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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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  1.  
    You ought to really catch all the plasterboard edges, not leaving 400mm gaps without fixings, but the world won't stop spinning if you don't.
    • CommentAuthorDur
    • CommentTimeJan 8th 2020
     
    Thanks Jeff
    That's all useful info. I'll have a look and think think this weekend.
    We have a pretty much blank canvas as the whole upstairs is stripped out at the moment as we have been putting in new beams and purlins so as it stands we can see where everything is.
    Some of the rafters are so far out that I will have to do a number of references to find the centres when covered but most will be fine.
    Here's a picture of one corner as it is at the moment...
      IMG_20191230_182616-s.jpg
    • CommentAuthorJeff B
    • CommentTimeJan 8th 2020
     
    <blockquote><cite>Posted By: GreenPaddy</cite>You ought to really catch all the plasterboard edges, not leaving 400mm gaps without fixings, but the world won't stop spinning if you don't.</blockquote>

    I thought the same before we started the job, but because the 400mm gaps are filled level to the battens with Celotex it honestly is not an issue. There are no signs of cracks in the plasterboard joins as yet (6 years on)!
    • CommentAuthorJeff B
    • CommentTimeJan 8th 2020
     
    <blockquote><cite>Posted By: Dur</cite>Thanks Jeff
    That's all useful info. I'll have a look and think think this weekend.
    We have a pretty much blank canvas as the whole upstairs is stripped out at the moment as we have been putting in new beams and purlins so as it stands we can see where everything is.
    Some of the rafters are so far out that I will have to do a number of references to find the centres when covered but most will be fine.
    Here's a picture of one corner as it is at the moment...<div class="Attachments" id="Attachments_277789"><ul><div><img src="/newforum/extensions/InlineImages/image.php?AttachmentID=7481" alt="IMG_20191230_182616-s.jpg"></img></div></ul></div></blockquote>

    I envy you with your blank canvas! We had to go with what we had as I did not fancy taking off all the existing plasterboard and stripping back to the timbers. I think it's all worked out ok except for around the Velux windows which still remain as significant thermal bridges, mitigated to some extent by DIY-type additional double glazing. But that's another story!
    • CommentAuthorDur
    • CommentTimeJan 8th 2020
     
    Making a decent job around the windows does look like it will be a challenge. I expect I will be coming back on that in due course!

    First I have to find a suitable expanding foam. Opinions seem to vary from Screwfix basics to something which expands a bit less and is more flexible. Both sound like sensible properties but how to find the right one? I never realised there were so many options.

    Tape looks to be the same problem. How do you know the one you choose will still seal in 20 years time?
  2.  
    Get the cheapest foam in boxes of a dozen. Or maybe something like this, if you don't already have a gun and cleaning solvent..

    https://www.ironmongerydirect.co.uk/product/soudal-gap-filler-expanding-foam-complete-trade-pack-gun-grade-kit-750ml-529125

    I have used lots of tape, and can guarantee every one of them to not still be sticky in 20 years time. My approach is to only apply tape (if at all humanly possible) where another layer or batten, or plasterboard will be on top of it, effectively clamping it in place. You do pretty much get what you pay for with tapes. Cheap will be cheap, and likely falling off by the time you've finished the roll.
    •  
      CommentAuthordjh
    • CommentTimeJan 8th 2020
     
    Siga and Pro Clima both provide a 10-year warranty, which goes some way towards the stated 20 years.

    Another possibility instead of foam for filling gaps around windows is compressible tape, of which Compriband is probably the best known name.
    • CommentAuthorJeff B
    • CommentTimeJan 8th 2020
     
    <blockquote><cite>Posted By: GreenPaddy</cite>

    I have used lots of tape, and can guarantee every one of them to not still be sticky in 20 years time. My approach is to only apply tape (if at all humanly possible) where another layer or batten, or plasterboard will be on top of it, effectively clamping it in place. You do pretty much get what you pay for with tapes. Cheap will be cheap, and likely falling off by the time you've finished the roll.</blockquote>

    I agree. Although the aluminium tape*I used was incredibly sticky, I only used it where there was going to be plasterboard screwed up tight against it.

