Does spray foam cause condensation and roof timber decay?
Our contractors: 15,123 verified reviews across 8 platforms, rated 4.9 on GoogleSometimes, and it is almost always about ventilation rather than the foam itself. Closed cell foam sprayed directly onto the tiles with no gap for air to move is the configuration that traps moisture, and it is the minority case, not the rule.
The mechanism, plainly
Warm, moist air from the house rises into the loft. With a ventilation gap, that moisture escapes. Without one, and with foam sealing the roof, it condenses against the cold underside of the tiles and sits against the timber. Given years, that causes decay. The Property Care Association's own inspections found this kind of defect in 35% of more than 500 properties, which means around two thirds had no such problem.
It is a ventilation problem wearing a foam costume
Every roof, foamed or not, has to deal with moist air rising from the rooms below. A traditional loft handles this with airflow: gaps at the eaves let air move across the underside of the roof and carry moisture away before it condenses anywhere it can do harm.
Spray foam applied with a ventilation gap preserved does not fundamentally change this. Spray foam applied flush against the tiles or sarking with no gap at all removes the pathway air would normally use, and closed cell foam in particular then acts as a vapour barrier that traps whatever moisture is already there against the timber behind it.
What to look for
Symptoms, what they usually mean, and what to do
| What you notice | What it usually means | What to do |
|---|---|---|
| A musty smell in the loft | Ventilation has likely been lost somewhere in the roof. | Get it inspected. Not always terminal, but worth assessing properly. |
| Condensation on the underside of the foam in winter | Vapour is meeting a cold surface with nowhere else to go. | This needs a moisture reading, not a guess based on looking at it. |
| Timbers damp or soft when probed | Moisture is being trapped against the wood, and decay may already be underway. | Removal plus timber repair is the likely outcome. |
| Dark staining or mould on visible timber | Moisture has been present for some time. | Inspection first, removal is likely. |
| No visible gap at the eaves | Closed cell was probably sprayed with no ventilation path. | Higher risk configuration, but still needs assessing properly rather than assumed. |
None of this replaces someone getting into the loft. Thumb pressure and a torch tell you what to expect, not a diagnosis.
What an inspection actually checks
How we assess a loft for trapped moisture
Identify the foam type and the spray pattern
Open or closed cell, and whether it was sprayed with a gap left at the eaves or flush against the tiles. This sets the baseline risk before anything else is checked.
Look for a ventilation path
Check the eaves, the ridge and any other points where air should be moving, and note whether that path still exists or has been blocked, by the foam itself or by something added later such as loft boarding.
Take moisture readings at several points
Not one reading in one accessible spot. Several points across the roof, including areas that look fine, because trapped moisture does not always announce itself where you can see it.
Probe accessible timber for softness
A physical check alongside the moisture reading. Decayed wood is not always visibly different before it is soft to a probe.
Write it all down, whichever way it points
The report records exactly what was found, with photographs and the readings themselves, not a verdict without the evidence behind it.
What a moisture reading actually tells you
A proper inspection takes a moisture reading directly from the timber with a calibrated probe, giving a percentage figure rather than a visual guess. Wood at a normal moisture content reads differently to wood that has been sitting damp for months, and the difference is measurable, not a matter of opinion. That reading, taken at more than one point, is part of the written report either way, whether the answer is that the timber is sound or that it is not.
No obligation, no deposit
Have someone look at it properly
An hour in the loft and you will know exactly where you stand, in writing, with photographs. If the foam is sound, that is what the report says and there is nothing to pay.
Book the free inspection
Two steps. No deposit, and nothing to pay if the foam is sound.
Why this does not mean every closed cell installation is a problem
Plenty of closed cell installations were done with a ventilation gap correctly preserved, or with a product and detailing that manages vapour differently. The Insulation Manufacturers Association stated in February 2026 that correctly installed foam is safe and rarely needs removing, and that applies across foam types, not to open cell alone.
The point of this page is narrower than a blanket verdict either way: no ventilation gap plus closed cell is the specific combination most likely to trap moisture, and that combination is identifiable and worth checking rather than assuming either way.
Common questions
Does open cell foam ever cause condensation problems?
Less often, because it is more open to vapour movement, but it is not impossible, particularly where ventilation elsewhere in the loft has been blocked for an unrelated reason, such as boarding laid over the eaves gap.
How quickly does timber decay once moisture is trapped?
It depends on how much moisture, how consistently, and the timber species and thickness. It is measured in years rather than months in most cases, which is exactly why an inspection rather than guesswork matters: catching it early keeps the repair small.
If there is no smell and nothing looks damp, is my loft definitely fine?
It is a reasonable sign, but not a certainty. Early stage moisture trapping does not always smell or show visibly from a hatch. A moisture reading during an inspection is the more reliable check.
Can ventilation be added without removing the foam?
Sometimes, depending on the roof design and where the foam was applied, but this is a job for a proper assessment rather than a guess, since adding ventilation incorrectly can create new problems rather than solve the existing one.
Is this the same issue that causes mortgage lenders to decline?
Related but distinct. Lenders most often decline because the timbers cannot be inspected at all, which is a paperwork problem. Trapped moisture is a genuine building problem that exists whether or not you are trying to sell or remortgage. See do you actually need it removed for how the two interact.
Read next
Do you actually need it removed?
Work out which situation you are in: a survey problem, a building problem, or both.
Read thisOpen cell vs closed cell
The type and the ventilation gap together predict the moisture risk. Identify which you have.
Read thisWill you need a new roof?
What happens after removal when timber decay or membrane damage is found.
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