Why is closed cel spray foam okay but not FB roll with VB?

[Gitter]

Hello,

We're in upper Western Oregon, we bought fiberglass insulation for our pole barn/shop ceiling - by advice of our big box store's contractor desk, It gets cold out there in winter and we want it to hold heat better. There are threads on the internet and none we've read nails it down. I've done tons of research before asking The Garage Journal. I'm just hoping you could help?

Our pole barn is 40' x 48', the ceiling has 2"x6" rafters @ 24" apart. There are no soffit vents or attic space. We were told to put fiberglass insulation into the cavities, cover the ceiling with a vapor barrier.

We already have a condensation barrier of fiberglass insulation roll under the roofing that does the job very well. It's the kind with the white vinyl vapor barrier that look like a pillow from the underside. So putting insulation up from the inside and then a vapor barrier on the ceiling afterwards, would make it a double vapor barrier. Right, we do get that.

We've talked to people asking advice around and narrowed it down to a few ways. Some say to poke holes in the old vapor barrier and then go as planned with fiberglass and a vapor barrier on the inside of the ceiling.

Others say 'No' absolutely not! As heat rises up and through the holes it will condensate against the cold metal roofing and you'll get moisture build up like a glass of iced tea. Cold air could also settle downward through the holes and hit the warm air in the celling too and do the same.

Blown-in foam insulation's nice. But it's not practical or in the budget. And neither of us care for it much anyway.

Several have mentioned using Rockwool to insulate. Leave the existing insulation barrier alone, putting Rockwool up and having it open faced so it can breathe, not hold moisture and the vapor barrier will be in a more neutral zone - not so extreme. And that would keep moisture from entering the ceiling from outside as well, if the old barrier remained in place.

one of the most popular methods for buildings without pre-existing insulation is to use closed cel spray foam or foam sheets against the metal from the inside. Then tape, spray foam or caulk it up air tight,.. then insulate and a vapor barrier over that.

For us, we would need to remove the old condensation barrier first if we did that. And that's where I want to understand things better, if someone would be kind enough to help?

Why take down the old insulation with a vapor barrier, and replace it with closed cel foam to act as insulation and a vapor barrier? Or foam board and making it air tight as we can to keep moisture and air out, before we insulate, So, why do do all that if we already have something that does that now and serves the same function?

What do you guys and gals think? Any thoughts to offer? Can anyone help us see the light here? Has anyone had to do something similar to this with good results? Thanks for reading!

Winter is coming...

 

[billconner]

The question is simply how much insulation is required to prevent condensation. I would have to look for that to formula, but it seems often 2" of xps against the cold surface - roof deck or exterior sheathing - is often enough. Then fibreglass or rock wool or cellulose inside that but not behind a vapor barrier so it can dry in.

I don't know if what you have is enough to prevent condensation. I'd guess adding 2" of rigid foam and sealing joints would be enough and the foam against vinyl should not be a a non-issue. Then on inside more insulation but no vapor barrier.

Or tear out the existing vinyl, lots of fuzzy stuff, and a air tight vapor barrier.

That's my take.

 

[theoldwizard1]

Insulating a metal (steel) panel building is a challenge ! You really need a vapor barrier between the metal and "traditional" insulation (fiberglass, cellulose or rock wool) because they will hold moisture for a very long time. Closed cel foam does not hold moisture.

If the interior is open to outside air (garage) or even normal living area these types of insulation can get moisture from the inside. Moisture conducts heat.

 

[Jeepster04]

Wizard, when you say vapor barrier, are you talking about a house wrap? Or plastic? I do not believe you want a vapor barrier between the metal and the insulation.

 

[Gitter]

Insulating a metal (steel) panel building is a challenge ! You really need a vapor barrier between the metal and "traditional" insulation (fiberglass, cellulose or rock wool) because they will hold moisture for a very long time. Closed cel foam does not hold moisture.

If the interior is open to outside air (garage) or even normal living area these types of insulation can get moisture from the inside. Moisture conducts heat.

