There is one obvious reason and two simple fixes of that problem.

The obvious reason is that you probably have messed around with the surface shading setting and disabled shading this way. No shading means no lightmap.

And the fix is either enabling lightmaps by marking the default 'Shaded' box again, or setting the diffuse component of the unshaded material (mat_flat) so that it does not reflect sun light if that is not desired.

The material does not care if your room is closed or not.