Metal Shed Condensation Prevention

John Sneddon, who hails from Glasgow, Scotland, in the United Kingdom, just purchased a metal shed and realizes he could have an issue with condensation.

"Hi there,

I have just purchased a Yardmaster 10ft x 13ft Metal Shed but have just found out that these are prone to condensation inside.

What I am looking to do is raise this shed with CLS treated timber around the sides with some foam insulation then clad them off etc. this would be about 1 to 1.5ft high and replace the door as it will not fit due to it being raised.

Can you tell me if this is correct please:

For a moisture barrier below concrete slabs, could I use rubble bags opened up or a tarpaulin to compete the underground area.  Then build a  wooden frame which would be treated for the OSB3 or Plywood flooring to go on.

I was going to put a vent under the floor so that any moisture could escape.

Funds are a bit tight and was looking to see what I hope you can understand makes sense."

John, howdy from across the pond!

You're on the right track if you want to prevent much, not all, of the condensation in your new shed.

First, the biggest issue is the invisible water vapor that's CONSTANTLY coming out of the soil. You need to stop that.

You can do that with a laminated vapor barrier or pieces of closed-cell foam rigid insulation. Read my past column about this.

If you decide to put the shed on a concrete slab - that would be my choice - then you simply need to place a great vapor barrier under the concrete. These are usually very inexpensive. If you use the cheap crap from a home center, then put down TWO OR THREE LAYERS of it.

Now here's the real issue. You can still get condensation in the shed from when the concrete slab is cold / cool and your weather warms up fast and some HUMID air gets into the shed.

This water vapor in the air will condense on metal tools, the concrete slab and maybe the shed itself.

Bottom Line: It's really hard to stop all condensation, but you can control most of it.