I'm confused, maybe that was a typo about the vapor barrier? If the vapor barrier is between the metal and insulation, and we can't leave the interior open to the inside then we should have two?

 

[Gitter]

The question is simply how much insulation is required to prevent condensation. I would have to look for that to formula, but it seems often 2" of xps against the cold surface - roof deck or exterior sheathing - is often enough. Then fibreglass or rock wool or cellulose inside that but not behind a vapor barrier so it can dry in.

I don't know if what you have is enough to prevent condensation. I'd guess adding 2" of rigid foam and sealing joints would be enough and the foam against vinyl should not be a a non-issue. Then on inside more insulation but no vapor barrier.

Or tear out the existing vinyl, lots of fuzzy stuff, and a air tight vapor barrier.

That's my take.

My research says 2" also.

If what was up there now wasn't working to prevent condensation against the metal, wouldn't the insulation inside the blanket or barrier be moist in the winter time? It was dry when we opened it up last winter. Or, sorry,.. was that not what you meant?

I don't know if it's worth mentioning, but it was mentioned to us that pressing foam board in would reduce the R value of the condensation blanket. But maybe that wouldn't even matter if it had foam board over it and then insulation over that?

I feel stupid asking but what did you mean when you said to 'dry in?' Dry inside? Should 'not' be a non-issue, or shouldn't be an issue? I just want to understand the jargon correctly so I'm not misunderstanding.

Thanks everybody! keep it coming if you can?

 

[billconner]

Apologies for clumsy posting.

Putting foam board against the existing vinyl should not matter. It's the same plane or zone.

Yes I meant with foam against roof deck, the insulation inside the foam (vapor barrier) needs to dry to inside.

Forgive me if you know this but the warmer interior holds a certain amount of moisture. As it cools the relative humidity increases. When it's cold enough it condenses. So the foam to outside has to stop that, stay above a certain temperature. But there is still air in the interior insulation with high RH, and that needs to be able to dissipate. I haven't gone through this in a few years.

 

[86turbodsl]

In general vapor barriers on both sides of insulation is a prescription for mold growth. It has to dry to somewhere.

 

[kj_mustang]

Confused by some of your comments. If you put a ceiling on the bottom chord of the rafters, you have created an attic space. That attic space will need to be vented unless you hvac the entire building and control it year round and then you can insulate at the roof line and leave the attic unvented. You have unvented soffits but does it have a roof ridge vent?

 

[theoldwizard1]

I'm confused, maybe that was a typo about the vapor barrier? If the vapor barrier is between the metal and insulation, and we can't leave the interior open to the inside then we should have two?

For best performance, yes, you need 2 vapor barriers on metal sided buildings if you are using fiberglass insulation. One outside, one inside.

 

[hippie2cams]

I'm not in an area that is really cold in the winter, but I have a 40 x 30 with the insulation as you describe. over the rafters then the metal sheets. 2x6 rafters spaced at 2ft centers with r19 batts w/vaper barrier. It was built in 2008 as a shell and I have done everything else. So far no leaks, mold, mildew so I guess it's good enough. mine is climate controlled by a 3.5T heatpump for heat in the winter and cooled in the summer. and did I mention that I love it. lol.

 

[Gitter]

Apologies for clumsy posting.

Putting foam board against the existing vinyl should not matter. It's the same plane or zone.

Yes I meant with foam against roof deck, the insulation inside the foam (vapor barrier) needs to dry to inside.

Forgive me if you know this but the warmer interior holds a certain amount of moisture. As it cools the relative humidity increases. When it's cold enough it condenses. So the foam to outside has to stop that, stay above a certain temperature. But there is still air in the interior insulation with high RH, and that needs to be able to dissipate. I haven't gone through this in a few years.

I'm going to try to do a multiple reply, hopefully I do it right...

