Insulating between rafters

[autolycusd]

I want to insulate (more for sound than heat) the upstairs room in a chalet house. What i believe i should do is create a 5cm air gap and then fill the remaining 5cm with 60kg/m3 rocksilk. My question is, what would happen if i did not leave an air gap and just filled between the rafters with 10cm rocksilk?

Above the rafters there is just featheredge boarding and tiles. Would the insulation/featheredge tend to rot? Could i combat this by polythene sheet above and below the rocksilk?

(I intend two sheets of soundshield plasterboard on resilient bars beneath the rocksilk.)

Any guidance much appreciated.

 

[^woody^]

If you fill it completely (ie no voids at all) and fit a vapour check board or barrier, then it will be OK

 

[autolycusd]

Woody - i appreciate the reply. Sounds like good news, but could you possibly tell me a bit more? Like what precisely i need to look for in the store as a "vapour barrier", (is a sheet of polythene OK or is thee something more specific)? how to fit? and whether i need one both sides of the insulation or just on one side? Thanks.

 

[^woody^]

The principle is to stop moist air getting into the area between the rafters, and to remove any voids which cause cold spots to the underside of the felt, which would allow air to condense

So you fit an impermeable barrier of polythene, or foil backed plasterboards to the underside of the rafters. Staple polythene and screw plasterboards

And you fully fill between the rafters to ensure that there are no voids anywhere.

For sound, you would use Rockwool or similar dense quilt.

But it may be better to use something more efficient to insulate for heat, such as celotex board. Or a combination of 50mm rockwool tight under the felt, and the 50mm celotex, and then the plasterboard

 

[autolycusd]

When you say "felt", i presume you mean roofing felt? But there is none - just featheredge under the roof tiles. Does this make any difference to the advice you are giving?

As i said, i'm planning on resilient bars under the rafters, this would produce a 17mm (approx) void between the vapour barrier and the plasterboard - i assume this would not be a problem?