How to seal but ventilate between fiberglass insulation and bitumen roof felt

Calibos

Renovated a Victorian House in 2010. Turned out the builder (Emigrated during crash a year or two after our build) did a few things wrong but most were rectifiable. (Nearly 100 grand cheaper than some quotes back then. The high quotes may simply have not wanted the job though I guess) I'm wondering if what I want to ask you about is one of those cases of a rectifiable mistake.

Here goes.

Part of the job was an attic conversion. Old Bitumen Roofing felt still in good nick from a Re-roofing in the early 80's. Not replaced for renovation. The problem is that I have since learnt that with non breathable roofing felt there should be a 2 inch ventilation space between the Bitumen felt and the ceiling insulation from the eaves to the apex of the roof and back down to the eaves on the other side??

However, it seems the builder did not use vent cards to create this 2 inch gap. Fibreglass insulation just put between the rafters right up under the felt and the rafters sealed with plastic sheeting as the vapour barrier. Not very well sealed tbh. Not taped and lots of punctures.

Only in the last 2 years have I actually moved into this bedroom. It was a Home Theater before that. I realised straight away when I moved into the bedroom in early 2017 that it was a cool and draughty bedroom but IIRC the Winter of 2017 wasn't too bad. However the bedroom showed its true colours in the Snow and easterly storms of Feb/Mar 2018. The room was an icebox.

Not surprising given that inspection of the eaves storage space along the room shows that the ceiling insulation is effectively open to the Vented Eaves and not sealed by the vapour barrier even. Warm air from the insulation being blown/sucked out and cold air being blown in through the insulation, into the eaves storage space and into the bedroom proper. Needless to say one can also listen in on conversations of people on the street below too.

On the plus side, at least this likely means that 10 years later there is no evidence of damage/mould from condensation on the insulation or rafters because its all getting sucked out/air dried!!

The question is though, how can I rectify this without removing the bedroom ceiling and re plasterboarding and skimming the bedroom ceiling area or replacing the bitumen roofing felt with a breathable one? I want to both seal the draughts, prevent warm air being sucked out of the insulation and block the streat noise by ultimately plasterboarding and tapping and jointing the inside of the eaves storage space.

After looking at the areas I have access to and photo's from during the renovation, its not like I have the option of retrofitting vent cards under the roofing felt anywhere other than in the accessible rafters in the eaves storage area. Most of the rafters from the knee wall to the apex of the roof have noggins or whatever the are called or the framework around the Velux. Even the one or two inter rafter runs all the way to the apex not blocked by a Velux or noggin simply run into the Steel i-Beam at the apex so there cant even be any cross ventilation between them or the dormer roof coming off the other side of the i-beam.

Would it serve any purpose for me to fit vent cards between the bitumen felt and the fibreglass insulation from the eaves to as far up the rafters/ceiling as I have access to from the eaves storage space? Re-install the fibreglass rolls, replace the vapour barrier plastic sheeting and seal properly this time and then install the plasterboard and tape and joint? Or is it a waste of time or counter productive if I can't do it all correctly?

A cost benefit analysis in my head tells me that for the few times in the Winter when the bedroom is truly uncomfortably cold, it'll be a lot cheaper to simply use my electric blanket and fit a draught excluder to the door into this room to prevent the heat from the rest of the house being lost out the attic conversion bedroom, than it would be to spend thousands replacing the roofing felt or re plasterboarding and re skimming the plasterboard ceiling to fix this once and for all. The payback would be decades.

So basically what I am asking about is whether there is anything I can do to prevent heatloss from the insulation, seal the draughts from the eaves and block street noise but without causing mold or rafter rot issues to develop.

Heres a link to an imgur photo album with some photo's of the room before it was slabbed, the eaves storage space and some shots of the old rafters and felt behind the fiberglass insulation. A photo of the top of the wall plate to the eaves and an opposite shot of the top of the knee wall and the ceiling insulation.

Calibos

More research and more confused.

Are vent cards only needed behind open cell spray foam insulation??

In the absence of any advice, i'm wondering about removing the fibreglass insulation between rafters in the eaves storage area and fitting DIY vapour permeable cardboard vent cards stapled to the rafters 50mm from the bitumen felt where I can access the rafters in the eaves storage area and DIY vent cards with 50mm spacers pushed over the knee wall as far as I can between rafters up into the bedroom ceiling space between the insulation and bitumen felt from my only access in the eaves storage space. There'd be no cross ventilation between rafters or to the apex or other side of the roof though.