@billconner Don't apologize we really appreciate you all helping us. I'm not complaining. So, you're saying to put up foam board (from underneath and from the inside), against the existing condensation barrier, then fiberglass but leave it open faced so it can dry out? And in doing this are you suggesting I leave the original barrier alone or poke holes into it? I'll admit I was concerned tearing out the old stuff becasue it fills the voids of the corrugated roofing better than than flat foam sheeting - maybe not by a whole lot but better than. We were worried it might condensate in the voids especially once we put foam boards up it would press the fiberglass firmly against the roofing and lose R value? Would that fiberglass left against the roofing also allow some circulation for moisture to escape? Not as good of an airspace as it was, but still better than nothing?

@86turbodsl Are you thinking the original barrier should be perforated or removed entirely? Which ever method we end up doing, if we put foam board up first, will moisture escape through that and then up and out of the roofing voids and gaps etc? I've noticed foam board has plastic on both sides and then you're supposed to seal all edges and cracks too.

@kj_mustang Sorry, yep I'm confusing myself to sometimes. Roof ridge vent? In a manor of speaking, sort of. The roofing comes to the peak with a metal cap on top that isn't air tight or anything. The corrugated voids of the roofing still have some air flow I imagine and that rises up and out of the top. It's not like the roof ridge vents I just looked up with the screen or actual vents, but it isn't air tight or anything. The corrugated metal creates an airspace under it too. But none of that is open or visible from the inside. It's covered by the condensation barrier like everything else. I hope I answered that right? Oh, and we're not putting in a suspended or lowered ceiling. We're just hoping we could fill the cavities between the 2x6's - like a cathedral ceiling and trying to figure out where the vapor barrier goes or how many to have?

Also, I'm not sure how well it measures up, but for temp and hvac year round you mentioned. It's heated in winter, when we're not working we keep it above freezing in the mid 40's. We've been shopping for a low-temp de-humidifier, so that will be in the works soon. In the summer there's a thermostat controlled exhaust fan that vents the building to remove pooling heat. In the winter that is closed up and cold air is kept out.

Thanks!!!!!!!!!!!!

 

[Gitter]

For best performance, yes, you need 2 vapor barriers on metal sided buildings if you are using fiberglass insulation. One outside, one inside.

I like your thinking, but would you have time to explain? Most say no, while you're saying we need it. Now I'm really curious.

@hippie2cams Jeez hippie, I am sooo sorry you have to put up with all that. You sound absolutely miserable!!

 

[billconner]

I suggested leave the original vinyl that the foam will go against as is. I see no difference between foam barrier and that vinyl. The general recommendation against two vapor barriers is if they are separated by an air space with a temperature gradient between them, and the neither side not letting the air between dry.

 

[Gitter]

I suggested leave the original vinyl that the foam will go against as is. I see no difference between foam barrier and that vinyl. The general recommendation against two vapor barriers is if they are separated by an air space with a temperature gradient between them, and the neither side not letting the air between dry.

Oh okay, i get it now. So if we go that route, leaving the original barrier in place, will it matter what type of 2" foam board we use over it?

 

[billconner]

Normally I'd say not to EPS, but with the vinyl, maybe it doesn't matter. I don't know. I'd personally fel safer with XPS but for $400-500 or more, some thought required.

 

[theoldwizard1]

I like your thinking, but would you have time to explain? Most say no, while you're saying we need it. Now I'm really curious.

Most types of insulation need to be kept dry. Metal "sweats". Interiors can hold a lot of moisture also.

 

[billconner]

Metal sweats when the temperature of the air around it reaches the dew point. Keep the moisture content down or temperature up and no condensation.

 

[kj_mustang]

You have a unvented roof with no soffit or a roof ridge vent. 5.5" of fiberglass, to fit in the 2 x 6 rafter, is only about R19-R21. Not much for a roof/ceiling and fiberglass will allow water vapor from the warm moist interior air to pass through it. It would need to have a vapor barrier on the floor facing side or use a different insulation that provides a vapor barrier.

 

[Gitter]

Normally I'd say not to EPS, but with the vinyl, maybe it doesn't matter. I don't know. I'd personally fel safer with XPS but for $400-500 or more, some thought required.

