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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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    I am doing some DIY EWI and the house is single story, 26 m long with 8 windows/doors on one side, so lots of cutting. Because I don't like the snow involved with using a saw with a few bits to hand I knocked up this hot wire cutter.
    The wire came from some left over trace heater cable (water pipe frost protection) the rest of the materials came from the 'might come in useful one day pile'
    The power supply is an old cheap battery charger which supplies about 2.5 A
    A bit of experimentation with the length of wire where too short -glowed red, too long - did not cut gave a reasonable result. it cuts a 1m x 100mm sheet in about 1 min. and the cut is indistinguishable from the factory edges.
    The tile batten (guide) adjusts to control the cut line. Internal angles done by the long cut using the guide, the short cut freehand following a pencil line
    Magic - I'll not be using the saw again !!
    Looks great Peter, show us the under side.
    Nothing really to see on the underside, just some tile battens to lift the OSB up a bit to give a bit of space for the bit of 25mm angle iron (about 5cm long) that I tapped a couple of 6mm threads into for two bolts, one to take the wire from the battery charger and one to take the hot wire. The angle iron is screwed to the underside of the OSB. There is a 12mm hole drilled through the OSB to pass the hot wire through to the connection bolt. The tension spring (seen at the back of the top arm) is adjusted by placing it in a different hole of the 10 or so I drilled as I had no idea where the spring would be best set when I built it.

    Once I sorted out the best length for the wire I haven't broken a wire.

    Yesterday I fixed the top 2 rows of EPS, of the 52 sheets for the two rows there were 3 that did not need cutting and overall about 80% of the sheets for the front wall needed cutting on one edge or for a corner. Luckily the back wall (north facing) only has 3 windows and no doors.

    Oh and the reason for the board to have the corner cut off to an angle is 'cos that's how the left over bit came!
    • CommentAuthorjamesingram
    • CommentTimeAug 6th 2019 edited
    thought Id share my own very cheap version
    30 minute to make and £7 for the wire, everything else off cuts and left overs
    How do you keep tension on the wire which I found expands when hot
    I made one of these years ago for a job. I used a bucket part filled with stones, hanging on the end of the hot wire, to keep it tensioned, but the spring is def a much better idea...like it.
    Peter, I tightened it under tension pushing the arm down, its ok , bends a tiny amount but if you keep the direction straight it doesnt seem to effect the cut. I have broke the wire several times though in a week of use. The spring seems a better idea though
    • CommentAuthorborpin
    • CommentTimeAug 9th 2019 edited
    Posted By: jamesingram£7 for the wire
    @jamesingram What wire did you use?
    • CommentAuthorCWatters
    • CommentTimeAug 11th 2019
    I once built a hot wire cutter powered by a 12v battery. I put the motor speed controller from an RC car in series so I could control the temperature. Yes remotely using a transmitter and receiver. Move the throttle stick on the transmitter and the wire heats up. The wire was also pulled through the foam around a template using weights and pulleys but that's another story.
    • CommentAuthorCWatters
    • CommentTimeAug 11th 2019
    Posted By: Peter_in_HungaryHow do you keep tension on the wire which I found expands when hot

    Model aircraft builders typically use a frame shaped like a bow saw. The wire is tensioned by twisting string or using a spring.
    • CommentTimeAug 16th 2019
    Hot wire cutters work fine on white EPS but make heavy weather of platinum EPS. It seems the carbon powder in the platinum conducts the heat away from the wire much quicker, so you end up trying to force the EPS 'through' the wire because it doesn't cut effortlessly. Result - wavy cuts and broken wires. Sadly.
    • CommentAuthorbarney
    • CommentTimeAug 16th 2019
    Sounds like an appalling product to use as insulation, Tom - I thought the "platinum" product has a higher value - so less conductive to heat, not better


    • CommentTimeAug 16th 2019
    It's bizzare, and I haven't fully understood it, but adding carbon black to some insulations actually improves their insulativeness. Aerogel's magic is due to a similar effect, tho without using carbon. It's something about heat within the airspaces being absorbed into the solid material, where it ends up in a maze of conductive fibres that leads nowhere and is difficult to escape from.
    • CommentTimeAug 16th 2019
    Can you not just run the wire a bit hotter?
    • CommentAuthorbarney
    • CommentTimeAug 16th 2019
    Hi Tom - it's graphite that's added rather than "carbon black" - -which is basically just burnt oil.