Then reinstall the fibreglass insulation between rafters in the eaves storage area. Then do a thorough job sealing all that with new plastic vapour membrane. Given that the rafters in the storage area are only 4.5 inches (bedroom ceiling is 6+) I would than install 50mm insulated plasterboard to the rafters in the storage area and on the uninsulated walls of the storage area with regular plasterboard on the inside of the kneewall. Tape and joint. With spare insulated plasterboard I'd fabricate a heat and sound insulating panel to fit behind the thin uninsulated MDF access door into the eages storage space.

[Meme]Change my mind![/Meme]

LOL.

BryanF

plasterboard or roof membrane to be lifted

Vapour/air-tightness

Ventilation of roof timbers

Calibos

BryanF wrote: »

plasterboard or roof membrane to be lifted

Vapour/air-tightness

Ventilation of roof timbers

So I've got 3 choices!

Joking aside (which I know is frowned upon in this forum having browsed it for the last 17 years), I am assuming you are implying all or nothing Bryan?

I dunno. Like I said, the thousands spent to redo it 100% right will never pay back in my lifetime in saved heating costs. Of course it would be worth spending if it prevented a new roof costing 30+ grand a few years down the road. However, I can't help but think if such a vapour leaky install has shown no evidence of condensation, rafter rot or mold in the insulation after 10 years, then it would appear its ventilated enough already, albeit over ventilated losing insulation efficiency.

Now that I think of it, even recent low single digit or sub zero temp nights haven't been cold in the room......because there was no accompanying wind driving air/draughts in under the eaves through the insulation and into the eaves storage and into the bedroom. With that in mind, I really can't see the problem doing what I mentioned in the previous post. Draughts and some sound into the eaves storage and room would be eliminated, vapour barrier would be improved and yet there would more ventilation further up the rafters than ever before which remember are fine after 10 years of the current state of affairs.

Is demanding all or nothing of an old building and not a new construction being a bit, 'Perfection is the enemy of the good'?

BryanF

Don’t see any other option

Lumen

I feel your pain, but you're essentially seeking validation for an unventilated roof, and nobody is going to tell you that it's a good idea. Reducing air flow around the eaves and knee walls might make the problem worse, who knows?

It might be OK now (although how would you know the state of the upper rafters, since they're covered?) but all you need is a bit of water ingress through a cracked slate and a small tear in the felt and you're in trouble.

Calibos

Someone elsewhere mentioned Ventilated Ridge Tiles. I ruled them out because there are actually only 3 clear inter rafter runs from Eaves/soffit to the Ridge without a velux and associated framework and noggins blocking the way. Maybe a Ridge tile at each of those 3 locations would be enough cross ventilation though?? I actually have safe DIY access to the Ridge.



Standing on top of the attic conversion En Suite Dormer in this photo. The Velux you see my brother popping his head out of are on the first floor landing. The are not the same Velux show in the earlier photo's. Those are on the other side of the main roof ridge to the Dormer.



This is the valley between the attic conversion Dormer and the neighbours early 20th century rear extension. I'm just demonstrating that I have safe access to the Ridge. I was up there to short term fix with Tec 7 another builder bodge job where the Dormer Zinc overlaps weren't soldered and ciphoned/ capilaried water into the dormer staining the ceiling. Builder doing the snaglist a few months after renovation painted on an ugly white rubberised sealant but that failed too last year. Will be getting it soldered sometime this year.

Calibos

Edit

When I've been saying knee wall, I've meant the stud wall that divides the eave storage space from the bedroom. In the photos of inside the storage space looking down the length of the eaves storage space, the stud wall I've been calling the knee wall is on the right side of the photos. The exterior wall of the house is on the left side in the relevant photos.

The more I think about it the more Ventilated Ridge tiles something like this



might be the answer.

ie. Remove existing fibreglass insulation in Eaves Storage and fit 50mm Kingspan insulation board between the rafters from the wall plate to the knee wall leaving 50mm gap between bitumen felt and the insulation board for ventilation. From the knee wall push a DIY vent card between the rafters with 50mm spacers as far up along the ceiling between the bitumen felt and fiberglass insulation as I can. Join another section of vent card to the first and continue pushing until I've got vent card all the way up to the ridge. That'll be in the 3 places where the rafters are clear from Eaves/soffit to Ridge. Where the Velux framework is, I'll just push a vent card as far as I can till it hits a 'noggin'.

