BLOG

Milling vs Build-Up Underfloor Heating in Existing Concrete Floors

For Dutch homeowners embarking on a major renovation, the switch from high-temperature radiators to low-temperature underfloor heating is no longer a luxury—it is a strategic necessity. Driven by the national ambition to phase out natural gas and the surge in heat pump installations, the question is no longer if you should retrofit underfloor heating into your existing concrete floor, but how. The decision boils down to two distinct techniques: milling (infrezen) directly into the structural concrete slab or installing a build-up (opbouw) system on top of it. Each has a profound impact on your renovation timeline, floor height, thermal performance, and budget. This article dissects both methods in detail, giving you the knowledge to make an informed, future-proof choice.

Understanding the Two Retrofit Approaches

Before a single pipe is laid, it’s essential to grasp what these terms mean in the context of a Dutch home. Both systems circulate warm water through pipes embedded in or on the floor, but their interaction with the existing construction is radically different.

What is Milling Underfloor Heating (Infrezen)?

Milling involves cutting precise grooves, typically 16–20 mm wide and about 18–25 mm deep, directly into the top surface of an existing structural concrete floor using a specialized diamond-blade floor saw. The heating pipes (usually 14–16 mm multi-layer composite or PEX) are pressed into these grooves, and the grooves are then filled flush with a self-leveling compound or a flexible filler. The result is a heating system fully integrated into the slab, with no additional height build-up. The original floor level is maintained, which is critical in spaces where doors, thresholds, and staircases cannot be altered.

What is Build-Up Underfloor Heating (Opbouw)?

A build-up system, by contrast, leaves the concrete slab untouched and places a new layer—or a series of layers—on top of it. This can take the form of a “dry” system using pre-routed insulation panels (often cement-bonded particle board or gypsum fiber boards with integrated pipe channels) or a “wet” system where a thin layer of screed (cementitious or anhydrite) is poured over insulation and pipe clips. The total added height typically ranges from 20 mm for ultra-low-profile dry systems to 70–100 mm for a fully insulated wet screed. While adding height, this approach allows for the incorporation of acoustic or thermal insulation that may not exist in the original slab.

Milling Underfloor Heating: The Deep Dive

For renovation projects where ceiling height is precious and the structural slab is sound, milling often emerges as the superior solution. Let’s break down every aspect you need to consider before choosing this method.

Installation Process and Equipment

A professional team arrives with an industrial floor saw connected to a high-performance vacuum system. The saw cuts multiple parallel grooves in a single pass, producing an enormous amount of fine concrete dust—hence the vacuum is non-negotiable. After cutting, the grooves are thoroughly cleaned, and the pipe is laid, usually in a serpentine or double-meander pattern to ensure even heat distribution. A specialist conducts a pressure test before the grooves are filled with a flexible, polymer-modified levelling compound. Once cured, the floor is ready for the final floor covering (tiles, laminate, parquet) after a final layer of levelling compound if required. The entire process for an average living room (30–40 m²) can often be completed in 2–3 days, with the heating operational a few days later for a dry-build covering.

Structural Integrity and Minimum Slab Thickness

This is the most common concern among homeowners: “Will cutting grooves weaken my concrete floor?” The short answer is no—provided the slab meets the minimum thickness criteria. Dutch structural engineers generally prescribe a minimum existing slab thickness of 80 mm for milling, and the groove depth must not exceed 25% of the total slab thickness. For a typical 1960s–1980s Dutch woning with a 120–150 mm concrete floor on sand or a kanaalplaatvloer (hollow core slab), milling is perfectly safe. However, you must always have a constructeur assess the floor. In apartments, the VvE (Homeowners’ Association) will require proof that the milling does not penetrate the compression layer of the hollow core slabs or damage the reinforcement—a professional company provides detailed cut depth and clearance documentation.

Thermal Performance and Response Time

Because the pipes sit directly in the concrete mass, the system has a very high thermal conductivity. Concrete is an excellent heat conductor and store; the slab acts as a thermal battery. This means it heats up relatively slowly—typically 2-4 hours to reach a comfortable surface temperature—but once warm, it retains heat for a long time, making it ideal for continuous heating with a heat pump. The low flow temperature (30–35 °C) works perfectly, achieving a high Coefficient of Performance (COP). However, this thermal inertia can be a drawback in rooms that are used intermittently, as you cannot rapidly adjust the temperature.

Cost Considerations in the Netherlands

Pricing for milling in the Dutch market ranges from €55 to €85 per square meter for the basic installation, including pipe, manifold connection, pressure testing, and filling. This does not include the heat source (heat pump), manifold itself, or final floor levelling and covering. For a 60 m² ground floor, expect to invest approximately €3,500–€5,000 for the milling alone. Key cost drivers include the number of corners (complex room shapes increase labour), the need for extra levelling compound if the slab is uneven, and temporary removal of furniture. Always request an offerte that separately lists the milling, the floor restoration, and the electrical connection for the manifold pump.

