The short answer
- Closed cell is dense, rigid and highly resistant to water vapour, with the lower lambda. Specify it for single-skin metal and fibre cement roofs, condensation control, asbestos cement undersides, damp environments and anywhere the foam adds strength. Examples: Purios H (λ 0.0224), Enverge Nexseal LE (λ 0.027), Purios HR for roofing.
- Open cell is light, soft and vapour-open, with a higher lambda. Specify it for lined internal spaces, timber-framed or rafter-level applications designed to breathe, and acoustic benefit. Examples: Purios F (λD 0.036), Enverge Sucraseal (λ 0.039).
- The deciding factor is usually moisture, not lambda. Where warm, moist air can reach a cold surface, a vapour-open foam can let condensation form behind it. Closed cell prevents that at the sheet.
- Thickness follows lambda. To reach 0.25 W/m²K, the foam layer needs about 86 mm of Purios H but about 151 mm of Sucraseal.
The difference in one paragraph
Both types are two-component polyurethane foams, mixed at the spray gun and expanding as they are applied. The difference is the cell structure. In closed-cell foam, most of the tiny cells are sealed. The gas inside does the insulating, and the sealed walls make the foam rigid and resistant to air, water and water vapour. In open-cell foam, most cells are broken open. The foam is softer, much lighter and much more permeable to water vapour, and it insulates with trapped air. That one difference drives everything else: lambda, vapour resistance, strength, water uptake, weight and where each foam belongs.
Published data compared
The figures below are typical published values from each product's technical data sheet. Manufacturers state that typical values vary and are not part of the product specification. Always work from the current data sheet and Declaration of Performance for a design.
| Product | Type | Thermal conductivity λ (W/m·K) | Vapour resistance factor μ | Compressive strength | Water absorption | Density |
|---|---|---|---|---|---|---|
| Purios F | Open cell (90–95% open) | λD 0.036 | ≥ 5 | ≥ 10 kPa | not declared | 9–12 kg/m³ |
| Enverge Sucraseal | Open cell | 0.039 | 8.8 | 34 kPa | 9.3 kg/m² | not stated |
| Enverge Nexseal LE | Closed cell, low emission | 0.027 (at 125 mm) | 62 | 78 kPa | 0.41 kg/m² | not stated |
| Purios H | Closed cell (≥90% closed) | 0.0224 | ≥ 46 | ≥ 200 kPa | ≤ 0.29 kg/m² | 35–40 kg/m³ |
| Purios HR | Closed cell roofing (≥90% closed) | 0.0233 | ≥ 54 | ≥ 300 kPa | ≤ 0.14 kg/m² | min 50 kg/m³ |
Sources: Enverge Sucraseal TDS V1 and Nexseal LE TDS V2 (EN 14315-1 declared methods); Purios F TDS 10/2018, Purios H TDS 20/2023 and Purios HR data sheet. Enverge Nexseal 2.0 is the closed-cell grade used for build-ups under 50 mm.
Reading the table
- Lambda: closed cell insulates better per millimetre. Purios H at 0.0224 needs well under two-thirds the thickness of Sucraseal at 0.039 for the same result.
- μ (vapour resistance factor): the higher the number, the more the foam resists water vapour. Open cell at single figures is vapour-open. Closed cell at 46–62 is highly resistant, and in practical thicknesses it acts as a vapour control in its own right.
- Compressive strength: closed cell is structural enough to stiffen sheets, and Purios HR is made for roof surfaces. Open cell is soft and must be protected from impact.
- Water absorption: open cell takes up water. Closed cell resists it, which matters wherever the foam might get wet.
Both Enverge and Purios foams are CE marked with published data sheets. Purios H uses a new-generation foaming agent with zero ozone depletion potential and low global warming potential.
How much thickness each needs
Lambda translates directly into depth. Here are indicative foam-layer thicknesses for TGD L 2022 targets:
| Target U-value | Purios H | Purios HR | Nexseal LE | Purios F | Sucraseal |
|---|---|---|---|---|---|
| 0.35 W/m²K (existing walls, Table 10) | 60 mm | 63 mm | 73 mm | 97 mm | 105 mm |
| 0.25 W/m²K (existing sloped or flat roof, Table 10) | 86 mm | 90 mm | 104 mm | 139 mm | 151 mm |
| 0.20 W/m²K (new flat roof, Table 1) | 109 mm | 113 mm | 131 mm | 175 mm | 190 mm |
| 0.16 W/m²K (new pitched roof, Table 1) | 137 mm | 142 mm | 165 mm | 220 mm | 238 mm |
Indicative only: foam layer with surface resistances of 0.14 (roofs) and 0.17 (walls), rounded. It ignores the substrate, framing, fixings and thermal bridging; your designer calculates the finished element.
Depth matters differently depending on the application. Between rafters or studs, the available depth may limit a closed-cell build. On the underside of a warehouse roof, space is rarely the constraint, but material cost and application time scale with thickness. Closed cell is applied in thinner passes: 10–25 mm for Purios H and 10–20 mm for Purios HR. Open cell goes on much thicker per pass, with Purios F at 60–100 mm.
Not sure which foam your build-up needs?
Send us the construction and the target U-value. The technical team will recommend a grade and arrange a free survey.
