The question is often asked: how is it best to install a heating system in a room with a joisted floor?
There are four approaches one could take:
- Remove the timber joists and replace them with solid concrete floor.
- Retain the existing floor construction and install an underfloor heating system between the existing floor joists.
- Retain the existing floor construction and install an underfloor heating system over the existing floor on top of a structural deck.
- Remove the joists and replace with breathable foamed glass aggregate construction.
Broadly speaking, properties with suspended floors suffer with poor thermal performance, the majority of this housing stock pre-dating the Second World War. Oftentimes, these floors are completely uninsulated and draughty when the air bricks are working as intended, ventilating the solum below.
Frequently, when an issue with a joisted floor presents, whether underfloor heating is being installed or not, it is ‘tackled’ by infilling with concrete. Further problems can then occur with any pre-existing moist condition and the introduction of a solid, impervious concrete infill. Moisture, seeking equilibrium, then takes the new path of least resistance via the walls causing even greater problems. The cause for any inherent damp is a subject in itself.
1. Remove existing suspended floor and replace with new insulated concrete slab.
For the reasons described in the introduction, this approach is generally not recommended. If it must be adopted then large diameter pipework or ducting must be cast within the concrete slab to ensure that adequate ventilation to any remaining suspended floor areas is retained. Access should be afforded to this ducting to ensure it can be kept clear over time.
2. Retain original floorboards and install underfloor heating system between existing joists.
This may be an option if the existing floor level needs to be retained. Raising floor heights can have an impact upon staircases, door thresholds and period feature such as hearths, skirtings etc. The depth of the existing joists is important both in terms of structural integrity as well as determining which thickness of insulation can be accommodated. Uninsulated suspended floors contribute dramatically to a building’s total heat loss and regardless of whether underfloor heating is to be installed, a suspended floor should be retrospectively insulated if possible.
Removing the floorboards which serve to tie the joists together reduces the strength and stiffness of the floor construction. If the original floorboards are not to be reinstalled, their replacement with a low thermal resistance alternative will seriously enhance the performance of a between-joist underfloor heating system.
Timber floorboards and plywood both impose a high level of thermal resistance. Unless a timber floorboard is to be installed as the floor finish itself, timber materials should not be used as these will necessitate a higher flow temperature through the underfloor heating system to achieve the same heat output. Historically one could simply turn their boiler temperature up, but as heat pumps and low-temperature systems become the norm, targeting the lowest possible flow temperature will ensure that system efficiency is maximised and that the joists are not exposed to potentially damaging temperatures. Cementitious and gypsiferous boards offer far less thermal resistance enabling even carpeted finishes to work effectively on top of an underfloor heating system.
Some shallow joists will not afford the installer enough depth to install both a meaningful layer of primary insulation and an underfloor heating system, and in these instances an alternative may be considered.
Installing floor heating between the joists requires the joists to be notched to allow pipes to cross from one void between the joists to the next–this may not always be permitted in the case of listed buildings and may not be safe from a structural standpoint if the joists are damaged or a little on the skinny side anyway.
Firrings are fixed to existing uneven joists with the topside of the new plywood deck positioned 30 mm lower than top of firrings to suit a 30 mm thick JUPITER Ideal Heating Panel.
When installing a between joist system over an unheated space below as is often the case on ground floors, the plywood deck should be set lower to accommodate the installation of a primary layer of insulation. In this case Kingspan K103 was chosen for its improved lambda value of only 0.019 W / mK when compared with normal PIR at 0.022 W / mK.
Standard 400 mm centre joists will normally accept three pipe runs between joist pairs, preserving the system’s high performance potential.
N.B. When installing between floor joists the layout of the joists dictates the underfloor heating design.
3. Install a heating system over existing suspended timber floor.
If raising the floor levels can be accommodated, installing a heating system over an existing suspended floor is likely the better option. Unless you have access from below, the floorboards will likely have to be lifted and replaced in order to install insulation. A foil-faced PIR insulation tends to be best with the foil facing acting as a VCL, this must be tightly fitted between the joists to be effective. The airbrick arrangement should be verified clear and ventilating effectively whilst access is easy. Additional airbricks may prove to be useful if there is any evidence of damaging moisture or condensation once the floorboards have been lifted. Replacing the original boards with tongue-and-groove plywood will also improve feel underfoot by further stiffening the floor.
The ‘overlaid’ approach will deliver a much improved result over a between joist system: for one, the working surface area of any between joist installation will be approximately 8-12% smaller than an ‘overlaid’ system owing to the surface area occupied by the joists themselves. Installing over a structural deck also affords the designer of the system much greater flexibility in pipe routing, often leading to an inherently better design.
The drawback with this approach is that the thickness of the underfloor heating system and its impact upon doors, staircases, thresholds and any historical features to be retained must be considered. A professional underfloor heating installation is in the order of 50 mm thick, comprising an insulated heating panel and a low-thermal resistance intermediate surface onto which the final floor finish is subsequently installed. Such an installation will likely necessitate planing doors down and careful modification of the staircase to accommodate the floor height increase.
Photo illustrating impact of raised floor and first step of staircase.
4. Removing existing floor joists & filling with Foam Glass Aggregate.
A less conventional but highly effective approach is the use of Foam Glass Aggregate rather than traditional hardcore and concrete.
Foam Glass Aggregate is commonly used in conjunction with a limecrete slab; however JUPITER have developed a floor construction that removes the need for limecrete and its notorious drying-out time by clever incorporation our dry-installed system. The solution not only expedites build-in time but also provides a heating system that is quick to respond to changing temperature demand.
Without the thermal mass of a limecrete slab, a JUPITER dry-installed system is so much faster to heat up and cool down, making the system much more comfortable to live with by avoiding the overshooting and undershooting characteristic of in-screed systems.
Foam Glass Aggregate is naturally insulating and is incredibly strong and stable. It mechanically binds together once compacted and its closed-cell structure prevents capillary action drawing moisture through the floor. In damp conditions, Foam Glass Aggregate acts as a buffer preventing transient moisture from entering the building envelope. Providing a flat and level finish comes courtesy of a fine granular levelling compound which is installed over the Foam Glass Aggregate to a typical thickness of 20-30 mm. Further rigid insulation can then be installed before the heating panels themselves followed by a load bearing surface suitable for the chosen floor finish.
Foam Glass Aggregate is easily introduced in a historic building, courtesy of its light weight it can be easily moved on site with wheelbarrows.
A 100 kg vibrating whacker plate is then used to achieve satisfactory compaction of Foam Glass Aggregate. Approximately one third of its fill depth is lost through the compaction process and a minimum post-compaction thickness of 150 mm can be achieved.
A breathable Geotextile membrane is then installed over the Foam Glass Aggregate to prevent the loss of the next layer, JUPITER’s dry floor levelling slate into the larger structure of the Foam Glass Aggregate below.
The non-capillary and insulating nature of the Foam Glass Aggregate keeps every element of the floor construction comfortably below the saturation point—relative humidity is depicted on the above graph as the black line with 100%RH, the saturation point depicted above in blue.