I think the existing one 'could' (maybe) do the job, but biting the bullet for 2" EPS in the long run seems like cheaper peace of mind than finding out in the worst way later. In our situation, we'd feel safer with it too. All the info I've read up on stated that 2" was typically the buffer needed stop the dew point or condensation. Once the EPS was pushed up against the underside of the roofing - from the inside - that original barrier will lose R value being compressed. But I also feel like it'll act as and air space as well and the temperature in that will likely be similar to outside without much R value and against cold metal. So the 'warm meets cold' won't be so extreme and it'll breathe a little through the voids too. Does that sound like science fiction? Or is there some small amount of logic in that thinking?

Once we put the foam up, we would use 2x4 insulation and that wouldn't be as thick and easier to remain dry. Or am I reaching here?

Metal sweats when the temperature of the air around it reaches the dew point. Keep the moisture content down or temperature up and no condensation.

With the original barrier and EPS under it, wouldn't the moisture be kept down / inside more and be okay? I'm not sure how you'd keep the metal roofing warmer than outside though. Unless heat was leaking through and keeping it slightly warmer? I may have misunderstood that part..

Metal sweats when the temperature of the air around it reaches the dew point. Keep the moisture content down or temperature up and no condensation.

And how do you feel about the 2" EPS under the roofing and a thinner 2"x4" insulation below that - open faced? If the inside of the wall was below dew point then it wouldn't condensate right? I wonder how effective a vapor barrier would really be in a pole barn where that's the only area it would have one until later when we were able to finish it. The walls haven't had anything but R-19 Kraft faced and OSB (original to the pole barn) with no vapor barrier and it's held up well but and very poorly done.

Hmmm,.. what I should've asked was how effective would a vapor barrier be in one room when nothing else has one and it's not tight and doesn't have such compartmentalized cavities like a regular house. If it'll help, we'll certainly do it. I just have to ask, to learn.


You're all awesome, thank you!!

 

[billconner]

I think first point is correct. I think the interior face of the 2" EPS will stay above dew point. I wondered if there was a way to install the EPS in contact with but not crushing original vinyl faced fiberglass. Sealing the joints around the EPS is important or else the bouyant air will pressure through. I'm sure this is often a failure point. I like spacing the foam that leaves room for the delivery tube of spray foam to fit in.

I must of missed something. What is the 2" x 4" insulation inside the EPS?

Second point - yes - that's what I meant. Warm moist air will contact the EPS and shouldn't condense. If it gets by, it will. I believe the system does have some tolerance for a very little leakage. I my view, you need to try for perfection at this later. It's probably why many would say don't try this.

Third point "And how do you feel about the 2" EPS under the roofing and a thinner 2"x4" insulation below that - open faced?" Confused. Below that as in on interior face or below that as on the walls, not roof?

I'd wish you had a vapor barrier under OSB, but I'm not sure OSB isn't a good vapor barrier. https://www.ecohome.net/guides/2289/interior-sheathing-as-an-air-and-vapour-barrier/ I guess it is, but seams/joints need sealing. I'd look at tapes (ugly, I know). It will probably be fine. Since floor is bottom of building (no basement) the neutral pressure plane is probably near top of wall - that's a big guess - so warm air isn't being driven by pressure through seams. I think this can be measured but pointless till roof/ceiling is complete.

Do remember I'm an amatuer. I was intrigued by the subject of weatherization for a number of years, when research was done at conferences, in hardcopy publications, the library, and expirementing in my house. So would Building Science people have more detail and tweak my recommendations, maybe assume you won't get EPS joints sealed, certainly.

I guess I'm still intrigued.

 

[Gitter]

I think first point is correct. I think the interior face of the 2" EPS will stay above dew point. I wondered if there was a way to install the EPS in contact with but not crushing original vinyl faced fiberglass. Sealing the joints around the EPS is important or else the bouyant air will pressure through. I'm sure this is often a failure point. I like spacing the foam that leaves room for the delivery tube of spray foam to fit in.

I must of missed something. What is the 2" x 4" insulation inside the EPS?

Second point - yes - that's what I meant. Warm moist air will contact the EPS and shouldn't condense. If it gets by, it will. I believe the system does have some tolerance for a very little leakage. I my view, you need to try for perfection at this later. It's probably why many would say don't try this.

