insulation and pb to ceiling joists

d o'c

What is the most common method here.
Note: I don't need to meet a 'spec' for compliance, in this case

1.attach insulation sheet, then pb separate, insulate between joists later, from above.

2.Attach composite board, insulate between joists later, from above.

Are there any downsides to either of above methods.


Secondly:


I'm laying ceiling joists soon, and plan spacing with 406mm centres.

But I'm trying to figure what logic is behind the 1200mm wide comp boards I measured recently at a friends place.
The length works fine, 2440mm, when running perpendicular to the joist runs.

I would have thought a 1220mm board would work better,for case when attaching boards parallel works better ( factory end to factory end)
ie 2 parallel will make up to 2440mm.
Or should this boards type not be flipped through 90degrees this way

davgtrek

using an insulation layer under the first floor joists is unusual and perhaps overkill. I'd address the insulation by using quilt rockwool in between joists from above. are you thinking of this insulation layer as an acoustic measure ?? the above applies when the floor is between say kitchen/dining and bed over. if you were very consious of acoustic then you could double slab with plasterslab on the ceiling.

usually the slab boards would be laid lengthways perpendicular to the joists. so that each board would be supported by 4 joists.
make sure you use MR ( moisture resistant plasterslab) board in wet areas ( GF Shower rooms etc. ) it cannot be skimmed though. just taped and painted.

d o'c

thanks for the input.

The acoustics is not a priority in these rooms, the attic space above will not be habitable, mainly for storage/services
I have precast slabs at 1st floor level , for habitable rooms.

I was thinking along the lines of having a better 'sealed' ceiling, if I taped the joints on the rigid ins board, then PB over this.
As you say maybe overkill, as its an internal-to-internal barrier

I think perhaps more gains could be had by putting the extra effort into my interior-to-exterior region at rafter level, ensuring the attic envelope is sealed for air tightness
( with a 2" air cavity between felt/rafter insulation)


Thanks for the pointer on the MR boards

davgtrek

i hadn't picked up it was a first floor room you referred to.
in that instance i would fix a layer of insulation to underside of ceiling joists and insulate in between. you could tape the joints and around the perimeter of room. if you used foilbacked insulation then with taping you would have a decent vapour barrier also.
The fact you are flooring out attic changes the insulation you should use in ceiling. Go with a 100mm thick roll of rockwool between joists. then cut and lay sheets of 50mm xtratherm foil backed yellow insulation in between joists. then counter batten the attic with 50x50mm battens running perpendicular to joists. then cut and place more 50mm xtratherm sheets in between the counterbattens. it should be nice and flush. then lay your decking OSB board or whatever on top.

this gives you a good level of insulation in the decked out ceiling/floor.

p

upstairs in store/attic if you want to insulate along rafter underside you could use rigid board insulation between rafters. theres a polystyrene board that has accordian type cuts for easily wedging between rafters or simply cut polyurethane ( yellow ) xtratherm or similar board in between rafters and slab over. if you were never going to use this space and it was for storage and didn't need to be highly insulated perhaps

cork1

why 406mm centres?? a 1.2m sheet should land on the centre of a joist when spaced at 400mm centres. that 6mm will push the position of every slab slightly and you could start missing joists and its alot slower when your cutting every sheet.

thunderRoad

406mm = 16'
this works better with ply above as ply is 8''x4''
that's the reason pb sheets are 2440mm (which = 8'')
so if sheets are fitted lenghtwise rather than widthwise this works perfectly

d o'c

Good detailed post davegtrek

thunderRoad exactly.
25.4 mm X 16 = 406mm

@ 406 centres: If flipping the sheets through 90 degrees, and two are laid this way to match the length of the perpendicular to joist laid sheet then at 1200 , its shy 36mm,over 2 shhets , due to loosing 18mm per sheet in width, .
This was the bit i couldn't figure out.

sheet perp (longways) attached to 7 joists, at 406mm centres ( 6 of) =2436mm covered ( sheet length is 2440mm), this is fine
sheet turned 90 degress attached to 4 joists, at 406mm centres (3 of)= 1218mm ( sheet length is 1200mm), this is what I was querying ,

406mm works well with length of 2440mm,
400mm works well with width of 1200mm

I'll continue with the 406mm ctrs, and aim to stagger joints when sheeting as much as possible, and the odd occation I have to turn the sheet I'l nail a batton to the joists as required........... or souce a 1220 X 2440 pb sheet.