Wall Insulation


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Apr 26, 2003
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Hi all,

I`m still continuing the conversion of my old brick outhouse in to a dedicated games / theatre room.

The old brick outhouse is only made of a single thickness of bricks. I have started to batten the wall to take plasterboard, and have just realised that I should really be thinking about insulation. My idea is to batten the wall then add Cellotex sheets to the batten and then plasterboard over the top. Will this be OK are there any cheaper ways of doing it ie. using foil backed / insulated plasterboard?

Also should I use any waterproofer on the wall either inside or outside? To stop the chance of any dampness coming through

As usual all ideas appreciated - whilst wanting to keep costs low I also want to do it correctly.

Many Thanks

If you want to add some soundproofing, you should build the wall one inch in front of the existing wall, preferably using 4" x 2" (for rigidity), then fill with 4" of Rockwool insulation. Finish with two layers of 1/2" plasterboard. A flexible wall can act as a bass trap and reduce the bass in the room. Using less than 4" x 2" wood will reduce rigidity, but sometimes isn't practicle.

Not touching the existing wall will also help prevent any damp. Anding plastic 'vapour barrier' sheeting to the wall after putting the rockwool in will stop any damp transfering to the plasterboard.

the insulation will help prevent any in-wall resonance and help reduce some higher frequencies from leaving the room. The two layers of plasterboard help reduce bass travel by adding mass.

Seal all edges with silicon sealant. Where air can go, so can sound.

If you don't want to change what you've done, Wickes do a 30mm High Density slab which you can cut easily and fit between your battons. The vapour barrier may still be a good idea to prevent demp getting to the plasterboard.


Gary thanks for your reply. Whilst I am worried about sound-proofing I`m also concerned about heat insulation. This building is actually in the garden and only attached to the house on one side.

What I`m worried about is the insulation getting damp as it is touching the wall and then transfering through to the plasterboard and ruining my hard work.
If I have brick - rockwall - vapour barrier - plasterboard , will this work or should I have an extra vapour barrier between the wall and the rockwall?

If I understand correctly is rockwall just like loft insulation or does it come in sheets?

Many Thanks

I don't think I've ever seen a vapour barrier between the brick wall and the new wall, only between the plasterboard and wooden battens. I think the rockwool is moisture resistant, but the vapour barrier will stop any moisture that may collect on the rockwool from touching the plasterboard.

Make sure the new wall is not touching the existing wall, then push fit fit the slab insulation in so that it goes to the back and touches the wall. This is to act as a fire barrier and stop it traveling up behind the false wall. You'll then have a bit of an air gap between the vapour barrier and the plasterboard.

Apart from what's already in the air inside the wall when you make it, you shouldn't get any extra moisture forming inside the wall if it's sealed in.

What you could use though, is the rockwoll that comes in (pink?) plastic bags - I've seen this used where the roll of rockwool (yes, it's just the same as loft insulation, though some is specifically designed for walls), is just pushed into the cavity between the 4 x 2s. Wickes and B&Q sell loft and wall insulation, and you could go and have a look at what they sell - they often have guides as to how to fit and apply the stuff on the bag. That would put your mind at rest about the moisture affecting the rockwool.

B&Q also do a book on how to do a loft conversion, and that may contain the info you need. Wickes has a lot of how-too guides, and I'm sure they'll have one about building a partition wall. Things change all the time, so getting up to date info on how to do these things propely is probably a good idea.

Some rockwool comes in rolls, and other come in slabs. The slabs are easy to cut and can be a push fit between the joists/battens etc so that they are a tight fit. That's what I used in my loft for the roof joists and new walls that I made.

Gary once again many thanks for your reply and the time it must of taken you to type!

My battens are only 2x1 as advised by the local timber merchant , are these too small if so could i screw another one on top to effectively have a 2x2 piece of wood , if not I`ll just have to pull all the stuff off.
This rockwall, (sorry to go on!) will it be OK actually touching the brick - I think that is what your saying? if so then I finally have it all sorted.

Many Thanks

Hi all....
Still pondering this!! (and giving myself a headache!)

If I put rockwool flexi in the gaps, and this stuff touches the wall, does anybody know if it matters if it gets damp? Will it sag or pass moisture through? I take it even if it does pass moisture through it will stop at my plastic vapour barrier?

Many Thanks in advance - if somebody could just help me out on this , I will be able to get on with my dedicated room (Yipee!!)

I don't think its moisture coming in from outside thats the problem, if it is then your building is not watertight ! What people are talking about is moisture travelling from inside the building to the outside ie to your rockwool.

This moisture is usually exhaled from people inside the room.


So by placing a vapour barrier between the plasterboard and rockwool should sort it out and be fine?

Sorry I didn't reply earlier - i always use 'quick reply' and forgot to subscribe this time. :(

Studd walls are usualy 2x1 or thereabouts, but the reasoning behind 4x2 is for a stiff wall. If a wall is too flexible, it can absorb bass. Having it not touching the existing wall means that you reduce room vibrations (sounds) from easily traveling into the room next door.

Two layers of plasterboard add mass, and reduce bass sound travel. It's nearly always impossible to stop bass traveling unless you have a completely isolated room ( a room within a room), as its the vibrations that travel via what they are touching (such ass the floor).

The rockwool helps reduce in-wall resonance, and attenuates some higher frequencies IIRC.

I believe that having the rockwool pushed to the back of the studding is to stop or slow the progress of fire behind it, so pushing against the wall is OK. Some slabs (the 30mm high density ones) are meant to be glued straight to the wall (using tile adhesive or similar IIRC), so that's fine too (for the reaons Parmenion gives).

I'm not sure if a vapour barrier is needed with internal walls, as I don't think they don't get cold enough, but external walls may be a different matter. Something to look into.... :)

I used 2x1 studding for my rear wall because I couldn't easily fix 2x4s, and I think I may have lost a little bass. I'm not sure if that's the reason, or if the sub isn't big enough. :)

The soundproofing seems to have worked though, as the neighbours don't hear anything (I've asked).


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