Rockwool in the ceiling - howto make an air barrier ?

Hello everyone.

I'm pondering the roof insulation for a house that will come to build time soon.
I'm having some issues figuring out the roof insulation.
I have eliminated strawclay due to the time needed to dry and strawbale due to the delicate water protection needed.
That leaves me with rockwool which, even if expensive, will be installed much faster and won't suffer from potential moisture infiltration as strawbale.

I leave in a cold climate therefore i also need to do an air barrier on the insulation, otherwise vapor will travel to the cold side and condense there, not good.
I am pondering a cold roof with rockwool but i'm uncertain as how to do the air barrier.

In the image there are 2 zones of "WHAT HERE?"
It's for the roof and habitat ceiling.

Right now, i have 2 questions:
1. Where to put more insulation, in the roof or in the ceiling ?
The attic will not be lived in, just storage.
2. What should be put in the "WHAT HERE ?"
An air barrier on the warmer, moist side is mandatory.
I was thinking of clay plaster or drywall on the underside of the insulation.
But what to put above it ?
For the habitat ceiling, maybe i'll just do a thin claycoat.
But what should i do for the upper roof insulation ?

It may happen that some condensation occurs on the back of the metal roof.
That will drip and maybe fall onto the insulation.
There needs to be some protection of sorts.

Any thougths ?

PS
My descriptions may be unintuitive and also drawings hard to read, so sorry for that.

Hello Ionel,

We generally try to have one solid thermal envelope/air barrier around the entire habitable area of the home. Since you are not going to live in the attic, you don't need any insulation in the roof and the air barrier should be at the ceiling plane. Since you are using a breathable wall assembly, you will only need an air barrier here, so builder's paper will work great. If you are going to install lath and plaster, then use asphalt impregnated paper, but if using drywall, then just red rosin paper or equivalent. It is easier to get a good seal on paper than to try and seal the drywall to the framing, but drywall can be used as an air barrier if installed perfectly. Either way silicone caulk is used as the primary sealing agent, installed at the top and bottom plate and around penetrations.
Don't worry about condensation from the roof since it should be close to the same temp as the air and the attic will be vented.

All Blessings,
Bill

Bill

Quite aside from moisture issues, doesn't rockwool pass air and thus need some kind of seal bottom _and_ top (and sides) to create a cavity and prevent convection air movement from killing its insulation value (if the top is left unsealed)? Wind washing. Not sure - been a while since I thought about this stuff.


Ionel

> storage

What access to the attic? That access, if from high in the living area, will need very careful detailing because thermo gradient in the building wants to push air up andout - it can be one of the major "leaks" in a building, allowing lots of atmosphere from the living area to go straight into the attic (along w/heat and moisture). W/a vented attic, an access from the living area is in fact a "door to the outside".

Also, what floor is in the attic?


Rufus

Field studies have shown most vented attics are not producing the correct CFM nor are vented properly. More of an issue in wet marine climates. There has been ongoing issues with condensation on the cold interior side up by the ridge vent from buoyancy so some have opted to open cell SPF spray foam insulation to get it below dew point yielding a food for fungi, rot, etc.....I'd personally seal the attic and not use any petro based papers or non-inert vapor retarders/barrier materials or house wraps, like most foams/wraps that don't work since they lack CFM or high perm nor are chemically stable. I design my envelop with two materials that mate well MW and 100% wood period. A natural inert plaster/stucco on that. My outer ventilation gaps get CFM verified by manometer: There is discussion on my "Air Sealing" thread along with ventilation gap values for different assemblies.

Roxul has r-value knock down factors from vapor penetration/transport that are the lowest in the industry if a tight fit, so unlike FG batt/loose cellulose/etc the need to air seal cavities to prevent r-value loss from moist convective loops is low to n/a. Rufus is correct most don't understand that it is the cavity and penetrations that have the most impact on r-value reductions and/or hvac loss do to loops not air sealing planes. BSC conducted test on 6 different wall cavities showing the r-knock down values on their site, FG the worst ~ 40%. That is the fault with blower door test that measures whole house CFM leak down at the doors, they do a poor job of measuring cavity loops. Again, using an insulation that does not degrade is the best defense and should take precedence over low whole house ACH or CFM lost. Air sealing products gaining popularity today will thermal/vapor cycle and break down in time.

