Hemp Insulation vs Mineral Wool
A natural fibre with a genuine technical advantage, and a price that means it has to be the right building.
At a glance
| Hemp insulation | Mineral wool | |
|---|---|---|
| Thermal conductivity | Broadly comparable to mineral wool | Broadly comparable to hemp |
| Density | Higher, so more thermal mass | Low |
| Summer performance | Better: slows heat entering through the roof | Weaker in a converted roof space |
| Moisture behaviour | Hygroscopic, buffers vapour well | Loses performance when wet |
| Fire performance | Treated; organic fibre | Non-combustible |
| Handling | No itch | Itchy; protective kit needed |
| Embodied carbon | Very low; hemp absorbs CO₂ as it grows | Higher; energy-intensive to produce |
| Availability in Scotland | Specialist suppliers | Every merchant |
| Cost per m² | High | Low |
Where hemp genuinely differs
As with sheep wool, the thermal conductivity of hemp insulation is broadly comparable to mineral wool, so at the same depth the two slow heat loss similarly. The interesting differences are elsewhere, and one of them is underrated.
Density and summer heat
Hemp batts are considerably denser than mineral wool. That gives them more thermal mass, which means heat takes longer to pass through the material. In winter this makes little practical difference. In a converted loft room in July it makes a very noticeable one, because the roof is the surface taking direct sun and a low-density insulation lets that heat through into the room over the course of the afternoon.
If you have a room in the roof that becomes uncomfortable in summer, this is the property worth paying for. In an unheated cold loft that nobody occupies, it is irrelevant.
Moisture
Hemp is hygroscopic, absorbing and releasing water vapour without a large loss of insulating value. That suits traditional breathable construction, which describes a great deal of Scotland's pre-1919 housing. Mineral wool has no such capacity: wet wool is compromised until it dries out.
Where mineral wool wins
- Cost. Substantially cheaper per square metre, and in an unoccupied cold loft it does the same thermal job.
- Fire. Non-combustible, which hemp as an organic fibre cannot claim even when treated.
- Availability. Stocked everywhere, in every depth, with no lead time. Hemp usually means a specialist supplier and a wait.
- Scheme acceptance. Funding schemes are built around conventional measures. Specifying hemp may complicate a grant application.
- Pests. Mineral wool holds no interest for insects or rodents. Natural fibres need treatment.
Where hemp wins
- Summer comfort in occupied roof spaces, through density and thermal mass.
- Moisture buffering in older breathable roofs where a membrane was never fitted.
- Handling. No itch, no fibres, no protective kit beyond gloves.
- Embodied carbon. Hemp absorbs carbon dioxide as it grows and needs little processing. On this measure it is one of the best insulation materials available.
- It holds its shape. Denser batts resist the settling that thins a wool layer over decades.
Which should you choose?
Choose mineral wool for an unoccupied cold loft. It performs the same thermally, costs a fraction as much, is non-combustible and is available tomorrow. For most Scottish homes this is the right answer and we will say so.
Choose hemp if you have a converted room in the roof that overheats in summer, if you are working on a traditional breathable building where moisture movement is part of how the roof functions, or if embodied carbon is a priority you are willing to pay for.
The mistake we see is people buying natural insulation for an empty loft on the assumption it will save more energy. It will not. Buy it for the properties that actually differ.
Frequently asked questions
Thermally they are broadly comparable at the same depth. Hemp is denser, which improves comfort in occupied roof spaces during summer, and it buffers moisture better. Mineral wool is cheaper, non-combustible and far easier to source. Neither is simply better; they suit different situations.
In an occupied roof space, yes. Its higher density gives it more thermal mass, so heat from a sun-facing roof takes longer to pass through into the room below. In an unheated cold loft that nobody uses, the benefit is largely academic.
Insulation-grade hemp is treated to improve fire performance and is used routinely in construction, but it is an organic fibre and cannot match mineral wool, which is non-combustible. Where fire performance is the priority, mineral wool is the stronger choice.
Funding schemes are generally built around conventional insulation measures, so specifying a natural fibre may complicate an application. Check with Home Energy Scotland before committing to a material if funding is part of your plan. We are not registered to deliver grant-funded work ourselves.