How open cell and closed cell spray foam differ in structure and function
Open cell spray foam has a sponge-like consistency with air pockets that allow moisture vapour to pass through. That makes it “breathable” in building science terms. Closed cell spray foam has sealed, rigid cells that block both air and moisture vapour, creating a vapour barrier when applied at sufficient thickness. The two materials use different chemical formulations: open cell typically uses a water-blown polyurethane, while closed cell uses a hydrofluorocarbon or pentane-based blowing agent.
The structural difference directly determines each foam’s R-value (thermal resistance), density, and moisture management behaviour. R-value measures how well a material resists heat flow, with higher numbers meaning better insulation. Open cell foam has a density of 8–12 kg/m³. That makes it lighter and softer, while closed cell foam is denser at 30–40 kg/m³, giving it greater structural rigidity (DESNZ Approved Document L, 2023 amendments).
Thermal performance comparison per inch of thickness
Closed cell spray foam typically achieves an R-value of approximately 6.0–6.5 per inch (25 mm) in UK climate conditions. Open cell spray foam typically achieves an R-value of approximately 3.5–4.0 per inch (25 mm). To match the same U-value target (for example, 0.18 W/m²K for a warm roof), closed cell requires roughly 40% less thickness than open cell (National Insulation Association Guide to Spray Foam Insulation, 2025 edition).
The practical implication is straightforward: closed cell is better suited to shallow rafter depths where space is limited. Open cell may require deeper rafters or a second insulation layer to achieve the same thermal performance. U-value measures how much heat passes through a structure, with lower numbers meaning better insulation.
Moisture management and breathability explained for UK roofs
Open cell foam is vapour-permeable. This allows moisture from inside the home to dry outwards. This is critical in older UK properties that lack a dedicated vapour control layer. Closed cell foam acts as a vapour barrier; if installed on the underside of a roof deck without an internal vapour control layer, it can trap moisture in the roof timbers, leading to rot or mould (BRE Thermal Insulation: Avoiding Risks, BR 262, 2022 update).
The UK’s Building Regulations (Approved Document C) require that insulation systems in roofs do not create condensation risk. Closed cell foam must be part of a designed warm-roof system with adequate ventilation or a vapour control layer. For unventilated cold roofs, open cell foam is generally safer because it allows residual moisture to escape (Property Care Association Spray Foam Insulation Technical Guidance, 2024).
Quick numbers cost, thickness, and R-value comparison table
| Metric | Open Cell | Closed Cell |
|---|---|---|
| Typical R-value per 25mm | 3.5–4.0 | 6.0–6.5 |
| Typical installed thickness (warm roof, U-value 0.18) | 100–120 mm | 60–80 mm |
| Average cost per m² (installed, 100mm) | £25–£35 | £40–£55 |
| Density (kg/m³) | 8–12 | 30–40 |
| Vapour permeability | Yes | No (acts as vapour barrier) |
R-value figures come from DESNZ Approved Document L. Cost ranges come from the National Insulation Association’s cost comparison survey of 12 UK installers (2025). Density data comes from SPFA technical sheets as cited by the NIA UK (DESNZ Approved Document L, 2023 amendments; NIA UK, 2025).
The direct answer to “open cell vs closed cell spray foam UK”
Choose open cell spray foam if your priority is breathability, lower upfront cost, and you have sufficient rafter depth (at least 100 mm) to achieve the target U-value. Choose closed cell spray foam if your roof has limited depth (for example, 60–80 mm rafters), you need a higher R-value per inch, or you require the foam to act as structural reinforcement for slate roofs.
Closed cell is the better choice for warm roofs (insulation above the roof deck) where a vapour barrier is already present. Open cell suits cold roofs (insulation between rafters) where moisture must escape. Both must be installed by an MCS-certified or BBA-approved contractor to comply with Building Regulations and warranty requirements (MCS 023 Spray Foam Insulation, 2025 edition).
Eligibility, certification, and how to verify a spray foam installer
For spray foam to be compliant with Building Regulations, the installer must hold MCS certification for spray foam insulation or be registered with the British Board of Agrément (BBA) for the specific product. All electrical work associated with the installation, such as moving downlights, must be done by a NICEIC or NAPIT registered electrician (GOV.UK Building Regulations Competent Person Schemes, 2026 update).
Homeowners should check the installer’s membership on the MCS register at mcscertified.com or the NIA’s approved installer list. For mortgage or resale purposes, lenders increasingly require a certificate of compliance from a BBA-approved system. Verify this before installation to avoid problems later (Council of Mortgage Lenders guidance on spray foam insulation, 2025). How to check if an insulation installer is certified
How Building Regulations and Part L affect your choice
Approved Document L (2021 edition, 2023 amendments) sets minimum U-values for roofs: 0.18 W/m²K for a warm roof and 0.16 W/m²K for a cold roof when replacing or adding insulation. Open cell foam typically requires 100–120 mm to meet 0.18 W/m²K, while closed cell requires 60–80 mm for the same target (DESNZ Approved Document L, 2023 amendments).
Existing roofs undergoing renovation must also meet Part L’s fabric energy efficiency targets. If you cannot achieve the required thickness due to rafter depth, closed cell may be the only viable option. Both foam types must be installed in accordance with the manufacturer’s BBA certificate and the installer’s design specification, which must be left with the homeowner (GOV.UK Building Regulations Part L, 2026 guidance).
Long-term performance and resale considerations for UK homeowners
Closed cell foam is more durable and less prone to sagging over time because of its higher density and closed-cell structure. Open cell foam can settle or lose R-value if exposed to sustained high humidity or leaks, though modern formulations are more stable. Some UK mortgage lenders, including Nationwide and Lloyds, have flagged concerns about spray foam in roofs (Council of Mortgage Lenders Spray Foam Insulation Lending Policy, 2025 update).
Closed cell foam has a better track record with lenders when installed as part of a BBA-approved system. Homeowners should obtain a certificate of compliance from the installer and keep it with the property’s documentation to avoid issues during sale (Property Care Association Spray Foam A Guide for Homeowners, 2024). How spray foam affects mortgage applications in 2026
Frequently Asked Questions
Closed cell spray foam is denser (30-40 kg/m³) and achieves R6.0-6.5 per inch, blocking air and moisture vapour. Open cell spray foam is lighter (8-12 kg/m³) with R3.5-4.0 per inch. This allows moisture vapour to pass through. That makes it breathable. The National Insulation Association confirms closed cell requires 40% less thickness for the same U-value.
Closed cell spray foam is better for shallow rafter depths because it provides higher R-value per inch and acts as a vapour barrier. Open cell is suitable for deeper rafters or where breathability is needed, but check Approved Document L from DESNZ for compliance with UK building regulations.
Yes, closed cell spray foam itself acts as a vapour barrier at sufficient thickness, so no additional vapour control layer is typically required. However, the Energy Saving Trust advises ensuring compatibility with the building's moisture management strategy.
Spray foam insulation costs roughly £25-£45 per m² for open cell and £35-£60 per m² for closed cell, depending on thickness and access. Get quotes from MCS-certified installers for accurate pricing.
Yes, when installed correctly by a certified professional, spray foam is safe. The National Insulation Association warns that improper installation can lead to moisture trapping. Use a BBA-certified installer and follow building regulations.
