Open-cell vs closed-cell spray foam in a loft

## The difference that matters to a homeowner Open-cell and closed-cell spray foam have different physical properties. That can affect how moisture moves through an installation, how firmly the foam adheres and how removal is approached. It does not tell you, on its own, whether your roof is damaged, whether a lender will accept it or whether removal is necessary.

If you have just discovered foam in a loft, start with the installation records and the reason you need an answer. An active leak, a mortgage valuation query and a routine insulation review are different problems. Each needs evidence about the actual roof rather than a verdict based on the words “open-cell” or “closed-cell”.

This guide concerns foam applied to existing pitched roofs. A designed new-build roof or a converted loft may have a different construction and specification. ## Open-cell and closed-cell compared The [Energy Saving Trust's spray foam guide](https://energysavingtrust.org.uk/spray-foam-insulation/) describes the broad material differences.

Individual products and installations vary; a product's technical documentation is more useful than a generic label. | Question | Open-cell foam | Closed-cell foam | |---|---|---| | Typical structure | Lighter, softer and more flexible | Denser, more rigid and strongly bonded | | Water-vapour behaviour | Generally more vapour-permeable | Generally less vapour-permeable | | Insulation per thickness | Usually needs more thickness for a comparable thermal result | Often provides more thermal resistance per thickness | | Inspection | Can conceal timber, underlay and junctions | Can conceal the same details and be difficult to separate from them | | Removal planning | Careful internal removal may be appropriate | Bonding and access may make a different approach necessary | “More permeable” does not mean an installation can never hold moisture against timber.

“Less permeable” does not mean a roof must be defective. Neither description proves that the roof has an appropriate air and vapour control layer, ventilation arrangement or water-shedding covering. ## Why the whole roof build-up matters A roof is a system. The covering keeps out rain; the layers beneath it, the insulation arrangement and the movement of air and water vapour all interact.

A competent assessment should establish, as far as access allows: - What the foam was applied to: an underlay, board, isolating layer or roof covering. - Whether the roof covering, flashings and penetrations are weather-tight. - Which timber surfaces and joints remain visible. - How moisture from the home is controlled and where extract ducts discharge.

- How the roof was intended to ventilate, where ventilation is required. - Whether the installed thickness and coverage match a documented design. The [Building Safety Regulator's moisture-risk research](https://www.gov.uk/government/publications/moisture-risk-of-spray-foam-insulation-applied-to-timber-sloped-roofs) modelled different roof assemblies.

Results depended on the foam, underlay, climate and vapour-control arrangements. Direct application to roof tiles was particularly concerning in the modelled scenarios. The study is not a diagnosis of an individual home, and it does not justify declaring every roof of one foam type safe or unsafe. [[IMAGE src="/images/spray-foam/evohome-roof-junction-inspection-detail.

jpeg" alt="Exposed timber and roof junction after removal" caption="Exposing a junction can make its construction easier to inspect. This photograph is a project record, not a moisture test or structural assessment."]] ## Can you identify the foam yourself? Start with paperwork: the invoice, product name, installer details, installation date, specification, suitability assessment and any warranty.

Ask for the manufacturer and product reference, not simply a description such as “breathable foam”. A certificate should relate to the actual product and use. Colour, texture and photographs can help explain what is present, but they do not reliably establish a full product specification. Ageing, lighting, surface treatments and mixed applications can mislead.

A surveyor or contractor may need closer inspection or further evidence before reaching a view. Do not pull foam away to test its feel. You could damage an underlay, disturb wiring or expose a roof defect without a plan to make it weather-tight. Only take photographs from somewhere you can safely access; you do not need to enter an unsafe loft to make an enquiry.

## What can an inspection establish? An inspection should answer your question and explain its limits. Useful findings distinguish between something directly observed, a test result, a possible explanation and an area that could not be inspected. For example, an accessible rafter may look satisfactory while its covered face remains unknown.

