Most people don’t give much thought to insulating between floors – at least not until it becomes a problem. The thud of footsteps, noisy TVs and the pitter patter of someone having a shower. The fix is often seen as simple – thick carpets, maybe more insulation, or learn to live with the nuisance.
The reality is a bit more nuanced than that, and intermediate floor insulation installed correctly will deal with three problems at once. It reduces sound transfer between floors, slows the spread of fire through the structure, and helps control how heat moves around the house.
If you don’t understand what intermediate floor insulation does, it’s easy to spend money without really solving the problem, but the way you approach it starts with understanding the problems at hand.
Article continues belowThe noise problem
Historically, most traditionally built older homes in the UK have a timber intermediate floor between storeys. Timber joists span from wall-to-wall, with floorboards or chipboard on top, and plasterboard ceilings underneath.
This leaves a hollow void which can act like a drum. Sound resonates within the void, amplifying footsteps, voices and music. If the floorboards are slightly loose, or there are gaps around the edges, the noise problem gets worse.
There are two distinct types of noise at play here, and both behave very differently.
- Airborne noise – This is what you hear when someone is talking upstairs or a television is on. It travels through the air and into the structure. The best way to deal with it is by adding insulation to absorb and disrupts sound waves, which would pass through the void.
- Impact noise is different – This is the sharp, physical transmission of energy through the structure: footsteps, dropped objects, kids running across a bedroom. You can almost feel the noise just as much as hear it. This is where people often get caught out, because stuffing insulation between joists does very little to stop it.
Fire safety
The building regulations and Approved Document B, set out the requirements for fire resistance between floors, and as such, fire-stopping between floors is often treated as a building regs compliance issue.
But as timber intermediate floors typically have the least level of fire integrity of any building element, compromising the fire integrity of an intermediate floor has real-world consequences.
Whilst the correct selection of materials, and filling the floor void with non-combustible insulation will help to slow fire spread, gaps are a weak point. Any openings around pipes, cables or at the edges of floors can allow fire to bypass the insulation layer entirely. Fire-stopping products and careful sealing around penetrations are essential, and where attention to detail matters.
In reality, building control officers tend to focus on whether the overall construction provides adequate protection, rather than the specific insulation product used. While this gives some flexibility, it also means responsibility sits with the designer and installer to get the details right. Therefore always consult a competent designer before making any alterations that may impair fire performance.
Insulating intermediate floors
It’s worth being clear that insulating intermediate floors is a very different exercise from insulating a ground floor in older houses.
Ground floors are primarily about controlling heat loss and managing moisture, particularly in suspended timber floors where ventilation is critical to avoid damp and decay.
Intermediate floors, on the other hand, are more about comfort and separation. You’re not dealing with ground moisture or significant heat loss, but with sound transfer, fire protection and internal heat movement.
This changes the priorities, and in intermediate floors, acoustic performance and airtightness often matter much more than thermal resistance.
Insulation options for intermediate floors
The joist void offers the main opportunity to improve performance, particularly in retrofit situations. And provided the work is done properly, it offers an easy win.
Mineral wool or glass wool insulation is typically used here. It’s relatively inexpensive, non-combustible, and dense enough to absorb airborne sound effectively. When fitted correctly between joists, it provides an acoustic break within the empty cavity that would otherwise allow sound to resonate through.
Sounds simple but for the insulation to be able to perform, it needs to sit snugly between joists without being compressed. If it’s squashed around pipes or cables, its performance drops. Equally, if it’s loosely fitted, it won’t do its job properly as air gaps will completely undermine its effectiveness. So don't make this insulation mistake.
Another detail that often gets overlooked is support. Insulation in floors that need to be suspended must be properly supported, typically using netting or timber battens. Without support, insulation will sag over time, creating gaps where sound can penetrate.
It's also worth remembering that electrical cables running through insulation can overheat if they’re completely surrounded. While this isn’t a problem in new homes, older circuits may not have the same current-carrying capacity as modern cables. This doesn’t mean you can’t insulate a floor void, it just means it needs to be done with a bit of care, especially in older properties.
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Insulating from above
If you have the option, insulating from above when insulating between floors is the easiest approach, depending on how the floor has been constructed. Working from above usually means lifting floorboards or removing chipboard decking, which isn’t always possible or convenient, especially if the boards are glued down, or walls are built off them.
If the floor can be lifted, insulation can be cut and fitted tightly between joists with no gaps. And, whilst the floor is up, it’s also possible to deal with another acoustic issue – air gaps.
Gaps around the perimeter of the floor, between floorboards, or where pipes pass through, allow air to move freely between floor levels. This airflow carries both heat and sound, bypassing whatever insulation is installed. Sealing these gaps with insulation, or a flexible acoustic sealant will make a noticeable difference.
Once the floorboards are back in place and sealed up, impact noise can be dealt with. This is where acoustic underlays are popular, with layers of rubber, foam or felt installed beneath the final floor finish. This creates a resilient layer that absorbs the energy of footsteps before it enters the structure.
Insulating from below
Where lifting floors isn’t practical, the only real option is to work from below, which usually requires the ceilings to be replaced.
With the plasterboard ceilings removed, insulation can be installed between the joists before new plasterboard is fixed. It’s a destructive and disruptive process, but possibly the only way to properly retrofit floor insulation.
When taking this approach, it’s worth upgrading the plasterboard ceiling to a different type of plasterboard with better acoustic, or fire performance properties. Acoustic plasterboard, or even doubling up on plasterboard layers will significantly improve airborne sound reduction. Fire-rated plasterboard is also worth considering, particularly in two-storey homes where separating floors form part of the fire strategy.
Insulating a floor from below doesn’t address impact noise in the same way as working from above, so acoustic underlays may still be required.
Choosing the right materials
The correct material choice isn’t about finding a miracle product, but is more about understanding what each material does well.
Mineral wool insulation and glass wool remain the go-to options for most inter-floor insulation. They strike a good balance between how much insulation costs, fire resistance and acoustic performance. High-density acoustic mineral wool products go a step further, offering better sound absorption due to their increased mass, but are without doubt harder to install, especially around existing services.
For impact noise, acoustic underlays are designed specifically to absorb vibration. These sit directly beneath floor finishes such as laminate, engineered timber or even carpet. Whilst specialist acoustic floor systems are available, these are rather technical, expensive and not easy to retrofit within an existing house, especially where headroom is limited.
The key point when choosing materials is that no single material solves everything. In practice, good results come from combining layers, filling mass within floor voids, installing acoustic resilience at the surface, and achieving airtightness with a good level of workmanship.
Insulating between floors is one of those jobs where small details make a big difference. It’s not complicated and is often one of the best ways to insulate a home, but it’s easy to get wrong.
Over the years I’ve seen many projects, and even new build ones where expensive materials were installed poorly and delivered very little performance. I’ve also seen relatively simple upgrades such as well-fitted mineral wool, combined with decent airtightness transform how a house feels.
In my experience, getting things right requires a focus on the system as a whole, and not just the insulation. Think about how sound, air and heat move through the floor, and deal with each of them, not forgetting that exceptional standards of workmanship and quality are essential to achieve the desired outcome.
