I have heard many different answers to that question, some say overlap is completely OK, others want to essentially fully weld all the joints.
You can't make it completely airtight no matter what you do.
I have used tape, mostly because my supplier thought it should be that way, I don't really have my own opinion on how it "should" be.
You can't make it completely airtight no matter what you do.
I have used tape, mostly because my supplier thought it should be that way, I don't really have my own opinion on how it "should" be.
Without being a professional or expert in any way, I believe the reason the tape is so expensive is that it is also age-resistant, meaning it won't come off in 10 years or so. Regarding overlap, it is usually recommended that the joints are also clamped between, for example, 2 studs or between a stud and drywall so that pressure is applied along the entire length. Airtight, so to speak.
If you use tape other than the age-resistant type, there is also a risk that the adhesive on the tape will dissolve the plastic after a number of years. There is research that shows that duct tape specifically does this. Unfortunately, you cannot see inside the wall if the tape comes off or dissolves the plastic, so I think it is best to be safe and buy age-resistant tape.
Sorry to resurrect an old thread, but here's something I don't understand.
At my local hardware store, there is both T-emballage Sealing Tape T-Flex for a bit over 200 SEK/25m and the aforementioned Tesa 4668 for just under 100 SEK/33m.
T-Flex seems very solid, adheres to various materials, and according to the product sheet, has an aging resistance of 50 years. But it's expensive!
The store staff reportedly claimed that the Tesa tape was not approved. According to the manufacturer, however, it is approved by SITAC. Exactly what type of approval this involves, however, is not specified. Given the staff's statement, perhaps the approval isn't that relevant. Even more confusing is that Tesa has a small sun symbol on the packaging, and underneath it says "180 days." What does this mean? Is the aging resistance only 180 days??? (Some retailers market it specifically for temporary joining, such as painting work). Or does it mean that it can withstand direct sunlight for 180 days, but otherwise lasts much longer?
At my local hardware store, there is both T-emballage Sealing Tape T-Flex for a bit over 200 SEK/25m and the aforementioned Tesa 4668 for just under 100 SEK/33m.
T-Flex seems very solid, adheres to various materials, and according to the product sheet, has an aging resistance of 50 years. But it's expensive!
The store staff reportedly claimed that the Tesa tape was not approved. According to the manufacturer, however, it is approved by SITAC. Exactly what type of approval this involves, however, is not specified. Given the staff's statement, perhaps the approval isn't that relevant. Even more confusing is that Tesa has a small sun symbol on the packaging, and underneath it says "180 days." What does this mean? Is the aging resistance only 180 days??? (Some retailers market it specifically for temporary joining, such as painting work). Or does it mean that it can withstand direct sunlight for 180 days, but otherwise lasts much longer?
But look there. On SITAC's website, it was possible to search for the approval for the specific product. 
There is, among other things, a comment about needing to avoid UV radiation when storing outdoors. Perhaps that's what is meant by the sun and "180 days".
There is, among other things, a comment about needing to avoid UV radiation when storing outdoors. Perhaps that's what is meant by the sun and "180 days".
When I had a building consultation, the guy at the municipality said that tape is to be disregarded. He argued that no one could know how long it would last and I kind of agree with him. He said the only right way is to clamp the plastic between two studs. That sounded good to me and I thought that's what I should do. Now I have started to think about how to do it. My walls will consist of standing 195mm with horizontal 45mm on the inside. The plastic should be placed between the 45mm and the 195mm. But since one is vertical and the other horizontal, I can't clamp the plastic without adding some material. Possibly, I could attach some form of strip that clamps the plastic against the vertical studs, but that feels really cumbersome.
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I also think tape seems a bit insecure. When you cross-brace and place the plastic between the vertical and horizontal studs, you should be able to brace the underlying studs at the plastic seams, if you know what I mean. Do you think it's worth the extra effort to do it that way?
I absolutely think it is worth the effort to tape where the joint is, then you know it will be tight and it doesn't take that much extra time.
You're right that some of the point is lost, but on the other hand, it's not often that you splice, so it should become negligible. But you can certainly add a thinner trim and clamp it if you want, it certainly won't be worse because of it.
Well, kortla might have been the wrong expression, now I've figured out how it should be! If you're going to put plastic on standing studs and have horizontal studs inside, you obviously put the plastic from top to bottom, resulting in a seam at every 2400mm. At these seams, you nail a thin strip instead of taping. And only full lengths of plastic are used.
That's how it will be!
That's how it will be!
But preferably, you should have as few joints in the plastic as possible, it's better if you roll the plastic along the wall, that way you minimize the joints. There are diffusion plastics up to 3.10 wide, for example, T-emballage has it.