Fire Safety & Real World Fire Performance: How Fibreglass, Knauf & ROCKWOOL Really Behave in a Fire

Fire safety is one of the most misunderstood topics in home insulation. People hear phrases like “non-combustible” or “fire resistant” and assume every insulation behaves the same in a fire.

It doesn’t.

When a real fire starts – not a lab test, but a genuine, uncontrolled house fire – insulation behaves very differently depending on:

• how dense it is
• how it reacts to rising heat
• whether it sags, melts or stays in place
• how it handles smoke and convection
• whether it continues protecting timber when temperatures surge

Most homeowners never think about this until after a fire – when it’s too late.

This guide breaks down, clearly and honestly, how fibreglass, Knauf Glass Mineral Wool and ROCKWOOL Stone Wool perform in real fire conditions… and why Knauf stands out as the best all-round choice for UK homes, even though ROCKWOOL holds the crown for extreme fire resistance.

How Fire Actually Moves Through a Home

Before talking materials, it’s important to understand how fires spread.

Fire doesn’t just burn what it touches. It spreads through:

• Flames (direct contact)
• Radiant heat (heating timber before flames reach it)
• Superheated air (hot gases moving through gaps and voids)

When a fire breaks out in a room below a loft, three things happen fast:

• the ceiling heats up
• insulation on the loft floor begins absorbing rising heat
• once joists and rafters reach ignition temperature, the loft becomes involved

So the question isn’t “does insulation burn?” The real question is:

Does the insulation stay in place and keep protecting the timber when the heat hits?

Any insulation that slumps, melts or shrinks away stops protecting the structure – even if the material itself isn’t flammable.

Air movement also matters. Poor airflow and uncontrolled convection currents can move heat and smoke rapidly through a loft. Our guide to
loft ventilation and Lap Vents
explains how air paths behave in normal conditions – and the same principles apply when those air paths are full of smoke instead of fresh air.

Fibreglass: Non-Combustible, But Poor Fire Performance

Fibreglass is often called “non-combustible.” Technically true – but very misleading.

Fibreglass melts long before it burns, and that’s the real problem.

How Fibreglass Responds to Fire

• softens at around 250–300 °C
• shrinks away from heat sources
• can collapse into a glass puddle under direct flame
• leaves joists and rafters exposed
• allows hot gases to pass through it easily

Because it’s so light and airy, heat moves through fibreglass quickly. So even though it doesn’t burn, it provides very little protection to the structure in a serious fire.

In downlight scenarios (a common ignition or heat build-up point), fibreglass can slump or melt away from hot fittings and create clear channels into the loft. That pathway allows heat and smoke to reach the roof space much earlier than most people expect.

Verdict: fibreglass doesn’t feed the fire – but it doesn’t defend your home from it either.

Knauf Glass Mineral Wool: The Best All-Rounder for Homes

Knauf Glass Mineral Wool performs significantly better than old fibreglass for one simple reason: it has more structure.

Thanks to higher density, stronger fibre bonding and a modern binder system, Knauf:

• holds its shape much longer under rising heat
• slows heat transfer more effectively
• resists sagging during early-stage fire exposure
• restricts airflow movement through the material
• maintains more consistent coverage over time

Knauf is still glass-based, so in a fully developed fire (600 °C+), it will eventually soften and slump – all glass insulation does. But there’s an important truth here:

Compared to ROCKWOOL, Knauf is:

• lighter and easier to handle
• more flexible around cables, pipes and awkward details
• less intrusive in tight loft spaces
• more cost-effective for standard domestic upgrades
• ideal for most UK lofts where extreme industrial fire ratings aren’t required

Knauf gives homeowners a strong level of fire performance and excellent warmth, acoustic performance and longevity. For 95% of UK properties, it’s simply the most sensible choice.

ROCKWOOL: The Fire Specialist (But Not the All-Rounder)

Here’s the honest part – and it matters for trust:

ROCKWOOL is the best performer in extreme fire conditions. It’s made from volcanic rock and stays stable above 1,000 °C. In pure fire testing, that puts it in a different league.

