Summer house floor condensation

[Mybrainismelting]

Hi folks,
I see a number of posts with similar issues but nene quite the same as my condensation is o ly on the floor.
Some background
We Bought and installed a basic 10' sq summer house last year (installed in October) on a plastic grid base backfilled with gravel. No issues with leaks, dampness or anything for a couple of months (it was sitting empty and not insulated or any thing internally). Then in preparation for starting to use it (and make it "cosy") my wife fitted interlocking foam tiles on the floor. A couple of weeks later I went out there to fetch something and as I walked across the floor (in my socks as had left Muddy boots at the door) my feet were getting wet. Water was squeezing up the joins in the foam tiles. I lifted the tiles and the floor was absolutely soaking (enough to saturate 2 old bath towels).

No sign of any leaks and it was pretty even across the entire floor so I assume it was condensation, but why only on the floor between boards and foam tiles? Also the summerhouse is not exactly air tight given the gaps in the eaves and loose fitting door so I wouldn't have thought ventilation was a major issue.

The other thing I was wondering is whether it could be due to lack of airflow beneath the floor, as the floor structure is framed on all sides and sits straight on to the grid with gravel. Do I need to think about lifting and having it bearers between the grid and the floor structure (framing is 44mm sq.

Really struggling to Figure this out and the Mrs want to insulate the walls too, but want to be sure I have this issue fixed first.

This is a pic of the build in progress for some context.
View media item 106180
Cheers

 

[akwexavante]

My thoughts are that airborne moisture will condensate on cold surfaces. The floor being the coldest surface in your case and the foam files are absorbing that moisture and holding onto it.

Others may have different thoughts but i'd say you should have assembled the summer house on a raised insulated platform or an insulated concrete base perhaps. An insulated concrete base could be OTT but it would make the summer house a more comfortable space to use in colder weather.... Spring and Autumn if not over winter.

I'd consider dismantling it and start again or put down a dpc sheet, a sheet of kingspan, another sheet of dcp. Make sure it's all sealed and then put down a plywood base then cover with flooring such as lino, carpet or tiles... whatever. You could dig out / remove the base inside so you finish no higher than the current floor level.