I've built one in th eloft between the rafters and have had no temprature issues. I cross battened the ceiling joist with 4x2 timber and used 18mm flooring chipboard to strengthen everything, most had up there at one time was three children and three adults. Then used 3x2 cls to form a room with 20mm insulated plasterboard for walls and ceiling, used insulating foil as used for timber frame housing on the outside of the room. Have been using it for about four years now, though am still building the layout pretty much only scenery and buildings left to do Leave a small gap about 1mm between joins. Even during the snow at winter temprature raely dropped much below 10c. I only put heaters on when I'm actually up there. Maximum temprature when in use was 36c this year, no issue with track. Have put a bathroom/kitchen extractor up there but would suggest more than one ore a Velux window in the roof, which is what I should really have done. Have fitted carpet remanents over the floor. /media/tinymce_upload/5c05ef4c5095391211075a379327effe.JPG/media/tinymce_upload/602c1114d99368e468666e06bfcde47d.JPG/media/tinymce_upload/1f3de43c08159182bd808d05806e7949.JPG/media/tinymce_upload/36a16cb72e228b241e62673d20a3d1f0.JPG