Hemp goes into a wall differently from mineral wool, and most of what goes wrong is a build-up designed for the other one. Here is the order of the layers, the arithmetic, and the four failures worth knowing before you buy anything.
A timber-framed wall or a studded-out masonry one, read from the room outwards. Thicknesses are typical rather than prescriptive — the frame you have decides most of them.
Inside · the roomOutside →
Lime finishes let the wall keep breathing. Gypsum board works too, but it is the layer most likely to be sealed with a vinyl paint that undoes everything below it.
Battens holding a gap for cables and sockets. Its whole job is to keep electricians out of the membrane behind it. Skipping it is the cheapest way to perforate your airtightness.
Not a polythene vapour barrier. A membrane that closes in winter and opens in summer, so moisture that gets in can get back out. This is the single most important choice on the page.
Friction-fitted, cut about 10 mm oversize so it holds itself without being squashed. Full depth of the frame, no gaps at the edges, no compression.
Keeps weather out while letting vapour pass. Board adds insulation and kills the thermal bridge across every stud, which is worth more than it sounds.
A drained and vented gap so anything that gets past the cladding runs out at the bottom instead of sitting against the wall.
Volume, weight and the insulation value you end up with. Batt figures assume a typical density of 35 kg/m³ and a thermal conductivity of 0.039 W/mK.
Add about 10% for off-cuts around openings, and buy the whole wall at once — density varies a little between production runs, and a wall built from two batches is a wall with a seam in it.
The insulation itself, and the two layers people most often get wrong.
Three properties that decide where hemp is the right answer and where it is not.
Hemp fibre absorbs moisture into the fibre itself and releases it again as conditions change, without losing much insulating value while it does. That is why a hemp wall is usually built vapour-open rather than sealed: you are managing moisture, not fighting it.
Hemp stores roughly 60% more heat per kilogram than mineral wool, so afternoon heat reaches the room hours later — in the evening, when it has somewhere to go. On a south-facing roof that is the difference between a usable room and an unusable one in August.
The plant took carbon out of the air to grow, and the wall keeps it there for as long as the wall stands. Manufacture is low-temperature. It is one of the few building products where the honest number is negative.
Typical ranges for hemp insulation products. Individual products vary; the figure that matters is on the pack.
| Depth | Insulation value | Roughly equal to | Weight per m² |
|---|---|---|---|
| 100 mm | R 2.6 | a warmer-than-nothing retrofit | 3.5 kg |
| 140 mm | R 3.6 | a standard timber frame | 4.9 kg |
| 160 mm | R 4.1 | current new-build practice | 5.6 kg |
| 200 mm | R 5.1 | low-energy territory | 7.0 kg |
| 240 mm | R 6.2 | passive-house territory | 8.4 kg |
Four failures, in the order they cost you money.
Specified out of habit from a mineral-wool detail. It traps in the wall exactly the moisture hemp is good at moving, and turns the material's main advantage into its main risk. If one layer on this page is worth paying more for, it is this one.
Squeezing a 160 mm batt into a 140 mm stud bay does not give you 160 mm of insulation. It gives you 140 mm of denser, slightly worse insulation, and a bowed board. Cut to the bay.
A 5 mm gap running the height of a stud loses far more than 5 mm of missing batt suggests, because air moves through it. Friction fit, cut generous, and check the corners by eye before the membrane goes on.
Hemp handles humidity; it does not handle standing water or a wall that is still wet from a leak nobody found. Fix the source, let it dry, then build. This is the failure that gets blamed on the material.
It chars rather than melts or drips, and behind plasterboard or lime it is protected the same way any insulation is. It is not a fire barrier and is not sold as one — the board in front of it does that job.
There is no food value in the fibre, and most batts are treated against insect attack. What actually attracts mice is a gap they can get through, so the answer is the same as for every other insulation: close the holes.
Yes, and it is one of the more pleasant materials to work with — it cuts with a bread knife and does not itch, so no mask-and-gloves ritual. The membrane and the airtightness taping are the parts worth being fussy about.
Per square metre it costs more than mineral wool and less than sheep's wool. The comparison that matters is the whole build-up, because a vapour-open wall usually saves a layer somewhere else, and the summer performance does not show up in a price per m² at all.
Uncontaminated hemp batt composts. In practice that depends on getting it out of the wall without the membrane and the plaster attached, which is a demolition question more than a material one.