A roof is not a wall lying down. It takes several times the sun, it has to keep breathing at the eaves, and the problem people are usually solving up there is August rather than January.
Almost every insulation on the market will hold heat in during winter. The difference between them shows up in July, in a room directly under the tiles.
That shift is the whole argument. Hemp stores roughly 60% more heat per kilogram than mineral wool, so the afternoon's heat takes far longer to cross the same thickness. It arrives late enough that you can open a window and let it straight back out, instead of at seven in the evening when the room is still warm and so is the air outside.
In winter the two perform about the same at the same thickness. Nobody should choose hemp for a roof on its winter numbers — they should choose it for the night of 3 August.
The first decision, and the one that decides everything after it. It is really a question about what you want the loft to be.
The loft becomes part of the heated house. This is what you build for a conversion, or when pipes and a tank live up there and you would rather they did not freeze.
The loft stays outside the heated house — cold, ventilated, for storage only. Cheaper, and the right answer for most houses nobody plans to convert.
Doing both is not twice as good; it traps the roof void between two insulated layers with no way to dry. Pick one.
Roofs carry more insulation than walls because there is usually room for it and because heat leaves upwards.
| Where | Depth | Insulation value | Notes |
|---|---|---|---|
| Between rafters | 140–180 mm | R 3.6–4.6 | Whatever the rafter gives you, less the air gap |
| Under rafters, second layer | 60–100 mm | R 1.5–2.6 | Crosses the rafters, so it kills the thermal bridge |
| Loft floor, total | 270–400 mm | R 6.9–10.3 | Two layers, second one laid across the first |
Three of these four happen in the last hour of the job, when someone is tidying up.
Insulation pushed right out to the wall plate closes the gap that ventilates the whole roof. The roof then has no way to dry and the first thing to suffer is the timber, not the insulation. Ventilation trays cost about a euro each and this is what they are for.
A warm roof needs a ventilated gap — around 50 mm — between the insulation and the sarking, unless the whole build-up was designed to work without one. Filling the rafter completely is the most common way to turn a good detail into a damp one.
On a loft floor, laying both layers between the joists leaves every joist as a cold bridge the full depth of the build-up. The second layer must cross the first. It is the same effort and a materially warmer roof.
Boarding straight onto the joists over 270 mm of insulation squashes it to the joist depth and undoes most of it. If the loft has to hold boxes, the boards need raising on battens above the insulation, not resting on it.
At around 35 kg/m³, 300 mm across a loft floor is roughly 10 kg per square metre. That is well within what a normal ceiling carries, but it is not nothing on an old lath-and-plaster ceiling that is already tired. Look up before you load it.
Yes, and crossing the joists with it is the best way to do it. The one thing not to do is put a vapour barrier between the two layers — that is a sealed surface in the middle of a build-up, and it is where the moisture will collect.
Batts cut about 10 mm oversize hold themselves by friction and stay put. On a steep pitch or a deep rafter it is worth running a few strands of twine or netting across as insurance, which costs almost nothing and is impossible to add later.
Noticeably, yes — rain on a roof especially. Fibrous insulation at this density absorbs mid and high frequencies well. It is a side effect rather than a reason to buy it.