Very OT! Roof insulation

My new house has no insulation in the roof, which is a pain. I’ve got some insulation, and the guy at the builders yard said to just whack it in there, but there’s just wooden slats between that and the tiles. Should I put roofing felt in forst, before the insulation? If so, should it be flush with the slats or tacked between the rafters?

thanks all!

What insulation?

The roof – you can see underside of the tiles and the battens across the roof that the tiles are on?

Generic, cheap synthetic (envrironmentally friendly) lagging stuff. and yeah, I can see tiles, and all the gaps in between them!

My house has plastic sheeting stuff under the tiles, so any water that might get through wouldnt get past this.

plenty of older houses don’t have any felt/plastic under the tiles/slates

Just put insulation on the attic floor

**** me, a whole forum on loft insulation;

loft insulation bores paradise

although it would appear they all gave up the will to live about 18 months ago.

Your main priority is to insulate the ceiling of the room below the loft. So lay the insulation between the ceiling joists up in the loft.

Insulating under the tiles directly is just going to keep your loft warm and your house cold.

That’s not quite true, but you will end up using a lot more insulation and have to find some means of fixing it up there.
Don’t put insulation over electrical cables or any recessed ceiling lights.

Probably help if I mentioned relevent factors, its a loft conversion! Thanks for the advice so far, I’m inclined to think some sort of plastic sheet or felt will be required?

To felt or sheet it properly though your going to need to remove the tiles and roof battens. Do you know how long ago the conversion was done? It seems strange not to have any felt unless it was done a long time ago or by cowboys.

If, looking at your roof from the inside and see slates, resting on battens (inch square maybe) which run horizontal, which in turn sit on rafters (four by two inch maybe) running from the head of the wall to the ridge board at the apex, then you’ve got a typical pre war roof. Felt or hessian sarking material as an additional layer is sometimes there, sometimes not. If you’ve got something like rockwool, you need a really big depth (200mm+) to get up to spec. To fit this you’ll have to add a big structure to the existing rafters so you can get that depth without compressing it. With most of this stuff you have to leave the last 50mm below the slates and battens clear to allow for airflow otherwise any moisture rising through the insulation or finding its way from above will not be able to get away.
So if you’re going for rockwool style stuff you ave to build out a frame to tuck it into which is rigid enough to plasterboard to.

Next option is if you’ve got high density rockwool slabs, easier this time as they’re only two inches thick. you could whack these straight in between your rafters if the rafters are bigger than 4" so they sit flush and still leave an airspace behind(you may need to add wire or netting to keep it from shifting). They don’t usually quite get you up to building regs part P, but if this a DIY renovation who’s to know when it was done, before or after PartP? To boost their u value thjough you could easily add rigid board such as Kingspan below before plasterboarding, or quicker still use plasterboard with insulation pre-bonded.

A rigid board such as Kingspan could be put between rafters (same as before you won’t get full depth reqd by regs just between rafters), but it’s less hassle to run it(70mm+) under rafters and attach plasterboard below. Because of the hassle of fixing plasterboard with huge screws it’s as easy to fix a run of battens in 2×1 or so underneath it to fix plasterboard to.

Easiest of all is Triso 10 which is a big roll of multilayer foil, just 30mm thick. Roll it out along the rafters, stapling as you go, overlapping and foil taping the edges. Add a layer of battens to make an airgap to the plasterboard and nail them up. You need noggins at the PB edges but that’s it. Oh I appear to have gone on a bit.

Someone who really knows the ins and outs (unless copy and pasted!) 😉

You say it’s a ‘loft conversion’ but by definition if you can see tiles etc the roof space has not been converted into a room per se. If you are not going to inhabit the loft space then simply insulate the ceiling per Anthony’s comments above.

If you are going to use the loft as a room (i.e heat the loft space)then the loft area needs to be thermally isolated from the (cold) roof or you will get major condensation issues. I’m sure a builder can correct me but for a loft conversion the build up is typically:

Roof tile
Tile batten
Felt
Truss
50mm air gap
50mm insulation
plasterboard

cheers midlifecrashes… that was very thorough!