    * https://www.sealantsandtoolsdirect.co.uk/manufacturers/everbuild/tapes/mixed_tape/everbuild_mammoth_aluminium_tape_100mm_silver_2alum100_P29649.html
    • CommentAuthorDur
    • CommentTimeJan 8th 2020
     
    Thanks all.
    Cheap foam and don't trust the tape - at least give it a backup.
    Seems that getting air tight and vapour seal is pretty critical so I will go belt and braces and maybe seal each layer.
    • CommentAuthorDur
    • CommentTimeJan 16th 2020
     
    So we have started fitting the insulation behind the knee walls...

    First as suggested the 100 mm block foamed onto the top of the wall and then the 75 mm between rafter insulation which has to be fitted in two or 3 pieces. I am still trying to get the measure of how much to cut the sides back to be able to get the foam in without overdoing it but will figure it out by the time we finish :)

    The underhung layer will then go in in two pieces and joints will be taped.

    Inside the room I am thinking to add a (belt and braces) poly membrane then 12 mm plasterboard.
    But behind the knee wall do I feel like I should add the number 2 vapour barrier but what about the plasterboard, the latter not for the finish but for fire retarding and presumably some insulation value. If not how should I finish that area?

    Secondly the blocks on the wall top have an exposed pir face which would be impossible to tape and I am not sure I can do much useful with poly either. Is there a paint which I can use to seal this?

    Third question (apologies) - the knee wall will have a plasterboard on the room side so the space behind will be within the new insulation "envelope" but actually not directly heated etc. There will be some loft roll type between the joists for the benefit of the room below but should there be some on the knee wall as well?

    Thanks!
      IMG_20200114_195329-S.jpg
    • CommentAuthorGreenPaddy
    • CommentTimeJan 16th 2020 edited
     
    Well done. It's a complete pain, but if you're going to do it, do it the best you can, and it looks like you're making a good job of it.

    Where does the VCL go? If you put it on the knee wall, and up the sloped roof, then what about vapour travelling through the floor from below? So I wouldn't personally, go with that.

    Logic tells me (well my logic anyway), that you should seal the VCL to the insul blocks at the wall head (where you started), and run up the rafters, (over the second insul board layer) which will be tricky to seal around the dwarf wall hangers. I normally would put a run of VCL before the the dwarf wall head goes in, but that's in new builds, not so easy for you.

    The question might be, what is the VCL doing for you in this case? You've got a fully vented roof, so if vapour gets out there, under the sarking, it's exactly as it has always been doing, and as it is designed to do. If you run the second layer of foil board, well jointed, and edges taped, that will give you a pretty good (not perfect) VCL. Your BCO may insist on a separate membrane, but I'd try the taped board bluff first. If he can't see that what you're doing is a much much higher standard than any other contractor would, then you're a bit unlucky. Also, since the "C" in VCL is "control", you are doing a goodly amount of control already.
    • CommentAuthorDur
    • CommentTimeJan 16th 2020 edited
     
    Thanks GP. I feel like we have a rare chance to bring this old place up to scratch so may as well try and do it as well as we can. The challenge is learning about the different materials and techniques and trying to figure out what is a sensible level to work to..
    BCO wanted the 25 mm ventilated space so we have this sort of hybrid half vented system but I think it a good idea to try and get areasonable VCL. I will take the VCL down to the top of the wall as you suggest . Sealing around the wall studs will be tricky but I'll give it a go.
    I'm still worrying if I should try and board over the insulation behind the wall for fire purposes. The void will be carrying wiriring ( and plumbing). There will be plasterboard on the knee wall but the space behind will be open from one end of the house to the other. (All notwithstanding that the place was previously clad in timber and the old wiring is dreadful- but it is still standing!)
    • CommentAuthorGreenPaddy
    • CommentTimeJan 17th 2020
     
    Honestly don't see any need to plasterboard that area, of underside of roof. Only thing might be, is to protect the VCL, but in protecting it, you'll put dozens of screw holes through it.

