ᐅ Interior insulation for heated basement: vapor barrier or vapor retarder?

Hello, I am looking for some advice for my building project involving a well-insulated room with sufficient moisture protection (it will be an aquarium room). I already have rough ideas and calculations, but I lack the practical experience to know if it will actually work.

My plan is as follows: I want to insulate the room from the inside using EPS panels (preferably with a thickness of 6-10cm (2.4-4 inches)). External insulation is unfortunately not possible because the room is completely underground, and I don’t want to undertake the work of excavating everything. With this approach, I expect to achieve a U-value that is sufficient for me (around 0.4-0.5 for the entire room). The problem is that the room probably won’t have very low humidity (I’m not exactly sure yet, but I could imagine it being easily 60% or higher). And as is typical with internal insulation, I will almost certainly have a very moist wall surface on the inside. To prevent this, I have been thinking about how to solve the problem.

A vapor barrier membrane sounds like a good idea, but I will need to screw things into the wall, which will likely make it difficult to maintain both stability and airtightness. OSB panels as a vapor retarder are currently my preferred option because I can screw things into them while still having some resistance to vapor diffusion. However, there would still be moisture between the insulation and the wall. This is my current situation, and I’m unsure how to proceed.

I hope my description is not too confusing, and maybe someone can offer a tip on the best way to approach this problem.
Best regards,
Paul

PIR or XPS insulation boards bonded with an OSB board and on the inside, attach and fill one layer of moisture-resistant gypsum board.

Install the insulation boards on the wall using flexible tile adhesive. They must be flush and fully adhered without any ventilation gap.

EPS is not vapor-tight, but PIR and XPS are. This allows you to drill or screw into the OSB board without any issues. You can build the floor in the same way, but do not use regular drywall; instead, use moisture-resistant dry screed or apply a waterproof membrane on top to prevent swelling.

If the basement exterior wall is not well sealed from the outside, first seal it with mineral-based waterproof slurry. Two-component hybrid coatings and bitumen do not adhere on the inside and will peel off.

First of all, thanks for the quick help. I have looked into the insulation materials and I think I will choose PIR with the aluminum facing. When I stick these to the wall, do I need to seal between the boards with tape or something similar? And another question I have is, why are the gypsum boards screwed onto the OSB panels? Do they also act as a vapor barrier or retarder, or is it purely for aesthetic reasons?

Regarding the dimensions: Does it make sense to use different thicknesses for the insulation boards, since my understanding is that the ceiling should be insulated the best and the floor insulation might even be negligible?

laul.laulsen schrieb:

First of all, thanks for the quick help. I looked into insulation materials and think I will choose PIR with aluminum facing. If I then stick these to the wall, do I need to seal the joints between the boards with tape or something similar? Also, one question I still have is why gypsum boards are screwed onto the OSB boards? Do they also serve as a vapor barrier or retarder, or is that purely for aesthetics?
Regarding the dimensions: Does it make sense to use different thicknesses of insulation boards, since as I understand it, the ceiling should be best insulated and the floor maybe even negligible?

There are also prefabricated panels made of PIR boards with OSB boards glued on. With tongue-and-groove OSB panels fitted tightly edge to edge, no additional sealing should be necessary if the boards are well fitted. If the cuts are not precise or there are gaps at the edges, using spray foam (rigid PUR foam adhesive or flexible PUR foam from a can) is recommended to fill any cracks.
I mentioned gypsum plasterboards because you mentioned consistently high humidity. OSB tends to swell under those conditions.
Is this a basement room, by any chance?
For thermal insulation, all exterior surfaces exposed to temperature differences to the room in question are relevant. This naturally includes the exterior walls in contact with the ground and the floor as well. Without floor insulation, you will have persistent condensation and moisture issues at the floor.
The ceiling (if there is a heated room above) can be neglected in terms of insulation. If the space above is unheated, you should insulate the ceiling as well.

For insulation thickness, I would recommend around 60mm (2.4 inches) for all surfaces. Routing electrical wiring on surface-mounted conduits is advisable; otherwise, you lose insulation underneath and risk thermal bridges with potential condensation.

I have now taken a closer look and created a detailed plan. However, I am still not completely convinced about the idea of using moisture-resistant boards. Would it be possible to make the OSB boards completely waterproof with something like paint? Epoxy resin comes to mind immediately.