I'm insulating one of the rooms in my house, having taken it all down to bare brick and roof rafters (no loft). I have some wood already that is the perfect size for what I need, but it's non-tanalised. Would this still be alright to use as joists on the walls the roof rafters (it will be behind plasterboard and kingspan), even though it's not treated wood? If not, can you just paint some kind of preservative treatment on them or somesuch?