Choosing Insulation for a Loft Conversion

By Party Wall or Boundary Dispute: Resolving Neighbour Issues
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20 Jun 2026

Choosing Insulation for a Loft Conversion

Why the roof becomes the hardest part of the job

In a loft conversion, the roof stops being a roof and becomes the wall of a habitable room. That single change makes insulation the most awkward decision of the project, because you are squeezing thermal performance into the depth of a rafter while also trying to keep enough headroom to stand up in.

In England and Wales, a converted loft roof is treated as a new thermal element, and Building Regulations typically look for a U-value of around 0.16 to 0.18 W/m²K. Scotland and Northern Ireland set their own figures, so confirm the target with your building control officer before you order anything. That number matters because a typical rafter is only 100mm deep, sometimes as little as 75mm, and a rafter depth alone will rarely get you there.

You have three realistic routes: insulation between the rafters, between and under the rafters, or above them in a warm roof build-up. From inside a loft, the middle option is almost always the practical one, and the choice of material then comes down to how much thickness you can afford to lose.

Rigid foam boards (PIR and PUR)

Rigid polyisocyanurate boards have a thermal conductivity of roughly 0.022 W/mK, which is the best performance per millimetre on the domestic market. Where headroom is tight, they are usually the only way to hit the target without dropping the ceiling line dramatically.

Thickness needed: around 120–140mm in total to reach 0.16–0.18 W/m²K, usually split between the rafters and a continuous layer beneath them.
Space: the winner. Losing 50mm under the rafters is far easier to live with than losing 100mm.
Fire: foam boards are combustible and will need covering with plasterboard. They should never be left exposed in a loft, and the roof void needs to be treated as a concealed space when you plan fire protection.
Installation: unforgiving. Boards must be cut tight and any gap foamed. A 5mm gap running along a rafter can knock a serious dent in the real-world performance of the whole roof.

Mineral wool

Mineral wool is the traditional choice and still the most forgiving. It is cheap, it is non-combustible, and it is excellent at absorbing sound — which matters in a loft, where rain drumming on the roof covering is a genuine quality-of-life issue.

Thickness needed: roughly 160–180mm, because its thermal conductivity sits nearer 0.035–0.040 W/mK.
Fire: the strongest performer. Non-combustible, with no contribution to fire spread. If your loft sits close to a party wall or beneath a flat roof section, this is a real advantage.
Space: the weak point. You need depth, and depth costs headroom.
Fitting: easy to push around pipes, trusses and awkward junctions, but it must not be crammed. Compressed mineral wool loses performance, and loosely packed wool slumps over time.

Natural and breathable options

Wood fibre, sheep wool, hemp and cellulose insulation are increasingly popular, particularly in older properties where you want the roof to manage moisture rather than trap it.

Wood fibre: rigid boards work well under rafters, adding a little thermal mass and buffering moisture. Conductivity is around 0.038–0.042 W/mK, so you need more thickness than PIR, and the boards are heavy.
Sheep wool: lovely to handle, excellent at filling irregular voids, and hygroscopic — it can absorb and release moisture without losing performance.
Cellulose: blown or damp-applied, ideal for awkward or irregular rafter bays that boards cannot fill neatly.
Fire: most natural products are treated to achieve a B-s1,d0 reaction to fire, and wood fibre forms a protective char layer. None of them are non-combustible, so treat them as you would foam.

The hybrid approach most lofts actually need

In practice, very few loft conversions use a single material. A common and sensible build-up is 100mm of rigid foam between the rafters, cut tight and foamed, with a further 40–50mm continuous board beneath the rafters before the plasterboard.

That continuous layer does two jobs. It reduces thermal bridging through the timbers — which occupy roughly 10–15% of the roof area and quietly drag performance down — and it reduces the risk of cold strips showing up as pattern staining on your new ceiling. Mineral wool between the rafters with a PIR board beneath is another sound combination, giving you fire and acoustic performance with a thinner overall build-up.

Ventilation, airtightness and sign-off

Get the moisture strategy wrong and the insulation is the least of your problems. If your roof has an old impermeable underlay, you will normally need a 50mm ventilated cavity above the insulation. With a modern breathable membrane, many systems allow insulation to sit tight against it, but that depends on the membrane and the manufacturer's approval — check, do not assume.

On the warm side, a continuous vapour control layer, taped at every joint and sealed around penetrations, is essential. So is airtightness at the eaves and around roof windows, downlights and loft hatches.

Finally, remember that insulation feeds into your fire strategy. A loft conversion needs a protected escape route, and how you build the roof — including what you wrap the insulation in — will be part of that conversation. Take your proposed specification to building control early. It is far cheaper to adjust a drawing than to strip out a finished ceiling.

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5 Comments

Robert Sonny says Jul 21, 2024 at 10:00am

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Reply
- John Abraham says Jul 21, 2024 at 10:00am
  
  I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
  Reply
  - Robert Sonnysays Jul 21, 2024 at 10:00am
    
    I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
    Reply
Robert Sonny says Jul 21, 2024 at 10:00am

I must explain to you how all this mistaken idea of denouncing pleasure and praising pain was born and I will give you a complete account of the system
Reply

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