ᐅ Roof insulation between rafters: Should you choose thin or thicker insulation wool?

Hello everyone,

we are dealing with a barn where energy-saving regulations and top insulation values are not a major concern. The interior roof area is about 26m² (280 sq ft), so slightly more expensive products would not significantly impact costs.

The roof construction consists of (from the outside in):

• Clay tiles
• Counter battens
• Parallel battens on rafters
• Underlay membrane
• Rafters

My questions:

• The rafters are only 8.5cm (3.3 inches) thick. So far, I have only seen insulation rolls with a thickness of 100mm (4 inches). Can you still use 100mm and compress it slightly? The hardware store advised against this. Otherwise, the only alternative would be 85mm (3.3 inches) insulation boards, which would cause more waste since I would have to cut them across (rafter spacing about 75cm (30 inches)). Later, a vapor retarder and a 24x48mm (1x2 inch) counter batten will be installed on top. I do not plan any additional insulation between the counter battens.
• When purchasing faced insulation batts, is there anything special to consider besides the insulation value? I noticed that the “ISOVER Integra ZKF 1-035 G3 touch” is intended for “unventilated” roofs. How does this differ? (Our roof would be ventilated due to the parallel and counter battens.)
• Is the “ISOVER Climate Membrane Vario KM Duplex UV” significantly different compared to standard vapor barriers from the hardware store, such as Probau, which costs only a third?

Best regards

What exactly do you plan to do with the barn? Will it be converted into a living space afterward?

No, it is not inhabited. However, we spend a lot of time there (barbecuing or sitting in the garden with guests, and when it starts to rain, some of the visitors go into the garden house, or sometimes to watch the Football World Cup).

Then, attach 4cm (1.6 inches) battens parallel to the rafters. This will leave you with a 2cm (0.8 inches) gap towards the back for the underlayment membrane.
And I hope you are not using a charcoal grill in an enclosed space.

Joedreck schrieb:

Then apply 4cm (1.5 inches) battens on top of the rafters at the same time. This will give you 2cm (0.8 inches) of space behind for the waterproof underlay.
And I hope you are not using a charcoal grill inside a closed room.

That would give me about 10.5cm (4.1 inches), and with 100mm (4 inches) insulation, I would theoretically have 0.5cm (0.2 inches) of air space. If I understand this correctly, you shouldn’t compress the insulation but actually plan for some air gap?

8.5cm (3.3 inches) plus 4cm (1.6 inches) is 12.5cm (4.9 inches), right?

Do not compress. Leave space. If the insulation wool touches the breathable membrane, moisture may condense there and cannot be removed.