Condensation control and vapor barrier for tiny metal building

We're building a tiny off-grid camp in the Southern US (USDA hardiness zone 8b) where it's extremely hot and humid.
We have a metal building with a wood frame and I'm very concerned about condensation and mold.

To combat that concern we're taking the following steps:

We have painted the interior wood frame with a mold resistant primer.

We'll be using a wood burning heater in the winter (likely not much as winters are mild here.)

No cooking or propane usage inside.

There is no bathroom, kitchen, or cooking inside, we're going to create separate structures for these in the future.

We have our solar and electrical wiring tested and nearly complete and very soon we're going to install insulation.
All the information I have found regarding our climate suggests that if we use insulation such as fiberglass or rockwool we should have a vapor barrier on the outside of the structure, this is not possible for us as the walls are already on the frame and have no vapor barrier (see attached photos.)

Another option I've found is to use closed cell spray-in insulation which is both a vapor barrier and insulation and is a good defense against condensation. Some of the shipping container builds I've seen have gone this route as well.

I have a few concerns with the closed cell spray foam insulation but I have more concerns with condensation and mold so will go that route if it's best. I just wanted to ask the community here if there were any other options. For example, would I be able to use some kind of paint or roll-on vapor barrier on the metal interior then use rockwool for the insulation instead of spray foam?

Thanks!

At this point, closed cell foam is the best cheapest solution, although not that green. You can do the minimum closed cell and then some rock wool for sound.

For those who haven’t built yet, it is cheaper to do foam board but it’s a pain.

For the roof, I would strongly recommend a "micro-attic" -- a substantial (3-4") air space between the metal roofing and the insulation layer (like Roxul), with lots of vents on both the low side and the high side of the roof.

This moves heat and moisture out of the roof and the insulation through a simple, passive convection flow. Yes, you'll need to add additional internal framing for your insulation and your ceiling will be a few inches lower, but the increase in comfort will be well worth it.

Yes, you need insulation.....

There is no alternative.

Spray foam will give great service for that "NEEDED" layer to your structure, but will be $$$$.

If you go get 3/4  or 1" foam board, (whatever the thickness of your purlin lumber), cut it to fit inside the rafters, AND caulk all the edges and seams,
you will pay $.

It would be easier to unscrew the metal, install foam sheets in full size (no cutting), re-install the metal (that already fits),
with longer screws to account for the thickness change due to insulation, and call it a day.
You should metal tape the seams of the foam as you install it. Foam Shiny side down. Tape on the living in side
You will not have a moisture drip problem.

Rain in the evening, inside the structure sucks.

Thank you all for the helpful replies!

R Scott wrote:At this point, closed cell foam is the best cheapest solution, although not that green. You can do the minimum closed cell and then some rock wool for sound.

That is the strategy that I've been considering and it sounds like that might be the best route.

Douglas Alpenstock wrote:For the roof, I would strongly recommend a "micro-attic" -- a substantial (3-4") air space between the metal roofing and the insulation layer (like Roxul), with lots of vents on both the low side and the high side of the roof.

I've been wondering about this to some degree but haven't considered creating a "micro-attic". Are you suggesting that I don't use the closed cell spray foam on the ceiling at all? When you say "vents on both the low side and the high side of the roof", where would the vents go? On the front above door and the rear? I've attached some additional photos of the outside of the building (before we had it delivered.)

It's worth noting that I've also looked into prodex and reflectix but have heard conflicting information about their efficacy.

I've also considered installing a solar attic fan such as this one: https://www.amazon.com/Solar-Roof-Vent-RVOblaster-Galvanized/dp/B075DVKMR3/ref=sr_1_14?crid=3SRMT7U0CYG7R&keywords=solar+roof+fan&qid=1661738762&sprefix=solar+roof+fan%2Caps%2C176&sr=8-14

But the thought of cutting into the roof is rather unnerving especially since the roof is ridged and looks like a pain to get installed properly but I think it would do wonders for air circulation.

Pete Arthur wrote:Yes, you need insulation.....

There is no alternative.

Spray foam will give great service for that "NEEDED" layer to your structure, but will be $$$$.

If you go get 3/4  or 1" foam board, (whatever the thickness of your purlin lumber), cut it to fit inside the rafters, AND caulk all the edges and seams,
you will pay $.

It would be easier to unscrew the metal, install foam sheets in full size (no cutting), re-install the metal (that already fits),
with longer screws to account for the thickness change due to insulation, and call it a day.
You should metal tape the seams of the foam as you install it. Foam Shiny side down. Tape on the living in side
You will not have a moisture drip problem.

