ᐅ Attic Conversion Exposed Beams / Proper Insulation Installation

Hello everyone,

All statements and questions refer to the attached drawing, which I hope is sufficient.

I have the following questions for the experts among you:

• Is the construction method I outlined basically correct?
• When leaving the beams visible from the inside, can you still achieve a tight roof with the vapor barrier/vapor retarder, or is it better to fully seal the roof from the interior?
• The insulation rests on the vapor barrier on the inside and is limited on the outside by the sheathing and the roofing membrane. Should there still be space left for air circulation here, or is this setup acceptable?
• Is the insulation thickness of 155 mm (6 inches) sufficient? It is a bio-based insulation (I cannot provide the exact name or manufacturer due to lack of knowledge) that has been stored unused in the attic for 15 years.
• Would you install one or two drywall boards (12.5 mm / 0.5 inch thickness) stacked between the rafters?
• If I decide against visible beams, I would have a total height of 205 mm (8 inches) to fill with insulation. Should this height be completely filled, or can I leave air space either below or above?

I look forward to helpful answers or any follow-up questions.

Best regards and thanks from Blautopf,
Galsan

In short: if you want the rafters to remain visible, the vapor retarder and insulation must be installed from the outside, that is, before the roof covering; anything else is ineffective and doesn’t work.
When doubling up, you do exactly what the name suggests: you add another layer on top, not just some plank work.
15 cm (6 inches) is far too little. “Organic stuff from 15 years ago” certainly has a poor insulation value, so you might need about 30 cm (12 inches) to just meet the energy-saving regulations.