Third point "And how do you feel about the 2" EPS under the roofing and a thinner 2"x4" insulation below that - open faced?" Confused. Below that as in on interior face or below that as on the walls, not roof?

I'd wish you had a vapor barrier under OSB, but I'm not sure OSB isn't a good vapor barrier. https://www.ecohome.net/guides/2289/interior-sheathing-as-an-air-and-vapour-barrier/ I guess it is, but seams/joints need sealing. I'd look at tapes (ugly, I know). It will probably be fine. Since floor is bottom of building (no basement) the neutral pressure plane is probably near top of wall - that's a big guess - so warm air isn't being driven by pressure through seams. I think this can be measured but pointless till roof/ceiling is complete.

Do remember I'm an amatuer. I was intrigued by the subject of weatherization for a number of years, when research was done at conferences, in hardcopy publications, the library, and expirementing in my house. So would Building Science people have more detail and tweak my recommendations, maybe assume you won't get EPS joints sealed, certainly.

I guess I'm still intrigued.

Okay, I just assumed we'd push the EPS or XPS up against it to avoid any air cavities on the sides. If we placed the EPS to where it was just touching the lowest point of sag, not compressing the fiberglass, there would be air pockets beside each 2x6 in the corners. My husband and I talked about cutting some fiberglass triangular pieces too fill those voids beforehand, but it would be really tricky. I just assumed we'd make it flush and compressed, my fault.

on your 2nd point === Thank you, I'm understanding much better. It's funny you say that. We went to our local big box store last night, to get 2" foam board. Peeking at options, the XPS had a better rating and we figured that would be better but it was un-faced, where EPS was faced. So we pulled out our phones for some in-store research. That was a can of worms ;-) We saw that moisture can pass through both materials and wondered if this was an issue?

3rd point + I must of missed something. What is the 2" x 4" insulation inside the EPS? === I was talking about insulating over the EPS after it was installed. From top to bottom it would go like this. Metal roofing, original roll fiberglass condensation barrier with vinyl vapor barrier, EPS and then fiberglass on bottom... the ceiling from the inside underneath, looking up. The reason I said 2x4 (x24") insulation is becasue that was what I thought would properly fill the remaining void after the the EPS board was put in.
EPS is more permeable and dries out faster, Where XPS is slow both ways and tends to hold moisture much longer. We had it in our heads that moisture wouldn't reach the back side (in a perfect world) and less chance of it condensing on top - facing up, the roof side. I kept thinking well if moisture can travel through it, it'll eventually hit the original barrier where it's colder now with all that EPS over it, and possibly condensate? I'm telling the story but I'm really more asking if that's right actually? Perhaps we panicked over thinking this and we should be making it more about temperature or dew points, than moisture penetration?? Humidity's always going to be present, right? We were there 2 hrs, finally left, agreeing we needed some final details before buying a truck load of sheets.

Sometimes I think we'd be better off putting 2 layers of sheeting and have no doubts about it. Neither of us are afraid to be perfectionist with this. What ever it takes to do it right. No amount of tedious caulking, taping or expanding foam will scare us off. So don't be shy.

I will admit a part of me wonders if it would be best we removed the original insulation barrier? But my original questions come right back when I do. I worry about moisture and air without it. It works very well for what it is. It blocks air for sure. It almost seems like we should poke holes in it to allow things to dry out. But what happens when that hits the back of the metal? Probably noting good.

I'd wish you had a vapor barrier under OSB=== Yes, so do we. Our plan is to re-do everything eventually and do it the way it should have been. We will be doing that next summer but we have to get this room finished so we can get back to work. We've removed many of the OSB wall pieces to inspect inside the walls and found very little to no issues so I feel like we have time to deal with this first. Then take what we've learned and apply it to the rest - thanks to all of you kind people. The walls were sheeted up 8ft and the rest has been left as the Kraft faced r-19 and badly done a that. Knock on wood,.. but when we looked behind that, there were no issues there either. In this work room though, it has completely re-done and done right. Vapor barrier included.