If you are using OSB or plywood in this assemble it is not beathable the vapor and air class is too low (perm 3-5) and moisture will accumulate at the glue fungi food. Roxul IS board makes a good sheathing. IIRC Roxul is perm 20+ it needs to be surround with similar perm to vapor breath it's max values. Vapor drive depends on vapor pressure levels primarily, so does thermal deltas or transport, air is not the primary transport vehicle. However, it is good design practice to pass some air through the walls especially mass walls so they can dry (~ .2/.3 ACH). The whole air seal trends & passive certification (PHIUS) are flawed.

"What here" ?: Put an energy heel truss at the soffits/fascias, lay down Roxul batts between joist on top of it a wet installed blown cellulose you install prior to the roof install let it dry in, follow the mfg install instructions. GreenFiber make an inert product that will not settle, expand, fill the heel.

Hi Rufus,
The thing with Roxul, which is the only brand of MW that I have used, is that it has a smoother, tighter finish on one side that helps prevent air washing. So if you prevent air from entering through the open sides of the batt, then air washing is as Terry points out negligible.

Ionel,
T&G boards will be your best attic floor option, with no air barrier on that side. If you are using trusses, then energy heels are recommended, but I've got a feeling that you will use rafters and collars in which case you just want to ensure there is sufficient insulation at the rafter/wall interface and install a 4' piece of cardboard in the cavity to allow air to pass for attic ventilation. Here in our dry climate, I see a lot more water come in through the ridge vent during storms than condenses and drips.
I would not use an air barrier on the bales, but remember to detail every penetration. You can use straw, stuffed in the hole and then mastic on that. Mastic is typically made with lime putty and linseed oil and applied on the last plaster coat before color, but you can use it on the color coat if your very careful.
When you draw the air barrier/insulation around your home, you should not have to lift your pencil.

All Blessings,
Bill

Thanks all for your feedback.

I imagined the whole thing as in the attached drawing.
Basically, the attic is not lived in but i want to bring it inside the envelope due to the reasons Terry mentioned.
So most of the insulation will be placed there.
However i also want to insulate (lightly) the attic floor (or living area ceiling).

For me it's very easy and cheap to just use drywall for any ceiling exposed to the inside.
Other materials (clay/lime plaster on lath) are getting complicated in both money and workforce/time needed.
I don't plan to have any roof penetrations, attic access is planned to be from outside via some form of staircase, most probably temporary.
Attic floor penetrations i think i might need 1 but if that will be the case, then some form of "airlock" type of hatch will be employed.

So, the main question i think boils down to, is drywall good enough as an air barrier ?
There are issues with penetrations tho. I don't plan to have any electric circuitry buried there and any lamp sockets or other stuff.
I want to place the very limited circuitry apparent. So the only penetration that might occur would be from the screws.
Or i could just glue the stuff but i'm not sure.

Ionel, two schools of thought and they don't really mix well like you are trying to do. HIGH INSULATED VS MASS....Two entirely different building design approaches the later being FAR more complex to get right that is why most (aka mainstream) sticks to the former.

As Bill said, the air barrier needs to be continuous. If highly insulated as in your ceiling attic proposal air tightness verified by blower door and cavity test by manometer is critical. Drywall paper will not handle vapor or air cycles without delaminating or voiding or a drying inward design w/o microbials. If you design the CFM in the ventilated attic properly you can dry out design. If you remove the drywall paper you can fully vapor "breathe", that has to do with the amount of cost reducing fillers in the and/or junk gypsum you purchase, has little to do with air sealing but, drywall in itself does not provide a good lasting air seal depending on some factors like grade of gypsum, especially over time from cycling.....that is why it is usually accompanied by a latex paint or poly with all sort of faults. Some like Heim or ROMA paints claim to transform the paper to have the same effects as a FG scrim cloth....not going to happen, use DenseArmor by GP, or MAG board like Forever Board new fire rate, be ready to pay more and installers to complain about FG fibers, lack of muding experience, and charge more.

Perhaps Bill will chim in and give some advice on removing the paper and putting a plaster on depending on the room......Drywall can make a good lath depending on cost of wood. Bill is a pro at this, I am not.

So if you want to be serious about highly insulated building's sweeping the nation and lots of layers of materials that may be junk like paper faced drywall and wood strand OSB you learn about blower door test or leak downs to .6 air changes per hour and if you get certified to that standard your utility bill will or should practically be nothing, not to be mistaken with life in a sick building and those low utility bills lasting forever, thats another story not captured by that cert nor blower door test. There are simple field test to determine the air seal levels of drywall fields (not seams) during a blower door test if you get there ask. Not to say that some mostly from DIYs earth construction is any healthier or will sustain.

