Extension drying out

[Greenlane]

Over the last few months have built a large single storey extension block with cavity and insulation on a currently empty property. We have recently had the flat roof done covering the entire area. The cavities got wet due to bad weather, so I've recently covered them and the external block work with sheeting as  I'm still awaiting the arrival of the coping stones to close the walls around the parapet. There is plenty of airflow inside due to the Windows not yet being fitted. Particularly in this cold, damp, wet weather Some of the interior block walls are getting damp/wet and the roof joists and Visible roof steels are the same, most of the moisture evident where the joists sit on the external walls. I can't see it being the weather getting through the block work or down the cavities as they are now covered with plastic sheeting.. Is it likely to be condensation? Should I cover all the window openings as best I can to try and get the inside dry? Any advice would be helpful.....

[Oz07]

It will dry from inside out once you get structure sealed and heat in there. Atleast that's how I understand it. Get copings on Windows in then start putting some heat into it

Edited December 18, 2017 by Oz07

[nod]

Also I wouldn’t rely on copings to close the cavity’s off

i fixed ply across mine then fibreglased over the ply and put the copings on top

[Russell griffiths]

As @nod said you really should have a cavity closer with a dpc under the coping. 

If the coping is made of any form of concrete/stone, it WILL let moisture through into your cavity. 

[Declan52]

You will need to close the cavity at the top with motar and some thing like a cement board or fire resistant board cut to suit your cavity width. If your build is a standard 100-100-100 then you cut the sheet 250mm wide and bed it over the cavity then you can bed your coping on top.

The walls will continue to absorb the rain that's on the floor coming through the openings. Once the openings are closed over then they will dry out.