    When fixing the VCL, if you go for that, I would run a timber batten on top of the VCL, where you've taped/sealed it a the wall head, so the batten clamps the VCL and seal to the insul behind it. Like wise at the VCL join at the top of the dwarf wall, or anywhere the VCL could come loose over the years.

    As to fire, once the flames are at the underside of the roof, it's all a bit late. If you wanted to put a little wall behind and perpendicular to the dwarf wall to make a fire compartment, to reduce the eaves being open from one end to the other (as you mention above), well maybe, but I would rather go to town on the fire detection system, and get you all the heck out of there, as a first principle, at smoke stage, rather than flames.
    • CommentAuthorDur
    • CommentTimeJan 17th 2020
     
    Thanks.
    The battens makes sense and nice and easy to do.
    I am sure you are right about the plasterboard - I was overthinking it and just needed telling!
    • CommentAuthorDur
    • CommentTimeFeb 14th 2020 edited
     
    So the between-rafter insulation creeps forward - such a slow, fiddly job behind the knee wall!
    I had been intending to put a 1200 g poly (as belt/braces) over this first layer, ie trapped by the next layer, but it is so fiddly working behind the wall studs that I don't think I will be able to get any sensible seal.
    So intention - I think - would be to get the second layer in and just do best to tape joints with eg Mammoth alu tape. Then use something like the Soudal liquid membrane to seal around the tops of the studs and probably the already taped joints etc having first done best to foam them in.
    Does this sound OK?

    Having looked at earlier comments suggesting that we don't strictly need the 25 mm gap behind the rafter insulation that BC asked for, I am wondering how much that gap would degrade the performance of the insulation. I had started off thinking I should be carrying the gap from eaves to the loft area but would I be better to put some horizontal blocks across from rafter to rafter at intervals to reduce vertical airflow? (At this stage it would involve poking a hole though the insulation and unleashing the foam gun for a couple of seconds.)
    (breather membrane over 16 or 18 mm sarking boards on front of house, no breather membrane on back of house).
    I guess its a case of, if moisture does get though the insulation layers, how easy will the it be able get away through that roof build up?
    • CommentAuthorDur
    • CommentTimeFeb 14th 2020
     
    Picture might help. Sorry about the mess!!
      HC insulation-S.jpg
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 18th 2020
     
    Whilst the highly breathable (or non-existent) membrane and 150mm sarking board do not need a vented cavity, the main benefit is to allow that void to have insulation in it. I personally would not go back and start to block that up. You won't have roaring gales blowing up that void, so wind washing is unlikely.

    For air tightness, one option might be to tidy up all the insulation joints, and tape over those. Then add your next layer of board, and foam up again. That would get you to a pretty good level of tightness, an the tape would have a mechanical fixing in the form of the next layer pressing against it. I'm never happy with tape as the last line of defence.

    I would think that you've done a job of work that is so very much better than any tradesman would have done, that you've probably milked all the benefit you're going to get from this, and perhaps not go completely mad trying to get from 98% to 99%. Use that energy on other areas that will get greater return.

    Just noticed what looks like a beautifully spliced beam/purlin, spliced in two planes. That's an original I take it? Some poor wee apprentice probably spend a week chiseling that out.
    • CommentAuthorDur
    • CommentTimeFeb 25th 2020
     
    Thanks GP

    I will back up the tape with the next layer. Should be pretty safe then.

    That picture is deceiving you. The beam under the window (1970s) is 2 pieces of 8 x 2" nailed together. There is one under the other dormer but the two didn't meet in the middle so I cut the scarf into one of the pieces on one side and another on the other side but with them staggered. Didn't do a wedged scarf on the basis that all the load is down on it. There is a corresponding beam up above the ceiling joists which I did a similar thing to except there I had to do them in two halves to get them in so they got hooks and wedges.

    On the back roof we took the old purlin out and put in a 10" flitch beam high and low. The steel plate (175 kg) took some getting in but in the end no problem with a block and tackle.
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