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Question regarding warm batten method for internal insulation of exterior walls

SuperHomes Revamp Forums General Discussion Question regarding warm batten method for internal insulation of exterior walls

This topic contains 1 reply, has 2 voices, and was last updated by  justin 4 years, 3 months ago.

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    Chris Murphy

    Hi ya

    I am renovating our 1892 vicorian semi detached house, it is built using lime mortar. It is freezing in the winter and noisy (next door). I have stripped back to the brick and have been reading with interest the various options of insulating the exterior walls internally. It has been suggested to dot and dab the foil back celotex (or eqivalent) onto the brick (these have been sealed with PVa to prevent the dust coming off all the time). Then the difference is to build a timber frame but this fixes into the floor and ceiling joists not the wall. This prevents screwing through the insulation. The celotex will be sealed using expanded foam as Martin did and then we had considered putting up a vapour barrier between the foam insultation and the timber frame, further sealing it from the wall. Then we had considered putting further celotex in the timber frame.

    So, the question is, does the above sound ‘ok’, if we did this are there any recommendations for the minimum timber batten size for the frame (?2″x1″, ?2″x2″, ?3″x2″) as the only weight it is taking is the plaster board and any pictures etc. There will be raidators near the floor and we can look at other alternatives for those or thicker battens.

    Any ideas / recommendations gladly welcome. I am keen to know if the above is safe in terms of moisture

    Many thanks in advance.




    Hi Chris.
    I’m not offering advice about the pros and cons of IWI for solid victorian walls. There is discussion elsewhere (Try the Green Building Forum). If I were doing so, I’d be looking very very closely indeed at ensuring perfect Vapour barrier, – bearing in mind the wall will definitely be below dewpoint for leakages of internal air.
    This gives you a significant challenge regarding what to do with floor joists to first floor (or ground floor?), since they will penetrate into a now colder wall and be a great conduit for humid air to condense round the joist end.
    Writers elsewhere have discussed amongst other things, removing the floor joists and re-building it on an internal (steel? frame, isolated from the walls, which can now be 100% sealed and insulated.
    My own (extensive) IWI work has been done on a 1970’s property with filled cavity wall, hence these risks are reduced. The effect has been absolutely dramatic, so done properly would be even more-so for a solid wall.
    Building an internal frame for the plasterboard and doing it sufficiently rigidly will eat up a lot of your internal space. You are already loosing perhaps up to 100mm. My own experience is that it’s not necessary, (although it would ease future wiring alterations somewhat).

    I wouldn’t recommend dot and dab since it encourages air to circulate behind the IWI due to the spacing of the dabs (or is it dots?). There are bound to be air leaks in the external wall in places.
    (It’s amazing how often even in 2000’s modern properties, doing my day-job, I’ll remove a wall socket from a dot-dabbed cavity wall, and gale of cold wind will blow out. The householder is inadvertently living in a ventilated cold tent. Dot-dab is not a great way to achieve energy efficiency IMO).

    I am an electrician also, and after some consideration, I decided to run my cables in 20mm conduit fitted to the original wall behind. There is no “model” for this type of wiring in BS7671, but the closest equivalent I estimate to be what we call in the trade somewhere between “Method 102” and “Method 101”. This entails a ~ 40%-53% reduction in the current carrying capacity of sockets cables. Don’t run cables even for short lengths surrounded by PIR foam without at least a conduit. The very important issue is that for UK “Ring Final” sockets circuits, your electrician must not fit 32A breakers, use 20A as a maximum. This is fine for 99% of users (especially those with a focus on low energy consumption). All other cables need to be equally carefully looked at and breakers or cables scaled as necessary.
    The method I used was to clean/PVA if necessary. (My walls were rendered internally and removal of the render would have destroyed the blockwork, so I was forced to leave it on. Importantly, since you will be using some adhesive, do ensure the plaster (if you leave any on) is in very good condition and will remain dry (ie no condensation at all is allowed to weaken it).

    I used separate PIR boards and plasterboard. This has several advantages over the combined boards, not to mention the cost benefit. It’s the only method you can use to ensure 100% vapour barrier, since you can foam and tape all the joints as you apply it You also tape screw heads (I’ll come to that). So I fitted the cable conduits, and I also fitted timber blocks carefully sized to mount standard metal wall boxes to make the front just below the final finish. I used oak blocks, you may prefer treated softwood. The blocks are the same size as the back-boxes so they are easy to split with screws- take care. Oak is good. Remember your cable “zones” as well. Before screwing in the boxes, add a sheet of polythene behind the box, which you can pull through the PIR and tape into place to make the VCL complete.
    Cable up the electrics, silicone seal the cable through the polythene into the boxes, and temporarily tape the polythene into the boxes to keep it out of the way.
    Cut out cable channels in the rear of the PIR (easier said than done) and cut openings for the back-boxes.

    I use adhesive foam gun, apply a full bead to the board, then divide it into “cells” about 18” to 1’ across and line every cable channel and back-box right round. My concept is to eliminate any cross-flow of air should there be brickwork gaps to the outside creating draught-points. It also helps prevent the spread of any warm/humid air which might find it’s way to the cold side. Fit the boards (use scrap timber lengths or ceiling-jacks from the opposite wall to push the board into place while the foam sets).
    On my solid ground-floor, I had a old Marley tile finish (No I didn’t dig out the concrete floor to insulate it..), and I also tape sealed the boards to the floor along that joint.
    I then added safety screws to hold the boards up in event of a fire. I would recommend stainless steel screws if you can get them, (£££), as thin as you can get, probably 6mm max, but try to allow ~ 50mm into the brickwork. Use about six or eight per board and drill right through into the wall, use the screw like a frame-fixing. ie push a plastic plug onto the start of the thread, push it right through and tap gently with a hammer where the plug meets the wall. Screw gently in. You may like to add a washer to the head. (I also add a scrape of bar-soap to each screw thread to help ease it into the plug and minimize risk of the latter rotating in the brick.). Pull out the plastic at the sockets and switches, tape to the internal VCL, foam and tape all joints and screw heads. You should have a 100% sealed foamed wall.
    Try to avoid pipes in the walls. If you must, I’d prefer to put them on the warm side and restore VCL with tape right into any cut-outs in the foam.

    When boarding, avoid lining up the plasterboard joints with the foam joints (which will be slightly uneven). I used tapered edge boards and dry-line finish tape and filler (I have learnt to be a dab-hand with dryline tools, you may prefer a full plaster skim for a better finish).

    I applied plasterboard the same way, cells of adhesive backed up with safety screws. Probably eight per board as a minimum. On these ones It obviously pierces the VCO, but I add a squirt of foam to each drilled hole before putting the screw in. With some care even ~ 6mm screws will pull flush into the plasterboard. (& If you actually calculate the cold-bridging effect of the screws, you find that it is minimal).

    Plasterboard is tough enough to take normal light loads. Go easy since every hole pierces the vapor barrier, but it’s sealed with a plug and the PIR is pretty vapour tight anyway. Where I fitted radiators (which I never need to turn on! – Shouldn’t have bothered..) I also added oak mounting blocks as with the electrics. Take care to measure exactly where they are before boarding.

    Take loads of photos as you go, especially of the electrics layouts.

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