There is no simple answer. In your design, listen to or ask Bill what he thinks is key. As he said, be continuous with your methods.

Ionel,

I think what you propose should work, but realize that an unvented attic as you've drawn here is sure to have problems in it's lifetime. All roofs leak, so I don't use vaulted, insulated, unvented roofs. You can install an air channel between the roof deck and the insulation to ensure leaked water could evaporate and your roof will stay cooler, making your home more comfortable as well.

Drywall can be used as an air barrier, but again I would not advise using only drywall as the air barrier of a highly insulated attic. The greater the insulation levels, the greater the risk of condensation in the cavity. Also, in this type of roof assembly, I would limit vapor permeability as there is nowhere for vapor to go once inside the cavity except back where it came from, which can cause issues. To limit vapor permeability on drywall, I would recommend a different plaster than on the straw. Try a mix of lime, gypsum and clay on a sanded latex primer. This means roll on primer and then roll sharp mason's sand onto the wet primer. This will give you a good key and limit vapor permeability without eliminating it.

A lot of people don't like drywall because if it does get saturated, the gypsum will disaggregate and the paper will delaminate, requiring demo and disposal. I've seen lime plaster on lath get soaked over and over with only staining and a few cracks as a consequence.

I hope you are able to find the balance of easy/cheap to durable/laborious for your own lifestyle and move forward with confidence.

All Blessings,
Bill

Ionel

I'm not trying to put forward "the" answer - can't. But there are basic rules of thumb that _can_ apply that may answer or eliminate some questions before moving on to the detailed conundrums.

1) Most experienced "house people" have bedrock faith that any space will get wet at times - so they usually try avoid things that might make "wet" a "tear down" experience.

2) Structures are complex and a lot of good ideas are good only when combined w/the right set of other good ideas. Much of what works does so because it's used w/in a specific "tradition" or system of complementary methods that work together. Start mixing and matching between build systems and _all_ bets are off, totally. Details matter, terribly, and stepping outside a proven system is _very_ hard to do w/out setting of unknown chain reactions. Even if you are a lifetime master builder.

3) KISS greatly improves your long term chances. Things always break and turn out "different", but the simpler the construct is, the easier it is to fix or modify it. Eg. a garbage disposal is a really great convenience but for most people, when it breaks, you have just lost your sink until you can find/afford a plumber. A plain drain needs respect and clogs if you don't put your scrapings in a separate bucket - but when it clogs or leaks you have a fighting chance of fixing it before the guests arrive. The plain drain is simple and fixable and lasts a long time. Sounds like you're on to this one judging from your plan to keep utilities out of your structural/barrier walls.

4) Get w/the _local_ community. Buildings are NOT an individual effort. Thousands of people thought, built, failed, rebuilt, talked it over, failed less horribly. <g> To get where we and you are at this moment. And you're dipping your tows in that huge river. The river bank is your local community, the dirty t-shirts at the lousy coffee shop (or maybe the coffee's good, if you're lucky <g>), the pick-ups w/3-foot tires, the volunteer fire chief who runs the lumber yard, his brother in law who does rat jobs w/the county road grader... These are _your_ building community and between them all you will find a lot of resources and actual, real knowledge that applies to your situation. In you community, you're wading in a lake of experience about your area and resources. You don't necessarily agree w/much of what any of them say - or maybe you do - but all together they represent a going concern, a live eco-system that you're joining. If nothing else, local experience can show you some of the rocky spots you might steer clear of.

Like Bill, I've seen 100-yr-old plaster shrug off fire and water w/just a few cracks; from the perspective of an owner. But part of the deal was wall cavities that passed significant amounts of air. Again, part of an old system that taken all together, leaky windows, little insulation, old growth wood, the whole works, functions very well. Recently we put in a new innovation. When a fire hose brought down the part of the living room ceiling which had been compromised for years before my parents bought the house, the plaster contractor (3rd generation, still trying to collect from Frank Lloyd Wright's estate - guy never paid his bills) used "blue board", instead of lath, under two coats of plaster. He said it was standard practice now and the insurance adjuster was already apoplectic about his price to repair the sculpted crown so we didn't "discuss" it too much. Good for 15 years so far, but it's an interior ceiling, not a demanding application. Don't know if it'll go 50-100 years but it might.

