Can You Use Expanding Foam Around a Fire Door?

Can you use expanding foam around a fire door?

For the perimeter gap between the frame and the wall, the answer is a straightforward no if you mean the ordinary polyurethane (PU) expanding foam sold in every builders' merchant. That product is a general-purpose gap filler and draught-stopper. It has no fire-resistance classification, it is not named in any fire doorset's certified installation instructions, and squirting it into the gap around a fire door frame replaces a tested, engineered seal with an untested one. The moment you do that, the doorset is no longer installed the way it was tested — and its certification no longer describes what is actually in the wall.

The confusion is understandable, because two very different products both get called 'foam'. There is ordinary PU expanding foam, which must never form part of a fire barrier, and there is a separate category of fire-rated (intumescent or fire-tested) expanding foam that is tested specifically as a linear gap seal. Even the fire-rated kind is not a blanket permission — it can only be used where the doorset's certified detail allows it and it carries the right test evidence, which we cover below. But the everyday can from the van is out, always.

Why is ordinary expanding foam a problem around a fire door?

A fire door is not just a leaf and a frame — it is a complete, tested assembly. As the certification body BM TRADA puts it, 'the door leaf, frame, glazing, associated hardware, intumescent and environmental seals, and any other components that make up the design, must all be tested together as a complete doorset' to a benchmark performance standard. How the frame is fixed and sealed into the surrounding wall is part of that tested picture. Ordinary expanding foam undermines it in several ways at once.

• No fire classification. Standard PU foam has no integrity (E) or insulation (I) rating under BS EN 13501-2. In a fire it softens, shrinks and burns rather than holding the gap closed, so it cannot be relied on to stop fire or hot smoke passing between the frame and the wall.
• Outside the tested field of application. Every certified doorset is tested with a specific method of sealing the frame into a specific type of supporting construction. Foam that was never in that test sits outside the doorset's field of application — there is no evidence it performs.
• It expands unpredictably and over-fills. PU foam keeps growing after it is applied. It can bow or push the frame out of true, which in turn opens up the leaf-to-frame gaps that the intumescent strips are sized to close. A distorted frame is a failed frame.
• It hides, rather than seals. Trimmed flush and painted over, foam looks tidy and passes a glance — but behind the architrave there is no mineral wool, no rated sealant and no evidence. The 'seal' is cosmetic.

Because installation is part of the tested product, using an unapproved material at the perimeter can invalidate the fire test evidence and the certification that rides on it. BM TRADA's guidance is blunt that changing a doorset's specification can 'completely undermine the fire performance of a doorset', and that third-party certification only assures performance 'as originally tested'. Swap the perimeter seal for foam that was never tested and you have changed the specification. Our guides to fire door certification schemes and the difference between a fire doorset and a fire door assembly explain why the whole assembly, installed as tested, is what carries the rating.

What is the perimeter gap, and how should it be filled?

Half the mistakes here come from confusing two different gaps around a fire door. They are sealed in completely different ways, and expanding foam belongs to neither.

The intumescent strips let into the leaf or frame deal with the small leaf-to-frame gap; our guide to intumescent strips and smoke seals covers those, and the acceptable leaf gaps are set out in fire door gap tolerances. The perimeter gap — the linear gap between the back of the frame and the structural opening — is a separate problem, and it is where installers reach for the foam gun. London Fire Door Consultants describe this linear seal as 'the protected gap between the back of the door frame and the surrounding structural opening', and the point of that protection is to stop fire and smoke tracking around the outside of the frame.

How the perimeter gap should actually be filled

The BWF Fire Door Alliance installation guidance is short and clear: 'ensure any voids between door frame and wall are tightly packed with mineral wool.' In most certified timber doorset details, that packing is then closed off on the visible faces with a fire-rated intumescent sealant or mastic, so you have a fibre backing that resists the fire and a sealant that closes the surface. Some certified details instead use proprietary pre-formed intumescent packers. The exact recipe is not for the installer to invent — it is whatever the doorset's own certified installation instructions specify.

What does 'fire-rated' or intumescent expanding foam actually mean — is it ever allowed?

Fire-rated expanding foam is a genuine product category, distinct from ordinary PU foam. It is tested as a linear joint seal — typically to BS EN 1366-4 (or historically BS 476-20) — and can carry a fire-resistance classification for the joint it fills. So yes, a fire-rated foam can, in principle, be an acceptable way to seal a perimeter gap. But 'fire-rated' printed on the can is not the end of the question; it is the start of it.

