What is spray foam insulation? Open-cell vs closed-cell explained

Spray foam is made on site from two liquid components that react, expand to fill the space and become tack-free within seconds^[1]. The Energy Saving Trust describes it as liquid polyurethane sprayed on to set into an insulating layer, quick to fit in awkward areas^[7].

The gas that makes the foam expand (the blowing agent) differs by type. Open-cell products sold in the UK are water-blown: water reacts with the isocyanate component and releases carbon dioxide^[3]. Closed-cell products moved from HFC to HFO blowing agents after new foams containing HFCs with a global warming potential of 150 or more were banned from 1 January 2023^[8][9]. Older closed-cell foam may differ: a 2011 BBA product approval for a closed-cell roof foam declared an HFC blowing agent^[10].

Spray foam is not a DIY product. The manufacturers' Code of Practice says only manufacturer-trained and approved installers should apply it^[1]. It is also different from factory-made PIR or PUR insulation boards: the standard for sprayed foam, BS EN 14315-1, covers foam formed in place and excludes factory-made products^[11].

Open-cell foam is softer and lets water vapour through easily; closed-cell foam is much denser, more solid and lets very little vapour through^[1]. The product approvals checked for this guide (the conditional documents issued for each product by bodies such as the BBA and Kiwa) give these typical figures:

The modelling done for the Building Safety Regulator used 7 kg/m³ for open-cell and 42 kg/m³ for closed-cell, taken from BBA and Kiwa data; the closed-cell figure is above the approval range in the table^[2]. The industry told the researchers that open-cell is now the most commonly applied type, and the report notes that spraying straight onto tiles was more common in the past^[2].

Not automatically. A common claim is that open-cell foam "breathes" and is therefore safe, while closed-cell traps moisture; the government-commissioned modelling does not support that as a general rule^[2].

The research, carried out by HSE for the Building Safety Regulator and published in March 2024, modelled moisture in timber roofs for both foam types, two kinds of underlay, and with or without an air and vapour control layer (AVCL, a sheet on the warm side that limits moist indoor air reaching the roof)^[2]. These are its risk ratings for London and Newcastle:

Source: HSE for the Building Safety Regulator, February 2024^[2].

Over a felt-type underlay without an AVCL, open-cell was the worse of the two^[2]. The same report flags closed-cell as likely to carry the highest condensation risk in imperfect roofs and at gaps, and found that a gap of just 1 mm between foam and rafter can push a roof into high risk^[2]. Leaving the old loft-floor insulation in place under rafter-level foam made the loft colder and raised the risk of mould^[2][12].

Closed-cell foam is not a sealed barrier either: the report describes it as giving "a degree of inherent vapour control", not as vapour-proof^[2]. Some well-known sources still give the simpler view: the Energy Saving Trust says the risk should be much lower with open-cell, and the Commons Library calls open-cell less prone to condensation^[7][13].

The regulator stresses that this is theoretical modelling of a limited set of scenarios, not installation guidance or a survey of real roofs^[12]. In the modelling, the type of underlay was the main driver of risk, and an AVCL lowered it^[2]. The full scenario results are on does spray foam cause condensation or rot?.

Mostly under the slope of a pitched roof, as an alternative to laying mineral wool on the loft floor^[7]. The Energy Saving Trust also lists empty cavity walls where standard fill is unsuitable, and the underside of suspended timber floors, often applied by robot^[7]. One open-cell product approval had separate sheets for pitched roofs, flat roofs, suspended floors and external walls^[14].

In a roof, what the foam is sprayed onto matters as much as the foam:

• A breathable membrane (a low-resistance underlay): the use the current open-cell approval for these roofs is written for, with conditions such as a vapour control layer and a condensation risk assessment^[3].
• Bitumen felt (a high-resistance underlay): the RICS consumer guide says not to spray onto bitumen felt^[15].
• The back of the tiles or slates, with no underlay: RICS says most products should not be applied this way^[15], and the modelling rated it high risk in every case^[2].

The full list of conditions is on how spray foam should be installed.

It insulates; it is not a roof repair. RICS says most products should not be used to remedy leaks, and the Energy Saving Trust says no insulation should go on a leaking roof until the leak is fixed^[15][7].

• Insulation value. Closed-cell insulates better per millimetre than open-cell^[2]. At 0.039 W/mK, 100 mm of open-cell foam is roughly equal to 113 mm of mineral wool at 0.044 W/mK, so the often-quoted "100 mm of foam equals 170 mm of wool" only fits closed-cell (a calculation using the approval's foam figure and a loft-roll data-sheet figure)^[3].
• Cost and efficiency. RICS says topping up mineral wool on the loft floor is usually much cheaper^[15]. The Energy Saving Trust says rafter-level insulation costs more and saves less, because gable walls, party walls and chimneys then need insulating too^[16]. The scoring method used for one government scheme instead assumed similar savings at either level, as a scoring assumption rather than a measurement^[17].
• Holding tiles on. Foam sold to stabilise a roof is a separate product. The one UK approval found, BBA 10/4771 Product Sheet 2 (first issued August 2010), covers re-fixing tiles and slates suffering from nail fatigue, makes no claim to strengthen the structure, and requires sound timbers and a weathertight roof first^[6].
• Structure. The manufacturers' Code says closed-cell adds only racking strength (resistance to the roof frame being pushed out of square), with no wider structural claim^[1]. The Energy Saving Trust puts it more broadly, saying closed-cell can help support the structure^[7].
• Airtightness. A 2011 closed-cell approval says only that the foam contributes to the airtightness of the building, and gives no figure^[10].
• Fire. RICS describes spray foam as combustible^[15]. Approvals require a fire-resistant board over it in habitable rooms^[3][5]. See is spray foam a fire risk?.
• Fumes. HSE calls spraying isocyanate products a significant cause of occupational asthma; its guidance is about the people spraying and anyone present until the area clears, not the cured foam^[18]. See spray foam fumes and health.

As long as the building, according to current product approvals, but only if it was installed and maintained as the approval requires and the roof covering stays weathertight^[3]. A Kiwa approval gives the same life as the building, which it assumes will stand for at least 30 years^[4].

Older documents are more cautious. A 2011 BBA approval for a closed-cell roof foam said the product should remain effective for "at least 20 years"^[10]. Claims that foam lasts 80 to 100 years or never deteriorates are covered on spray foam claims we checked.

The modelling also notes that foam may shrink away from the timber, and that even a 1 mm gap at the rafter greatly raised the predicted moisture risk^[2].

Mainly because foam covers the roof timbers. RICS says a foam-covered timber roof "cannot be fully seen or inspected", so leaks may go unnoticed^[15]. It also says foam may affect a surveyor's opinion of value, and that the surveyor follows the lender's instructions^[15].

The research then showed that some ways foam has been installed, such as spraying it straight onto the tiles, carry a high modelled risk^[2]. The government's position, set out by the housing minister in May 2024, is that the presence of spray foam should not in itself rule out lending^[19]. For the lending picture see spray foam and mortgages, and for how it unfolded see the history of spray foam in the UK.