I am making a lean-to pantry into part of a room. It has an uneven single-skin external brick wall, rendered on the outside. I had thought to build a stud wall an inch or so off the wall. The would comprise studs with 2 inch Celotex between, and wainscotting (old floorboards) over. Depending on how I deal with the eaves, the air space behind could be ventilated or be dead. Should I put anything between the stud wall and the brick wall, e.g. tanking or Tyvek, and if so how should I attach it?
I had a customer with a similar situation which was resolved by fixing a special plasterboard to the wall which incorporates a 20mm thick layer of foam to insulate the wall and prevent moisture penetration. Would be easier than building a stud wall.
Best to put half the thickness between the studs, the rest in front of them, taped up to create a vapour barrier. Better still 50mm between studs, 25mm over, or insulated plaster board screwed into the studs. If there is a gap behind the studs, in theory it should be ventilated, or some kind of cavity tray at the bottom and weep holes to the outside. Depends how thorough you want to be. Simon.
If the battens are pre-drilled, you can use the fixing screws to adjust the distance from the wall and pack out at intervals with filler. Spending time getting the battens level usually means you don't have to skim
I've got a similar situation but with restrictions on how thick I can have the internal building.
I'm looking at dotting and dabbing 50mm celotex onto the wall, aiming to leave 12mm ventilated behind, then cutting channels into the front of the celotex at 600mm intervals (at the joins and in the middle of each sheet) to take thin 20x80 battens. Battens fixed though the celotex with frame fixings at regular intervals with 12mm plasterboard over the top screwed into the battens.
The channelling of the insulation is a faff but the benefits I see are, no wood on the wet side and something to fix shelves to on the outside.
Also, if the channels are foil taped before fitting the battens (under rather than than on top) then a decent stud finder will pick up the lack of foil at the battens for future fixing opportunities.
Probably less than you think, the batten can guide a stanley knife to for a perfect depth cut and the cutouts at a the edges are dead easy if done before fitting. The centre sheet ones are aren't as easy but celotex is so easy to sculpt that 5 mins per is an over estimate.
Nope, figure of speech, but you can't assume that a the inside of a single skin brick or stone wall will be bone dry for life so it makes sense to cater for the worst case.
Fred, I think you'd find it easier to fix a sheet of 25mm celotex to the wall first (making sure to get it flat), then stick additional sections of 25mm over the top, leaving gaps for your channels. Easier than carving them out.
Your battens would be stronger, however, if screwed direct to the wall. If you're worried about wood on the wet side, you could use treated timber and/or put DPM behind it.
My problem with screwing battens to my wall is that the wall goes in and out by an inch or more, which is why I'm thinking of building a stud wall an inch or so out. Thinking about it, the air space behind it will inevitably be dead, not ventilated, so I'm wondering about filling it with expanding foam, behind the celotex. I searched the group for opinions on how moisture-resistant expanding foam is, and found asnwers ranging from "not very" to "completely waterproof" ;-)
Yes, that works too but 50mm celotex is cheap as chips as seconds whereas 2x25mm is unlikely to be as cheap or as readily available as seconds.
The main reason for having the battens close to the plasterboard side is to provide direct support for shelving and with the bonus of being easily detected (or undetected) if they're not foil taped over.
Yes, I agree it would be stronger overall with the battens closer to the wall but I planned to have a hefty fillet of goop/fill'n'fix-foam between the celotex and the wall directly under the battens to firm it up and then have the benefit of stronger support for shelving on the inside.
I'm not concerned about the wall being properly wet, keeping the wood on the definitely-non-damp side was a secondary advantage.
So why not take up the variable gap with fill'n'fix foam between the celotex and the wall and then build a thinner frame structure on the inside. My plan is to get strength by creating a composite structure.
Also, my feeling would be to leave the space open behind the celotex, an air gap will always provide a barrier to damp penetration but if you put something in the gap then is may pass damp to the inner leaf. FWIW, my experience is that expanding foam is not closed cell and so could, in the extreme, conduct moisture.
I'm not sure you aren't over-estimating the rigidity and levelness of Celotex - if you jut stick it to a wonky wall, you;re liable to end up with wonky Celotex. It's normally it's the wooden studding that will provide a wall that's flat in all directions, with Celotex between and/ or over that. I think if I were you I'd use thicker battens, either attached to the wall and packed out for levelness, or built as a separate stud wall if your wall is too uneven. The foil on both sides of the Celotex provides a vapour barrier from the inside as well as a damp barrier from the outside - if you're worried about timber in contact with the wall, use a DPM as well. Treated timber isn't such a bad idea either. I just think this arrangement is more likely to give you a flat wall. It's what I'm going to do anyway ;-)
I've discovered today, through research at my local TP, that although bog-standard expanding foam isn't claimed to be waterproof, the stuff designed for fixing windows and doors is, although it doesn't expand as much.
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