Underfloor heating in renovation projects: myth or reality?

Why does everyone think it’s impossible?

The idea stems from a real-life experience on a building site. Fifteen years ago, installing underfloor heating as part of a renovation meant gutting the whole house, planing down the doors, rebuilding the staircase and living without a floor for six weeks. Many Belgian homeowners saw this happening at a neighbour’s or a relative’s house, and came to regard it as a universal truth.

The problem is that this description applies to just one type of system: the traditional screed poured to a depth of 10 to 15 cm, designed for new-build properties. Since then, manufacturers have brought out solutions designed specifically for existing properties, with thicknesses reduced by a factor of three or four. The technology has advanced. The reputation, however, has not.

The three real constraints, and how we deal with them:

There are three common obstacles that crop up in every renovation project. None of them is a deal-breaker in itself, but each one is worth considering before signing a quote.

The available height

This is the factor that determines everything else. A traditional hydronic underfloor heating system consists of a layer of insulation (30 to 60 mm), pipes (15 to 20 mm) and a screed 60 to 80 mm thick: 10 to 15 cm in total, once the floor covering has been laid. On a new concrete slab, this clearance must be allowed for in the plans. On an existing floor, it reduces the ceiling height, raises the floor level above the thresholds and means the bottom of doors must be planed down.

Thin-layer systems are a game-changer. A thin fibre-reinforced screed laid over thin insulation is just 30–45 mm thick, excluding the floor covering. Dry systems go even further: Comap’s Biofloor Dry, distributed by Facq, is laid directly onto the existing floor using aluminium diffusion profiles, resulting in a total thickness of 25 mm, with no screed required. This results in a total depth of 3 to 5 cm, including the floor covering, compared with 10 to 15 cm in a conventional system.

In practical terms: measure the headroom beneath your French windows and internal thresholds. If it is less than 4 cm, a slim system is still a viable option. If it is less than 3 cm, only an ultra-thin dry system or electric underfloor heating will meet the requirement.

The load on the structure

A traditional screed weighs between 100 and 120 kg/m², which is around six tonnes for a 50 m² room. On a solid concrete slab, this is not a problem. However, on an old wooden floor, a mezzanine or a floor supported by joists – whether in Brussels or in the workers’ houses of Hainaut – this weight may exceed the permissible load.

A dry system weighs 15 to 20 kg/m², which is ten times less. This is what makes underfloor heating a viable option on the upper floor of a 19th-century mansion, where a screed would have required the joists to be reinforced. Before obtaining any quotes, a simple load calculation carried out by a professional will help avoid any unpleasant surprises.

The duration and cleanliness of the building site

A wet screed requires a drying time of three to four weeks before the floor covering can be laid, during which time the room remains unusable. A liquid anhydrite screed shortens this time slightly, but it is still in the same ballpark. A dry system, on the other hand, can be laid and covered within a few days, with no drying time required as there is no screed to pour.

When renovating a house one room at a time whilst it is still occupied, this difference carries as much weight in the decision as the price per square metre.

Three specific examples from Belgium

Terraced houses built between the 1930s and the 1970s. This is the most common scenario in renovation projects in Wallonia and Brussels. The ground floor often rests on a concrete slab, which allows for a thin screed system or even a conventional screed if the headroom permits. Upstairs, on wooden joists, a dry screed system is the logical solution.

The flat in a block of flats. Ceiling height is a factor, and the block’s management regulations sometimes limit the loads permitted on the floor slab. A 25 mm dry-lay system, installed room by room without affecting the neighbours below, is best suited to this situation.

An extension or conservatory attached to the existing building. Ensuring the floor level matches that of the existing structure is a key technical consideration: a slim, precisely calibrated system allows the new floor to be aligned with the old one, without an unsightly step between the two rooms.

When is underfloor heating a bad idea?

It’s also worth noting that some situations are not suitable for underfloor heating, even with a dry system.

A room that is already cluttered with fixed furniture and floor-level storage (walk-in wardrobe, built-in bookcase) does not make the most of the investment, as the heat only radiates onto the unobstructed surface. A poorly insulated property, with original walls and a roof lacking insulation, derives no benefit from a low-temperature underfloor heating system: the heat produced escapes faster than it is distributed, regardless of the system installed. Finally, a one-off renovation of a single small room, such as a toilet or a 3 m² storeroom, rarely justifies the additional technical cost of a hydronic underfloor heating system: a low-temperature radiator or an electric underfloor heating system can be installed more quickly, whilst delivering an equivalent result over such a small area.

In all three cases, it is better to prioritise spending on insulation first, or on another heat source.

The whole house, or room by room?

Renovation in stages remains the norm for most building projects in Belgium. You don’t renovate an entire house in a single month, due to a lack of budget or alternative accommodation whilst the work is being carried out.

A compact or streamlined system lends itself well to this approach. There is nothing to stop you fitting out the living room and kitchen this year, and then the bedrooms the following year, provided that the heat generator (heat pump or boiler) and the manifold are sized from the outset to cater for the entire final project. It is this starting point – which is often overlooked – that prevents you from having to redo the whole installation two years later.

Combining it with a heat pump, even in a compact system

An underfloor heating system, whether slimline or traditional, remains a low-temperature heat emitter: water circulates through it at between 30 and 40 °C, compared with 60 to 70 °C for an old-style radiator. This is precisely the temperature range in which an air-to-water heat pump achieves its best efficiency. The slim design of the system does not affect this compatibility: a 25 mm Biofloor Dry system operates at the same temperature as a conventional 12 cm screed.

If you haven’t yet decided between a heat pump, a boiler or underfloor heating for your project, our guide Heat pump, boiler or underfloor heating: which to choose? sets out the basics before going into detail about the installation systems.

How much does a renovation actually cost?

The price per square metre often comes as a surprise, as thin-screed systems cost more to purchase than a conventional screed. The technical complexity of the diffusion profiles and the precision required for installation account for this difference, which is offset by the shorter construction time and the absence of additional structural work.

For a 20 m² room using a dry system, expect to pay between €1,800 and €2,800, including installation but excluding the heat generator. For a 100 m² house using the thin-layer system, the cost ranges from €8,000 to €12,000, compared with €6,000 to €8,000 for a traditional system, assuming the ceiling height allows it. The difference is mainly justified by what it avoids: no need to raise thresholds, no need to alter staircases, and no weeks of drying time that bring the building site to a standstill.

As regards financial support, it is worth noting that Belgian energy-efficiency renovation grants are primarily aimed at the heat generator – with heat pumps being the main focus – rather than the underfloor heating system itself. Our article on  hydraulic versus electric underfloor heating sets out this scheme in detail, along with the current grant amounts by region.

Facq’s suggestions for renovation:

Facq distributes Comap’s Biofloor system, in both its traditional version and the Biofloor Dry version for installations with limited headroom, as well as Radson’s Rolljet system. When it comes to control, Danfoss and Honeywell Home smart thermostats regulate each room independently – a real advantage when the installation is carried out in stages. For the associated heat sources, the range includes heat pumps from Vaillant, Bulex, Remeha, Bosch and Atlantic. Where underfloor heating is not feasible – even in its slimline version – low-temperature radiators and towel warmers from Vasco, Radson and Zehnder take over without compromising on comfort.

A Facq adviser at one of the 17 EXPOcentres will work with you to measure the available height and assess the type of structure before recommending one system over another. This is a technical matter that needs to be decided on site, not from a catalogue.