sand and cement mix is packed tightly
    around the pipes level with the top of the joists.
  • This option is best suited when a higher outputs
    is needed and works well alongside in-screed
  • NOTE: this system adds up to 25kg/sqm
    additional load to the joists. Seek advice and
    approval if you’re unsure before proceeding with
    this type of system.
  • PRO TIP! Use an 8:1 sand to cement mix for the
    best performance and output.



Which kit should you choose?
Now you have decided on your floor construction and learnt about the different fixing methods, is your project for a singe room or multiple rooms?


Single Room System >
Materials to heat one room controlled by one thermostat


Multi Room System >
Materials to heat multiple rooms, controlled by a thermostat in each room



We only use high quality European standard Pert-Al-Pert pipe in all our water underfloor heating systems.



Our range of pert-al-pert pipe is manufactured to ISO9001 European standards ensuring a lifetime of outstanding performance & reliability.

Five layer 16mm pert-al-pert pipe is used in all of our warm water underfloor heating kits and offers unrivaled suitability for underfloor heating by combining the best elements of a plastic and metal pipe into one product.

The aluminium core provides strength and importantly acts as an oxygen barrier to stop the ingress of oxygen into the system causing premature wear (corrosion) to the manifold. The aluminium layer also makes the pipe form-stable, making it very easy to work with by eliminating spring back for easier installation.


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