insulation in ceiling / floor over a garage

[tonytonys]

hello

there is a new room over the garage in the extension.
plans show this as having 100mm kingspan in the ceiling/floor.
this is a room over a cold space below.

its got 225mm floor joists and one inspector said i could put 200mm insulation quilt / rockwool style insulation in instead of the kingspan. is this correct?

i know a floor on an extension slab has to meet 0.22 W/m2 /, but im not sure on the value of a room over a garage? is this the same being a room over cold space?

joist spacing is 400, chipboard 18mm floor, 225x50 joists, galv netting underneath, 12.5mm fireline plasterboards.

also, it it worth putting a vapour barrier under the chipboard floor over the top of the joists? its not on the spec, but would help seal the room.

 

[tony1851]

200 quilt would be roughly equivalent to 100 board and of course a lot cheaper.
Wouldn't bother with a vapour barrier in that position.

 

[tonytonys]

thanks.
some guides do suggest a polythene sheet over the joists. its not required but some recommend it?

 

[dhutch]

thanks.
some guides do suggest a polythene sheet over the joists. its not required but some recommend it?

I would be wanting something in there to acheave decent airtightness/draftprevention if nothing else.


Daniel

 

[tonytonys]

hi
yes thats what the recommendation is for, to achieve a bit more of an airtight room. if the sides are lapped up the walls and under the plasterboard, or even taped to the walls before its dot and dabbed then this will help a little i assume.

some may argue that its a bit of a waste of time?
the cost of the polythene sheet isnt expensive and its a little bit of messing about to install it.
im sure most dont bother at all because its not required in the regs....

 

[^woody^]

The polythene is a nonsense idea

The skimming of the ceiling and the perimeter seal will give it airtightness along with the fire and smoke resistance

 

[tonytonys]

woody - thanks, will leave it out.