How can I improve the insulation in my loft?

Look at the recent thread "Loft Insulation - 270mm?"

Yes, you could put celotex between the joists. It is about twice as good (or perhaps a bit more) as glass fibre.

Putting celotex/kingspan up on the rafters will be the best solution, provided you maintain 5cm ventilation space between it and the tiles (and it is vented top and bottom to the outside). The work to do this is likely to be considerably more extensive, but it will result in insulation to modern standards and give you a warm loft, which is better for any plumbing and storage up there.

Christian.

A warmish loft, as it is still vented to the outside air, which may be entering at sub zero temperatures.

The ventilation is outside the insulation, between it and the tiles, the loft itself should be pretty airtight to make the insulation worth installing! The ventilation requirement is to prevent the roof rafters/purlins/etc. rotting in a damp atmosphere, and requires a 2" gap between foam and tiles and sarking, and vents at the ridge line and in the soffits to allow a through-draught. . Hope that helps, Dave H. (The engineer formerly known as Homeless)

I'm considering doing the same/similar but can't get my head round the ridge vents. Do you have to install one vent per 2 bays i.e. half the vnet does one bay the other the other or is there a more obvious solution to linking the tops of all bays together?

"Dave H."

"Dave H."

The message from Rob Horton contains these words:

Yes you can - you just add another set of rafters at right angles and put the floor back on top.

I've just done it and though it wasn't a very pleasant job it's worked fine.

Sorry, I was answering the poster who wrote of venting the (now warm) loft to the outside with ingress of "sub-zero" air - and the insulation should of course have a vapour barrier on the "warm" side to prevent condensation (water vapour's not much of a problem until it condenses!) - the ventilation between insulation and tiling is required unless one has "breathable" vapour-permeable (or no) sarking.

Dave H. (The engineer formerly known as Homeless)

"Dave H." ventilation between insulation and tiling is required unless one has

Even then you need cross batons and the tiles proud so air flows between membrane and tiles and out the roof tile.

A full warm roof/warm loft is fully breathable. The Spanish have a concrete roof with tiles over and nothing else. Vapour does not cause problems in the concrete as the room below is ventilated as normal. They have never heard of insulation in Spain and Italy.

You are confusing my solution with something completely different. I am proposing a warm loft room, with only the level of ventilation required for a habitable room. The eaves vents will vent to the space between the tiles and the insulation, not the room itself.

Christian.

If you do a little flat ceiling bit at the top, you can produce a cross ventilation space at the top. This would only need a few ridge/tile vents, rather than needing each bay to have access to its own vent. Speak to the BCO to see what they will accept.

Christian.

Most houses only need three ridge tiles. Some put ridge tiles all along the apex to keep the roof clean looking.

That is a bit over the top for the OPs request. Your solution gives a habitable room, he is not after that.

Putting insulating between the rafters will make the loft "warmer", but at times it will be quite cool in there as the eves are still vented allowing cold auir inside.

I need to do that also, not just to place boarding on for storage but also so that I can get around the loft to wiring, central heating pipes etc. if necessary. So what is the best way of fixing the new set of rafters to the old? I don't want to cause any damage to the ceilings below by heavy hammering.

Drill a hole in the top joist. Then place in position and use 6 inch screws using a suitable powerful drill. No need to hammer these days.

Personally, I would say that insulating the rafters with Kingspan and then allowing a howling gale through eaves ventilation would be a complete waste of time. Either insulating the floor and keeping the room ventilation open, or insulating the rafters and closing the room ventilation are the only sensible options.

Christian.

The message from Broadback contains these words:

Many of mine just sat there. The boards you can screw down and that'll hold it all in place like a raft. It's not as if there are any lateral forces at work. If you're bothered, noggins screwed to the tops of the old rafters, either side of the new ones would stop them sliding.

Not a full waste of time as in a sheltered position wind through the eves may be minimal. The best is to seal the loft off and allow ventilation through ridge tile.