Plaster really has no peer for building quality, but it's not appropriate for every set of circumstances. However, if you consider the cost over 15 years or more it becomes more interesting for it's ROI. Unless you want to move your walls around every few years. My parents got 60 years out of their walls w/basically no expense except painting. Kids, movers, parties - all just bounced off. Stuff is HARD. You get cracks when the structure moves and you need to understand what to expect and how to repair it efficiently. It's not hard and once the structure stops moving, you're left w/nothing much to do.

Have fun.

Rufus

Thanks all of you for your inputs.

I will try (hard) to make myself transparent so you get what goes inside my thinking.

So in a pseudorandom order :

1. I know insulation vs mass issue. I'm not trying to mix them at a conscious level.
I have chosen strawbale because, after all analysis, it was the only thing that made sense in my location/situation.
I have some experience in building with it, both how to and how not to do it for my inlaws house build some years ago.
So, while i know what i want (roughly), i also know what i don't want (and why).
Insulation of the walls is SB. Mass will be provided by the plaster and floor (earthen, stone, etc).

2. I have bad experiences with vapors getting everywhere so i know for a fact that moisture will get everywhere, you just have to find ways to deal with it.
So an air barrier that keeps the permeability low is needed.
I can't make it perfect and continuous, i'd be a fool to think so.
But what i have access to (in time/labor/raw materials) is limited so i have to pick something.
I might go the extra mile to get a better (best?) solution but that is upto to decision.
As i said, i do have easy/cheap access to drywall.
I'm not a fan of it but it's fast.
I don't mind doing a wooden lath and making a lime plaster, but that will take a lot of time so that's a minus.

3. I don't intend to live in the attic.
But in the future, necessity could rise to have this space temporarily or more permanently lived in.
So, doing small things now (that are harder or impossible later) that will take this into account is what i'm trying to do.
Also, the attic can act as a buffer zone between outside and inside.
Ventilation of this space is necessary indeed, just have to decide how to do it as it won't be connected to the living spaces.
Or just drop this ball entirely.

Anyway, all this requires insulating the roof.
I intended it to be a cold roof, the space between metal roofing and insulation being between 5-10 cm (2-4in) so there's air moving.
I would so much like for the roof to not leak but ain't gonna happen.
That's why in my first post i put a "what here" on top of the roof insulation.
A thin clay plaster would do it but again, details on joints ...
One of the reasons i have chosen MW is that it does not decay if water somehow gets in.
I would have used SB (due to extremely low cost compared t MW) but i really can't protect them up there (not with the current timeframe).
Also, if there's not a lot of water, some might flow on top of the MW if faced properly.

4. I would like to get the community involved, really would.
It's just that the community is:
- you're insane, don't know what you're doing;
- we all use concrete everywhere so should you;
- all the conventional garbage and more (plastics to the rescue);
- no labor for what you want;
- will be more expensive ...

So my best bet is to involve people from away, faaar away.
Or locals, but on very clear, small, precise stuff.

I'll also talk to a PE in the design/permit team but they are very conventionalists and i better have a very convincing story to tell.
On some aspects i will have much better understanding than they have.

So, to wrap it, i know the principles, i just need some ideas as to how to do it and maybe choose one option or get inspired by another.
Or just weep in misery ...

Bill and I differ on designs, and there are some popular principles that need clarity.

Insulation vs mass principle has been an on-going debate. Air sealing, which can be different than vapor sealing, is verified by a blower door test that measures CFM. When the air change per hour (ACH) rate gets below some controversial number a need for a ventilation system arises such as an ERV/HRV. High mass air sealing can degrade the effect of or you need alot of density and a good binder to obtain. Let's referred to that as principle # 1 AIR. ASHRAE 62.2 is a decent guide but fails to analyze vapor transport modes or microbial risk. Most get the seal from continuous tapped drywall or sheathing. I'd never seen a hybrid, be interesting to see what ACH could be achieved. My guess is not that great due to mass and interfaces. Having interior drywall and mass both is contradictory imo if your goal is to achieve low ACH (.5 - 1).

Note due to solar panels many are ditching the guess work behind mass as a passive efficient HVAC system. I'd have to agree that sizing it like thickness, windows, SHGC, shading, sun angles, orientation, should be left to the pro's, models, and solar panels, too high a chance for error and over heating.

Another Air Principle is cavity as described above that a blower door test does not measure well, that would require a volumetric leak down test of each cavity. Most just use a caulk or sealant. If you read my air seal thread and Joes publication you see how complex these convective flows are and how little we in America understand them. There is a link to moisture here as in moist convective loops that knock down r-values. Lets call this Air Principle # 2.

