Deck above heated space, (above living room)

[Seerised1]

Hello

I am considering building my house with a deck above the living room and my question is whether an air gap
is necessary between the roof insulation above the room and the rough boarding, felt, batten, and treated decking.

What could happen if I skip it?

Erik

 

[justusandersson]

It depends. If you have a diffusion-open waterproofing layer on top of the råspont, no air gap is needed; otherwise, it is necessary for the rådpont to dry out if it becomes damp.

 

[Seerised1]

Above the raw timber, there will be base paper + roofing felt laid in asphalt, as the roof should be waterproof under the decking floor.

 

[justusandersson]

A material can be waterproof and breathable at the same time. Compare with Gore-Tex in clothing and shoes.

 

[Rickard.]

Roof felt is of course airtight, the only waterproofing layer that I know is breathable is PVC sheet but I'm unsure if it's open enough to be suitable in such a construction.

What is the purpose of skipping?

I have worked on a job where I made a roof with felt without an air gap using an unusual construction, and whether it will work or not remains to be seen.

 

[justusandersson]

The experiences with diffusion-open underlay fabrics are not entirely positive, however, there are studies that show it is possible to build without an air gap even though the insulation material is mineral wool. With a low roof pitch, it can be difficult to have an air gap directly under the roof decking function well. It is possible to build more complicated roof solutions where ventilation occurs between the insulation layers, with the insulation closest to the roof decking being moisture-transporting. However, more space is needed for such a solution.

 

[Seerised1]

The purpose of skipping the air gap is that the deck height with wedges in two directions + the air gap builds approximately 450mm, which results in a height difference from the floor on the upper floor with an internal staircase of 3 steps as a result, which means both extra costs and the unwanted height difference.

Thank you for the response.

 

[Rickard.]

I don't see a diffusion-open underlay as a solution since they require a ventilated roof structure that is bulkier and unsuitable (complicated) for building a deck on.

I would argue that 20mm ventilation is better than nothing, and I can't quite accept that it would ruin that much. On the other hand, I still don't quite understand the situation.

 

[Seerised1]

I don't see diffusion-open underlay fabric as a solution since they require a ventilated roof that adds more height and is unsuitable (complicated) for building a deck.

I would argue that 20mm ventilation is better than nothing, and I can't quite accept that it would cause so much damage. But on the other hand, I still don't quite understand the situation.

We'll probably have to go with the solution of an air gap and wedges to be on the safe side. What is it that you don't understand, Rickard?