Can I just replace the sawdust with insulation?

[boohoo]

and what do you achieve with that then? If you have an old house, you will probably get the best result by complementing what you have with more sawdust or spraying in some modern cellulose or similar. Then you can just refill as they did in the past, and you don't need any air gaps or other fuss, and everything breathes perfectly fine anyway.

"closed-cell polyurethane foam to achieve a diffusion-tight layer?"

Much of what is discussed above are problems that can arise if you add too much insulation without having a vapor barrier/brake.

You achieve the same effect as if you would plastic wrap the whole thing, i.e. minimize the moisture traveling from the living space to the cold attic, which risks condensing.

 

[Stefan1972]

"closed-cell polyurethane foam to achieve a diffusion-tight layer?"

In many ways, what is discussed above are problems that can arise if you add too much insulation without having any vapor barrier/brake.

You achieve the same effect as if you were to plastic the whole thing, i.e., minimizing the moisture that goes from the living area up to the cold attic and risks condensing.

yes, but then that layer is on top of any subroof. That just means it ends up there and it starts to mold there........If you plastic wrap a room correctly, it's at a level downwards.

 

[boohoo]

yes, but then that layer is above any under-roof. That just means that it ends up there and starts to mold there........If you seal a room correctly, it's one level down.

I'm not really sure if we're misunderstanding each other, but for my part, it would end up basically where you would have sealed, i.e., between the lower parts of the roof trusses. The only difference is that where the lower parts come on the roof trusses, you get it on the "top side" instead of under, the under-roof. Between the lower parts, you get it exactly where the plastic would come.

It shouldn't have any possibility to condense unless you have an indoor RH that allows it, and then you will have condensation on the walls as well.....

 

[Stefan1972]

I don't really know if we're misunderstanding each other, but for my part, it would basically be where you would place the plastic, i.e., between the lower sections of the roof trusses. The only difference is that where the lower sections come on the roof trusses, you get it on the "top side" instead of underneath, under the roof. Between the lower sections, you get it exactly where the plastic would come.

There shouldn't be any possibility for it to condense unless you have an indoor RH that allows it, and then you should have condensation on the walls as well.....

Yeah, no, we're thinking alike, but since you mentioned wood chips, I assumed you might have an old house. In that case, it's often raw wood against the roof truss/construction and maybe even some old underroof. It's rare to tear all that away, and if you spray foam on the raw wood, it will be higher up than if you plastic from inside the room. But of course, you can tear everything away and start from scratch.

 

[boohoo]

Yes, we have råspont. The råspont is nailed directly onto the underarms. If you were to correctly apply plastic in this case, you would have to tear away the råspont and then apply plastic against the underarms.

If you were to spray PUR, it would be done from the attic, right at the place where any plastic would have ended up. I just don't see how it becomes "one level up" except where you can't reach on the underside of the underarm.

 

[Stefan1972]

Yes, the difference is plastic on either the underside or above the raw plank. But if it's in the cards to possibly remove it in your comparison, then of course there's no difference, but then you might as well use plastic instead of doing something that no one else does. The Americans and English like foam everywhere, but here it's not really a recognized method. I don't really understand what the advantage would be. If you have an old house with existing raw plank and shavings in the attic, it's much easier to leave it as is and just add a little more. You don't need to add half a meter directly; a few centimeters of fresh insulation covering everything will likely make a dramatic difference if there's not much there now. My house from 1900 has about 30-40 cm of shavings in the roof against the attic, so I think the problems we create in our heads are mostly theoretical.