ᐅ Insulating a cold roof with compressed fiberglass insulation, cost

Watch out, don’t get worked up. ---- Take another look at my photos. You’ll see tongue-and-groove boards instead of OSB. And those are only in the standing area; otherwise, the insulation is open to the top. Do it like that for your build. Then just ventilate from time to time, and that’s fine. Also, vapor barriers are not sealed, but vapor retarders, which makes a difference. It’s not completely airtight. Karsten

Hmm, now you’re making me doubt. Just like with Nordlys, I was given exactly the same recommendation by the professional. The material is already at the house, and on Sunday we started installing the rough boarding in the attic. The plan is as follows:

Ceiling upper floor/attic: 23.5 cm (9 inches) rough boarding, 22 cm (9 inches) insulation wool, vapor retarder, battens, drywall sheets. So far, no problem.

Attic roof (triangle window with tilt function in the attic present): 24 cm (9.5 inches) insulation wool, vapor barrier, battens.

The material costs for the nearly 90 sqm (970 sq ft) roof were under 1,000. The attic will of course not be heated, but it can be ventilated. Seems like this is not the best idea then. So cancel everything? Basically, I trust the professional, but the arguments here do make sense to me^^

boxandroof schrieb:
But it is also possible that the mechanical ventilation system is pushing humid air upstairs somewhere.

The mechanical ventilation system should (normally) be a closed system and therefore would have nothing to do with the ventilation of your attic.

Nordlys schrieb:
Also, vapor barriers are not glued, but vapor retarders, which is a difference.

True vapor barriers are hardly used anymore; the terms are often used interchangeably. But of course, you are right—we are talking about vapor retarders here.

What apparently hasn’t quite sunk in yet: vapor retarders are not supposed to be completely “airtight” (not even the vapor barriers of the past were), but are meant to allow some controlled diffusion so that any moisture that gets in (in summer, for example) can dry out again.
The problem, especially with poor workmanship like in @boxandroof’s case, is leaks through which more moisture can enter than is healthy for the structure—that is, more than can dry out naturally. This issue is made worse when materials with a relatively high sd-value, like OSB, are used. Drying back through OSB is usually much less effective than through a vapor retarder membrane.

@pffreestyler

I would avoid it. Either you want to use the attic as a living space in winter and heat it accordingly, or it should remain a pure storage or, at best, a hobby room for the warmer months. In that case, you can also leave the roof structure open.

We have insulated an attic with drywall; it is not separately heated.

There is also no insulated or any other type of hatch. The ceiling to the attic is insulated and has a vapor barrier. On top of the insulation, there is wood paneling (tongue and groove boards); up to 2.5cm (1 inch) from the edge.

This has been in place for almost 20 years.

So, should an uninhabited attic always remain uninsulated? I don’t find that ideal either. Some items stored there do not tolerate large temperature fluctuations well. That would never be possible then?! Hmm

Edit: @Caspar2020 How thick is the insulation on the sloping walls, and is there a vapor barrier included or not?

@Caspar2020

Dr Hix schrieb:
This kind of thing can hold up well for decades, but it can also lead to rotting ceiling joists within a very short time.

This falls into the category of "Smoking isn’t harmful, my grandfather smoked four packs a day until he was 103 and only died in a car accident."

Of course, these are always just theoretical considerations. Why it works out well for you in reality, but doesn’t for someone else who has invested a lot of effort into the ‘correct’ execution, would have to be examined on site.