A stain can show where water has travelled without establishing whether the timber has decayed. Moisture readings need interpretation alongside the material, conditions and test method. An attractive “after” photograph is useful evidence of exposure, but is not a substitute for those findings. The [RICS consumer guide](https://www.

rics.org/consumer-guides/spray-foam-insulation) emphasises assessing the property's circumstances and obtaining suitable professional advice. Before commissioning a report, explain whether you need a condition assessment, advice about removal or evidence for a particular lender. Ask who will inspect, what qualifications are relevant, which areas they can reach and what the report will cover.

[[BEFORE_AFTER before="/images/spray-foam/october-project/loft-6286.webp" after="/images/spray-foam/october-project/loft-6507.webp" caption="The same EvoHome project before and after removal. The exposed view helps inspection; it does not, by itself, certify the whole roof."]] ## Does the type change the removal method?

Yes, it can. Softer foam may allow careful removal from inside the loft where access and the roof construction are suitable. More rigid, strongly bonded foam can be harder to separate from an underlay or covering without damage. On some roofs, work from above, removal of coverings or a wider repair and reinstatement plan may be necessary.

That is a possibility to assess, not an automatic requirement for every closed-cell installation. Conversely, describing foam as open-cell is not enough to promise an easy or damage-free internal removal. A written proposal should explain the intended method, protected areas, accessible scope, likely limitations and how discoveries will be handled.

At EvoHome, selected areas are opened at the start of agreed work, with checks continuing as the roof is exposed. Those checks inform the job; a small opened area cannot prove the condition of all hidden timber. ## What happens to the insulation afterwards? Agree the future insulation arrangement as part of the scope.

Removing foam is not the same as completing a roof upgrade. The next arrangement needs to suit how the space is used and the roof's construction, with ventilation and moisture control considered together. Ask which areas will be insulated, what product is proposed, how access to services is retained and which repairs need to happen first.

Roof repairs and replacement insulation should be itemised or clearly identified as separate work. You can compare the commercial details in our [removal-cost guide](/blog/how-much-does-spray-foam-removal-cost-uk). ## What about a mortgage or house sale? Do not assume a lender treats every installation of the same type alike.

Send the relevant paperwork to your broker or lender and obtain the requirements for your particular application. If a report is requested, confirm the required professional qualifications and scope before paying for it. Removal, a product certificate or a survey cannot guarantee a lending decision. Our [mortgage guide](/blog/spray-foam-insulation-mortgage-problems) sets out a practical sequence for gathering the right evidence without commissioning work twice.

## A useful next step Write down the product details you have, any signs of water entry and what prompted the enquiry. Keep the original documents together. That is a better starting point than trying to decide the outcome from cell type alone. EvoHome's own teams undertake removal. Every EvoHome removal includes an [independent professional final survey](/services/spray-foam-removal/independent-survey).

We can explain the proposed removal scope, any further assessment needed and what remains uncertain. [Tell us about your property](/quote?service=spray-foam-insulation-help&source=open-cell-vs-closed-cell-spray-foam); photographs are helpful but optional. Is open-cell foam always safer for timber? No. Moisture behaviour depends on the roof assembly, water entry, ventilation and vapour control as well as the foam.

A cell-type label cannot establish timber condition. Does closed-cell foam always need a new roof? No. Strong adhesion can make removal more involved, but the appropriate method depends on the substrate, access and actual condition. Ask for a roof-specific explanation before agreeing wider work. Can I tell the type from a photo?

A photograph can assist an enquiry but is not a reliable product identification. Installation records and a suitable inspection may be needed. Should I remove foam before arranging an assessment? First establish the question you need answered. Unplanned removal can destroy evidence or create repair needs. Agree any opening-up with a competent professional and confirm lender requirements where relevant.

Is the final survey the same as a guarantee? No. An independent final survey records findings within its scope and access limitations. It does not guarantee the future condition of a roof or a mortgage decision. Do I need photographs to ask EvoHome for help? No. Explain the property, the issue and the documents you have.

Photos are optional and should only be taken from a safe position.