"Just top it up" is the reflex answer to a cold loft. For a bare, unused void it is a cheap win. But for the way most families actually use a loft, adding another layer of floor wool often pours money into a material that cannot do the job you need done. Here is how to tell which camp your loft is in, and why replacing rather than topping up is frequently the better long-term buy.
By Adam Gough, Founder & Lead Adviser, Insulation Advice Centre
Last verified: 9 September 2026. We supply and arrange certified SuperFOIL installations, and we will still tell you if standard mineral wool is all your loft needs.
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One of the most common questions on our advice line is a version of this: "I have got some old insulation up there, should I just add more on top?" The standard answer, and the one most quotes assume, is yes, top it up to around 270mm and move on. Sometimes that is right. But for a large share of the homes that ring us, topping up is treating the wrong problem, and the money would work far harder on a different approach. Here is the honest version.
Floor-level mineral wool does one job: it slows heat escaping upward from a cold, empty loft into the sky. If that is genuinely your loft, more depth helps. What it cannot do is anything for the way most people actually use the space above their heads. It cannot keep a boarded, stored-in loft insulated once you put weight on it. It cannot warm a room in the roof, because the insulation is under the floor of a space you live in. And it does almost nothing about the summer heat that pours down through a hot roof. So before you pay to add more of it, the real question is not "how thick," it is "is this even the right material for how I use the loft?"
Adding a second layer of wool doubles down on a material with three well-known weaknesses, and paying more for those weaknesses is where the false economy creeps in:
None of this means your existing insulation has failed or must be binned. Dry, intact wool still works. It means that adding more of the same is often the wrong direction if your loft is used, boarded, converted or hot in summer.
Be realistic about what a top-up actually returns. If your loft already holds a decent 120mm, taking it to 270mm saves only in the region of 17 pounds a year (Energy Saving Trust, 2026). The large savings people quote, around 360 pounds a year for a detached house, 190 for a semi and 140 for a mid-terrace, apply only when a loft goes from little or no insulation up to full depth (Energy Saving Trust, England, Scotland and Wales). So paying a few hundred pounds to top up a loft you will then board and crush, or to add depth that cannot touch the room or the summer heat you are actually bothered by, buys very little back. Money spent on the right system, by contrast, keeps paying you in comfort and usable space.
We would be selling you something, not advising you, if we pretended a top-up never makes sense. If your loft is a true cold void, never boarded, never stored in, well ventilated and simply short of depth, then bringing it up to around 270mm with fresh wool is a sound, low-cost improvement, and we will tell you so plainly. The 270mm convention exists for a reason. If that is honestly your loft, do not let anyone upsell you past it. Our guide to loft insulation depth covers hitting the right total.
If you store things up there, plan to board it, have a room in the roof, or find the top floor unbearable in a heatwave, the stronger investment is to insulate at the roof slope rather than pile more wool on the floor. That is where a certified rafter-level system earns its place. We supply and arrange certified SuperFOIL installations, whose multifoil products carry independent BBA certification, which is the figure to trust with any insulation, ours or anyone else's. Insulating at the rafters keeps your loft floor free to use, its reflective faces send summer radiant heat back out, and correctly installed it does not slump or compress the way floor wool does, so the performance you buy is the performance you keep. Our comparison of multifoil versus mineral wool lays out the trade-off in full, and if you have a converted or half-converted loft, see insulating a room in the roof.
Whatever you decide, do not simply bury a loft that has a moisture problem. If the existing insulation is wet, mouldy or has been fouled by vermin, it should come out and the cause dealt with first, not sealed under a new layer. Most loft damp that reaches our advice line traces back to insulation choking the airflow at the eaves, so keep that ventilation gap clear whatever material you use. If your loft shows any sign of damp, read our loft condensation and damp guide before spending a penny on insulation.
The bottom line: topping up is not automatically the answer, and it is often a false economy for a loft that earns its keep. Match the material to how you actually use the space, and spend where the money keeps paying you back.
It depends how you use the loft. For a bare, unused cold void that is simply short of depth, yes, topping up to around 270mm is a cheap, sensible win. For a loft you board, store in, live under or that overheats in summer, adding more floor wool is often a false economy, and a certified rafter-level system is the better long-term buy.
Less than people expect. Going from around 120mm to 270mm saves only in the region of 17 pounds a year (Energy Saving Trust, 2026). The big savings, up to around 360 pounds a year for a detached home, apply only when a loft goes from little or no insulation to full depth. If you will then board and crush the wool, the spend rarely returns.
Not because of its age. Dry, intact mineral wool still works and does not expire. Remove it only if it is wet, mouldy, vermin-fouled or crushed, and fix the cause first. The more useful question is whether adding more floor wool is the right approach at all, or whether a rafter-level system suits how you use the loft.
Because floor wool cannot solve those problems. It does nothing for a room in the roof, little for a boarded loft once the wool is crushed, and almost nothing against the summer radiant heat that comes through the roof. Those need insulation at the rafters, not more depth on the floor.
Only on raised loft legs or a platform. Laid straight onto the joists, boards crush the insulation you just paid for and leave you under-insulated. If storage or using the loft matters to you, a rafter-level system keeps the whole floor usable without that trade-off.
For a loft you actually use, a certified rafter-level multifoil system such as SuperFOIL. It keeps the floor usable, reflects summer heat, and correctly installed it does not slump or compress the way floor wool does. Certified SuperFOIL carries independent BBA certification, which is the figure to trust.
We supply and arrange certified SuperFOIL installations, and we will still tell you if standard mineral wool is all your loft needs. Tell us what is up there now and how you use the space, and we will give you the honest answer for your home, then, if you want, match you with vetted, accredited installers so you can compare no cost quotes.
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Insulation Advice Centre