Insulating block outbuilding (Man Cave)

[Mpb10]

Hi, After some advice / assurance of the method Im'm looking at meets some of the 'best practice' for insulating a single brick wall ensuring (if done correctly) shouldn't be any issues surround damp etc. Any advice would be appreciated or just a "yes sounds fine" or "Nooooo"
I propose to use 25mm Celotex against the block wall (on the inside) joints taped with foil tape. I'll then use 3x2 battens which will be drilled through into celotex and block wall. I was then looking to install 50mm sound slab insulation between the battens to help reduce sound transfer and increase insulating properties. Finally plasterboard over the sound slab insulation to th battens.

Thanks in advance..

 

[oldbutnotdead]

You might want a vapour control layer between the plasterboard and the battens/sound insulation stuff (so water vapour doesn't end up in the sound insulation). Apart from that it sounds fine to me- with a caveat about don't expect the sound slabs to reduce noise pollution that much because your inside wall surface (plasterboard) will be mechanically coupled to the outside wall (by battens and screws).
You'd get better acoustic performance if you don't drill the battens to the walls (just fix top and bottom and nog at frequent intervals) but with 3 x 2 you might get a floppy wall if it is much higher than 2400 or so.
Also (if noise is really important) consider acoustic plasterboard (denser than standard).

 

[John D v2.0]

agreed with above about as much mass as possible isolated from the wall helping the sound - so plenty of heavy plasterboard. Also if your humidity/heating is lightly to be strong, be careful of your vcl With on 25 celotex and 3 inch/75mm wool, you'll have your vcl/taped celotex closer to the cold side of the insulation than the warm. Really you need more permeable insulation towards the cold side. So batten, wool between, and the celotex on the warm side.

 

[Mpb10]

Thanks John D
Was concerned that the wool would be more likely to retain moisture if against the outside wall? (Current gets damp on the inside if heavy persistent rain driving gains it.. Also would the battens would be more suseptible to rot even if using tantalised timber?
Would it be an option to put an extra vcl on th battens as OBND described?

Appreciate the advice !!

 

[John D v2.0]

Hi MPB so you have two problems for moisture, one is penetrating damp as you mentioned, and one is condensation.
Tanalised timber shouldn't be in an unventilated space. Normally that problem is got around with a rainscreen/cladding with ventilation behind. Then any driving rain will fall down the cavity harmlessly and be directed outside.
The other condensation problem is got around by making sure the layers are right. But for an outbuilding it won't be heated as much and you won't be drying washing and having showers. So worry more about damage from rain from outside.

 

[Mpb10]

Thanks again John.
Yes you are right thinking about it being two separate potential problems!!

Don't think the rainscreen is an option due to space, expense and the look I was trying to achieve. Was going to get it Rendered - Will this help/ prevent the Water ingress? Lots of buildings are Rendered so presume this is ok?
The reason I was going to put the Celotex against the wall then batten was to counter both the ingress and the possible condensation..

Was thinking could seal the outside wall for the time being which would hopefully stop water penetrating the block work then when the brickwork has dried out / weather improves get it Rendered!?

 

[John D v2.0]

Render will help a lot but not completely guaranteed. It can also affect the breathability. I'm sure it'll be fine for an outbuilding.
Generally rendered walls are still cavity, but you don't expect much penetration.
The condensation comes from inside so the celotex needs to be the warm side to have minimum risk.

 

[Mpb10]

Would using DPM on walls (internally) or maybe Synthaprufe be an option? This should solve the water penetrating!?

 

[John D v2.0]

Would using DPM on walls (internally) or maybe Synthaprufe be an option? This should solve the water penetrating!?

Yes it would work too stop the penetrating damp as long as it's not going to get any moisture buildup. Traditionally in roofs where an impermeable membrane is used before the tiles, the inside has to be very well ventilated to stop condensation. In your case the risk will be much lower, but still be careful

 

[oldbutnotdead]

My block garage suffered slightly from external water penetration at one end (it catches the wind and I should have put a 4 x 2 in as a standoff for the capping). A good heavy coat of masonry paint (rollered it on) hasn't eliminated it but it now only shows damp in extreme rainfall.

 

[Mpb10]

After ALOT of deliberation I'm thinking of using the following method:

i) Block Wall painted (Internally) with Liquid DPM to stop the water penetration - Outside will eventually be rendered too.
ii) Fix tanalised battens to the wall (spaced at 600mm Centres) with the 50mm Rockwool between.
iii) 25mm Foil back Celotex on top to ensure no thermal bridging through timbers, foil taped on joins and at edges to ensure continuous VCL.
iv) 12.5mm Soundbloc Plasterboard skimmed

Does anyone see any issues with this?

My only real concern at the moment is there not being an air gap between the DPM on the wall and the Rockwool..

Thanks in advance..