Insulating between joists

barrowvian

Thank you for your replies.

Taking down the ceiling underneath isn't an option so it has to be taking up the floor boards. It's an easy enough job because we're redecorating and already have the carpets and skirting boards removed.

If I was to lay rockwool directly on to the plasterboard, would this not cause it to sag over time?

malc_b

There are 2 different types of Rockwool, one is for heat insulation and one is for sound insulation. The difference is the density. You need denser stuff to stop sound. However, the dense stuff also is a good heat insulator too. Anything over 10kg/m3 is consider a sound insulator but the denser the better. For example, RWA45 is 45kg/m3 R=0.035 W/mk, 100mm between floor joists should give you 40dB (rockwool have a leaflet on this BTW). But noise is tricky. You need to seal all the air gaps and if the problem is noise from people stamping around upstairs then that is going to transmit through the joists. The only solution to that is a floating floor or maybe think carpet and underlay.

barrowvian

Eventually the floor upstairs will have thick underlay and carpet on it. We won't be leaving it as exposed floor boards.

I've measured and the depth of the joists is 180mm. The biggest issue for us is heat loss going upwards from the living room ceiling so heat insulation is our main targets. Maybe a good thickness of celotex on battens between the joists. I'd read that either expanding foam or tape should be used over the seals as an additional option.

We could then look at laying some form of sound proofing; this is mostly just to prevent the noise from the tv being heard in upstairs too.

malc_b

I would have thought a rockwool RWA45 would work better for you. Celotex is hard so it will be tricky to cut to fit between the joists. You'd have to cut it loose and then seal with acoustic sealant around the edges. Rockwall RWA45 is in stiff bats. Cut to slightly wider and push into place. A lot simpler. Celotek has go a lower R value but I would have though 100mm or rockwool would be adequate to cut down the heat loss. You still want to heat the upstairs don't you? It's not like it will be at the same temperature as outside is it? If upstairs is at 10C, outside 0C, lounge at 20C then 100mm of rockwall in floor will be equivalent to 200mm of rockwall in a ceiling under a roof.