Help with new loft project and loft conversion

Hi all,
Please bear with me and I'll explain my project.
I have an end terrace house with a two story extension. We are booked in to have a full loft conversion, but the local council (Thurrock), will not let me have a dormer over the extension side so the dormer and conversion can only be under PD on the original side of the house. This means I will have a separate side for storage. It will be sectioned off with a door once the conversion is done by the builders.
This leaves me my project of creating a nice storage space over the extension.
When the extension was built (before we moved in), the builder made it conversion ready (as you can see by photo's below).Bigger joists and no structures floor to ceiling.
When I moved in I quickly did some loft boarding on the floor and that's how it's stayed for over 10 years.
What I would like to do though is create a warm loft instead of the freezing cold one. Also want to create in the eaves each side some storage cupboards.
Now my couple of questions:
1. the space between each joist on the ceiling is 350mm wide and 150mm deep. I've researched and think that the best insulation would be insualtion boarding (celotex for eg), but have costed this up and for just between the joists its over £500, plus from what I've read ideally another thinner layer would go over that!!
I don't really have that extra cash and for a storage area was wondering if normal insulation (earthwool) would be sufficient, or would it still be cold?
2. If I can use normal insulation, should I get 100mm dep for an airgap, or squeeze 200mm into it.
3. Would it be best then to get sheets of 25mm insulation board as a cover before I plaster board? or can I just plaster straight over insulation between joists?
4. What could I do on the wall to insulate (with the white breeze blocks-Picture #1)
5. As you can see, there is a steel running through the middle which is higher than the loft boarding. I intend on purchasing 15 scaffold boards and laying them down to raise the height, so I could then underlay and laminate flooring at the end. Would this be correct?
6. Lastly, as you can see in photo #2, there is an extra wide random joist sticking out. Could I cut this flush to the floor? Can't see any reason for it being there. I intend on hiding the pipework in the storage that side so that shouldn't be an issue
Thanks in advance for any help...I'm not a builder, but not afraid to put my hand to anything, just not knowledgeable in this area at all (as you can probably tell )
Also, if there's anymore advice or tips you can give me that'd be great

 

use this insulation instead of celotex type,

https://www.roofingsuperstore.co.uk/browse/loft-insulation/insulation-slab.html

think you'll need a vapour barrier also, though i'm not clued up on such matters.

the lump of wood in the middle, need to know what its connected to, in order to advise if it can be cut.

was the original intention to have this room as an ensuite or something?

 

Thanks for reply. Would the slab insulation go over all the joists and still use the fibre glass stuff between the joists?
As for the vapour barrier, would that go under the slab insulation (fitted directly onto joists)
I think the lump of wood is connected to a joist
I don't know what they planned to do when they built the two story extension as it was done before we moved in. The pipework there at the moment is above the master bedrooms en-suite below.

 

this video from knauf explains what needs to be done.

obviously this is for knauf products, but i believe the principles essentially the same

 

Priced up the insulation slabs and it's double the price of the Celotex slabs. Could I not use the Fibre glass stuff? so much cheaper...would it cause condensation problems (or something else)

 

Yes you can think the slabs just make it easier but at double the price not worth it

 

There is an argument that insulation won't make an unheated space warmer (you need heating to do that). Personally I think you'd get a bit of heat from the adjacent heated parts of the house. Insulation really needs to be PIR (celotex), 100mm between rafters with a 50mm airgap above and whatever you can afford under. Gable needs to be celotex as well (minimum 100mm stuck or battened on and covered with plasterboard). Tape all joints with foil tape. You just need to find it cheaper: ebay, seconds suppliers, Facebook etc rather than proper suppliers. You could get away with cavity batts between the rafters if you see any cheap but ensure they don't interfere with the air gap.
If you're short of cash and it's just storage I wouldn't worry about the different floor levels.

On a more general note, I know there isn't much height there but that space could make a nice ensuite bathroom.

 

Cheers for info mate. I'll keep my eye out for sheets of 100mm Celotex.
We're having a games room in the converted part (Pool table Cinema room etc) as we don't need an extra bedroom, so no need for the en suite, good thinking though.

 

Have had a look around and is so much more expensive than the fibre glass roll stuff (earthwool). Looked on FB and ebay and none around my area which are that much cheaper than retail. Obviously the earthwool stuff is not as good I understand, but If I ended up using that , with 25mm celotex sheets underneath, would that be ok?
Would I use 100mm still and leave the air gap or put full depth of 150mm?

 

Better than nothing, you still need the air gap and a way to stop the earthwool blocking it.

 

would the 100mm depth in the 150mm gap be ok for the airgap? How would I stop the earthwool blocking it otherwise? Sorry, I really know nothing about this

 

Yes. Normally you'd use battens fixed to the sides of the rafters to keep rigid insulation in place. Rockwool isn't designed for this so I don't know - maybe chicken wire mesh or plastic netting.

 

thanks mate

 

Have made the decision and gone the 100mm pir with 50mm airgap, 25mm pir sheets to cover then vapour barrier over that. I'll then plasterboard on top of that. all came to £715 minus plasterboard (will buy that once the rest is done)
Right decision do you think?