Bit of background. got a house in SW Scotland on kero heating its a traditionally built solid stone, slate roof with a big orangery thing. its not terrible but could be better and as I am doing jobs on it this ones next ish!

So the living room ceiling, its currently sagged due to poor fitment of the plaster boards not staggered joints no noggins etc. so I am going to drop it and replace the boards with 12.5m and noggin it properly stager joints etc. that bits fine no issues but above the boards is insulation of the roll variety and then the lofts boarded/floored over the top direct to the joists so I think I have 100mm maybe 150mm void between celling and loft floor

my plan is to rather than use rockwool etc. is Celotex/Kingspan etc. board instead. this will give me one an easier time fitting it as its rigid and secondly better performance.

I want to then fix 100/150mm + plus boards in the loft against the pitch and then board that out and then fill the eaves with insulation. Idea is to get more insulation in with out loosing the roof space.

the other loft I want to do the same but I don’t think I will be bringing the ceilings down so I am looking at boards against the roof.

as its tiled in the trad manor booth roofs have solid timber planks then a membrane then tile.
I am planning on leaving 25mm gap and then foaming the boards in against the joists/trusses.

have I missed anything?

I am not wanting to Gut the place in one go but just improve its credentials every time I do some necessary work

Cheers

I’m not familiar with the detail of Scottish Building Regs, if they are similar to English then if you’re upgrading insulation you should, legally, be doing it to the current regs, if that makes any difference to you. To fit it in the ceiling void you would not have enough insulation with rockwool, so Kingspan etc is the way. In some ways it’s easier but you have to cut it to fit exactly between the joists, which are probably not exactly the same distance apart.

As I understand it you’re insulating the floor of the loot and the pitched roof – so that’s neither a warm loft nor a cold loft, but somewhere between. I considered doing that, but the Approved Docs for English regs don’t cover it – in the end, because I have trussed rafters, I didn’t do it as cutting the Kingspan around all the diagonals would have been a pain.

Are there any cables in the void? If you insulate round them the current rating drops.

Before you take down the ceiling, are you sure it doesn’t have asbestos? Anything like Artex will almost certainly have it, ordinary plaster before about 1980 might.

I don’t follow what you’re planning for the second loft. I would guess you need to leave a ventilated space between the insulation and the sarking boards, ventilated meaning it has vents in the eaves and at the ridge, not just a space.

Yeah it seams you leave 25mm between sarking and insulation.

Ceilings 90s and just board n plaster.

I’m not set on building regs. I’m just after improving what I have

if they are similar to English then if you’re upgrading insulation you should, legally, be doing it to the current regs,

Not the case in Scotland unless your doing notifiable work under a warrent.

Not to say it would be a bad idea but I wouldn’t get bent out of shape about it.

Is the plasterboard on the living room ceiling dropping down from the ceiling joists?
If not then my guess is that the joists themselves are undersized and not able to take the weight of the boarding that’s been put in the loft (plus anything on top of that boarding).

If you want to use the loft space then you may need to replace the joists first.

It’s bowed across the shortest span like a wave mate there’s no noggins in there so it’s level on the joist and drops in middle.

Loft floor is fine as is the direction on the joists.

if your meaning what i think noggins don’t wouldn’t make any differance to the plasterboard sagging between joists,you don’t fix the plasterboard to noggins, only the joists.
when i’ve seen that happen its usually been wider joist centers and the thinner 3/8ths plasterboard.
no harm adding noggins while its down though.

Yeah it’s on 3/8 or 9.5mm board.

Not measured the centres but I think you’re right.

If you insulate the rafters 50mm gap sounds better than 25 mm to me. Don’t block the soffits, also think about air flow. Now the air can’t com in via the soffits, swirl around and maybe blackout the otherwise depending on the wind that day. You now have a dead end channel. This needs to be delt with.

When I was placing Kingspan between joists at a previous house I used spacers to hold the boards, which I undersized by ~25mm on each dimension, before foaming the boards in, removing spacers and then foaming up those little gaps remaining. Joints were all foil taped to span the joists with really wide foil tape of ~75mm.

The spacers were the below. Totally indispensable to me whilst dong this job!

I undersized by ~25mm on each dimension

Crickey….. why?
When I did our I just cut each one to size, foamed any gaps then foiled taped everything.

OP don’t underestimate just how messy/dusty it is to cut all the kingspan!
Use one of these and the Screwfix no nonsense foam – lasts for ages.

Oh and the air gap requirement is 50mm – 25mm wouldn’t allow enough circulation.

It’s very satisfying though!

In my two flats I have stuffed insulation everywhere I can – no vapour membrane.  No issues with condensation or anything else – just a nice warm less draughty old flat

No issues with condensation or anything else

And you know this how?

Cheers all.

I was concerned over warm roof cold roof malarkey as it’s warm ceiling cold loft plus more insulation so a luke warm loft.

main issue on oil spends the aga but the kitchens built around it and it is kinda nice!

My next topics are going to be electric combis. Not for here mind and solar…

Many thanks

And you know this how?

Because occasionally I have to go into the eaves space and when I do I look for signs of it.  No signs there nor any signs anywhere else

I believe its because that there is huge air circulation outside of the insulation because when I had some work done building control made me put in ridiculous amounts of roof vents

why?

An old house where joists were all relatively irregularly spaced and not always parallel to their neighbours, exactly, mean that a 12.5mm gap all the way around made the whole situation workable. I started trying to be ‘ideal’ with my approach but this soon changed. Undersize, wedge, foam, de-wedge, foam, tape.

I believe its because that there is huge air circulation outside of the insulation

Sorry I read your post as though you’d not bothered with ventilation and just filled everything with insulation!

An old house where joists were all relatively irregularly spaced and not always parallel to their neighbours

Fair enough…. I just measured each gap fairly accurately and cut to size.

You realise the foam doesn’t perform as well as celotex?

Anyway don’t cut the PIR sheets with a saw, use a big ass knife. It cuts easily and cleanly. If you use a saw you will breath in all kinds of junk.

No worries sharkbait.

Anyway don’t cut the PIR sheets with a saw, use a big ass knife. It cuts easily and cleanly.

It does for thinner sheets, but I found that when cutting thicker stuff it’s faster and easier to cut a straight, accurate line with a saw as a knife tends to stray off course.
But I did wear a mask (this was just before they became really fashionable 😉).
And yes, the kingspan/celotex will perform much, much better than some spray foam.

or

Pitched Roof – Under Rafters

used at about 7:00 in

You have to remember the normal scottish roof is built very differently to England.  You do not have batons to nail slates to – there is full boarding across the whole roof and the soffits do not have gaps for air circulation.  the outer roof is the windproofing.