I apologize, That got longer than I wanted. I will say that we understand everything's taken with a grain of salt here - no worries at all. And billconner, you may humbly call yourself an amateur but your opinions and determination is priceless to us. We've really appreciated those who have pitched in offering advice and even more those who have tolerated our questions staying with it. It's becoming less common and refreshing to see that hasn't been lost. We've dived into research of things like you did with weatherization, so I can relate. We're always glad to unconditionally help anybody, especially those asking questions that are within our wheelhouse. So,.. 'Thank You'. Thank you all!

 

[billconner]

I'm going too have to reread in am but are you planning to fit all insulation between top chords of trusses? Not a major thing, just didn't see that aspect before.

 

[Gitter]

I'm going too have to reread in am but are you planning to fit all insulation between top chords of trusses? Not a major thing, just didn't see that aspect before.

My apologies for the slow response. There was a family emergency, we had to leave town without notice. Just got back today.

But yes, we just need advice or guidance on the safest way to add fiberglass insulation or Rockwool in the ceiling, from the inside, underneath - filling the cavities between the celling joists. I'm happy to draw a diagram if needed?

 

[olytdi]

It seems to me that if you are going to add insulation, that you must mimic the 2 inch vinyl-backed insulation you have in there currently. By mimic, I mean making your 2 inch vinyl-backed insulation 6 inches or whatever depth you seek but doing so keeping the layout of the insulation the same -- batt with single vapor barrier.

That said, I don't know how you can do that without removing the vinyl, adding more batt insulation and then installing a new vapor barrier. Sounds pretty difficult to pull off.

The problem is, the current construction method you have, is mated with the current insulation type that has been installed. They work because they're a match. Altering one may require altering the other.

If by increasing your insulation, your roof trusses go from being exposed inside, to being encapsulated with the additional insulation by the new vapor barrier, you may have moisture problems with thermal bridging through those trusses.

I don't know that any of this is so, I'm just reasoning out loud....

 

[billconner]

how deep are the top chords/rafters? 2x4, 6, 8.....?

 

[Gitter]

how deep are the top chords/rafters? 2x4, 6, 8.....?

2"x6"

It seems to me that if you are going to add insulation, that you must mimic the 2 inch vinyl-backed insulation you have in there currently. By mimic, I mean making your 2 inch vinyl-backed insulation 6 inches or whatever depth you seek but doing so keeping the layout of the insulation the same -- batt with single vapor barrier.

That said, I don't know how you can do that without removing the vinyl, adding more batt insulation and then installing a new vapor barrier. Sounds pretty difficult to pull off.

The problem is, the current construction method you have, is mated with the current insulation type that has been installed. They work because they're a match. Altering one may require altering the other.

If by increasing your insulation, your roof trusses go from being exposed inside, to being encapsulated with the additional insulation by the new vapor barrier, you may have moisture problems with thermal bridging through those trusses.

I don't know that any of this is so, I'm just reasoning out loud....

That's a very reasonable observation. We were afraid to bring up too many variables with people and have them give up on us. I tried finding that out too, but haven't come up with anything certain. The only option I found to rule that out was to laminate every board surface with EPS sheet before insulating. Phew! (Might as well use spray foam)

There's a point at the eves where walls and ceiling meet. A 2x6 wall cavity meets a 2x6 ceiling cavity,.. right? The temperatures through summer or winter at that junction aren't much different from each other inside or out. Except that one can hold snow and the other can't. So why can the wall be closed up and not the ceiling? I'm sure there's an answer, we just haven't found it yet. Something to think about though. There are 2x6's in the wall that are conducting the cold too.

We have considered using a soldering iron to perforate nice neat holes all over the original barrier, so the cavity could breathe. That would mimic the original insulation, just thicker. And then put a vapor barrier on the ceiling interior after insulating. BUT, that would introduce air to the back of the metal it didn't have before. I know we're trying to make a stick house ceiling out of pole barn celling. but surely there's some way to insulate better,.. maybe not?

