Insulating and boarding an attic with lath and plaster ceilings below

Your loft joists will most likely be 100mm. Remove all the old insulation and then purchase some 100mm loft insulation role. If it's 75mm joists then buy 75mm thick loft insulation. Some older properties might have different size joists so you just need to check first. If you do it correctly then no weight or pressure will be applied to the ceiling plasterboard. It's recommended to have between 200 and 300mm of insulation in the loft so if you're installing floor boards you will also need to add another level to the existing joists running in the other direction in a criss cross. In either of the above cases you can add some 150mm timber then add 150mm loft insulation. It would take you to either 225 or 250mm which would give you inbetween the recommended thickness of loft insulation in the UK. Also use screws instead of nails to avoid vibration and check to see if the ceilings have any damage. If they don't have any damage you will be all good. I hope this helps and good luck with your project.

As per previous answer raising the height 150 / 200mm timber to accommodate the 150/200 mm loft insulation is the best option. Be aware of cable entrapment , ensure there is plenty of ventilation around any electrical wiring. Especially on the heavier supplies for cookers and showers. Good luck

Right, let’s get the facts right and clear here: The recommended loft insulation thickness currently stands at 275mm So, remove the old insulation and add 100mm base layer (parallel with the joists) then 200 super top up (lay across) would bring you just above the recommended guideline. Please make sure that the insulation has nothing resting on top, as it’ll loose the insulating properties and may also cause condensation. When it comes to boarding up, there are loft legs you can buy and fix them up to the original joists then board over. Just make sure you are above the (300mm) insulation If the loft legs are not to the required height, then an extension on the joists is a must then proceed with the rest. It is important to keep that 300mm space where the insulation is. Also, a vapour barrier would be needed just under the boards (loft boards) so, if and when condensation happens, it doesn’t come through the boards and creates wet/damp patches. Lastly, with all this work required, expect the lath and plaster ceiling below to crack as there will be a lot of movement happening. Hope this helps and good luck

✅ What Will Work (Safe + Cheap Plan) 1. Insulate Properly You’ll want 270mm total insulation: First layer (100mm) between the ceiling joists (rockwool/mineral wool) Second layer (170mm) cross-laid on top, perpendicular to joists ⚠️ This will rise above the joists — so you can’t lay boards directly on top. 2. Raise a Platform Above the Insulation Here’s the safest & cheapest method for storage: 🛠 Use Loft Legs (or similar plastic risers): These are cheap riser posts that screw into the joists and allow you to lay chipboard above the insulation, without compressing it Gives you a stable platform and protects the ceiling below from weight 💰 Approx cost: Loft legs: £25–30 per 12-pack Loft boards (chipboard): £10–12 per 1220×320 panel 3. Don’t Overload the Storage Area Use it for lightweight items only (boxes, clothes, books, etc.). Avoid: Heavy furniture Large gym equipment Water tanks (should be independently supported) ✅ Optional: Add a Vapour Barrier Not required in every case, but: A foil-backed insulation or polythene layer can help reduce condensation risk if the attic is poorly ventilated