ᐅ Roof insulation between rafters – 200 or 220 mm

Good evening,

it's me again...
But I’m really impressed with the help here, no matter the topic.

One more question, this time about insulation between rafters:

- Rafters are 220mm (9 inches) thick
- Can / should / must I fill the entire rafter thickness?
- So, do I use 220mm (9 inches) insulation with a thermal conductivity of 0.035 W/(m·K)
- Or 200mm (8 inches) insulation with 0.032 W/(m·K) and leave 20mm (1 inch) of air space?

Thanks,
Regards and stay healthy!

Haha! You can, should, and are allowed to choose the maximum value; the price difference is marginal.

Okay, I just meant that I thought I had read that the rafter depth should not be completely filled in order to avoid direct contact between the insulation and the vapor barrier.

If that’s not the case, then the matter is quite clear.

Well, the matter is not that clear-cut. I am currently dealing with the same question.

Ventilation is necessary if the underlayment is either missing or not vapor-permeable. Moisture needs to be able to escape.

I have 240 rafters and am planning a full rafter insulation with 200mm (8 inches) 035. Initially, I planned a partial rafter insulation with 220mm (9 inches) 035 and ventilation.

Which underlay membrane is meant? The one on the living space side? That will be vapor permeable; a climate membrane is planned....

hgerike schrieb:

if the underlay membrane is missing or not vapor permeable...

...then someone has done a pretty poor job.

Insulation should cover the full rafter thickness.