Finishing the inside of a metal frame storage shed as naturally as possible

I'm in the very early planning stages of turning an outbuilding into a workshop.

I finally got some input from a local (non-permie) carpenter who told me how he would do it. It was tremendously valuable input because I was at a loss due to moisture and mold problems.

First of all here's a sketch-up I did of the structure. Everything is pretty much to scale here I think I'm within a few mm of tolerance in my measurements, but I did them with a tape measure so they could be off a bit.



Explanation of structure as it is
The walls are a sort of corrugated sheet metal. The frame is steel. The roof is metal. The underside the roof is a sort of styrofoam-like layer that adheres to the contour of the roofing. The floor is concrete slab that slopes downward past the two lines visible on the left side. Those two lines show where a concrete gutter runs under the structure and is covered with concrete slabs (complete with gaps and holes letting moisture in from below). The only door to the structure is a rolling shutter and the only other ventilation is tiny slits in the back-upper wall under the contour lines of the roof.

Carpenter's advice for finishing
The carpenter said, given the moisture problems he would lay a sheet of plastic directly over the concrete and then put plywood flooring down directly over the plastic, screwed or pinned into the concrete. He would build an inner wall with a door and window(s) behind the concrete gutter line. To finish the walls he would fill in between the metal frame with insulation, then construct a wooden lattice (maybe better word for that) attached to the frame to support wall and ceiling boards/shelving, etc. Then use plywood wall panels over the lattice and sheet rock ceiling panels. He would cut out a window in the back wall as well, so that windows could be opened for a cross breeze. He would also install a ventilation fan. He also suggested cutting down my chestnut tree and clearing all plant life out about 50 cm from the structure!

Permie adjustments
I don't like plywood very much though I respect it's virtues in terms of stability and strength. I also don't like plastic, but in this case the plastic sheet might be necessary to keep moisture out from below. I have absolutely no intention of cutting down my chestnut tree, but I understand his point about plants being a contributor to moisture. So I need to keep up my garden maintenance, especially keeping the shed roof clear.

Flooring
I wonder if I can use a clever tongue and groove or korean style flooring that just floats over the plastic/concrete, possibly wedged in at the edges, and thus avoid puncturing holes in the plastic at all. I believe a lot of flooring is done in cedar here which is cheap and easily available.

Walls
Insulation is an option, the carpenter said why not. The why not would be that conventional insulation is against my natural building goals. Also I hear horror stories of insulation molding and becoming more of a problem than a solution. It really doesn't get that cold here in the first place and when it is cold it's also usually a great time to work outdoors, so this might be left out. Instead of plywood I was thinking of overlapping batten boards that can expand and contract without causing a problem. I will probably intentionally provide some marking showing where the metal frame is to make it easy to do wall-mounted installations.

Ceiling
I haven't really thought through this one yet.

Inner door wall
I imagine this will be the biggest challenge for me, and I have no idea how I'm approach it yet. I will admit I am intrigued by truly round or seemingly round doors.

Electricity and wiring
Right now I have an extension cord that I run to the house when I need power (rarely). In the future it would be great to have lights and I probably need to power a ventilation fan (and/or wall-mounted heat pump). Unfortunately I can't legally do electric work here, so I'll need to factor in hiring an electrician into the planning.

Purpose and use
I need a sheltered workshop. I work outside when I can, but mosquitos, horseflies, glaring sunlight and heat all make that a challenge during late spring to early autumn. It would be nice to have lights to turn on so I could work after dark as well. I also hope to make some wall-mounted lumber racks. It would be great to have a place to store and dry the wood I collect.

I do mostly hand-tool woodworking so I don't need much in the way of space for things like table saws, sanders, planers, etc. I will probably build a western workbench with a vice, but beyond that I do most of my work Japanese style, on the floor with a planing board. Since I don't make a lot of fine dust a broom will probably suit well enough for dust collection. I may opt to do most sawing and sanding outside, since those two produce the finest dust particles.

I get annoyed when people suggest cutting trees down!!
I would not start sheeting the shed until I had sorted a few things out.
- The use of natural materials inside a steel shed is pushing your luck
- Korean flooring or plywood I believe would be similar in their properties
- A concrete floor would be adequate for wood working by itself
- perhaps improve the insulation on the wools with wool insulation.
- Is the water off the roof captured and kept away from the building at all.
- then fit battens to the walls and line them with timber
- I doubt the gutter would be contributing to moisture inside, that may be a result of condensation from the walls.
- Perhaps heat the place when its cold, that will help with condensation.
- As for doors, will you be able to move things through a round door easily?