I'll then fit 50mm insulated plasterboard to the exterior walls and undersides of the exposed rafters in the eaves storage space, with plain plasterboard on the unslabbed inside of the knee wall. That'll give me 50mm of insulation on the 18in thick exterior long wall and 24in thick bit of exposed gable wall. 100mm of insulation on the 'roof' of the eaves storage area and because the new wood ceiling joists between knee wall and ridge are 6 or 7 inches deep, and I can assume from that last photo the fibreglass insulation in the ceiling is filling the space top to bottom, so even when I push vent cards with 50mm spacers up and compress the insulation a bit as a result, I'll still have an effective 100mm+ in the bedroom ceiling.

Then when I remove the old Ridge tiles and before I replace with the vented ones I'll have access from the ridge to push down vent cards to the velux framework noggins from above.

For the Diy Vent cards they need to be stiff enough to push up and compress the fiberglass as they go, so I'm thinking 2-5mm fiberboard. They need to be vapour permeable though, so I'm thinking of scatter-shotting it with 1 inch holes with a forstner bit and then gluing a vapour permeable membrane to the holed fiberboard vent card. To prevent it catching on the bitumen and tearing it as I push it up, I'd kill two birds with one stone and make the 50mm spacers furniture wheels which will roll/run up the bitumen felt as I push the vent cards up between the rafters.

I'd rather try and solve this with some ingenuity rather than 4 or 5 figures!

sydthebeat

Is there insulation in the knee walls? Or does the insulation continue down the rafter line?

Calibos

sydthebeat wrote: »

Is there insulation in the knee walls? Or does the insulation continue down the rafter line?

By that question I realise I must have used the wrong terminology. When Ive said 'knee wall', I've been refering to the dividing stud wall between the eaves storage space and the bedroom on the right in the photos from inside the eaves storage space which as you can see is insulated with fibreglass rolls between the studs with a vapour sheet stapled over.

The short wall on the left of the storage space is the 18in thick solid exterior wall. The fiberglass rolls between the rafters end at the top of the wall and dont meet up with any cavity insulation inside that wall if thats what you mean. That wall is 18 inches of 1860's solid stone and mortar. This aint no victorian redbrick. Looks like it was thrown up with whatever was at hand and cheap at the time. That wall is more akin to a farmers wall in a field. Still, it hasn't fallen down in 150+ years.

Calibos

Also upon further inspection of the photos I now realise that while the new frame work/6inch+ rafters ties into the I-Beam only the old rafters continue to the point of the ridge. So there is indeed a void above the I-Beam that the 50mm ventilation gap between the rafters all run to and share. In fact I also now remember that the non gable wall end of that I-Beam is visible in the cold roof attic on the other side of the stud wall from the attic conversion bedroom. In other words its 'open' to the actual attic. So once I open up a 50mm ventilation gap under the bitumen felt by one of the methods discussed earlier it already be cross ventilating with the rest of the attic space.

an_fathach

Excuse me if it is an ignorant question and if I misread here but there was mention of the cold air blowing in from the roof venting onto the fibreglass batts. But isn't this always the case in a cold attic? Even if you had an airtightness membrane, wouldn't it be sitting under the fibreglass, while cold air also comes in through vapour membrane as they're supposed to be holes in it for vents?

Calibos

an_fathach wrote: »

Excuse me if it is an ignorant question and if I misread here but there was mention of the cold air blowing in from the roof venting onto the fibreglass batts. But isn't this always the case in a cold attic? Even if you had an airtightness membrane, wouldn't it be sitting under the fibreglass, while cold air also comes in through vapour membrane as they're supposed to be holes in it for vents?

How I understand it but I could be wrong as that the first line of defence is a vapour barrier on the warm side of the insulation. Any moisture that gets through that and into the insulation should be able to pass through a vapour permeable vent card or cross battens that holds the insulation 50mm from the underside of a vapour impermeable bitumen felt when that's what you have. This 50mm of cross ventilation dries and carries off any vapour that condensed on the underside of the cold bitumen felt before it can drip off down into the insulation and soak it or condense on the cold rafters and soak And ultimately rot them.

If you have a vapour permeable roof membrane you don't need this 50mm ventilated gap. I presume in that case you don't need an open vented eaves/soffit either and thus outside cold air blowing warm air out of the insulation is never an issue.

