A quality insulation system ensures not only thermal performance but also moisture control. Moisture that infiltrates a wall or roof and condenses can lead to mould and structural damage. Polyurethane foam (PUR) can behave either as a moisture regulator or a vapor barrier, depending on its type.
Before choosing a material, it is essential to understand the differences between open-cell and closed-cell PUR foams and to consider where the dew point lies in the assembly.
Open‑cell vs Closed‑cell PUR
Open-cell PUR. Low density ~8.0–9.6 kg/m³. Cells are interconnected, so they absorb sound well but allow water vapor to pass through. Typical water-vapour permeance ≈ 860 ng/(Pa·s·m²) at 50.8 mm thickness, and thermal resistance RSI ≈ 0.62–0.67 per 25 mm. Suit interior walls and acoustic uses where the assembly should “breathe”.
Closed-cell PUR. Much denser and stronger ~27–32 kg/m³. The sealed cell structure yields very low vapour transmission—permeance < 57 ng/(Pa·s·m²) at 50.8 mm – and higher thermal resistance RSI ≈ 1.06–1.23 per 25 mm. Acts as an integral vapour barrier, making it ideal for basements, exterior walls, flat roofs, and other moisture- or cold-prone areas.
Dew point and vapour barrier
The dew point is the temperature at which water vapour condenses. If too much moisture diffuses into an assembly or if there is no vapour barrier, the dew point can shift into the insulation layers. To prevent this, install a vapour barrier or use an insulating material that functions as one.
Closed‑cell PUR foam restricts moisture, generally keeping the dew point outside the insulation. Open‑cell PUR allows moisture to move and must be combined with a separate vapour barrier or used only in dry conditions.
Choosing a solution for the Nordic climate
In Northern Europe’s cool, moist climate, both thermal conductivity and vapour control matter. Open‑cell foam suits interior partitions, lofts, and acoustic upgrades where the construction is protected from rain and humidity. Closed‑cell PUR is preferable for external walls, foundations, flat roofs, and floors because it provides both insulation and a vapour barrier.
Conclusion
Choosing the right PUR foam is not just about R‑value; you must also consider moisture control and vapour barriers.
Open-cell foam is lightweight and breathable, but it needs a vapor barrier, whereas closed-cell foam is firm and moisture-resistant.
If you need help selecting the right solution, contact the Polimaris team. We’ll help you find the best insulation in Scandinavia or the Baltics, based on local weather conditions, and ensure proper installation.