Critical Limitations

Floor structure: Not possible on wooden floors or very thin concrete. Unsuitable for floors with embedded underfloor insulation beneath the slab—the milled grooves could compromise the damp-proof membrane.
Insulation retrofit: Milling alone does not improve the insulation value of the floor. If your ground floor lacks under-slab insulation (common in pre-1980 homes), you will lose significant heat downwards. In such cases, you may be required to add side-edge insulation and accept higher heat loss, or choose a build-up system with integrated insulation.
Height gain: While no build-up is added, the final floor after levelling may be 3–5 mm higher than original, which can still affect doors if tolerances are tight.

Build-Up Underfloor Heating: The Layered Approach

When the existing floor is structurally unsuited for milling, you need insulation, or you value rapid response, a build-up system is the pragmatic choice. Here we examine both dry and wet variants in a Dutch renovation context.

Dry Construction Systems (Droogbouw)

Dry systems utilize prefabricated panels, 18–25 mm thick, into which aluminium heat distribution plates and pipe tracks are factory-cut. Brands like Fermacell Therm25, Knauf GIFAfloor, or WTH Vloerverwarming are widely available in the Netherlands. The panels are laid directly on a levelled concrete slab (with a thin PE-foil separation layer). After pipe installation, the floor can be covered immediately with laminate, parquet, or vinyl—no drying time. Total height build-up: from as little as 20 mm if using integrated pipe-in-panel solutions. This makes it the fastest route to a finished floor, with installation times of 1–2 days for a team.

Advantages

No wet trades, no drying time;
Total height increase typically 20–35 mm including final covering;
Low weight—suitable for timber joist floors (with additional considerations) or apartments with load restrictions;
Fast thermal response: heats up in 30–60 minutes, ideal for bedrooms or spaces heated occasionally.

Disadvantages

Limited thermal mass: floors cool down quickly once heating stops;
Aluminium layers can creak if the substrate is not perfectly flat;
Higher material cost per m² than wet screed (€60–€100/m² for premium panels with aluminium);
Poor performance with high-temperature heat sources—optimized only for low-temperature.

Wet Screed Build-Up (Natbouw)

This traditional method applies a bonded or floating screed over the concrete slab. For floating screeds, a layer of rigid insulation (PIR, EPS, or mineral wool) is laid first, then the pipes are clipped onto the insulation, and finally a cementitious or anhydrite screed of 30–70 mm thickness is poured. Total build-up: 70–120 mm. This completely transforms the floor’s thermal properties. For a ground floor, the Dutch Bouwbesluit requires an Rc value of at least 3.7 m²K/W for new-build; in renovation, you are strongly recommended to achieve ≥ 2.5 m²K/W. A 50 mm PIR insulation layer (Lambda 0.022) gives Rc ≈ 2.3, often sufficient when combined with the existing slab’s residual insulation.

Process and Drying Times

After placing the edge insulation strip and laying the insulation boards, pipes are fixed with tacker staples or attached to a studded panel. The screed is poured and must dry for at least 28 days for cementitious screed (or 7–14 days for anhydrite with forced drying protocols). This renders the space unusable during that period and delays the final floor covering. However, the massive thermal mass delivers supreme comfort and storage capacity, perfectly complementing a warmtepomp.

Cost Structure

A wet screed build-up with insulation, pipe, and screed typically costs €80–€130 per m² including materials and labour (excl. vloerafwerking). The wide range depends on insulation thickness, screed type, and accessibility. For a 60 m² floor, budget €5,000–€8,000. Dry systems are slightly more expensive in material but save on screed labour, often coming in at €90–€120 per m² for the complete underlayment ready for covering.

Direct Comparison: Milling vs Build-Up

The table below synthesizes the key decision-making parameters for a Dutch renovation. Use this as a quick guide, but always validate with your installer and the specific conditions of your project.

Criteria	Milling (Infrezen)	Build-Up Dry (Droogbouw)	Build-Up Wet (Natbouw)
Added Floor Height	0–5 mm	20–35 mm	70–120 mm
Thermal Inertia	Very High (slow response, long storage)	Low (fast response, quick cooling)	High (moderate response, good storage)
Ideal Heat Source	Heat pump (continuous low temp)	Heat pump (modulating, low temp)	Heat pump or HR-ketel (any low temp)
Insulation Upgrade	None (relies on existing under-slab)	Possible if special insulated panels used	Integral and essential
Structural Load	No added dead load	Low: 10–15 kg/m²	High: 80–150 kg/m² depending on screed and insulation
Installation Speed	2–4 days (plus floor prep)	1–2 days (immediate covering)	5–8 days (plus 4-week drying)
Floor Suitability	Only solid concrete ≥80 mm thickness	Concrete, timber joists (with caution), existing tiles	Concrete (with load assessment)
Typical Cost (per m² installed)	€55–€85	€90–€120	€80–€130
Suitable for VvE (Apartment)	Often allowed with structural report	Usually allowed (low impact)	Rarely allowed due to weight and height

