Choosing the Right Filling Material for Heat and Sound Insulation
Insulation performance in steel doors largely depends on the quality of the internal filling material. A poorly chosen material can lead to both energy loss and the intrusion of external noise. In this article, we compare common filling types and their features on the market.
Polyurethane Foam: High-Performance Insulation
Polyurethane (PUR) foam is the most preferred filling material for steel doors. Thanks to its closed-cell structure, it reduces thermal conductivity (λ ≈ 0.024 W/mK) and significantly blocks sound transmission. Its fire resistance class is typically B2 or B1 (EN 13501). However, cheap, low-density polyurethane can shrink over time; therefore, we recommend a density of at least 40 kg/m³.
Rock Wool: Fire Safety Focused Solution
Rock wool (mineral wool) stands out especially in projects requiring fire resistance. It is a Class A1 non-combustible material and does not melt even above 1000°C. Its thermal insulation is slightly lower than polyurethane (λ ≈ 0.036 W/mK), but it offers similar soundproofing performance. Its disadvantages include a tendency to absorb moisture over time and its weight. It is ideal for areas not exposed to moisture.
XPS Rigid Foam: Resistant to Pressure and Moisture
Extruded polystyrene (XPS) is preferred especially for doors facing the exterior due to its water-resistant structure. Its thermal conductivity is close to polyurethane (λ ≈ 0.030 W/mK). High compressive strength helps maintain the door leaf's shape. However, its sound insulation performance is slightly lower than other alternatives, generally moderate.
Air Gap or Filling?
Some manufacturers leave the door interior empty or use cardboard honeycomb to reduce costs. While cavity structures provide some sound insulation, they are insufficient for thermal insulation. Cardboard honeycomb burns quickly in a fire and has low insulation value. As a professional choice, we recommend using a filling material with a thickness of at least 40 mm.
Installation Details and Additional Insulation Measures
- Frame seals: Magnetic or EPDM seals should be used to prevent thermal and acoustic bridges.
- Under-threshold insulation: The gap under the door can account for up to 10% of total heat loss; polyurethane tape should be added under the aluminum threshold.
- Panel interruption: Insulation material should be continuous in hinge and lock areas, avoiding metal contact.
- Layer order: Outer steel sheet → insulation filling → inner steel sheet, each layer must fit tightly together.
Recommendation: When buying a new door, choose models compliant with TS EN 1125 and TS 8173 standards, with a thermal transmittance coefficient (U-value) below 1.2 W/m²K. For sound insulation, a value of at least 30 dB (Rw) is sufficient; for commercial areas, 40 dB and above is recommended.
Making the Right Choice: Decide Based on Need
Each filling material has its advantages and limitations. If fire safety is a priority, choose rock wool; if thermal insulation is a priority, choose polyurethane; if moisture resistance is needed, choose XPS. But remember: insulation does not end with the filling material; seals, thresholds, and installation quality are at least as important as the material itself.
