Cellotex over, under or between joists

[Hl.house]

I am planning to use 50mm Cellotex insulation boards in between ceiling joists and then 12.5mm plasterboard. IIRC BCO mentioned something about fibre glass. Can't remember if it was for roof or ceiling in addition to 50mm cellotex.
Questions:
1. I do not plan to use loft space at all. Should I put cellotex boards over my ceiling, under or in between?
2. Where do I put fibre glass (Is it rock wool ?)
I am totally confused and very new to DIY. Please advice soon.
Ps. It doesn't have to pass thro' BC.

 

[garyo]

Assuming this is a standard loft scenario then using cellotex is an unusual choice, and 50mm is only the equivalent of 100mm 'normal' fibreglass insulation, and far more work to fit. You don't describe your particular circumstances in detail, but why not just lay 300mm of standard fibreglass?

 

[Hl.house]

This is for a first floor bedroom ceiling with pitched roof and two external walls. Currently it is all open upto the roof. There are ceiling joists but no ceiling at all. Previous broken fake ceiling is completely removed.
I bought the Small 45cm x 120cm cellotex boards as that is the size I can fit in my car and carry upstairs on the windy stairs.
Let me know if you need any other details.
Cellotex is not my preferred option as it is a pain to fit in. Decided to use it because BC mentioned it. 100mm for wall and 50mm for ceiling. Don't know if he meant 50mm equivalent of cellotex.
What is the normal way of doing it? Rock wool between joists + cellotex above joists)
Ps. Loft is not habitable. I don't have good access from top.

 

[Hl.house]

Please will you also suggest which fibreglass I use? Any particular brand, product link will be extremely helpful.

 

[JohnD]

Knauf make mineral wool treated with Ecose, which prevents it shedding irritant dust and fibres, and makes it easier to handle. It is brown. I would never use yellow fibreglass again.

Somd own-brands sold by the big chains are the same, look for the word Ecose on the packaging before you buy. It may show if you do an online search.

Insulation is usually on special offer, but you have picked the time of year when demand is almost at its peak. It will get worse as soon as we see snow or ice.

 

[garyo]

Will you have access to the void after the plasterboard is fitted? If not then perhaps that's why BC advised cellotex?

 

[Hl.house]

First of all thank you so much for your advice. You are a godsend. You will help me save £££s and also the bully plasterer that we were planning to hire.
I will still have access to the space above but a crawling space. If this helps, this is a rear bedroom of a 2up 2down Victorian mid terraced house with kitchen extension. The said bedroom is above the kitchen. It has slanted(pitched?) roof. Will see if I can attach pictures.
Joist depth is 10cm only. Will I have to compress 300mm wool? Whilst there is no floor or covering above joist there is some timber perpendicular to joist in the central area running across full length of the room. Don't know whether to call it stud, plank, 2x4 ? I believe this may be for access.
BC wants me to get a u value of 0.28 or under. I will use 60mm Kingspan or 70mm thermalise super gypsum. Will 100mm wool help me achieve it? Also, this will be a child's bedroom so would want it as warm as possible.

 

[geraldthehamster]

Pictures made it clearer. I am not a builder but have done similar insulation in a dormer bungalow, to building regs.

My advice - use Celotex on the sloping bits where the depth is limited. When I did mine (2008), Building Control required 125mm. You also need a ventilated 50mm air gap between the Celotex and the roofing felt, if the latter is the older impervious type. In theory you don't need an air gap if the roofing felt is the modern breathable type, though a gap is often recommended. So to get your 125mm you would need either to fix battens to the sloping rafters to increase the depth, or add more Celotex over the face of the rafters before fixing the plasterboard through it. In either case, I would fix at least 25mm Celotex over the face of the rafters, and tape the joints, to reduce cold bridging across the rafters, and to provide a vapour barrier (the foil layer) to prevent warm moist air getting into the cold space and causing condensation.

For the flat ceiling joists at the top, I would chuck 300mm rock wool in there as you plasterboard. Rock wool is more usual to insulate above a flat ceiling. In theory you need a vapour barrier there as well, so you could use foil backed plasterboard, or 25mm Celotex under the rafters (I don't know what trades would do - I was putting my ceiling above the joists, to leave them exposed, and working upwards I used plasterboard, 25mm Celotex and then rock wool, but that's probably unconventional, though Building Control were happy).

Maybe run what I've said past your building inspector and see if this the kind of thing he was getting at.

Cheers
Richard

 

[Hl.house]

Thanks GTH. I am thinking of:
1) screwing a standard 12.5mm plasterboard to the flat joist
2) put rock wool over the 12.5mm plasterboard, in between joists as I board. As my joists are only 100m deep can i still use 300mm rock wool?
3) do i need to cover rock wool with anything on top?
4) sloping bits and walls can I use 60mm kingspan or 70mm thermaline super gypsum, preferably dot and dab.

Knauf has several products which one do I use? Three main categories being:
1. Glass mineral wool roll
2. Glass mineral wool slab
3. Rock mineral wool

What is the difference between glass and rock mineral wool. Then there is also earth mineral wool. This is so confusing.

 

[garyo]

Go with mineral wool as John advises.

Picture speaks a thousand words! The 50mm Kingspan between the rafters makes sense now, or if you can afford to lose the head height then you can save some hassle and put as much as you can get away with under the rafters instead, as Gerald says.

Yes you can lay the mineral as you go, but don't compress it. Put 100mm between the joists then lay 170 over the top in the opposite direction (unroll it as you perhaps)

The space won't be usable after you're finished : the won't be the room and even if there was your need some form of lift stilts and boarding so that you don't compress your mineral insulation.

Good luck!

 

[garyo]

The slabs are for cavity walls so forget those. Don't get too tangled up in the detail... Just get the nicest feeling loft insulation wickes have that suits your wallet.

 

[Hl.house]

Thank you so much for your advice. I will add knauf 100m eko roll from b&Q as base layer in between the ceiling joists as I plaster board. The top it with another 170mm top up roll. I suppose I don't need to bother about vapour barrier because my house has air bricks and the materials are pretty breathable.