If there's a lot of moisture coming through the concrete, sweating, or possible flooding, I would be concerned with screws or nails anchoring the floor into the concrete. They may rust through since they are so thin. A floating floor on a sheet of plastic will wear holes in the plastic unless it's much thicker than I'm thinking. Here, laminate floors and such are placed on a layer of thin foam.

Windows are nice. Maybe think about possibly working on very long pieces some day and place them where it can stick out of them in relation to where you will be working.

Thanks for your response John, I was hoping you would chime in!

John C Daley wrote:I get annoyed when people suggest cutting trees down!!
I would not start sheeting the shed until I had sorted a few things out.
- The use of natural materials inside a steel shed is pushing your luck

Do you say this because steel isn't permeable and tends to create condensation? Or for another reason?

John C Daley wrote:
- A concrete floor would be adequate for wood working by itself

I have been told this before, and that is how I am using it now. I am mainly worried about the concrete contributing to moisture inside. Do you think that might not be so much of an issue?

John C Daley wrote:
- perhaps improve the insulation on the wools with wool insulation.

I have had that thought as well, or perhaps rice straw, or rice hulls. My main concern is that they will absorb the moisture from the steel and become moldy.

John C Daley wrote:
- I doubt the gutter would be contributing to moisture, that may be a result of condensation from the walls.
- Perhaps heat the place when its cold, that will help with condensation.

So if the moisture problems are likely coming from the walls, wouldn't I want to focus on getting the indoor ambient temperature to remain relatively similar to the outdoor temperature? That way I could avoid condensation entirely.

Honestly year round the need for heating and cooling for basic work is very minimal. The humidity we experience is a bigger problem. We see 70-100% humidity for large parts of the year. I'm hoping that improving the ventilation will do a lot for this. I may install a small AC unit, or at least leave that as an option if it seems necessary later. Our other, smaller shed, has no real moisture problems, but it is finished conventionally by the aforementioned carpenter.

John C Daley wrote:
- As for doors, will you be able to move things through a round door easily?

That's a good point. Since I'm building the entire inner wall I was thinking about having either a separate a lumber door, or a way to open the door wider than typical when I want to move in large pieces.

You’re experiencing similar dilemmas to me - insulation, plastics, plywood. I’m working on finishing a basement in a totally different environment. I eventually compromised with very thin foam insulation with a R value of 15. It’s only 4mm thick.

Does the shed get direct sun during the summer? You might want insulation to stop it turning into a massive solar oven. I’m guessing that’s what the foam and tiny slits are for, any hot air just under the roof, flows out through the slits. Our house in Singapore had slits above the windows for this purpose.

Have you considered a dehumidifier? Maybe solar powered with batteries? Or is that something that’s also governed by local laws?

Concrete floors are very unforgiving on knees and ankles if you intend to spend a lot of time in there. I’m guessing tatami isn’t practical even though it would be the ideal ‘anti-fatigue’ flooring. What about rubber mats made from recycled rubber?

It looks like a lovely outdoor space. I’ll be building my workshop in the garden some day, so I look forward to seeing how you get on and what you eventually make in the space.

This thread has already given me some really good food for thought...

I think I'm going to take some measurements of temperature and humidity inside and outside for the next year (It's unlikely I will start building before that anyway).

Having some data on seasonal relative temperatures and humidity in its current state should give me a good basis for how I can take precautions and such.

I don't think it's getting a big solar oven effect because it's fairly shaded. A dehumidifier is definitely a thought I had, and if I install an AC unit that will be an included feature.

Concrete slab -> Plastic sheet -> thin foam layer -> floor boards huh... could do that compromise. The foam could add some cushion. I wonder if I could address the floor afterwards and make judgements as I go as to its necessity.

Hereis more feedback MY COMMENTS IN CAPITALS SO THEY ARE SEPARATED
Thanks for your response John, I was hoping you would chime in!
BACK GROUND TO CONDENSATION
"Condensation occurs when warm air collides with cold surfaces, or when there's too much humidity in your home.
When this moisture-packed warm air comes into contact with a chilly surface, it cools down quickly and releases the water,
which turns into liquid droplets on the cold surface."

