How can I avoid damp or staining via wood on metal?

[jono2.0]

Hello,

I'm starting to batten out my sail-away.  It's been spray foamed to death and it's going to be an interesting job getting it level. Above the gunnels has been done so I'm just talking about below gunnels.

I have vertical metal U-shaped uprights below the gunnel welded to the hull outer.  These obviously form the structure and I will be adding wooden 2x1" battens horizontally to these using self-tapping screws.  I have needed to take some of the spray foam off the uprights to reveal the metal so the batten can fix to it.  Is there anything I should do to stop damp coming through the wood from the cold metal hull?  Also, what about longer lengths where metal and wood are joined?  I just don't want stain banding or marks on walls 6 months down the line.

 

Any ideas? 

[Robbo]

I used the foam that goes under wooden floors that's around 3-4mm thick to make thermal barrier between the wood and metal.  Damp shouldn't be an issue, use expanding foam to cover any exposed metal back up.  Your trying to avoid condensation that's cause from water vapour inside the cable from forming so thermal barriers between the metal hull and the cabin lining.  Adding the foam for the battenings to fix to the metal hull is one method but most don't use it.  You are also trying to stop the water vapour from getting to things that will absorb the water vapour (like rockwool) and cold surfaces (like the metal hull) so these need a water proof barrier between the wool and the cabin lining.  Spray foam doesn't require this.

Edited March 29, 2017 by Robbo

[Drayke]

Take a look at this link it may give you some idea how to trim your spray foam. http://laughingsquid.com/how-to-build-a-simple-diy-plastic-foam-cutter-using-a-nine-volt-battery-and-some-wire/

[Robbo]

I've used a orbital sander to get large areas of spray foam level.