PV fire safety: Can mineral wool stop a PV roof fire?

The fire reaches the roof before it reaches the insulation

In a conventional insulated pitched timber roof, mineral wool is typically installed between the rafters.

Above it are several other parts of the roof construction. Depending on the design, these may include an underlay or wind barrier, battens and a ventilated cavity, timber-based roof decking and the external roof covering.

The PV system is located above all of these layers.

If a fire originates in or beneath the PV installation, the mineral wool is therefore not the first layer separating the fire from the building.

The fire meets the roof first.

Mineral wool can remain completely non-combustible while timber rafters, battens, decking and other materials above and around it become involved in fire.

The mineral wool has not failed.

But because it is positioned between the structural members, it does not necessarily form a continuous fire barrier between an external PV fire and the roof construction.

That distinction is fundamental.

Recent full-scale research confirms that photovoltaic installations can change the fire dynamics on pitched roofs.

The UK Building Safety Regulator commissioned 11 large-scale experiments on pitched roofs at 45 degrees. Every tested configuration incorporating PV showed greater flame spread than the control roof without PV.

The cavity beneath above-roof PV was an important factor. The geometry created between the panel and the roof influenced flame development and allowed the fire to extend further along the roof surface.

Norwegian research from RISE Fire Research has reached similar conclusions.

RISE conducted 29 experiments investigating fire spread behind PV modules on pitched roof surfaces. The researchers found that PV modules mounted parallel to the roof could affect the fire dynamics, and that reducing the distance between the module and the roof increased both the damaged area and the temperature exposure further into the roof construction.

This does not mean that every PV fire will penetrate a pitched roof.

It means that the roof is exposed to a different fire scenario after PV has been installed above it.

This difference between material performance and system performance is important.

An A1-classified mineral wool product provides very strong information about the reaction-to-fire properties of that material.

It does not, by itself, tell us how the complete roof assembly will perform when the fire originates outside the insulation layer.

The UK research demonstrates the same principle from another perspective.

One tested roof covering had achieved BROOF(t4) classification without PV. When PV was installed above it in the experimental configuration, the fire behaviour changed dramatically and almost the entire roof area beneath the PV array became involved.

This does not make the original classification invalid.

It demonstrates that:

The classified component and the complete PV roof assembly are not necessarily exposed to the same fire conditions.

The same reasoning applies to insulation.

This question has also been central to Bridgehill’s own development work.

We have conducted several comparative full-scale fire tests using approximately 2 × 5 metre pitched-roof constructions built as complete roof assemblies.

The test roofs incorporated the key elements of a conventional pitched roof, including timber structure, mineral wool insulation, ventilation layers, roof decking, bitumen roofing and photovoltaic panels with their mounting systems.

The objective was straightforward:

What happens when a severe PV fire develops above a mineral-wool-insulated pitched roof?

In the reference configuration without an additional continuous fire barrier, the mineral wool itself did not burn.

But that did not save the roof.

The fire developed through the upper roof construction and the roof was extensively destroyed despite the presence of non-combustible mineral wool.

This was an important observation.

The mineral wool behaved as expected.

It was its position in the assembly that prevented it from stopping the fire before other parts of the roof became involved.

We then tested the same principle with a continuous FireBlock membrane installed on top of the bitumen roofing, immediately beneath the PV installation.

The fire severity above the roof was substantial.

Approximately 95% of the PV installation and mounting system was destroyed, leaving primarily glass and steel components.

But the result beneath the fire barrier was fundamentally different.

The underlying roof construction remained intact. Even the bitumen roofing beneath the FireBlock membrane was preserved.

The fire had enough energy to destroy almost the entire PV installation.

But under the tested conditions, it did not continue down into the roof.

These are Bridgehill’s own documented comparative development tests and should be considered as such rather than as a substitute for independent classification. But they demonstrate the engineering principle very clearly.

The mineral wool was non-combustible in both roof configurations.

What changed was where the continuous fire barrier was positioned.

Our review of real PV-related fire incidents has repeatedly raised the same issue.

Severe fires on pitched roofs can progress beyond the PV installation and involve the roof construction itself.

Data from the Netherlands Institute for Public Safety, NIPV, also provides an interesting perspective.

Of the incidents involving affected PV panels where roof geometry was known, 23 occurred on pitched roofs. NIPV also recorded the insulation material. Six of the investigated roofs contained stone wool or glass wool, and in two of these six cases the fire was recorded as having originated in the PV installation.

The published dataset does not cross-reference those two mineral-wool cases with roof geometry, so they cannot be presented specifically as pitched-roof cases.

But the data reinforces the broader point:

The presence of non-combustible insulation does not, by itself, determine the fire performance of the complete PV roof assembly.

This is perhaps the simplest way to describe the issue.

After severe building fires, mineral wool can remain visible among the remains.

That is precisely because it does not burn.

But if the timber rafters, battens, decking and surrounding roof construction have been destroyed, the survival of the mineral wool does not mean that the roof was protected.

The survival of the insulation and the survival of the roof are two different measures of fire performance.

This does not reduce the value of mineral wool as a fire-safe insulation material.

It highlights a different issue:

A non-combustible material can perform exactly as intended and still be positioned too far inside the assembly to stop a fire originating outside the roof construction.

For decades, the construction industry has correctly focused on reducing combustible materials and using insulation with excellent fire properties.

PV introduces another dimension.

The potential fire originates outside the thermal insulation layer.

It develops above the roof.

And the PV installation itself can influence the fire conditions beneath the modules.

That means another question should become part of the fire-safety discussion:

Where is the first continuous fire barrier between the PV installation and the combustible parts of the roof?

Our comparative testing indicates why this question matters.

With mineral wool positioned between the rafters, the roof construction above and around the insulation could still become involved.

When a continuous fire barrier was instead positioned at the top of the roof, before the fire could gain access to the underlying construction, the outcome was completely different under the same type of severe PV fire exposure.

This is the principle behind FireBlock.

But the broader issue is not about one product.

It is about how we evaluate fire safety when photovoltaic systems become part of the roof.

Perhaps we need to move beyond asking only:

“Is the insulation non-combustible?”

and also ask:

“Is the non-combustible protection positioned where it can actually stop the fire?”

Because mineral wool can be an excellent non-combustible insulation material and still not necessarily solve the fire problem created by PV on a pitched roof.

Building Safety Regulator / Health and Safety Executive. Fire Spread Over Pitched Roofs Fitted with Solar Panels, 2025.

RISE Fire Research. EBOB – Solcelleinstallasjoner på bygg: Brannspredning og sikkerhet for brannvesen, RISE Report 2022:82.

Stölen, Fjærestad, Mikalsen & Jomaas. Research on fire propagation beneath BAPV installations on sloped roofs.

Netherlands Institute for Public Safety, NIPV. Gebouwbranden met zonnepanelen, 2024.

Bridgehill. Comparative full-scale pitched-roof PV fire testing and internal test documentation.