I've a top floor apartment in India which gets almost like an oven during the summers which is like for 9 months out of 12. I've decided to apply under the deck XPS 50mm (nailed to the ceiling). I dont want to do a gypsum false ceiling after that, since I like to keep the height and cost low. The supplier (Supreme Petrochem) suggests to apply putty + acrylic paint over the XPS board, but I'm skeptical and dont know if that will crack. I'm thinking of just sticking wall paper over it. Any experience of such application & suggestions will be appreciated.
Dow, a manufacturer of XPS products, on their India web site, does not recommend using XPS under a roof deck. This is due to the possibility of condensation occurring between the insulation and the structure.
As far as painting goes, US codes require a 15 minute thermal barrier over an interior foam, in the event of fire. This could be satisfied by gypsum wallboard, obviating the need to paint. For noninsulation interior uses, Dow permits latex paint.
Instead, they recommend using an "inverted roof" where the panels are placed OVER the waterproofing and covered with a ballast material that will weigh the panel down and protect it from sunlight.
Yes of course XPS board can be used under a deck for insulating the room. When on the ceiling 50mm XPS is used, its joints are set due to shiplap design. After fastening them to the ceiling, their joints are covered with the tape and then fibre mesh is put over it, then the putty is applied over it, and finally the paint is applied over it. The joints never crack due to this process applied on XPS.
For better insulation, the south or west wall of a room should also be insulated as they gain the maximum time of heat during the day. The walls can be insulated with 25mm XPS board. The joints should be grooved in a v-shape to fill in putty and then the same above procedure is applied.
what part of india do you live at (aka, do you get horrible long monsoon rains), and are there any sort of cracks, etc, on the roof that may cause water to seep in? i think these are more important questions than whether you can paint XPS.
i lived most of my life in bangalore (and several years of that on the top floor of a 13story highrise), and a little judicious application of ceiling fans and air conditioners goes a longer way than trying to insulate your roof from the underside.
i fail to see how 2" of any sort of insulation under your ceiling would help in heat control. now, if you're trying to dress your ceiling up, i understand; and there are several options. not for heat control, though.