Underfloor Heating Construction Design
Underfloor Heating on Suspended Timber Floors
Suspended timber floors consist of timber floorboards being laid on floor joists, which are supported above the subfloor on small pillars or walls, called tassel walls, or sleeper walls.
Suspended timber floors can be drafty, cold, and potentially damper than a solid floor, and they do not conduct heat as efficiently as a screed concrete floor. This means that the heat output is less; however, many older properties were constructed with timber suspended floors.
Underfloor heating with a timber floor therefore tends to heat up, and cool down, rapidly, more like a conventional radiator system. These systems, then, rely on the conductivity of components fitted within the floor to transfer heat efficiently from the pipe surface to the underside of the floor finish.
Laying the Underfloor Heating
It is crucial to have an underfloor heating plan – this is both timesaving and cost efficient. Decide where the controls need to be situated for ease of use, and where the pipes will run through the joists. Your plan should include every zone, or area, even if the installation is being carried out zone by zone; this way, your whole system will link up and work efficiently.
There are several ways to install your underfloor heating with timber floors, depending on time constraints and budget. This first method uses screed, however this accounts for less than 10% of installations.
In this method, the air gap between the insulation and the floor boards is filled with a lightweight screed mix. The infill is normally 10:1 sand/cement mix at a height of 25 mm. This, in effect, turns it into a screed concrete floor, which acts like a giant storage heater.
There are situations where a screed mix cannot be used such as old buildings where, maybe, the joists are not strong enough, lightweight materials that provide thermal mass, such as vermiculite, can be considered.
First, the insulation is laid down – 20-50mm is the recommended underfloor heating insulation thickness, however, with some of the recent developments in insulation technology, thinner layers of a modern insulation material, can be equally efficient. This should be cut to fit between the joists, and run up to the base of the walls. Continue the insulation up the walls, to a height that includes the depth of floor insulation, and, if you plan to include it, screed. Any gaps or joins should be taped to resist any seepage or infill of the screed.
Some proprietary brands of insulation board for underfloor heating are manufactured to include the pre-formed pipe work routed inlays, negating the need for another layer. This is, obviously, both time and labour saving.
Always check your insulation material and its depth complies with the most recent underfloor heating insulation regulations.
If your insulation does not have the preformed inlays, then these underfloor heating panels are laid now, and fixed to the insulation, taping over any gaps to avoid any screed infilling. Some panels have a mesh covering to allow attachment of the pipe work, and with others, holes must be drilled and staples or cable ties used. Again, these panels are cut to size, fitting between the joists.
Referring to your plan, holes, or gaps, must be cut in the joists to allow the pipe work, and any cabling, to pass through.
There must be room for the pipes to be expanded, as they would be when in use, as they pass through the joists, and this must be tested before laying screed or any other covering.
The final stage, before laying the floor surface is pouring on the screed to infill the empty space around the joists, and underfloor heating pipes.
Most people, however, opt for the far easier option of using proprietary products; these are readily available, cost effective and time saving, and whilst underfloor heating requires very little maintenance, the following systems have the benefit of accessibility, if it ever does becomes necessary.
Suspended Floor Panels for Underfloor Heating
One popular method is a suspended floor panel; these are foil lined for insulation, and are cut to fit between the joists. Timber battens are fitted along the sides of the joists, assuming the joists are deep enough, and the panels fixed to these battens. The pipe work is then clipped directly into the pre-cut routed grooves in the panels, so that it lies flush and level, and the floor surface is laid on top.
The standard sized panels may need to be cut into smaller sections to cover the entire floor; however, very small sections along the floor perimeter should be avoided, as that could hinder the final floor’s stability.
Since the mass of a timber floor structure can be up to 50% less than a screed floor, the response time of a timber floor system is much faster than that from a solid floor system and effective control can be made simpler.
Aluminium Diffuser Plates for Underfloor Heating
As an alternative to the suspended floor panels, aluminium spreader plates, pre-cut for secure pipe work positioning, will enable homogenous heat transmission. These are generally laid over the joists, and screwed or stapled in place. The panels should be staggered to avoid corner joints between four adjacent panels.
Insulation can be placed in the space below the plates to minimise any downward heat loss and dissipate the heat even across the floor, avoiding cold spots.
Both suspended floor panels and aluminium spreader plates have several advantages:
- Easy to install for the DIY installer as they set the pipe spacing
- Lightweight and manoeuvrable
- Clean and mess free installation
- Quick to fit
- Spread the heat evenly when used with insulation alone
- Suitable for either new builds or retro fitting
With either method, to maintain an even floor thickness in areas where underfloor heating will be omitted, such as under baths, kitchen units or fitted cupboards, the level should be maintained by replacing the panels, or plates, with plywood or chipboard.