OSB board instead of sparse panel under slanted roof cc 120 cm?

[xyle]

That's how many houses have been built; adding an installation layer with 45-70mm of insulation between the wall panels and the plastic is a fairly new concept. The installation layers were introduced not because of moisture issues but because the houses became more airtight by avoiding making holes in the plastic during installations, like wiring.

Now, I'm talking about exterior walls, but the principle is the same for roofs.

But just think logically. Isn't the plastic meant to be airtight? What do you think happens when you screw a panel with about 30 screws? Yes, 30 holes per panel in the plastic. Then it might be better to skip the plastic. It's like pouring water into a colander and expecting it to be airtight.

 

[vectrex]

what is a modern house for you? got a job last week where the private individual had chosen to put directly on the plastic. mold across the entire roof. can't remember if it was paroc, isover or t-täk fancy plastic

Yes, in Norrbotten, 35cm insulation in the pitched roof is a reasonably modern insulated house. If you have that (and there are no gaps or other issues in the insulation) and also heat up your house, you don't risk mold in the roof even with boards against the plastic.

It's not the brand of the plastic (or how modern the plastic is) but the amount of insulation.

Of course, it can't leak water from the outside either, but I probably don't even need to say that.

If the roof is insulated, the inside becomes significantly warmer than the dew point, and thus condensation does not form. Simple physics.

 

[hempularen]

Completely apart from plastic/non-plastic etc.

The OSB board will sag if it is installed with such a large span. Insulation weighs quite a bit.

 

[Gabbe1]

Just think logically. You probably have plastic to make it airtight? How do you think it turns out when you screw a panel with about 30 screws? That's right, 30 holes per panel in the plastic. Then it's almost better to skip the plastic. It's like pouring water into a colander and expecting it to be airtight.

Either you haven't read my post or have misunderstood it.
Just as I mentioned, the problem with punctured plastic is the main reason why installation spacers between panels and plastic have been introduced.
However, my point in this context was that thousands of houses have been built with plastic positioned directly behind the wall panels, and it does not lead to mold problems. Naturally, this is under the condition that the construction is properly insulated so that humidity does not condense on the plastic.

Now, SBH claims that this problem apparently only occurs in the ceiling (which the thread is about) and not in walls, and I find this strange. I maintain that if panels lying directly against the plastic begin to mold, there are other problems with the house/construction. If it is the case that spacing resolves the problem, then only the symptoms have been addressed, not the underlying issue.

 

[xyle]

However, several houses have been built with crawl spaces without moisture problems even though it is a risky construction. That doesn't mean that a crawl space always works. It was just an example.

I can imagine it gets worse in the ceiling since moisture rises upwards and not sideways in the same way. If you have a little negative pressure in the house, it will manage better with holes in the plastic, but if you have overpressure, it will want to go up again through leaks. So why take the chance? Sure, I agree with you but if you are so sure there won't be any moisture/overpressure, then you might as well skip the plastic?

So if you are going to use plastic, it should be airtight. As I am currently working on a renovation in the extension, I have plastic in the ceiling, but not in the old part because I will screw the plasterboard directly to the ceiling. There, I skip the plastic since I won't be able to make it airtight. And then it will be much more concentrated through those holes instead of the whole structure taking it.

Haven’t you seen an attic where there can be a dark spot on the plywood? That is often because there is a hole in the plastic underneath. Now there tend to be slightly larger holes than a screw hole, but still.

 

[xyle]

Then it's probably like hempularen says. It will bulge. I used OSB on the walls where I had a bit too much insulation and they were only cc60. I had to press the board between the studs when I screwed for it not to bulge. So I can imagine it gets even worse in the ceiling plus that it is cc 120.

 

[Gabbe1]

I agree with you.
But now it was about mold on wooden boards directly against the plastic. (or actually it was about whether OSB is suitable as a substitute for gles and there I tend to agree with hempularen when I think about it)
Regardless of whether the plastic is dense or not, I don't understand why the air would condense against the plastic just because a board is lying directly against it, it should be the same problem if you use gles. And above all, there should be no condensation against the plastic if the construction is properly insulated and heated year-round.

 

[xyle]

I agree with you. But now it was about mold on wooden boards directly against the plastic. (or actually it was about whether OSB is a suitable replacement for a spacing board and there I probably agree with hempularen when I think about it) Regardless of whether the plastic is sealed or not, I don't understand why the air would condense against the plastic just because a board is lying directly against it, it should be the same problem if you use a spacing board. And above all, it shouldn't condense at all against the plastic if the construction is properly insulated and heated year-round.

Oh, then I misunderstood. So the problem would be that the board is flush against the plastic, not that it gets punctured? Well, I don't know anything about that. In that case, it would be just as bad with an installation wall. When you have the insulation on the plastic, then the board?

 

[xyle]

Unfortunately, the thread got a bit off track. But if you go with gles, that's probably the best from every perspective. Or, well, if you don't have plastic, you could always "kottla" and put drywall directly on the rafters? A bit more work since there will be quite a few and you'll have to cut away some insulation.

It's not something I would do since it's extremely a lot of work just to save a few cm. But if height is important, then that's the best way to gain the height you need!

 

[vectrex]

To give a more concrete tip.

Forget the plaster and install wood paneling instead. Use tongue and groove or shiplap and paint it a solid white to create a modern impression.

Personally, I think a sloped ceiling becomes incredibly boring if it's just smooth plaster. In this case, you also get the advantage that it takes up considerably less space. You probably need to use >20mm thickness on the paneling to ensure it doesn't sag between. Maybe it works with 15mm paneling, provided it's tongue and grooved, but a bit uncertain.

 

[SBH]

Yes, in Norrbotten, insulation in slanted roofs is at least 35cm in a somewhat modern insulated house. If you have that (and there are no gaps or other issues in the insulation) and you also heat your house, you won't risk mold in the roof even with boards against plastic.

It's not the brand of the plastic (or how modern the plastic is) but the amount of insulation.

Of course, water must also not leak in from the outside, but I probably don't need to say that.

If the roof is insulated, the inside becomes significantly warmer than the dew point, so condensation does not form. Simple physics.

you're not quite getting it. and I have no desire to explain. I've worked at both high and low altitudes in our country, and the same problem remains.