Insulating loft between rafters

11 Sep 2012
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United Kingdom
I am looking for a bit of advice on how to insulate my roof between the rafters. As a bit of background my loft has previously been used for storage and we are about to put some walls up so I can put a desk there. The intention is that we will also have a radiator in the room. The loft already has a good layer of insulation beneath the floor.

The roof is tiled with a thick felt behind the tiles. There is ventilation at the apex of the roof and also down at the eaves. I got a deal on some Knauf Earthwool Flexible Slab (50mm) insulation so figured we could put that up prior to putting gyproc on. I have a few questions:

1. Should I place the insulation directly against the felt or do I need some type of membrane between the two?
2. My rafters are 95mm so there will be around 45mm excess by the time I place the insulation in. That means I need to pull the insulation tight against the roof. My plan was to install screws into the rafters then wrap twine in a zig zag pattern between the rafters to pull the insulation boards onto the roof. Does that sound reasonable?
3. My plan was to still leave the ventilation gap at the apex of the roof and also to keep a gap down at the bottom of the eaves as I assume the ventilation there for a reason. Is that the correct approach?

Thanks in advance. Hopefully my approach isn't total nonsense.
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Hmm, all sounding a bit Heath Robinson, anyway, Building Regs, fire regs etc aside, if you are intent on using your paltry 50mm earthwool I would fix a rudimentary network of wire (think green garden wire) suspended at the bottom of the rafters with staples or similar to support the earthwool, This will leave a ventilation gap between the insulation and the felt which should be open to the vents at the eaves and the ridge, where you conveniently have a vent already. Then board over the lot with foil backed plasterboard.

The gap is essential and it must be between the insulation and the felt to allow any moisture that does get through the plasterboard and insulation to escape though the vents at ridge and eaves. Do not press your insulation up to the felt.
I assume it has a separate suspended / joisted floor and you are not just using the ceiling beams as the loft floor to put your office on top of?
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If you are only going to use the room occasionally having such little insulation kind of makes sense.

But if you plan to use it regularly, 50mm is anemic.

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