It can probably be said that there is no absolutely correct answer to the question, at least not a known answer.
In my thirty years in the industry, the theories have varied.
I have also recently done a thesis on the subject.
It is not certain that bigger is better, for example on a rainy/humid and windy autumn day, the best might be to be without/have it closed.
A suitable middle ground is probably to make the air gap out of 28 gles.
Of all the houses I have demolished, with different constructions of the air gap, I have never found any damages caused by faulty construction of the air gap.
In my thirty years in the industry, the theories have varied.
I have also recently done a thesis on the subject.
It is not certain that bigger is better, for example on a rainy/humid and windy autumn day, the best might be to be without/have it closed.
A suitable middle ground is probably to make the air gap out of 28 gles.
Of all the houses I have demolished, with different constructions of the air gap, I have never found any damages caused by faulty construction of the air gap.
The need for an air gap is mainly related to the desire to ventilate away the moisture that is always present in freshly cut lumber, such as in newly laid tongue-and-groove boards on the roof, which are also covered with relatively diffusion-tight paper on top. The ability to manage this moisture is also affected by the choice of insulation material. Mineral wool, for example, lacks hygroscopic properties, and if a diffusion barrier, such as plastic sheeting, is also placed on the underside of the insulation, the moisture cannot escape any other way than through the air gap. Theoretically, one could build without an air gap if using cellulose fibers as insulation and having a diffusion-open windproofing underneath. However, few dare to do it. In the past, people took their time and allowed all materials to dry before building them in.
I would like to emphasize what larsbj says, that too much ventilation is not always good. With a shed roof, there are conditions that the air gap can become quite effective from a ventilation point of view. 28 mm therefore feels like a good compromise.
I would like to emphasize what larsbj says, that too much ventilation is not always good. With a shed roof, there are conditions that the air gap can become quite effective from a ventilation point of view. 28 mm therefore feels like a good compromise.