Another air principle is cladding flow where furring strips are used to produce the CFM noted in Joes publication. It is the flow that needs to be verified here most fail at checking, not the gap. There is a vapor and liquid water relationship to air here, the air will evaporate vapor or liquid water if the volume of the air vessel and rate of flow is designed right. Boundaries or conditions are as critical as flow. Note: These same principles have been used in theory for vented attics and crawl spaces for a long time, all kinds of fluid flows, some work some don't, now being challenged. Field observations have shown a lack of air flow and materials to be the issues.

Vapor Principles, at least three basics - Vapor drive can be in the opposite direction of air flow depending on where high heat or humidity is.

Permeable or Perm Rating is used in high insulation designs. It gets more complex with mass since such things as sorption rates both from vapor diffusion and capillary action, material holding or moisture contents, enthalpy, etc. I think there is a failure to realize all it's benefits and it goes beyond heat storage to heat and humidity buffering and comfort, and like many HVAC systems enthalpy. Lots of guessing here, very risky, yes to not over heat and design robust moisture management systems that last the test of time from fatigue.

IMO, a sealed attic can be just as effective as a properly ventilated attic or crawl. Sealed is not for the average DIY. Joe's or BCS test show that the proper cladding gap and CFM produces 1 % liquid water penetration form high wind driven rain that should not last long before it is evaporated. Would be a good idea during the build to verify that < 1%. I think there is an old word of caution out there that says liquid will get it. However, today, there are designs that don't care or can handle that 1%. Here in CA dry deserts I would not think twice about sealing but, my definition of sealing is still vapor breathable in all directions for all modes.

I like to call it quality control or assurance. The only way to obtain that is be the manufacture. Of course that means more labor.

> design/permit team...

Don't know what exactly you're referring to, BUT. Assuming some kind of permitting process, a lot of your options may be determined here.

Get the broad outlines approved in principle before spending much time/money brainstorming options that, well, are _not_ options. A PE on your side is golden so if you can get a stamp on you plans approved, then no worry, back to planning. But, if you're dealing w/legal permit stuff w/out a "stamp" it would seem to make sense to do as quick and dirty an overview design as possible and run it by "Gawd" in the form of the AHJ. Then after recovering from shock, outrage and despair, do it again, making adjustments or using Plan B, w/an eye to getting it OK'd, in principle, ahead of time. IMHO it can be worth paying for their official time, if they require it, in this regard. I have the greatest respect for the inspection field but I know that it can be painful when they don't have the experience, resources, or in some cases interest to make sensible calls on your "new" design. This process up front may make a lot of your choices for you and save you from wasting resources on not-to-be's.

> community...

Well, hopefully you can stay friendly. How do you feel about being the entertainment? Anyway, every crowd has individuals and you'll probably find genuine value in the strangest places. Maybe they'll buy you a beer if you let them "told you so". Look upon it as putting a "weathering" on any thin skin you might have. <G>


Cheers

Rufus

Hi Rufus

It has come to be that lately you pay a team to get everything nice and approved for you. They'll do everything needed to get your permit, you just have to shell out the cash.
Permits mainly care for the building to fit local council rules (size, height, etc) and a sound structure. So you could put great unconventional stuff if dressed in a conventional garment. The dressing part is the team's job.

From what I understand many nations are looking at energy efficient designs, some are mandating them testing the waters in some locals to reduce the 40% CO2 emissions from building's. In my local the B&S office has been reluctant for years to enforce them due to the fear of bankrupting builders. In some areas of my state it is mandatory, soon the federal government will mandate all states comply to some levels over time, just like utility companies, so my area is hiring consultants, which IMO have no clue. The Passive Homes that have been built have been built to the old German standard PHPP and are highly insulated with toxins, no or little mass. Design to it if Passive air sealing is desired, just watch the toxins. I just went through a bidding process that shows a close to net zero internal mass passive solar design cost @ or below market @ $140 USD/SF. I have the same mid-high end finish as competitors, but a much better healthier design and the same SF. Due to the renewable energy much more sustainable. I proved the rumor wrong that it cost 10% or more and bankrupts builders and, most of what I did is move HVAC money to the breathable envelope. My HVAC loads are very low.

