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  • justin

    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.

    in reply to: Insulating a bedroom floor above a garage #27320


    Get the asbestos removed (or do it yourself – very carefully!).
    The best place for a polythene vapour barrier would be directly under the floorboards.
    Re-lay the floor.
    Fit between and under with PIR board, foamed in.
    If you do a tight job with the insulation, I don’t see why a gap would matter. It’s just a pocket of warm air. – But a gap IS BAD if you have other air-gaps to the sides of the joists, and cold air can circulate through to the warm side. This is probably why the advice to keep insulation touching the boards was given.
    The space beneath the boards comes from standard roofing practice, which is to allow condensation under impervious felt somewhere to go. 50mm gap is normal beneath ordinary old felt. (Less or zero if you have breathable membrane..). In this case it’s a garage, not a roof exposed to the cold night sky. Also the plasterboard you will re-line the garage with is breathable. – So I’d say the gap beneath was unimportant.
    Replace garage ceiling with standard fireproof plasterboard.

    in reply to: Smoke/mvhr/insulated plasterboard? #27319


    Hi Sarah. Not much on specific fixes but one thing is certain, the condensed cigarette smoke products are volatile – or you wouldn’t be able to smell them. Ergo some day the smell will go of it’s own as there will be none left so to speak.
    So ultimately, time will heal the problem. That’s not reassuring in the short term, and I have no idea how long that would be. Quicker in the summer when it’s all warmer of course.

    If you are considering internal dry lining/remedial insulation, then the vapour barrier you must install (behind the plasterboard) would prevent the contamination reaching the living space. Even if you are not internally insulating, you may see new foil backed plasterboard as a great opportunity to cover up old artex.. Just a thought.

    in reply to: Can I use cork for internal thermal insulation? #15845


    Hi Ruby.


    Well I stand to some extent corrected then, sorry. (Thanks Gordon).
    My previous biased viewpoint had come from the comparatively huge price of cork used as flooring (It was so common and cheap, and good to use, back in the 80’s). I put it down to shortage of cork trees and assumes insulation would share the same fate.

    If cork insulation is really available 100mm thick for £25 then that’s a very good price compared to ~6mm thick cork flooring which can work out the same price per M^2. Maybe it’s the manufacturing method or the quality, I don’t know.

    Wood fibre board may perform just a little better than the cork.




    in reply to: Can I use cork for internal thermal insulation? #14514


    Cork (very sadly) is an enormously expensive product these days. In terms of performance it’s only about half as hood as PIR foam commonly used for internal remedial insulation. In real terms, to bring a solid wall up to the minimum UK standards requires about 70mm PIR foam. Using cork you would need about 140mm thickness.

    Now since as a product to buy, just 10mm thickness (say a cork board wall) can cost you perhaps £30 per square metre, pro rata at 140mm we are onto £400 per M^2 and that’s assuming you could get product (you can’t), and assuming you are prepared to loose half a foot of your room.


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