Roof Insulation - Advice Appreciated

[defsdoor]

We had our roof re-tiled a couple of years ago as it had no underfelt and the tiles were beginning to show their age.

The finished result looks great as we kept with clay tiles but unfortunately the loft space is absolutely full of fallen lime cement from the underneath of the tiles (I did ask for it to be tarp'd but thats another story). I've filled over 15 sacks just clearing around the loft hatch and have decided that it's going to be much simpler just to skip all the fibre glass insulation and lime cement and insulate it anew.

What I'd like to do though is instead of insulating the ceiling (loft floor) - which is partly boarded with fabulous narrow tongue and grove planks - I'm thinking of putting celotex in between the roof rafters.

The rafters are 4" deep and 16" centres. (14 inch gap).

So, I'd like to know what thickness celotex I should use. Should I leave a gap between the felt and the celotex ? and, eventually should I need a vapour barrier before we board it (long time away).

Anyone got any experience of this or advice ? Perhaps I should get it spray foamed instead - if thats an option ?

 

[Phil Pascoe]

Have a look at Seconds & Co for prices - If you have to cut to fit you might pick up a bargain/mixed lot.

 

[RobinBHM]

I would go to the manufacturers website, kingspan, celetex, recticel or ecotherm and look for pitch roof or loft conversion, ventilated. Insulation between and under rafters.

Insulating at rafter level means you are including the loft space within the heated envelope of the home, so you will have a larger volume to heat.

Probably you will need to leave a 50mm air void at the top of the rafters if your tile battens are fitted straight to the rafters and tyvek breather memrane directly onto rafter. Its difficult to fit insulation tight so you may need to foam any gaps, foil tape all joints. Then fit another layer of insulation below the rafters. A vapour barrier should be fitted on the warm side to stop vapour reaching the cold side and forming condensation.

Detailing of the insulation at the eves may be awkward since the insulation needs to come down to ceiling level but keeping soffit ventilation open to air space at tile level.