Designing an air tight roof - Truss, I Beam, ???????

[Renegade105]

Hi all,

 

please can anyone advice what I should do….

I thought attic truss would be the fastest and cost effective way to get watertight, looks like we will go wood fibre insulation but the biggest problem is going to be trying to get an air tight roof….I mean trying to get in between the trusses etc is going to be a nightmare with OSB…

 

any guys here with experience mind sharing their thoughts?

 

pherhaps steels and cut rafters or I. Exams and posijoist floors might be the way forward?

 

thanks

962306781_PPG-978_300ASectionAABBCopy.pdf

[Iceverge]

If making it airtight from the inside with trusses will mean an extraordinary amount of taping. Unlightly to be a good result. 

 

Some glulam ridge beams and a cut roof would be easier but still require diligent workmanship. 

 

Given the complexity of the design I'd be tempted to move the airtight outboard. 

 

A layer of OSB over the trusses. Taped at all joints and then the airtightness layer returned to the wall airtightness layer would work. You'd need a good layer of external insulation like woodfiber boards or PIR. You may run into issues of buildup thickness and long fixings through the insulation are not cheap. Taping the OSB would need dry weather. 

 

Alternatively a breathable but airtight woodfiber board outboard like @IanR and blown cellulose between a cut roof should produce a good result.