• It must be in the doorset's certified instructions. The doorset is what is certified, and the installation detail is part of that. If the manufacturer's instructions specify mineral wool and mastic — and some explicitly rule foam out — then foam is not permitted, however good its own separate rating.
• The test evidence must match your wall. Foam is tested in a particular supporting construction (for example, a masonry wall of a stated minimum thickness) and a particular gap range. Evidence gained in dense masonry says nothing about a metal-stud plasterboard partition. BS 8214 makes clear the use of a mastic or foam seal is conditional on that product having test evidence for the actual structure it is used in.
• The gap width must be within the tested range. A fire-rated foam is only classified between a minimum and a maximum joint width. Too narrow or too wide and you are outside its evidence.
• Untested combinations are not permitted. Layering a foam and a mastic that were never tested together, or using a foam with the wrong backing, creates an assembly no one has tested.

For that reason this guide does not — and cannot — recommend a particular product. The correct seal is defined by the doorset you have bought and the wall you are fixing it into, and the answer is written in the installation instructions that came with the doorset. If those instructions are missing, treat the specification as unknown and get them from the manufacturer before you seal anything.

What do BS 8214 and BS EN 1366-4 say about sealing the gap?

BS 8214 is the UK code of practice for the specification, installation and maintenance of fire-resisting and smoke-control doors. It was substantially revised in 2026, and the new edition, published by BSI, widens the scope to cover timber, steel, aluminium and composite doorsets and tightens the installation guidance to cut non-compliant work. Among the areas it addresses more directly is exactly this one: how to seal the interface between the door frame and the surrounding structure, covering stone wool packing, fire-rated mastics and foam sealants. The consistent thread is that these materials are acceptable only when supported by test evidence, not by assumption.

The relevant test standard for the seal itself is BS EN 1366-4, the fire resistance test for linear joint seals. It assesses whether a seal in a linear gap maintains the integrity — and, where classified, the insulation — of the construction under fire exposure, including the rapid temperature rise associated with flashover. It is the standard by which a fire-rated mastic or foam earns a classification for the perimeter gap. A linear seal is classified for integrity (E) and, separately, insulation (I) under BS EN 13501-2 — a reminder that a rating is only ever for a defined performance; an FD30 doorset, for instance, corresponds broadly to E30, not to the insulation-plus-integrity EI30.

Sitting behind both of these is the doorset's own fire test — to BS EN 1634-1 or the older BS 476-22 — which fixes the doorset's field of application, including how the frame is sealed into the wall. That is why the perimeter seal is not a free choice on site: it is part of the tested assembly. Our guide to BS 476 vs EN 1634 fire door testing explains how those doorset tests define what a given fire door is actually certified to do.

Why do builders get this wrong, and how does it fail an inspection?

Expanding foam is fast, cheap and forgiving. A gap that would take real time to pack with mineral wool and finish with sealant can be foamed, trimmed and hidden behind the architrave in a couple of minutes. On a general building site, foaming around a frame is muscle memory — it is simply how ordinary internal door frames get fixed. The trouble is that a fire door frame is not an ordinary frame, and the two-gaps confusion does the rest: an installer who correctly fits intumescent strips at the leaf still reaches for the foam gun at the perimeter, not realising that is the seal that matters most.

The result is one of the most common findings on a fire door inspection. Typical failures include:

• Ordinary PU foam in the perimeter gap, trimmed flush and painted, with no mineral wool and no rated sealant behind it.
• Over-filled foam bulging past the frame or architrave, a tell-tale that the gap was foamed rather than packed and sealed.
• Foam where the certified detail calls for mineral wool — the right idea (fill the void) executed with the wrong, untested material.
• A fire-rated foam used with no evidence for the wall type or gap width, or combined with an untested mastic.
• No installation record or manufacturer's instructions to show the perimeter was sealed as tested.

Fixing it is not cosmetic. The remedy is to rake out the offending foam, repack the void with mineral wool and finish it with the fire-rated sealant the certified detail specifies — or otherwise reinstate the manufacturer's tested method — and to record what was done. Because this is skilled, evidence-driven work, it belongs to a competent installer, ideally one working under a third-party installation scheme; our guides to who can install fire doors and fire door inspection set out what good looks like. And if you are tempted to treat the gap as 'just a detail', remember that the perimeter seal, the leaf gaps, the strips and the hardware only add up to a fire door when every one of them matches the tested specification — the same principle that governs whether you can trim or paint a fire door without wrecking its rating.