Open cell vs closed cell spray foam is not a quality ranking. The two are different materials made from the same polyol and isocyanate chemistry. They solve different problems in the building envelope.
Open-cell foam is light, soft and vapour-open. It is mainly an acoustic and thermal fill for interiors. Closed-cell foam is denser and rigid. It resists water and vapour and carries some load.
This article compares them using real values from Krypton Chemical's Rayston Spray Foam technical data sheets (TDS). It does not use generic "typical" numbers.
How spray polyurethane foam works
Spray polyurethane foam (SPF) is made on site by mixing a polyol blend (component A) with an isocyanate (component B) in a plural-component spray gun. A blowing agent expands the mixture within seconds.
Two things decide whether the cells end up open or closed:
- The blowing agent. Water reacts with isocyanate to make CO₂, which tends to escape and leave open cells. Physical blowing agents such as HFO stay trapped in closed cells.
- The formulation and density. Low-density foams are generally open-cell. Higher-density foams are closed-cell.
The trapped gas is why closed-cell foam insulates better per millimetre. For a general overview, see the Wikipedia article on spray foam.
Open cell vs closed cell spray foam: TDS data
| Property (TDS) | Spray Foam 8W (open cell) | Spray Foam HFO 35 | Spray Foam HFO 40 | Spray Foam HFO 50 |
|---|---|---|---|---|
| Cell structure | ~95% open cells (ASTM D2856) | above 90% closed | above 90% closed | above 90% closed |
| Blowing agent | CO₂ from water reaction | HFO | HFO | HCFO-1233zd(E) |
| Applied density | 8–10 kg/m³ | 30–40 kg/m³ | 42–47 kg/m³ | 48–55 kg/m³ |
| Declared aged λ at 100 mm | 0.038 W/m·K | 0.027 W/m·K | 0.027 W/m·K | 0.026 W/m·K |
| Thermal resistance at 100 mm | 2.63 m²K/W | 3.70 m²K/W | 3.75 m²K/W | 3.80 m²K/W |
| Vapour resistance factor μ | 4.6 | ≥ 80 | ≥ 80 | 80 (CE code MU80) |
| Short-term water absorption | — | ≤ 0.2 kg/m² | ≤ 0.2 kg/m² | 0.2 kg/m² (CE code W0,2) |
| Compressive strength | — | — | — | above 200 kPa (stated application) |
Sources: Rayston Spray Foam 8W TDS, HFO 35 TDS, HFO 40 TDS, HFO 50 TDS. A dash means the value is not stated in that TDS.
The closed-cell grades carry CE marking to EN 14315-1 and are classed Reaction to Fire Class E for all thicknesses.
What the numbers mean in practice
To reach about 3.7 m²K/W:
- Closed-cell HFO 35 needs about 100 mm (3.70 m²K/W per its table).
- Open-cell 8W needs about 140 mm (3.68 m²K/W per its table).
Where depth is limited, such as a rafter zone or a retrofit wall, that difference often decides the choice.
When to choose open-cell foam
The 8W TDS lists internal thermal and acoustic insulation of ceilings, attics, timber walls, masonry and steel structures, and lightweight buildings.
Its strengths are:
- Acoustics. The TDS reports sound insulation of 40 dB at 195 mm plus one 13 mm plasterboard, and 44 dB with two boards (EN 10140-2).
- Vapour openness. μ = 4.6 lets the construction dry, which can suit timber.
- Low density. 8–10 kg/m³ adds very little weight.
- Blowing agent. CO₂ from the water reaction only (ODP = 0, per the TDS).
- Indoor air. VOC emissions Class A (ISO 16000).
Its limits: it is not a water barrier, and it should not be left exposed on a roof.
When to choose closed-cell foam
The HFO grades are intended for direct spraying on surfaces that do not carry permanent loads: interior walls and ceilings, and façades. The TDS notes that on façades the foam "provides waterproofing to water, maintaining some transpiration".
Choose closed-cell foam when you need:
- More R per mm. About 0.026–0.028 W/m·K aged.
- Moisture control. μ ≥ 80 and water absorption ≤ 0.2 kg/m².
- Rigidity. HFO 50 is designed for applications needing compressive strength above 200 kPa.
- Adhesion and air sealing on irregular substrates, including metal roofs.
A water-blown rigid option also exists. The Rayston Spray Foam 50W TDS declares a thermal conductivity of 0.030 W/m·K and no HFC or HCFC content.
Protecting foam on roofs
Spray foam on an exposed roof needs a waterproof, UV-stable skin. Krypton's membranes list adhesion to polyurethane foam in their data sheets. For example, the Polyurea Rayston TDS gives more than 1.5 MPa on 150 kg/m³ high-density PU foam (foam failure). Impermax 2K lists "protective finish of polyurethane insulating foam" among its applications.
See the systems for thermal and acoustic insulation of roofs, metal roofs and industrial roofs.
Application conditions checklist (HFO grades, TDS)
- [ ] Component temperature 30–60 °C (HFO 35/40), 25–45 °C (HFO 50)
- [ ] Working pressure 60–120 bar
- [ ] Ambient 0–40 °C, RH below 90%, wind below 30 km/h
- [ ] Substrate 5–40 °C with no surface condensation
- [ ] Mix ratio 100:100 by volume
- [ ] Respiratory protection and ventilation, since component B contains isocyanates
The Spray Polyurethane Foam Alliance publishes industry guidance on safe SPF installation and re-occupancy.
Key takeaways
- In open cell vs closed cell spray foam, closed cell gives more R per mm (0.026–0.028 vs 0.038 W/m·K aged).
- Open cell (8–10 kg/m³, μ 4.6) is a light, vapour-open thermal and acoustic fill for interiors.
- Closed cell (30–55 kg/m³, μ ≥ 80) controls moisture and adds rigidity.
- Exposed roof foam needs a waterproof, UV-protected membrane on top.
FAQ
Which is better, open cell or closed cell spray foam?
Neither in general. Closed cell is better where space is tight or moisture control matters. Open cell is better for acoustic fill and vapour-open timber construction.
What is the R-value of closed cell spray foam per inch?
The TDS values are given in metric. At 100 mm, HFO 35 declares 3.70 m²K/W, which is roughly 0.94 m²K/W per 25 mm. Always use the declared aged value from the product sheet.
Can open cell spray foam be used on a roof?
Only on the inside, as insulation under the deck. It is vapour-open and not water-resistant, so it cannot act as an exposed roof layer.
Does spray foam need a vapour barrier?
It depends on the climate and the build-up. Closed-cell foam has high vapour resistance itself (μ ≥ 80 here). Open-cell foam (μ 4.6) often needs a separate vapour control layer, so run a hygrothermal check.
Further reading
- Insulation and roof systems at Krypton Chemical Solutions.
- Product catalogue at kryptonchemical.com.
About the author
Krypton Chemical, S.L. Marketing Department, Nidal Malkawi
Krypton Chemical has manufactured construction chemicals in Spain since 1999. Technical resources: kryptonchemicalsolutions.com · Products: kryptonchemical.com