John C Daley wrote:
I get annoyed when people suggest cutting trees down!!
I would not start sheeting the shed until I had sorted a few things out.
- The use of natural materials inside a steel shed is pushing your luck

Do you say this because steel isn't permeable and tends to create condensation? Or for another reason?
BOTH, THE OTHER REASON IS THAT MOISTURE FROM THE AIR WILL COLLECT ON THE INSIDE OF BARE STEEL, LOOK AT ANY CARPORT ROOF ETC.

John C Daley wrote:

- A concrete floor would be adequate for wood working by itself

I have been told this before, and that is how I am using it now. I am mainly worried about the concrete contributing to moisture inside. Do you think that might not be so much of an issue?
I BELIEVE IF YOU PUT DOWN PADS AS EDWARD SUGGESTED YOU WILL BE OK

John C Daley wrote:

- perhaps improve the insulation on the wools with wool insulation.

I have had that thought as well, or perhaps rice straw, or rice hulls. My main concern is that they will absorb the moisture from the steel and become moldy.
WOOL DOES NOT HOLD MOISTURE AND IS NATURAL. ITS USE WILL PREVENT CONDENSATION


John C Daley wrote:

- I doubt the gutter would be contributing to moisture, that may be a result of condensation from the walls.
- Perhaps heat the place when its cold, that will help with condensation.

So if the moisture problems are likely coming from the walls, wouldn't I want to focus on getting the indoor ambient temperature to remain relatively similar to the outdoor temperature? That way I could avoid condensation entirely.
PARTIALLY CORRECT HERE, BUT INSULATION TO PREVENT THE TEMP. DIFFERENCE WILL BE EASIER AND MORE COMFORTABLE AND IT WILL HAVE NO ONGOING COST.

Honestly year round the need for heating and cooling for basic work is very minimal. The humidity we experience is a bigger problem. We see 70-100% humidity for large parts of the year. I'm hoping that improving the ventilation will do a lot for this. I may install a small AC unit, or at least leave that as an option if it seems necessary later. Our other, smaller shed, has no real moisture problems, but it is finished conventionally by the aforementioned carpenter.
VENTILATION IS NEEDED AS WELL

John C Daley wrote:

- As for doors, will you be able to move things through a round door easily?

That's a good point. Since I'm building the entire inner wall I was thinking about having either a separate a lumber door, or a way to open the door wider than typical when I want to move in large pieces.

I THINK YOU NEED TO REALISE ITS A WORKSHOP AND IT NEEDS TO BE FUNCTIONAL.
KEEP THE FANCY DESIGN FOR THE MANCAVE OR THE HOUSE!!

I'm now looking more in depth at insulation solutions.

Wool seems to be a good solution, unfortunately being Japan it is not readily available.

Rice hulls also seem to be a good solution which are readily available. I'm trying to figure out a couple of points though...
1. Without a container they will a pain to deal with. If I were to go with no container I need some way to keep them in the wall and not just spilling out everywhere.
2. Plastic containers introduce plastic... which is an option but less desirable
3. Paper, hemp, or burlap bags seem likely to be a weak point in terms of mold/rot.
4. I worry they might become a nesting ground for rodents.
I had a novel idea to build narrow wooden frames filled with rice hulls to fit into the steel frame spaces. It would be extremely time consuming and labor intensive though, and no guarantee it would work.

Blue jeans/cotton batting is apparently another common natural solution... but I don't see that it will handle moisture and mold particularly well. Also used clothing is not as readily available in Japan as it is in the USA... this would probably turn into an absurdly expensive solution.

XPS foam seems like one of the better conventional options, as it is less dangerous to install and handle. When I pulled some semi-exposed XPS out from another outbuilding it had some mold, but not a lot considering how long it had been there - probably 5-10 years at the bottom edge of a tarp covered metal frame hoop house.

I don't really want to do fiberglass batting. I don't want to use hazmat gear to finish my shed/mancave/workshop.

There is a product made from recycled drink bottles is not itchy.

So what are you aiming to do with the insulation? Is it there to control the temperature and thus your comfort or the humidity and thus the building integrity?

I would be more interested in high airflow than insulation.
How big are the eaves of your roof? Can you leave a big air gap between the walls and the roof and not have rain drive in?

Does water drain from your concrete floor or just sit and slowly evaporate?