My renovation install was the worst of all worlds. No 50mm ventilation gap between my attic ceiling insulation and impermeable bitumen felt, leaky un taped vapour barrier, open vented eaves. A strong wind blows in from my soffit/eaves, blows warm air out of my insulation and also escapes into the eaves storage area and out the access door of the storage area into the bedroom.

Unfortunately the lads here haven't advised on any course of action except rip it all out and start again whether that be removing all the roof tiles and fitting a vapour permeable roofing felt or pulling down the plasterboard ceiling of the bedroom, pulling out the insulation and fitting vent cards, reinstating the insulation, re slabbing and then re skimming the ceiling. Problem is that I don't have and wont have the thousands it might cost to do that.

Because I cant do what they advise, they might not be interested in commenting further probably due to liability concerns which is fair enough I suppose but it means no advice either on my potentially promising idea about how to create the 50mm gap from eaves/soffit to Ridgeline without removing the roof tiles or bedroom ceiling. An opportunity presented by the combination of somewhat unique factors in my case where I have access to the ceiling space between rafters from the eaves storage area at one end of the rafters and safe working access to the Ridgeline where I can fit ventilated Ridge tiles. Of course it could also simply be because its the weekend and they've more important things to be doing than reading every reply on boards and aren't even aware of my new findings or idea yet

BryanF

an_fathach wrote: »

,

while cold air also comes in through vapour membrane as they're supposed to be holes in it for vents?

Breathable felt on the cold side of insulation, let’s air in

The vapour barrier /air-tightness membrane on the warm side of the insulation, stops warm moist ‘house’ air (vapor) from entering the roof

Calibos

Seems like everyone switched off and put the thread on ignore in a manner of speaking once I said I wouldn't nor couldn't afford to lift the roof slates to fit a vapour permeable roofing felt/membrane nor remove the ceiling, fit the ventilation cards and re-plasterboard and skim.

In case anyone hadn't noticed, I think I've come up with a pretty darn good plan for retrofitting wind proof but vapour permeable 'vent cards' to create the 50mm gap between bitumen felt and fiberglass insulation in the attic bedroom ceiling between the rafters from eaves/soffits to the Ridge without needing to remove the roof or ceiling. Made possible because of safe easy access to the eaves and the Ridge. From the eaves storage space from which I can push my DIY Vent cards up inside the bedroom ceiling all the way up to the ridge between most rafters except those blocked by the Velux framework. Even those I now realise I can vent because I can reach the lower cross rafters/noggins with a sawzall from the eaves storage space and cut out 50mm of them under the bitumen felt and after measuring the ceiling I reckon that when I'm up replacing the ridge tiles with vented tiles I can probably lift the first course of slates, roll back the felt cut similar 50mm slots in the upper noggins and fit the DIY vent cards before reinstating the felt, course of tiles and then fitting the Ridge vent tiles. I also have safe access to the Ridge for removal of Ridge tiles and associated works.



Basically all I need advice on now is the insulation/vapour barrier detailing I should do inside the eaves Storage space.

Assumptions:

My DIY Vent cards are Strips/sheets sized to fit between the ceiling rafters, of 2-3mm thick fibreboard perforated all over with a Forstner drill bit. That thickness and fiberboard so that the sheets are stiff enough to be pushed up into the ceiling space over the in situ fiberglass insulation and stiff enough that with the 50mm spacers affixed to the fiberboard, the fiberglass will be compressed and the 50mm spacing maintained all the way up. The spacers will actually be plastic furniture wheels which will roll up the underside of the bitumen felt insuring the DIY vent cards don't snag on or tear the bitumen felt as I push the DIY vent cards up inside the ceiling. The perforated fiberboard sheets will have a vapour permeable wind proof membrane glued to them that lets the moisture through to the 50mm ventilation space under the felt but doesn't let the wind flowing through the 50mm ventilation space blow the heated air out of the insulation.

So, I've got my eaves/soffit to Ridge 50mm ventilation sorted.

In the ceiling space even taking into account the 50mm ventilation gap I'll have about 150mm of fibreglass insulation due to the 8 inch attic conversion newer rafters. However those 8in rafters terminate at the stud wall dividing the bedroom from the eaves storage space. From the Stud wall to the exterior wall plate its just the old Victorian 4in rafters 2 inches of which will now be given over to the 50mm ventilation runs to the Ridge.

So....

