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Steffen_3415 Apr 2026 18:41Hello,
My roof and dormers were newly constructed. From the inside, I installed insulation between the rafters and screwed a three-layer board, which also serves as a vapor retarder, onto the rafters from the inside.
Now, the interior sides of the dormers (roof and cheeks) are to be painted white, so I will line the inside of the dormer with gypsum plasterboard. My question: How should I connect the plasterboard to the PVC window and the wooden window sill?
I do not want to use acrylic. Therefore, I plan to proceed as follows:
My roof and dormers were newly constructed. From the inside, I installed insulation between the rafters and screwed a three-layer board, which also serves as a vapor retarder, onto the rafters from the inside.
Now, the interior sides of the dormers (roof and cheeks) are to be painted white, so I will line the inside of the dormer with gypsum plasterboard. My question: How should I connect the plasterboard to the PVC window and the wooden window sill?
I do not want to use acrylic. Therefore, I plan to proceed as follows:
- Stick a separating tape onto the window frame
- Bevel the gypsum board edge at 45 degrees and press it against the separating tape
- Fill the gap between the separating tape and the gypsum board with joint compound
- Cut off the separating tape
- Is this correct, or should I use an expansion joint tape with a thickness of 3 mm (0.12 inches) for stress-free plaster connections instead of separating tape to better absorb expansions of the gypsum board, window, and window sill?
- Are there any special window plastering profiles for interior use, similar to those used outdoors, that can be used with gypsum plasterboard? What are they called exactly?
Hi,
your approach is already quite close, although I winced a bit internally at “no acrylic,” but that’s fine, it’s a matter of preference. Separation tape basically works because it creates a clean separation and prevents a rigid connection to the window frame, which is not unimportant with PVC. The 45° edge is acceptable, although many put more effort into it than necessary; the joint mainly relies on movement absorption, not perfect geometry.
A 3mm (1/8 inch) expansion joint tape really only benefits you if you expect actual movement, which on the interior is usually below 1mm (0.04 inches), unless you have extreme temperature fluctuations. In this case, separation tape is often the more practical solution, even though some consider it “too simple.”
Regarding plaster profiles, yes, there are also ones for interior use, especially for drywall connections at windows, with a flexible lip and a tear-off edge. In my opinion, these result in cleaner lines because you get defined edges instead of feathering by feel. Especially on white surfaces, every imperfection is visible after 2–3 months.
How do you plan to connect to the window sill? Will you butt the drywall panel straight against it, or will you intentionally leave a small shadow gap?
your approach is already quite close, although I winced a bit internally at “no acrylic,” but that’s fine, it’s a matter of preference. Separation tape basically works because it creates a clean separation and prevents a rigid connection to the window frame, which is not unimportant with PVC. The 45° edge is acceptable, although many put more effort into it than necessary; the joint mainly relies on movement absorption, not perfect geometry.
A 3mm (1/8 inch) expansion joint tape really only benefits you if you expect actual movement, which on the interior is usually below 1mm (0.04 inches), unless you have extreme temperature fluctuations. In this case, separation tape is often the more practical solution, even though some consider it “too simple.”
Regarding plaster profiles, yes, there are also ones for interior use, especially for drywall connections at windows, with a flexible lip and a tear-off edge. In my opinion, these result in cleaner lines because you get defined edges instead of feathering by feel. Especially on white surfaces, every imperfection is visible after 2–3 months.
How do you plan to connect to the window sill? Will you butt the drywall panel straight against it, or will you intentionally leave a small shadow gap?
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Steffen_3416 Apr 2026 09:00Thank you, Nauer, for the detailed response.
Gypsum board joint at the window – plaster profiles
I need help identifying the exact profiles and how to install them. I’ve already asked at the hardware store and spent hours searching online – so far, I don’t understand it.
Gypsum board joint at the window sill
The window sill (oiled glued laminated core beech, 26mm (1 inch) thick) rests on the ends of the rafters and the header beam (see photo) and is screwed multiple times to the rafters and header beam using angle brackets. Here, I would also use separation tape or 3mm (1/8 inch) expansion joint tape and apply a silicone bead after painting.
Best regards
Gypsum board joint at the window – plaster profiles
I need help identifying the exact profiles and how to install them. I’ve already asked at the hardware store and spent hours searching online – so far, I don’t understand it.
Gypsum board joint at the window sill
The window sill (oiled glued laminated core beech, 26mm (1 inch) thick) rests on the ends of the rafters and the header beam (see photo) and is screwed multiple times to the rafters and header beam using angle brackets. Here, I would also use separation tape or 3mm (1/8 inch) expansion joint tape and apply a silicone bead after painting.
Best regards
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