    This is BASF claim on Neopor regarding the radiant scattering due to the graphite matrix - and a few other claims that the thermal resistance increases as the temperature drops

    I've only ever seen a small section cut with a hot wire cutter - which seemed to work fine to my untrained eye.

    I was just surprised that cutting the grey stuff was physically harder than white when using a thermal cutter


    • CommentTimeAug 16th 2019 edited
    Posted By: djhCan you not just run the wire a bit hotter?
    That means either a higher resistance (thinner) wire at given voltage, or higher voltage. Both options seem to make the wire much more liable to snapping. The balance of wire strength vs electrical resistance vs temp vs EPS mechanical resistance seems to arrive at a 'sweet spot' with white EPS.

    Wire cutters for EWI seem to mostly expect plastic foam insulations - apparently they are very easy-cutters (never tried, myself), compared to graphite EPS.
    • CommentAuthorjamesingram
    • CommentTimeAug 18th 2019 edited
    no issue with my 200mm G-EPS ( in 3 sheet packs from Poland) other than wire tension previously mentioned, I am using a nice big battery though.
    the other issue is just getting a nice smooth sliding motion.
    I have noticed the G-EPS Ive currently got appears less dense than stuff I got made to measure from a uk manufacturer for a job a while back
    • CommentAuthorRobL
    • CommentTimeSep 8th 2021
    Thought I'd add my own diy EPS cutter to this old thread. It's made just big enough to fit the EPS sheets I used with external plywood I had, and has spent a lot of it's life outside. Tensioned with perforated steel at the top, which is long enough to be nice and springy, and the perforations allow different angles to be set.
    There's a right angle of wood as a guide - it has magnets glued into blind holes on the back, so it holds in place to some metal side pieces. Maybe this was unnecessary, as I found it best to put a brick on it to make sure it doesn't move in practice.
    I use 0.4mm wire at 5A constant current from a bench psu.

    I find it cuts platinum/white eps fine - it's harder with any eps left out in the rain though. A bit of damp on the surface, or inside, and it takes 3* longer to get through it. Some bits of the eps cut easily, the wire catches on the waterlogged parts - then starts to melt away any dry parts. It's not awful, but best put eps out to dry for a while in the sun if it has got wet before cutting.
    • CommentAuthorRobL
    • CommentTimeSep 8th 2021
    Zoom in of the wire fixings. There's a butterfly nut at the bottom, ideally at the top too(only had 1), allowing for easy wire attachment if it breaks. It only breaks if manhandling rig / glows too bright / pushing too hard. The wire is cheap, thicker the better - if you have a power supply that can cope.
    • CommentAuthorEd Davies
    • CommentTimeSep 8th 2021
    Posted By: RobLI use 0.4mm wire at 5A constant current from a bench psu.
    Thanks, that's a useful data point. Any idea what the voltage needed is (volts/metre, watts/meter or resistance of the wire at the operating temperature)? My bench PSU goes up to 16V at 5A so just wondering if that would be sufficient.
    • CommentAuthorRobL
    • CommentTimeSep 8th 2021 edited
    I wanted to thin down some of my EPS sheets. Our house had some brick bits, and some blockwork bits with cladding, while I wanted the finished result to be smooth. I've also got a garage roof project, where I'd like to improve the slope of it.
    Anyway, this cutter has metal with guide holes every 10mm drilled in it to set the cut thickness, and a spring to set the tension. I used 0.3mm wire at about 2.5A - my PSU couldn't cope with 0.4mm wire for this unfortunately.

    If the insulation is bone dry it takes 30s to slice a 1.2m x 0.6m sheet of platinum EPS into two.
      SliceOverview - Copy.JPG
    • CommentAuthorRobL
    • CommentTimeSep 8th 2021
    Zoom in of this one

    This looks like just what I need for my 130+m2 job next year. Do you mind if I pick your brain further if need be? Please whisper me your e-mail if that would be OK.


    • CommentAuthorbhommels
    • CommentTimeSep 9th 2021 edited
    I can testify that all RobL's EPS cutters work very well :bigsmile:
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