How ROCKWOOL Behaves Under Extreme Heat

• it does not melt
• it does not shrink
• it does not sag away from timber
• it keeps full contact with the structure

That’s why ROCKWOOL is widely used in:

• commercial firestopping
• high-rise compartmentation
• industrial fire-barrier systems

However, ROCKWOOL is not always the ideal product for a typical domestic loft. It is heavier, more rigid, more expensive and often more complex to work with in cramped roof spaces. It’s also not designed to do everything that modern hybrid systems like Actis Hybris can do for airtightness and thermal control.

In other words:

ROCKWOOL wins in the “extreme fire specialist” category. Knauf wins in the “real-world domestic all-rounder” category.

30-Year View: How Fire Performance Changes Over Time

The biggest differences between materials often show up decades later.

Again, ROCKWOOL wins on pure long-term fire stability. But for most homes, the goal is not to install an industrial fire barrier – it is to achieve a safe, warm, cost-effective and practical loft upgrade. This is where Knauf is hard to beat.

What Fire Investigators See in Real UK House Fires

Across the UK, fire investigators regularly report patterns that line up with how these materials behave in theory.

Homes with Fibreglass

• rapid heat penetration through ceilings
• visible melted or missing sections of insulation
• early loft involvement and fast spread across rafters

Homes with Knauf Glass Mineral Wool

• slower heat transfer into loft spaces
• more time before timber reaches critical temperature
• fewer direct air channels for smoke and hot gases
• a better structural survival window in many cases

Homes with ROCKWOOL Stone Wool

• insulation still in place after the fire is out
• fire slowed significantly as it approached the loft
• roof structures often better preserved than expected

There is one other huge factor that shows up across real fire investigations:

Underfloors, Hybrids and the Bigger Picture

While this article focuses on fibreglass, Knauf and ROCKWOOL, the same principles apply under suspended timber floors and in hybrid systems.

Cold voids under floors can move heat and smoke quickly. Our
underfloor insulation
guide explains how we use modern systems to make suspended floors warmer and more stable – which also helps the structure behave more predictably if a fire ever reaches those spaces.

Hybrid systems such as Hybris insulation also play a role in overall building performance, improving airtightness and thermal control in situations where mineral wool isn’t the best option. Fire behaviour differs between materials, but the underlying goal is the same – consistent coverage, stability and controlled airflow.

Why Knauf Is the Right Choice for Most Homes

Homeowners don’t choose insulation based on industrial fire-test charts. They choose based on:

• warmth and comfort
• energy bills
• long-term performance
• airflow and ventilation
• safety around electrics and downlights
• cost and practicality
• the quality of the installation

This is exactly where Knauf shines.

Knauf offers:

• strong domestic fire performance
• excellent all-round thermal comfort
• low dust, low itch and better install experience
• good long-term stability when correctly installed
• solid acoustic performance
• a very good balance of cost versus performance
• flexibility for real-world UK lofts with services and awkward details

In other words: Knauf is the insulation that gives homeowners the best overall outcome – not just the best single number on a fire test.

The Honest Verdict: Who Wins Where?

The honest summary is simple:

• ROCKWOOL is the fire-performance specialist.
• Knauf is the best all-round insulation for UK homes.
• Fibreglass is outdated and offers the weakest real-world fire protection.

Next Steps: Fire-Aware Insulation for Your Home

Fire performance should never be an afterthought. It sits alongside warmth, cost, moisture control and ventilation as a core part of choosing the right insulation system.

If you’d like a fire-aware look at your loft or underfloor insulation, we can carry out a detailed survey, explain what’s currently in place and recommend a modern, safer upgrade using Knauf and other appropriate systems.

Related reading:
learn more about our
Knauf loft insulation upgrades,
underfloor insulation,
Hybris hybrid insulation,
and why we specialise in safe spray foam removal
to protect both your home and its structure.