Renovation - approach to sloped ceiling

[Olf]

First of all - what is the proper name of this feature, where a wall morphes into a ceiling due to roof height restriction?

Anyway - this is the photo of the current state once the only element separating inside and roof space (a single layer of plasterboard) has been removed:

 

 

So the said plasterboard was held by chamfered battens - my plan would be to remove them and attach to rafter to space out, so the insulation board would not have to be trimmed in a weird way to fit between the wall plate and rafter, but rather go against the wall. In picture something like that:

The blue object with weird hatching (one doesn't get the right tools with free cad software...) is 100mm K108 (or equivalent) phenolic board: at 450mm standard width this board should only need chamfering to fit wall/ceiling, where 1200mm PIR board cut in half and then butchered would produce so much waste, that in this case phenolic happens to be cheaper - and protrudes less into the room.

25mm from the top of the rafter (surface of the felt) would be a ventilation tray, the remaining 65mm space filled with Thermofloc - I'm planning to use it on the ceiling (70mm between joists, then 9mm osb to support, then up - to 400mm in total), so it would probably spill in the void anyway. A layer of plasterboard would be placed directly on the phenolic board.

The walls are concrete, so for airtightness I'm planning to sandwich a layer of membrane between the insulation board and the plasterboard and tape it to the wall. I'm assuming the walls are airtight enough (concrete + plaster): the vent seen ot the first photo  - an original feature, present in each room - must be there for a reason. Ultimately MVHR will take their role, hence the trouble of sealing.

Not sure yet what to do at the ceiling: whether to stick membrane to existing ceiling and board over, or go over to the loft and lay it there

 

Questions:

- hand laid Thermofloc comes with 'it is designed to install by hand in horizontal spaces' clause - but as its blown brother lives happily inside vertical walls, what problems could I potentially expect from stuffing it into this sloped void? Has anybody done that before?

- any ideas how to attach the new plasterboard? Loking at the drawings my best chance it to screw at an angle into the wall plate and roof joists. Easier would be to go into the rafters, but I'm worried about the edges sagging.

- can anyone point me to loong ventilation tray - for this I'd need something at least of 1m. Or should I simply stagger 2 or 3 standard ones for required length, just making sure that air channels align?

- any recommendations for airtight tape that seals well to old, rough concrete? Or how to prepare the concrete so the tape binds better?

 

 

[nod]

You need to buy a sheet of 62 mil insulated PB 

Cut it to size measuring from the bottom of the timber wall plate 

Then further up the sloping timber measure down 62 mil Draw a pencil line between the two points up against the flat part of the ceiling and you have your size 

Then cut the foam back with a saw so it sits flat 

Fix with 100 mil drywall screws 

[Nickfromwales]

5 hours ago, Olf said:

Not sure yet what to do at the ceiling: whether to stick membrane to existing ceiling and board over, or go over to the loft and lay it there

Deffo apply to the internal if you stand any chance of getting some integrity there. Forget the attic as that'll be an absolute nightmare to detail successfully.

Have you also thought of over-boarding the ceilings joists with 30mm PIR boards, foil faced, and then use foil tape to complete the barrier. Two birds one stone.

 

[Olf]

7 hours ago, nod said:

You need to buy a sheet of 62 mil insulated PB

 

I like the idea of bonded insulation, found https://www.xtratherm.com/products/sr-pr/ that gives me 100mm phenolic, and seems to be rougly the same price as K108+plasterboard (even with offcuts), saving on hassle of mounting 2 layers separately.

 

2 hours ago, Nickfromwales said:

Deffo apply to the internal if you stand any chance of getting some integrity there

That's what I'm worried about: that contrary to what makes sense, shmbo won't let me play with the board hoist (oh she hated it so much last time), especially that this time the celings aren't that bad and skimming alone would be enough...

[DevilDamo]

18 hours ago, Olf said:

First of all - what is the proper name of this feature, where a wall morphes into a ceiling due to roof height restriction?

Skeiling ?

[Ferdinand]

One question.

 

This is  non-standard construction (albeit Laing Easiform is one of the more accepted methods and relatively mortgageable).

 

Since you are planning EWI is it actually going to be a better solution to insulate under the entire roof under the tiles ("the full tea cosy") if you are  planning  extensive works?

 

Ferdinand

Edited August 10, 2020 by Ferdinand

[Olf]

1 hour ago, Ferdinand said:

it actually going to be a better solution to insulate under the entire roof under the tiles

Unless I lifted the level of tiles, there would not be more room for insulation (at least at the junction with the wall)?

I need to open the soffit to see exactly what is happening there, but in principle I'd put EWI all the way up, to meet (get as close as possible) with the above skeiling layer discussed here.

 

I think part of my problem is timing: missus wants to start painting/decorating asap (especially now that Multifinish is finally stocked and ready to go), I'm already in her way with need to access this part of the room and the only thing that keeps me alive is the fact that lights need moving and that ties with roof insulation (with timing bad again, heatwave stopping me from occupying the loft).

EWI is way further down on to-do list...

But I've seen idea like this, and I'm not saying that it may not happen down the line

 

Speaking of decorating- I think I'll have to go with airtight membrane over to the loft, but I thought as a backup I could use one of paint-on membranes on the ceiling (and all the junctions around).

The ceilings need painting anyway, so 2 jobs at once (although at a price). But the question is: how white are the 'white' solutions, like https://www.intelligentmembranes.co.uk/product/airtight-white/ or https://www.blowerproof.co.uk/product/blowerproof-liquid-brush-white/

Would it be good enough to act as a ceiling paint ?

 

 

[Ferdinand]

Speaking of painting, have you considered spraying your undercoats?

 

May be too late for this time! But worth knowing about as a technique.

 

 

Edited August 10, 2020 by Ferdinand

[Olf]

2 hours ago, Ferdinand said:

have you considered spraying your undercoats

Yes! That was actually also the plan on the previous renovation - I invested in £5 hand pumped Aldi sprayer and once it started spitting like a drunken llama I threw it away and grabbed the roller...

So I'm a bit discouraged, also missus is a wallpaper aficionado, so by democratic vote each room has only 2 walls (and ceiling) available for painting (those with window/door, where wallpaper cutting would make her sweat) - so less of benefit here. 

But looking at the linked post (thanks!) the clue seem to be to have the right equipment. And having EWI on mind to paint later, I shall better start a research, in case the hardware and skills could be useful throughout the project.

[Ferdinand]

24 minutes ago, Olf said:

Yes! That was actually also the plan on the previous renovation - I invested in £5 hand pumped Aldi sprayer and once it started spitting like a drunken llama I threw it away and grabbed the roller...

So I'm a bit discouraged, also missus is a wallpaper aficionado, so by democratic vote each room has only 2 walls (and ceiling) available for painting (those with window/door, where wallpaper cutting would make her sweat) - so less of benefit here. 

But looking at the linked post (thanks!) the clue seem to be to have the right equipment. And having EWI on mind to paint later, I shall better start a research, in case the hardware and skills could be useful throughout the project.

 

it gets easier on the second house ?. Or the fifth house.

 

So perhaps practice on the garage.

Edited August 10, 2020 by Ferdinand