I'm glad that you mentioned the option of unscrewing the metal. I considered it but really prefer to not take the walls off, I also don't have much of a roof overhang and have already installed gutters which might get in the way.

I second the creation of a micro attice for venting. Generally you have some vents on the bottom of the eaves on the side, to let air in, and then some at the peak on the ends to let the warm air out.

I'm going to highly suggest you DO NOT use spray foam. It sounds like this great product, but in reality there are many issues. If you use a company that mixes the foam, there is a small chance it will go wrong and create a horrid smell. If you ever need to move anything around in the wall or remodel down the road, the spray foam is terrible to get out. Things stick to it, and it is a pain. As has been mentioned, it is not very "green". However, my biggest personal reason for not using spray foam is that it creates an vapor tight seal. Without active air controls (venting and bringing in fresh air) you end up living in a plastic bag. This will keep what little vapor there is, inside... and make mold heaven. Since mold is one of the things you specifically are worried about... please don't go that route. Spray foam also puts out nasty smoke if, heaven forbid, there ever was a fire. The traditional way is better, but not good. You put a plastic vapor barrier over regular insulation. You still have to be super careful about where you put the vapor barrier. It needs to be on one side of the wall in cold climates and the opposite in warm climates... and it just generally gets complicated.

My personal suggestion is to get some insulation and wall materials that allow for vapor permeability both directions. Rockwool insulation comes to mind, but there are some others too. This helps insulate, while allowing the humidy to dry to whichever side it wants to. Rockwool also won't lose its insulative value if it gets wet and then dries out. Please forgive my poor artistry skills, but below is an example. The purple line is the inside wall. Plaster or sheetrock or something. The black rectangles are the studs, the large brown boxes are rockwool bat insulation, the long thin brown rectangle is their equivalent for those 4x8 sheets of foam. That extra layer helps break the thermal bridging that studs have. Then the brown line is your metal exterior. There are different ways and materials to achieve the ability to let your wall dry either direction... this is just one example.

The walls are not that critical, many of the insulation methods will be fine.

The steel of the roof needs to be sealed off and temperature insulated from any air inside the building.

The strong cheep way:  top to bottom,       steel, tar paper, plywood.

better:    steel, tar paper, foam sheet insulation shiny side down, plywood. Plywood keeps the foam from moving/sagging.

Steel pole buildings have had 1" fiberglass with white vapor barrier rolled over the purlins for years,
to keep moisture in the air from condensing on the underside of the steel.
If your teenage kids later pierce the thin white poly layer with BB guns, the moisture that collects inside the plastic makes
puddles that droop inside the structure, and rust the steel from underneath.
Plywood and tar paper is cheaper and BB guns don't bother it. Looks better than an unfinished white poly stretched over rafters.

I would encourage you to look at Petes original suggestion.
Walls do not usually have condensation its the ceiling.
By lifting the roof in sections and doing a good job now it will be prefect.
In australia there is a blanket pf rockwool and reflection sisalation that sits with the rock wool against the metal roofing and everything is screwed back down.
You may beed to create a thicker space for the rockwool.

Standing back, the biggest challenge (as I see it) is that this structure was designed as a storage shed, not as a residence for humans. So, we're retrofitting.

Somehow, we need to vent heat and control moisture to make it a livable home -- neither a mold factory nor a convection oven.

- - -
Just so I understand clearly, is the roofing material coated steel (both sides) or coated aluminum? Up here we see both.

- - -
While I am skeptical of spray foams, I can tell you that the thin, hard foam/fiber mix they spray on the inside of shipping containers and large quonsets is completely effective at preventing condensation through a massive range of temperatures and humidities. I have not been able to smell any hint of offgassing in these structures over time, even when well sealed. Totally unscientific, but first hand experience.

Just a minor update here. We went with 2 DIY spay foam kits, then put up fiberglass insulation over that (couldn't find rockwool anywhere.) We then used rigid foam boards to insulate under the shed as it was too tight of a fit (and messy) to attempt the spray foam there. The heat is coming soon, and we'll get to see how well it holds up to condensation we've not put up the ceiling yet so still have a chance to peak and feel behind the fiberglass insulation on the ceiling.

As far as smell, I've not noticed any and I'm pretty sensitive to that sort of thing. I think I have some pictures if anyone is interested but it's nothing spectacular IMHO.

Also, for anyone with a beard attempting to do the DIY spray thing yourself, google the "Singh Thattha" technique for wearing a respirator thanks to COVID there have been a lot of research and fit tests showing it's effectiveness.