Should I save some money and use my DIY vent cards between the rafters here too, compress the fiberglass batts that are already there and then affix 50mm insulated plasterboard to the underside of the rafters.....or.....pull out the old fiberglass batts, don't use more DIY Vent cards in this location and instead batten the rafters and fit 50mm EPS between the rafters which gives me my 50mm ventilation gap between the EPS and the bitumen felt and then fit 50mm insulated plasterboard to the underside of the rafters.

The 50mm insulated plasterboard along the exterior wall mitred to mate with the rafter panels?? Fix battens shallow battens to the wall and then the insulated plasterboard?? Should I knock off the remaining lime plaster on that wall (half of its gone anyway). Should I fit vapour sheet to the wall, then batten then plasterboard or vapour sheet over battens with air gap between it and wall?? Tape and mud all the joins obviously. What to I use to seal the Insulated plasterboard to the subfloor of the storage space.

I took advantage of family out shopping earlier today to climb into the cold attic to inspect the Steel I-beam that penetrates the partition wall between attic conversion bedroom and the cold attic space over the vertically offset bedroom on the first floor. Think I have a good mental picture now of what I'll find when I lift the Ridge tiles.



Getting up there reminded me that theres plenty of DIY work for me in this cold attic too. Photo's from when we moved back in in 2010 but before we filled the attic with crap.



First job will be to pull the highly compacted Fiberglass from the Eaves rafters. Vaulted ceiling in the room below so I've only got the 4in old rafters worth of a gap. I'm thinking that 50mm of EPS or PIR shoved down there to the wall plate leaving a 50mm gap out to the soffit is better than 100mm of compacted Fiberglass roll with wind blowing through it thats in situ at the moment. I'll lift the current attic floor and replace with an insulated EPS T&G attic floor to add more insulation and detail it around the attic hatch such that an DIY EPS Hatch Box over the hatch/ladder assembly will T&G tightly with the insulated flooring. Will make a Box and Lid out of EPS for the Cold Water tanks. Remove the Fiberglass crossing the joists at the other end and fit insulated T&G attic flooring there too. (Need the Storage Space) last but not least, remove the plasterboard on this side of the partition wall with the adjoining attic conversion bedroom, go bananas with the expanding foam, refit the insulation and refit and properly seal the plasterboard again.

Interesting Fact

I DIY fitted a Hive Multizone system I picked up for €250 during their Summer sale to replace a broken non thermostat Sunvic Multizone timer controller during the summer thanks to the great help I received from DGOBS over on the plumbing forum who talked me through the wiring. With the browser Hive app you can look at temperature graphs from the downstairs and upstairs thermostats. While the This Week, Last Week, This Month, Last month and This Year tabs show averaged temperatures, the Today and Yesterday tabs give a half hourly breakdown of actual temperature readings. I check it a lot. Anyway, given we've had a very large temperature differential between the inside temperature and the near zero outdoor temperatures these last few days, its very interesting to note that the house is still only dropping 1ºc overnight when the Heating is completely turned off for 8.5 hours because of the schedule I set. (Goes totally of at 11pm and comes back on in thermostatic mode at 7:30am every day)




Can't be too much wrong with draughts or insulation if the temp is only dropping 1ºc in 8.5 hours can it?? Well, outside of this attic conversion bedroom there ain't anyway. I assume this is down to our large solar gain during the day because 18 of our 20 DG Windows, Velux, Skylights, Hall doors and French doors are South and West facing and because of the huge thermal mass of the exterior walls which are at least 18-24in thick Stone and Mortar.

Lumen

Are you intending to raise the temperature of the eaves storage space by insulating it? i.e. bringing it inside the heated envelope?

Calibos

Lumen wrote: »

Are you intending to raise the temperature of the eaves storage space by insulating it? i.e. bringing it inside the heated envelope?

It already is inside the envelope for all intents and purposes because of a simple thin painted MDF access door to the space not even draught proofed. I already understand that the insulation in the stud wall between the bedroom and the eaves storage area that the builder installed serves no useful purpose as a result.

The fact that moisture was likely hoovered up from the bedroom/house into the eaves storage area for 10 years and yet there is no evidence of damp, mold or rafter rot to the insulation or rafters that are inspectable from the storage area and neither was there any evidence of same from my admittedly limited inspection of the i-beam penetration in the cold attic next to the conversion at the Ridge line where I could feel significant airflow coming out of into the cold attic. Well it seems to me that while the insulation in the ceiling of the attic conversion was effectively useless in terms of retaining heat, at least it was getting ventilated out into the cold attic adjoining the attic conversion.