Most got not clue what I'm talking about so to build a spec the banks want 20% down for the construction spec loan, so I decided to take down the SF from 2800 to ~ 1500 working on it now. The Architects job is to run the energy or moisture models or hire it out. I run my own and model the design. The PE does structures. Most have no clue about energy which includes inspectors and trades. The next scary's are realtors selling something they don't understand, insurance, appraisers, false competitor rating's (Passive House, LEED, HER's, Energy Star IAQ Plus etc....)....been down this long road not fun. Energy Efficient mortgages are tied to a HERs rating, and critical, so are tax credits @ 30% of cost for approved designs. We have all this in house.

IECC - is the international code for energy. In it and IRC for residential you will find three conditions that determine if you need a vapor barrier and that includes natural or any mass. Most adopt some portions of IECC since it is prescriptive - proven, including vapor barriers that ruin hygrothermal mass. It does that since mass is too dynamic to provide code for, otherwise a good design guide. If your AHJ has local adopted code follow it unless it makes no sense, then you need a stamped drawing and engineers, hired inspectors, that blows most budgets and schedules.

Teams of engineers, the days of builders designing homes with the new risk of errors and omissions are coming to an end. I have heard word of law suits. To get to mandated levels or efficency without killing people will take engineering, several sorts. I been in corporate for decades among these teams, still am, but I see a growing market for me as a Natural Design-Build Architect so I am looking at retiring in it soon I hope, just for kicks and grins and keep me occupied

Terry, don't worry, i understand you very well.

I'm waay below your expertise level and most people around here (including pro's) stare with an empty look at me when i start talking.
That's why i pretty much stopped doing it (talking).
I may do it once in a while if the other person looks interested but odds are against me.

Regarding permits, it's simpler around here (for some parts).
You have the architect ding the "looks" and local regulation mambo jumbo (position, size, window no. and sizes, mains sockets and light breakers, blah blah) as i said earlier.
Then you have a structure engineer that makes sure your structure will endure the forces of nature (whatever that means).
That usually means an anti atomic, to the proverbial frost line, reinforced concrete foundation (1m deep) so everything is nice and dandy (except it's not).
After that is confined hollow bricks (confined in RC).
Then goes some EPS slapping on the walls, glass wool in the roof (maybe EPS or sometimes MW) with the habitual plastic sheet.
That's it, high comfort.
To be honest, it's more thermally comforting than the old, thin full brick walled houses, or heavy concrete flats of the old.
But i think that's just for a while until issues come up ... not to mention real habitat quality (air quality).

Of course, i don't want (nor can afford this style of building).
As you said, going natural makes sense from too many pov's (i've done my math).


So, i'll do some more heavy thinking, make some sketches to scale, think some more, and if i come up with something, come back here.
Either way, think 1000 times, do it once.

Ionel,

Please in your reconsiderations, consider placing the insulation and air barrier at the ceiling plane. This will simplify everything and the cold attic will still be usable for sleeping and storage. Also this enables you to harvest heat from the attic in the shoulder seasons with a small fan and a couple of thermostats. This can be a significant amount of heat!

All Blessings,
Bill

Hello everyone, been on a long short vacation.

Bill, your suggestion makes sense.
You get to inspect when needed/wanted the insulation/air barrier from both sides (inside - by looking at the ceiling and attic by looking at the floor).
So you can catch possible issues before they get nasty.

But this inspectability also raises another option.
I could use strawbales (with the weight taken into account) since they would be 3 times cheaper than MW layed in 20cm thickness.
Clay render on top (attic floor) and lime underneath (room ceiling).
So one of the major problems with SB could be eliminated since you get to see/inspect all of it's sides.

Much to crunch ...

I'm back, large as life and twice as ugly (more or less).

The decision has been made to use MW in the ceiling, probably 30cm thick.
Much safer for any water leak.
In the attic there will probably be a wooden floor (in one form of T&G or another, depending on price) and no air barrier.
The ceiling is still up in the air, no exact idea on how to do it.
I could do a lime plaster as that will go very well with the lime plaster on the walls, making it a continuous lime plaster skin on the inside.

I'm however a little scared of this because i have no hands on experience with this.
I know it's historically well established but i lack the knowledge.
Also, the amount of labor is not to be ignored.

Am i to proceed with this ?
How to do it ?
What lath should i use ?
Wood strips may or may not be economical. Are there other options ?
How are they attached to the ceiling (nails or screws) ?
What lime type (plain lime or mixed with some brick dust to give it some hydraulic properties and less permeability) ?
Should i put the lath and plaster first and inspect the keying and only after install above it the insulation boards ?

Probably other questions i can't think of right now.