If the floor was solid I would just have it as concrete. But if water is getting in. I would cement single bricks in a grid pattern over the concrete (air flow!) and build a conventional, joist and tongue & groove floor. I would then build up tongue and groove walls with the framing on the inside (not touching the metal!) and then a mozzie net roof.

Essentially an untreated wooden shed within the metal shed, with lots of airflow around the wooden shed to prevent the build up of humidity. Cedar would be good, just don't let any part of the wood sit in water.

This perspective may be too heavily influenced by the weather where I grew up in the UK.

I will look into recycled drink bottle insulation, never heard of it but it sounds intriguing.

The moisture problems are not rain or flooding per se. In fact drainage on our property is fairly decent. We have to considering our yearly rainfall is 2457.6 mm (96.75 inches) I believe we get a lot of mold and damp because of 1. year round high humidity, 2. low to no air-flow, 3. condensation. There may be other reasons I'm not aware of. I have heard of concrete wicking moisture from the ground, but I don't know the truth of that.

The roof on the building slopes to the back into a gutter which drains well as long as I keep it cleared, it had been clogged up in the past. Even if I were to cut down my precious chestnut tree, the trees on the mountain above drop enough leaves that I'd still have to do twice-yearly roof clearing.

As I mentioned above I wasn't sure that insulation was necessary, but it sounds from helpful comments that if I do the insulation well it should contribute to preventing moisture from condensation. I still need to manage air-flow, but I believe that opposing windows, a ventilation fan, and potentially an A/C unit would greatly improve that situation.

What I don't want to do is a crappy choice of insulation materials/set-up that introduces mold or other problems.

Here is the Japanese web site for Earthwool https://www.knaufinsulation.jp/ja
Who manufactures Earthwool?
the Knauf Group
Earthwool ® is part of the Knauf Group, which was originally founded way back in 1932.

I found a wool insulation manufacturer selling in Japan. It's expensive, but I think I like this route the best: https://www.itnjapan.com/product/

The performance aspect of sheep wool insulation is impressive to read. It does look like I'll need to be extra thoughtful about preventing rodent entry if I use any kind of wool (sheep, mineral, or other)

That product you have come up with looks good.
I see it has between 20-30% recycled polyester, I think that is the material made from drink bottles. But I will check.

I came across your interesting project while looking for tips of fitting out an existing shed in my yard, also with colorbond walls and roof and concrete floor. Do you have any gaps between the roof and walls, and, again at floor level due to the colorbond grooving?   If anyone knows is there is an as natural as possible way to fill these up or will I be destined to buy that expanding foam stuff?

B Zalm wrote:I came across your interesting project while looking for tips of fitting out an existing shed in my yard, also with colorbond walls and roof and concrete floor. Do you have any gaps between the roof and walls, and, again at floor level due to the colorbond grooving?   If anyone knows is there is an as natural as possible way to fill these up or will I be destined to buy that expanding foam stuff?

I think I understand your question. It looks like "colorbond" is a brand of metal wall material. It sounds like it has similar corrugations or grooves as you put it that mine does.

There aren't any gaps between the walls and the roof, but if I remember correctly the walls had small gaps, I actually need to go out and check up on that point since I'm concerned about rodent entry in my case. To that point, one solution I read was to use steel wool to block the gaps.

In terms of insulation I think as long as everything has a point of overlap your bases should be covered. Maybe not for critter control though.

I think it would depend on your plans for the interior insulation and wall material, also your climate and particular plot, any pests that might be a problem, etc.

Take all that with a grain of salt... I'm doing this finishing job for the first time myself and have only just begun research, plan, and design.

B Zalm, there is a product in Australia for your situation., I will get some details for you.
https://shedblog.com.au/buy-corro-corners/
and
steel-emberseal-and-vermaseal-all-in-one-banish-vermin-embers/

Alas my techno knowledge is very low, so thanks for these links John. The gaps intersecting the roof and sides are bigger where there is an outside vertical beams.                                                                                                                                                                                                                                                          
That vermaseal looks suited if shed is being constructed (?) I will take a closer look at the other one.                                                                                                          
In the interim I have made up lavender bon bons  (lengths of aluminium foil rolled with lavender flowers scrunched and wedged along the gaps at the bottom seams). I won't kid myself it has bought me a little time to find a longer term solution, the past week was nice no signs of rodent activities.