Help with loft conversion please

[dogbower]

we are renovating a victorian property, big thick stone walls, slate roof etc.
We are converting the loft and this was the area we first started on. following advise from building regs we installed kingspan type insullation in the joists and then, because we were short of head height covered this with a tri iso type multi layer foil.
I got as far as plasterboarding some rooms and then moved downstairs to do jobs in the warmth. the loft has a staircase so is linked to downstairs but we blocked up the bottom of the stairs to keep warm downstairs.
Going back upstairs to fit some heating and get on with the rooms i noticed some damp issues.
I think i have made a silly mistake and not allowed enough ventilation around the joists so condensation is building up.
although a bit peed off I just want to make sure any fix I do is the right one.
I believe a picture might help so

The rafters are 75mm thick, with another 25mm to internal tile face. The kingspan is 50mm.

I am thinking i needed to batton on the underside of the rafter, 40 mm or so, and then attach the foil stuff? I would then batton again then attach the plasterboard. Dont know why I didnt do this the first time.
Sound sensible?

 

[Deluks]

Is it an unfelted roof? What's the condition of the roof tiles? Looks more leak-ey to me.

 

[dogbower]

Hi
yes roof is unfelted.
it is in very good condition and any leaks delt with before this process.
it is 100% an issue with condensation.
having had a night to think about this I believe my desire to keep the head height has meant I have compromised all the air spaces and this has caused the problem. But I need some advise as to how to put it right.
Anyone?

 

[jeds]

I don't see an easy way to solve this. Top and bottom venting might help but the gap is so small it will be hit and miss whether it works or not. Vented slates might also help but probably even more hit and miss than the top and bottom venting. Otherwise you are into stripping and relaying.

 

[dogbower]

Thanks John
I am resigned to the fact i will have to strip it all out and start again, just glad i didnt get to the plastering stage.
I am thinking what I need to do is-
1. drop the kingspan down to allow min 50mm gap between it and slates.
2. then allow another 40mm to the foil stuff.
3. and again 40mm to the plasterboard.

So I will end up with, slates, 50mm air gap, 50mm kingspan, 40mm air gap, multi layer foil stuff, 40mm air gap, then plasterboard.

I assume as my roof isnt felted no additional ventilation needs to be added on fascia or ridge?

If you look at the pictures you can see my roof has horizontal beams every 1200mm or so, these would now go below the plasterboard however surely these will stop the roof to fully breathe. Each section will be seperated by the beams, so do I just cut some holes in the beams to allow the air to flow through the whole roof?
Your thoughts
Stuart.

 

[dogbower]

Hi
I have just been upstiars and wondered could I do it another way-

Foil straight onto underside of joists then knigspan, and then plasterboard.
this would give a nice big airspace around the joists.
Would I need to create an airgap below/over the kingspan? If so how big?
Thanks Stuart.

 

[dogbower]

Anyone got any answers?
I am on holiday next week and I am looking forward to sorting this out then.
Stuart.

 

[jeds]

You know how it works. Warm vapour travels through the roof layers and when it cools to x temperature it condenses to water - which in your case then drips back down onto the internal layers. There are two standard ways to avoid that happening. (but both involve a membrane)

One is to provide a vented cavity above the outer insulation layer. the cavity should be a minimum of 50mm and the ventilation must be continuous up and over the roof from eaves to eaves or from eaves to ridge. It is very important that the cavity is clear 50mm - not 50mm with battens in the way. Here's where it falls down though; that system is only safe when the cavity runs UNDER a membrane - which you don't have! Otherwise vapour will still reach the underside of the slates and will still condense and drip down.

Second option is a breathable membrane. This allows vapour through which condenses on the slates and drips down but is then caught by the membrane.

So there is no way to be 100% safe without a membrane. Everything you do without a membrane is a compromise which might work - but might not. Or which might work most of the time - but some of the time it won't.