Ventilating my rafters?

[Lobster]

I've spent a happy(?) weekend ripping down the old lath and plaster from the slopey ceilings of one of our upstairs bedrooms, which is built into the roof (like a loft conversion, except it's our 1st floor and that's how the house was built).

As there's currently nothing between my daughter's bed and the sky except for one layer of lath and plaster and one layer of rooftiles, I'm fitting Kingspan between the rafters and overlaying the whole lot with insulated plasterboard with a view to reducing both the whingeing next winter, and my fuel bills.

Anyway - I'm mindful of the need to maintain an airgap between the Kingspan and the tiles to allow a through draught... but having exposed the rafters today I find there is no gap to the air at the bottom as I'd expected; it's totally sealed. There's a wallplate on top of the solid brick wall, and the rafters sit on that; clearly the house must have been built that way (1912). No sarking; tiles are bedded in original mortar which is crumbling somewhat.

I can't see any way I can introduce any ventilation in there; there's no eway through to the soffits. All I can think of is to fit ventilation tiles in between every rafter (that would be about 20); it would be awkward, expensive and look pretty hideous on a period property.

Is there any other way of doing this? Or should I just ignore it? Gut feeling says that although it's been fine for 100 years and all the timbers are fine, it might be a different story once I introduce 21st-century insulation and vapour barriers...

Any thoughts please?

Thanks
David

 

[CHRIS]

I have seen it many times when people try the kingspan method to get a warm loft insted of the normal insulated ceiling, and all I can say is sometimes it works and sometimes it does not. You must try and get air flow behind the kingspan but without vents this is hard. I tried using some "Free" kingspan that I once had but ended up pulling it out as it started to feel very stuffy in my loft and had a funny feeling it could go very wrong very quickly!

You may want to try using breathable membrane (on cold side)stapled to rafters then 100mm x 2 sheeps wool under rafters with a clay board and a lime finish to retain breathability (I know your daughter would rather wear a gortex jacket than a nice PVC one) you may loose some head room but would be much better than insulated plaster board , also you could drill a few soffit vents to allow air flow?

http://www.secondnatureuk.com/where.htm

and look at the various methods of instalation

PS the pulling down of a lath and plaster ceiling may not go down too well with some on this site so go easy on that poor old building!!!!!! but whats done is done - but the old ways are normaly the best ways!!!!!

 

[biffvernon]

Spot on, Chris. Thermafleece is lovely stuff to work with and much better for both planet and loft than Klingspan. Replacing lath and lime plaster the old way is an absorbing pastime.