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Straw bale insulation under steel roof

 
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Hello, I want to build A frame house with wood beams, straw insulation and steel roof, can I avoid condensation from the steel? Thanks
 
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Hi Vakhtangi, welcome to Permies!  I think you can do that, but you do need to plan for the condensation.  I think most roof insulation systems would have issues with metal roof condensation as well so I'd start by doing whatever is normally done in your area for metal roofs.  Here in the US it might be a plywood roof deck with tar paper or "ice and water" shield on it and then the metal over that.  My greenhouse has a really steep roof (almost a wall) so I just used housewrap between the metal roofing and the insulation.
 
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Steel roofs always condense.
Tarpaper is your friend, tarpaper will last forever if kept out of U.V.
Ensure there is an adequate runoff channel at the end of the tarpaper, so the condensate doesn't find its way back to the straw.
Ensure the straw insulation can expirate to stop dew point condensation from building in your straw
 
Mike Haasl
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Hi Bill, how do you allow the straw to expirate while also having tar paper as a layer in the roofing?  Is a 2" air gap that's vented at the top and bottom sufficient?
 
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In steel roofing construction there’d normally be a sheet of insulation placed under it – the common stuff here is silver on one side and insulated on the other. Google ‘installation of sarking under metal roof’.

The sarking is installed length wise from the ridge to the top plate. With straw bales, it would extend over them so any moisture falls outside near the eaves. There should be sufficient insulation in the roof cavity to prevent too much condensation forming.

Also, that is why straw bales are usually coated with a lime or earth render – it breathes.

 
Bill Haynes
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If his design is such that the tops of the bales can be left open / porous covering and evaporation can occur through normal roof / gable vents, will be adequate,
Any method that allows steady slow transpiration of air through the  straw column.
The worst would be tarpaper laid directly against / sandwiched between steel and straw.
The straw would be protected from the condensate, but would rot from dewpoint condensation from the inside.
 
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