Thanks as always!

 

[billconner]

If you want to stay flush with 2x6, just put in 4" of XPS and I'd put a vapor barrier across it. ~R23-24 with the existing fibreglass squished to 1 1/2 ". Seal gaps between foam and 2x6 with a non-expanding foam in a can.

But I'm not finding it but you bought some fibreglass? 2"? Sorry I'm missing that piece.

What did I miss?

 

[Gitter]

If you want to stay flush with 2x6, just put in 4" of XPS and I'd put a vapor barrier across it. ~R23-24 with the existing fibreglass squished to 1 1/2 ". Seal gaps between foam and 2x6 with a non-expanding foam in a can.

But I'm not finding it but you bought some fibreglass? 2"? Sorry I'm missing that piece.

What did I miss?

Yes, we originally bought 24" 2x6 faced fiberglass R-21 batts - par advice of the big box contractor desk - for the job. Would XPS collect moisture over time? I read EPS & XPS are vapor permeable but often feel like we're over thinking it. If we had a solid plan not needing the fiberglass, it can be also used to upgrade insulation in the walls later on. So not a total loss. We'd hoped for fiberglass, so to help for sound absorption. We also have 3.5" thick Rockwool bats we picked up a year ago on sale, also to help with sound absorption.

Would the non expanding foam need to be closed cel?

Our original plan was to fill the celling's 2x6 voids with fiberglass bats, and cap the celling off with 1" EPS and a Vapor Barrier and then Tin Corrugated roofing over that to protect the ceiling from being a fire hazard. It's been at a stand still since then... To answer your question, nope, we don't have to stay flush with the ceiling as long as it's relative to the ceiling. And we're unable do a drop ceiling so that's off the board. At this point we're so desperate we'd gladly pack it 10" thick as long as it would work.

 

[Gitter]

If we removed the fiberglass condensation barrier and vinyl vapor barrier, and then mimic what we have now with a version, will the metal roofing condensate where it's corrugated (ridges)?

 

[Firebrick43]

You do NOT add fiber glass insulation to any rafter bays (which it will be after you add the ceiling) without adequate ventilation in the bays. Period.

If the bay is not ventilated, the fiberglass will be saturated over time and can not dry out. It will mold and rot the rafters, more than likely rust the roofing.

Google "unventilated cathedral ceiling" and see the results.

You can NOT adequately ventilate a bay that is only 2x6 deep unless you use full length vent trays.

You need to either make the rafters much deeper by hanging false rafters below the original or put purlins across the bottom of the rafters and ventilate the soffits/ridge.

Or rip the fiber glass out and place xps foam or spray foam between the rafters. Poking holes is not a solution. Those fiberglass blankets used as a condensation barrier is a bad idea to begin with but if allowed exposure somewhat works.

 

[billconner]

I don't think you can "vent" this with vent trays, as I think the 2x6s are purlins, parallel to the ridge and sitting on top of trusses. Maybe someone has a plan for venting horizontally but never heard of it myself.

Filling the rafter space with fibreglass and then sheathing inside with rigid foam can work but is risky as it depends on a near perfect seal of the foam. Unless polyiso, the foam also needs to be covered for fire safety. And of course no acoustical benefit, maybe a negative

The 2" foam sealed against the existing thin fibreglass and vinyl should prevent condensation but the remaining depth is likely insufficient for enough fibreglass for proper insulation, and iirc probably not deep enough to use what you have on hand. (correct?)

Sorry, not offering any good or useful ideas. Probably why the usual solution is a solid ceiling at lower chords and piling blow in on top of it - deep.

If you're going to hang a ceiling, either high within trusses and parallel with roof or at bottom chords, be sure the trusses were designed for it.

Just spit-balling, hang 2x4s a couple feet under purlins, probably perpendicular to purlins, install on hand batts, sheath (rough sawn 1 bys?), and then blow in more fibreglass or you could lay unfaced batts on top of the faced batts. Or skip sheathing and just cover with a continuous vapor barrier. And add vents for this "attic" - ridge and soffit or fan and louvers in gables.