Moisture: why it usually decides the choice
In Ireland's damp climate, most spray foam problems are moisture problems. The question to ask is: where will warm, moist air meet a cold surface, and what stops it?
Single-skin metal and fibre cement roofs: closed cell
The sheet is cold, and there is nothing behind it. A vapour-open foam sprayed on it lets moist air diffuse through to the cold sheet, where it can condense behind the foam. That leaves wet insulation, corroded steel and drips. Closed-cell foam bonds to the sheet, leaves no void and resists vapour, so the problem does not start. This is why closed cell is the default for warehouses, farm buildings, workshops and asbestos cement undersides.
Lined, heated spaces designed to breathe: open cell can fit
Where a roof or wall has an outer layer that lets vapour out, such as a breathable membrane or ventilated void, and an inner finish, open cell can form part of a vapour-open design. The build-up still needs a condensation risk assessment to the EN ISO 13788 method. The data sheets note that an air and vapour control layer may be needed, depending on the construction and internal humidity.
High-humidity buildings: closed cell and careful design
Pools, food processing, livestock housing, laundries and cold stores push a lot of moisture at the envelope. Closed cell is normally specified, and the design is checked, not assumed.
The rule of thumb: if the foam goes directly onto a cold, impermeable surface with no ventilated gap behind it, specify closed cell. If you are designing a breathable construction, open cell may suit it, but only after a condensation risk assessment.
Where each type belongs
| Application | Usually specify | Why |
|---|---|---|
| Underside of single-skin metal roof (warehouse, workshop, shed) | Closed cell: Purios H or Nexseal LE (2.0 under 50 mm) | Condensation control at the sheet, air sealing, stiffening |
| Underside of asbestos cement sheets | Closed cell: Nexseal 2.0 / LE | Self-bonds with no fixings through the sheet; encapsulates and insulates |
| Livestock and grain buildings | Closed cell | High moisture; drips must stop at the sheet |
| External roof foam under a waterproof top coat | Closed cell roofing grade: Purios HR (or Gaco roofing foam) | Compressive strength, low water absorption, roof-surface duty |
| Metal containers, tanks, cold surfaces | Closed cell | Cold impermeable surface; vapour resistance |
| Lined internal walls and ceilings in heated buildings | Open cell: Purios F or Sucraseal (subject to design) | Lower weight, fills voids, vapour-open design, acoustic benefit |
| Timber rafter or stud cavities in a breathable build-up | Open cell (subject to condensation risk assessment) | Vapour-open, flexible, cost per volume |
| Acoustic separation of internal spaces | Open cell | Soft open structure absorbs sound |
For commercial and agricultural buildings, most of what we see in Ireland is the first four rows. That is why most of our spray foam work is closed cell.
Common myths about open and closed cell foam
"Closed cell is always the better product"
Closed cell is better per millimetre and on cold, impermeable surfaces. It is not automatically better for a breathable, lined construction, where a vapour-open layer may be exactly what the design needs. Better means "right for the build-up", not "denser".
"Open cell is cheaper, so it saves money"
Open cell covers more volume per set, but it needs much more thickness for the same U-value. On a bare metal roof it can also cause the condensation problem you are trying to solve. Compare products on the cost of reaching the target U-value with the moisture risk controlled, not on the cost of a set.
"Spray foam makes roofs rot"
Foam applied to the wrong build-up can trap moisture. Usually that means open cell on a cold, impermeable sheet, or any foam over an unresolved leak. Closed-cell foam on a sound single-skin sheet does the opposite: it removes the cold, damp void where rot and corrosion start. The problem is specification, not the material.
"Foam will glue the roof so it can never be replaced"
Closed-cell foam does bond to the sheet, and that bond is what gives it air sealing and stiffness. When a roof eventually reaches the end of its life, the sheets and the bonded foam come away together as the roof is stripped. Plan the waste route for that at the time, as you would for any insulated roof.
"Any foam will do as long as it is thick enough"
Thickness only answers the U-value question. Vapour resistance, water absorption, strength, fire classification and application conditions all differ between grades, and each matters for a particular building. That is why a specification names the product, not just "spray foam".
Fire, site conditions and writing the specification
Fire
Both types are organic, combustible insulants. The reaction-to-fire classification of each product is declared to I.S. EN 13501-1, and TGD B – Fire Safety decides whether the foam can be exposed or needs a lining or thermal barrier in your building. Purios H declares Euroclass E. Check the current Declaration of Performance for each product you specify, and do not compare classifications from different test standards as if they were the same.
Site conditions
Spray foam is a chemical reaction on site, so conditions matter. The substrate must be clean and dry. Purios HR's data sheet requires substrate and air temperatures of at least 15 °C and relative humidity no higher than 60%. The installer checks conditions before and during application, and in an Irish climate that sometimes means heating a building or waiting for the right day.
What a good specification states
- the product by trade name, not "closed-cell foam"
- the thickness for each element and the U-value it achieves
- the areas, including profile allowances for corrugated sheets
- substrate preparation and masking
- any lining or thermal barrier required by the fire strategy
- the documents at handover: data sheet, Declaration of Performance, installation record and manufacturer-backed product warranty
If you are writing a specification and are not sure which grade fits, send us the build-up. The technical team will review it before anyone quotes. Surveys are arranged through approved installers across the Republic, including County Dublin, County Cork, County Galway and County Kildare.