Insulation/renovating a cupboard

Sadly, polystyrene is a non-starter in a rented property. You and I know that it is perfectly usable in our own homes, however landlords are held to different standards and my understanding is that polystyrene (including the one that used to be available on a roll with the brand name 'Cortina') which is basically an under lining paper insulator, are not allowed.

 

No issue joint taping and/or filling the joints smooth before applying a lining paper. It can't be more than 4ft wide, 5-6ft deep and no more than 8ft high (not even, I'd say); of course before ordering, I do need to measure properly.

 

No, it goes from the bedroom over the top of the stairs - so juts out from the room rather than into it. I don't think it is just as wide as a chest of drawers, but admittedly I haven't measured it. Shelving in there will be more useful than anything though as Mrs C has a seemingly endless supply of lotions and potions...

 

It does - with the loft above. Currently there is only one shelf, about 18-inches down from the ceiling and perhaps half the depth of the cupboard; the cupboard door itself does have louvre doors, which may be a bit of an attempt at ventilation. I do like the idea of drilling holes in shelves to promote airflow though - definitely one I hadn't considered.

I did wonder if it might be an idea to fit one of those tubular heaters at the back (maybe a 60w one) and put it on a timer to be unplugged in summer - but then pondered if one so small may be somewhat 'farting against thunder'