Moisture issues with wood ceiling / metal roof ?

[gjbuilder]

I have a pole barn with sheet metal roofing and it's all in very good shape.

I am converting some of the space to interior, finished space, and so the ceiling of these interior spaces is currently the original plywood with the sheet metal lying on top of it.

In that current configuration there is basically zero insulation and it's also a bit ugly.

I am thinking of doing a very simple 2x2 skeleton frame underneath that original plywood and then putting up *another* layer of plywood over the top of that 2x2 skeleton. This gives me two pieces of plywood plus 2 inches of air as insulation and also gives me a small space to hide the junction boxes for the lighting behind.

It all seems simple and sensible, but then someone mentioned moisture/condensation/vapor ...

Do I have anything to worry about there ? I am basically trapping 2" of air between two sheets of plywood that has metal roofing on top of it. I was NOT planning on venting that space in any way ...

I am not sure where the moisture would come from or how it would be a problem, but I would like to hear any comments/suggestions.

THanks.

 

[Al Bundy]

Condensation is where it will come from. You want the air moving in enclosed spaces so the moisture doesn't settle out of it and collect on every available surface. Especially when you have large temperature variation on either side.

 

[gjbuilder]

Condensation is where it will come from. You want the air moving in enclosed spaces so the moisture doesn't settle out of it and collect on every available surface. Especially when you have large temperature variation on either side.

Ok, so there is indeed an issue here - the top piece of plywood is hot since it's next to the steel roof, the bottom piece of plywood is not hot since it's inside, and that 2" space is where I will get condensation ... correct ?

It's easy enough to vent that small space, so that's an easy fix, but won't I lose a lot of my insulation value by venting it ? I don't need much insulation since this is northern california, but we do drop down to 18-20F in the winter here and I was depending on 2 sheets of plywood with 2" air between as my insulation ...

If I put some little vents in each ceiling section, will I still get some insulation value from it ?

 

[gjbuilder]

It's easy enough to vent that small space, so that's an easy fix, but won't I lose a lot of my insulation value by venting it ? I don't need much insulation since this is northern california, but we do drop down to 18-20F in the winter here and I was depending on 2 sheets of plywood with 2" air between as my insulation ...

If I put some little vents in each ceiling section, will I still get some insulation value from it ?

Any thoughts on this ? Appreciated.

 

[Jess]

In my climate, vapour barrier goes on the warm side of any insulation. When we build my big shop, we used engineered trusses, strapped the framing with 2x4, placed 15lb roofing felt on top and then installed the metal roofing sheets. The felt keeps any drips from condensation from dropping into the attic space. In my case, its a vented attic.

I would be concerned about creating any space above the warm side that does not have a way for moisture to exit the building. A lot of issues were created around here and the PNW by closing up tight wall and roof assemblies and later finding they were rotting from inside. Moisture from vapour condensing in the exterior sheeting and from lack of detailing on the exterior finish.

For your case, you could strap the ceiling to allow air flow up and out a ridge vent, then place whatever grid under that. In my area, that would be filled with insulation but you may not need it. If you did use insulation, you would then sheet over it.

There are lots of good ideas here, and I'm sure you will get a suggested solution that will work for you.

 

[gjbuilder]

OK, sorry to be dense but I still haven't wrapped my head around this...

Structure, from top to bottom (outside to inside) is:

- sheet metal roof on top of ...
- plywood, on top of ...
- 1.75" of empty space, on top of ...
- interior plywood, on top of ... the empty room

The 1.75" of gap between the exterior plywood and the interior facing plywood (the ceiling) is there not only to give us space to run wires and boxes for lights, but also to give us some tiny amount of insulation (the 2" of air).

This is northern california so temps can go as low as 15F, but only briefly at night and just 1-2x per year. So two layers of plywood with 1.75" of air between is as much insulation at the ceiling as anyone needs (in a barn).

OK, anyway ...

So the danger here is that it's cold outside and the sheet metal is cold, and the plywood underneath the metal roofing is sort of cold, and the 1.75" of empty air is sort of warm, and I will get condensation on the downward facing side of the top sheet of plywood.

Do I understand that correctly ?

Let's say I were to solve by adding a single sheet of plastic/visqueen to the layers I have just described ... where would I put that single sheet of plastic ?

I'm thinking that I put the plastic sheet underneath the top layer of plywood - the exterior layer that is under the metal roof ... but then would moisture just condense on the underside of the plastic and produce the same problem ?

Maybe a well ventilated working space (the room) with the 1.75" gap I am talking about, combined with mild nor-cal temps means this isn't a problem at all and I should leave my layers just as I described them ?

Thank you again.

 

[readhead]

What is holding up the plywood?

 

[gjbuilder]

What is holding up the plywood?

The sheet metal roof is on top of the plywood, and that plywood is held up with 4x6 rafters.

So if we plywood over the underside of the rafter, we would end up with a 6" empty space.

Instead, we outlined each ceiling space with 2x2 sticks and will attach the ceiling to those, which is why the empty space is 1.75" thick.

So if you look up, you'll see the exposed 4x6 rafters, and in each segment of ceiling there is a "sub ceiling" framed just below the original ceiling.

Make sense ?

 

[readhead]

Something is missing here. Pictures?