I imported a shell, formed the upper half of a turtle shell, light can only enter through the hole for the turtle head. But from the outside the inside is lit just as if the shell were translucent. L/W/H ca. 30/20/10 m.
Also, I built a box-like room out of Twinmotion primtive cubes, size 10 x 10 x 10 m, wall + flat roof thickness 30 cm with a small opening, like a door. Looked int from the outside it should be nearly black, but it´s bright because the roof seems to be translucent (see picture) until I make it much thicker.
Both is rendered correctly dark with the path tracer.
I played with light + GI. The cube became a bit better but the 30 cm ceiling is still glowing.
No difference between one- or two-sided, imported material or roughcast coating 01.
What's happening here is due to limitations of the raster rendering engine. The reason your roof looks transparent is due to a combination of the Global Illumination, exposure, and ambient lighting values that the raster engine uses to fake the lighting for interior shots. Twinmotion's raster lighting uses a combination of ambient lighting and global illumination to approximate what lighting should look like in interiors. which creates a fake lighting effect that makes it easy to see interiors. This effect is why you are able to see the sun through the ceiling of your enclosed box, which isn't possible realistically.
For best results, Path Tracer will give you the most accurate lighting. If you only plan to render images with Path Tracer, you do not need to worry about the glowing lighting from the ceiling when Path Tracer is disabled, as these won't show up in your final renders.
If you want to get accurate lighting with rasterized rendering, then in your Renderer settings disable GI, which should get rid of the sunlight appearing through the roof. In the Lighting settings, lower the Ambient lighting value to 0 to remove any false lighting. You can also adjust exposure values, and choose to disable or enable auto-exposure, depending on your needs.