Dutch Building Regulations and Practical Considerations

When renovating, you must be aware of the following points that specifically affect Dutch homes:

Insulation Requirements (Bouwbesluit)

If you replace or renovate more than 25% of a floor, the minimum Rc-value for that floor must be brought up to modern standards (currently Rc ≥ 3.7 m²K/W). However, the “renovation exception” allows a lower value of 2.5 m²K/W when full compliance is technically unfeasible. When milling, you are not replacing the floor, merely modifying it; thus the insulation requirement often does not apply. But if you choose a build-up system with a new screed, the municipality may consider it a thermal renovation, triggering the insulation obligation. Always check with your gemeente or a building inspector before starting.

Door Thresholds and Ceiling Height

Dutch homes, particularly apartments in cities like Amsterdam or Utrecht, often have ceiling heights of 2.50 m or less. The Bouwbesluit does not prescribe a minimum ceiling height for existing buildings, but removing 10 cm for a wet build-up can make a room feel oppressive and violates practical comfort norms. In such cases, milling is the only way to preserve headroom. Measure your existing door thresholds; they are typically 20–30 mm. A dry build-up of 25 mm will eat that entirely, requiring all doors to be trimmed, which adds €50–€80 per door to your budget.

VvE Approval for Apartments

In an apartment complex, the concrete floor is often shared structural property. Milling requires explicit written consent from the VvE and usually a structural engineer’s report. The argument you must make is that the grooves are well within the top cover layer and do not compromise the floor’s load capacity. A dry build-up often raises fewer objections because it adds minimal weight and can be removed without damaging the structure. Always present your plan with technical drawings and an installer’s declaration.

Practical Tips for a Successful Retrofit

Commission a structural assessment first. Hire an independent engineer to measure slab thickness, locate reinforcement, and test concrete quality. This €300–€500 investment can save you from costly damage or VvE disputes.
Verify floor flatness. For milling, the slab must be reasonably level (within 5 mm over 2 meters). For dry build-up, the substrate flatness tolerance is even tighter (±2 mm) to prevent panel movement and noise. Budget for a self-leveling underlayment if needed.
Do not ignore under-slab insulation. If your ground floor has no insulation, milling alone will result in 15–25% downward heat loss. Mitigate by adding high-performance edge insulation at the floor perimeter (at least 100 mm width) and accepting a slightly higher flow temperature. Better yet, consider a build-up with integrated insulation if height allows.
Plan the manifold location carefully. Both systems need a manifold, preferably centrally located to minimize pipe runs and pressure loss. In a milling project, the manifold can be wall-recessed; in a build-up, it may be placed in a new technical cabinet. Ensure a dedicated 230V power supply within 1 meter.
Choose your floor covering with heat resistance in mind. The total thermal resistance of the floor covering should not exceed 0.15 m²K/W for optimal performance. Avoid thick carpets with high tog ratings. Use a suitable underlay for parquet or laminate specifically designed for underfloor heating.
Pressure test before covering. Whether milled or built-up, insist on a 24-hour pressure test at double the operating pressure before any filler, screed, or panels are placed. Document the results for your peace of mind.
Engage a BENG-aware installer. With the shift to nearly zero-energy standards (BENG), your heating system must work at 35 °C or below. An installer experienced in low-temperature design will calculate the exact pipe spacing (10–15 cm for milling, 10–15 cm for build-up), flow rates, and manifold balancing to ensure your heat pump achieves a SCOP above 4.0.

Conclusion: Which Method Should You Choose?

The choice between milling and building up does not have a universal answer; it is a function of your home’s physics, your renovation boundaries, and your long-term energy plan. Summarizing:

Choose milling (infrezen) when you have a sufficiently thick, structurally sound concrete floor, no height to lose (low ceilings, many doors), and your ground floor already has adequate under-slab insulation. This method preserves your existing thresholds and is usually the more cost-effective option when structural conditions allow. It integrates seamlessly with a warmtepomp and delivers robust, stable heating.
Choose a dry build-up if you need a lightweight solution, your floor cannot be milled, you require a fast installation without drying times, or you need a responsive heating system for intermittent-use rooms. It is the method of choice for apartments where weight constraints are critical, and it can be executed with minimal disruption.
Choose a wet build-up with screed when you must correct floor insulation levels significantly, you accept the longer construction time, and you have the ceiling height to sacrifice. The massive thermal storage is excellent for night-setback strategies with a heat pump, and it provides the most comfortable, even heat distribution over the long term.

Whichever path you select, never compromise on the quality of the pipe and manifold, engage a certified installer (preferably a member of the Vloerverwarming en Koeling branchevereniging), and ensure the design is hydraulicly balanced. A well-executed system will not only warm your feet but will turn your concrete floor into a silent, invisible, and highly efficient heater, aligning perfectly with the Netherlands’ sustainable future.

Usefull Articles