When I do this job myself and as part of it plasterboard the back of the dividing stud wall between the bedroom and eaves storage and fabricate a solid insulated plasterboard panel to fit into the opening/framing of the access door/hatch to the eaves storage area, that will be in order to better sound isolate the bedroom from the street, not to make the eaves storage area a 'cold attic' type space outside the envelope with the envelope being the stud dividing wall or anything like that.

Lumen

I find this confusing. Either the eaves is warm or cold. You can't have it both ways.

If it's warm, the airtightness and insulation layer goes on the roof running down to the eaves, and you don't need to insulate/draught proof the partition between the room and the storage area.

If it's cold, the airtightness and insulation layer goes on the knee wall/whatever you're calling it between the room and the storage space, and you don't need to insulate the roof of the storage area.

Calibos

Lumen wrote: »

I find this confusing. Either the eaves is warm or cold. You can't have it both ways.

If it's warm, the airtightness and insulation layer goes on the roof running down to the eaves, and you don't need to insulate/draught proof the partition between the room and the storage area.

If it's cold, the airtightness and insulation layer goes on the knee wall/whatever you're calling it between the room and the storage space, and you don't need to insulate the roof of the storage area.

Its not so much that I want it both ways. The builder effectively did it both ways during the renovation in 2010 albeit very poorly and downright wrong in some respects. EG. He did fit a vapour control layer sheet of plastic between the ceiling rafters/ceiling fiberglass and the plasterboard and skimmed bedroom ceiling. I know this because when I reach over the stud wall dividing the bedroom from the eaves storage area and push my arm into the bedroom ceiling space under the fiberglass, I can feel the vapour control layer on the back side of the ceiling plasterboard.

ie. He knew to fit a vapour control layer on the warm side of the insulation yet didn't seem to know that when you have a vapour impermeable bitumen roofing felt you can't just fill up the space with insulation right up to the underside of the felt, you need to incorporate a 50mm ventilation space with the help of vent cards. This he didn't do. Most of the early thread was me trying to discuss vent card retrofit ideas that didn't require lifting all the slates and fitting a vapour permeable roofing felt or ripping out the internal ceilings and insulation to fit conventional vent cards the conventional way. I think I've come up with a solution to that particular problem on my own which I want/need to do DIY because I can neither afford the thousands it would cost to 'Do it Right' by lifting the roof tiles or pulling down the ceiling and because somewhat uniquely I have safe access to the bedroom ceiling rafters from the eaves storage space on one end and safe access and working area to the Ridge on the other end.

I am perfectly happy to have the envelope cover the the eaves storage area. The fact that the dividing stud wall between that area and the bedroom happens to have fiberglass between the studs and a vapour control sheet stapled to it is simply a legacy of the incorrect job the builder did 10 years ago which I assume I can leave in the wall cause whynot? When I talk about fabricating a removable plasterboard panel behind the simple thin MDF access door thats there now, thats not me trying to create heating or moisture envelope within an envelope, its simply me wanting to create a secondary sound barrier from outside noise.

Lumen

If you have an insulated space that doesn't have any heating, it will be cold. If warm moist air leaks into that space, it will condense.

That's why the air tightness layer should be coplanar with, and inside, the insulation layer.

That's why it's important to be clear about whether a space is inside or outside.

Calibos

Lumen wrote: »

If you have an insulated space that doesn't have any heating, it will be cold. If warm moist air leaks into that space, it will condense.

That's why the air tightness layer should be coplanar with, and inside, the insulation layer.

That's why it's important to be clear about whether a space is inside or outside.

OK Gotcha.

So your advice if I wanted the eaves storage space inside the exterior wall and roof envelope would actually be to remove the existing insulation from the stud wall between the bedroom and eaves storage space in order to let heat from the bedroom penetrate into the eaves storage space so that its at the same temperature as the bedroom or certainly much closer to it. In that case, if I found that the insulated plasterboard on the exterior wall and rafters in the eaves storage space was not blocking enough of the sound from the street coming in the soffit/eaves, then I should try a second layer of plasterboard screwed into the insulated plasterboard on the exterior wall and rafters.

Would that be correct?

Lumen

You're best placed to judge what's achievable. As long as you're clear and consistent on where you need ventilation, air tightness and insulation respectively then I'm sure you'll pull off this project.