Does spray foam cause condensation or rot roof timbers? What the evidence says

It found that the risk to roof timbers depends mostly on what the foam is sprayed onto, with foam bonded directly to tiles or slates the highest risk in every case modelled^[1]. HSE did the work for the Building Safety Regulator (BSR), publishing it on 28 March 2024^[1].

It simulated pitched timber roofs in a milder (London) and a more severe (Newcastle) climate, rating each build-up on the timber loss predicted in the outer layer of the rafter over a five-year simulation: low (under 1%), medium (1–25%) or high (over 25%)^[1].

"HR" below is a high-resistance underlay (bitumen felt and similar sheets that slow moisture escaping outwards), "LR" a low-resistance or "breathable" membrane, and "AVCL" an air and vapour control layer on the room side (glossary).

Old insulation left on the loft floor made the loft colder and more humid, which is why mould risk rose; gaps at the rafter were linked to materials that shrink or do not move with the timber^[1]. More loft ventilation lowered mould and condensation risk, but all but removed the energy benefit of foam at rafter level^[1].

Because the layer under the tiles decides how easily moisture can escape outwards, and the modelling found a high-resistance underlay was the main driver of risk^[1].

Foam straight onto tiles or slates. Both foam types were high risk in every climate, with or without a vapour control layer, with possible risks to battens, tile fixings and the tiles themselves^[1].

Foam on bitumen felt or another HR underlay. The report treats felt as slowing moisture escaping rather than blocking it outright, modelling it at an Sd value of 5 m against 0.04 m for a breathable membrane (Sd measures resistance to water vapour; higher means more)^[1]. RICS's consumer guide says some products must not be applied to bitumen sarking felt^[4].

Foam on a breathable (LR) membrane. With an LR membrane, every combination was low risk in London and three of the four were medium in Newcastle^[1]. BBA product approval 23/7062 Product Sheet 1 is written around this build-up: it covers breathable low-resistance underlays only and requires an AVCL behind the lining^[5].

The model's AVCL (foil-backed plasterboard with a 25 mm service void) reduced risk in most cases but not every case, and made no difference where foam was bonded to the tiles^[1].

BS 5250:2021, the British Standard on managing moisture in buildings, is behind a paywall^[6]. HSE's report summarises it for rafter-level insulation^[1]:

• Under an HR underlay: an AVCL on the warm side, plus a ventilated gap of at least 25 mm below the underlay (with 15 mm more allowed for its sag between rafters), ventilated high and low^[1].
• Under an LR underlay: an AVCL plus an air gap in the counter-batten zone above the membrane^[1].
• LR underlay where an AVCL is not practicable: treat the roof as if it had an HR underlay^[1].

NHBC's standards, citing BS 5250:2021, put the minimum clear ventilation path under an HR underlay at 25 mm, not 50 mm^[7]. Approvals also require a condensation risk assessment before spraying^[5]; HSE notes the simple Glaser method (BS EN ISO 13788) may underestimate risk^[1]. See how spray foam should be installed.

It is a computer study of a limited set of roof designs, not a survey of real lofts^[2]. It covers pitched timber roofs at a fixed 35° pitch in two English climates, with foam 100 or 200 mm thick^[1].

It is not:

• Field measurement: the report describes no surveys of real roofs and no observed failures^[1].
• Installation guidance, an assessment of current practice or a survey guide, and the BSR says it should not replace the industry's inspection protocols^[2].
• A risk assessment of your house: the report says every household is unique^[1].
• A model of roofs that are already wet or damp, which were not modelled^[1].
• Government policy: it gives the authors' opinions, not necessarily DLUHC or HSE policy^[1].

It also does not call any build-up "incorrect" installation^[1]. For the "25% more decay" claim, see the FAQ below and spray foam claims we checked.

The Property Care Association (PCA) published field figures in May 2025, from a sample of homes already under suspicion^[3]. As reported:

• 15 of the 21 firms on the PCA's spray foam surveyor register reported on more than 500 inspections since January 2024^[3].
• In the PCA's May 2025 figures, 35% of inspected homes had a defect linked to the installation^[3].
• In 27% of all inspected cases, major works were recommended: replacing the insulation or re-roofing^[3].
• 79% of properties diverged from the PCA and RPSA's Sprayed Foam Insulation Inspection Protocol (March 2023)^[3].

Because the homes were already under suspicion, these percentages cannot be applied to every home with spray foam^[3]. Six of the 21 register firms did not report. This guide could not reach the PCA's own release or data, so the figures rest on press reports. A joint HomeOwners Alliance and PCA letter in May 2025 repeated the 35% figure^[8].

Not automatically: the modelling showed no general advantage for open-cell foam, because the underlay and vapour control mattered more^[1].

A common claim, made by the Energy Saving Trust among others, is that the risk should be much lower with open-cell foam^[9]. Yet over an HR underlay with no AVCL, open-cell foam was high risk in both climates while closed-cell was low in London, and on tiles both were high^[1].

Closed-cell foam is not vapour-proof either: the report says it gives only "a degree of inherent vapour control", and flags it as likely to carry the highest condensation risk in imperfect roofs and at gaps^[1]. The PCA's homeowner advice says closed-cell foam can trap moisture, leading to timber decay, mould and corroded fixings^[10]. See open-cell vs closed-cell explained.

From the loft hatch with a torch you can check the risk factors the research names, but not the condition of timber the foam covers^[11]. Stay on boarded areas or look from the hatch, and don't step between joists.

• Foam sprayed straight onto the back of tiles or slates^[1].
• Foam on bitumen felt or an unventilated plastic sheet^[4].
• Eaves or roof vents filled with foam: approval conditions say existing vents must be kept clear and never foamed over^[12].
• Old insulation still on the loft floor below foamed rafters^[1].
• Foam pulling away from the sides of rafters^[1].
• Signs of roof leaks: RICS warns an undetected leak beneath foam can lead to rotting timber^[4].
• Mould, condensation or rusting fixings, which the PCA lists among the effects of trapped moisture^[10].

Thick foam hides the rafters, trusses and boarding it covers, so rot, cracks and leaks behind it cannot be seen^[11].

Because one number describes moisture at one spot on one day, not whether the timber is sound.

Product approvals require timber below 20% moisture by mass (BBA 23/7062) or below 19% (Kiwa BAR-19-132) before spraying^[5][13]. The HSE modelling uses 20% for solid timber and 18% for timber-based boards as its decay threshold^[1]. The industry inspection protocol sets no moisture percentage at all^[14].

Readings can mislead both ways. RICS cautions that timber can give a low reading on its visible face and still be wet behind the foam^[4]. A low reading can also come from timber that has already failed: in buying with old foam on the slates: the lowest reading came from failed timber, the wall plate read just 11.6% yet came apart under light finger pressure where examined.

So a reading needs its context: which timber, where, how close to the foam, the time of year, and how the timber looked and felt. An inspection can report what can be seen and tested on exposed timber; foam hides the rest, and no one can tell you the condition of concealed timber without exposing it.