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    Detailing at floor/external door or patio door junction to reduce thermal bridging etc

    Anybody got a detailed section showing the best way to deal with this junction for a slab/insulation/screed floor on a block/cavity/block wall construction
    I've seen it where you leave the cavity and bridge over it with slate/floor finish or window board
    but surely it make sense to just run the dpm/insulation/screed up to the threshold with a insulation upstand at the inside of the outer wall
    Anyway i'm unsure of the best solution

    cheers Jim
    like this one?
      Threshold detail.jpg
    that looks good to me . I can crack on tomorrow now

    thanks Jim
    • CommentAuthorwookey
    • CommentTimeFeb 4th 2011 edited
    I've been pondering this problem for a while, and come up with what I hope is a useful detail. Rather than extend the concrete slab part-way across the cavity (as in DavidF's detail) or all the way across as above, how about using a fibreglass treadplate to provide the necessary rigidity across the cavity, whilst retaining the full insulation thickness?

    According to http://www.racolin.co.uk/catalogue.pdf 6mm treadplate spanning a 300mm gap, fixed both sides will take 47KN/m2 uniform load with 1.5mm deflection. 10mm 183KN/m2. If we take the door to be a point load in the middle, then around 60% of those loads will be OK. Which suggests that a 6mm plate will take an 80Kg door and a 10mm plate a 300Kg door. Which seems the right ballpark.

    Probably easier than fixing the plate into the brickwork at either end is to use a bit of angle pultruded fibreglass across the cavity, bolted to the plate. This slightly increases the thermal bridge, but enormously increases the rigidity and means that the plate can just be dropped into place (after slotting the insulation below the angle.

    This is a variation on the foam-filled box that denby dale had made up, but stock plate cut to wall width and angle to make it a little more rigid spanning the cavity width are very easy to make up on site with a saw, drill and a few CSK bolts.

    This stuff should be totally rotproof and can be used as the bottom of the airtight box. It's not very beatiful so putting a 30-40mm timber cill (or an aluminium one) on top of it to provide door sealing and a draining slope and something that looks nice seems a good idea.

    The detail below puts the seals on the bottom of the door, with just plain weatherbar set into the cill to avoid the aluminium threshold bar condensation-collector, and to provide a smooth part M nearly-flat threshold.

    Can anyone see any problems with this design? I don't know how much this treadplate costs - I assuming it's not unreasonably expensive.

    The thermal conductivity is quoted as 0.6W/mC, which is basically the same as timber.

    There are various suppliers: Racolin, Captrad, and Dura Composites.
    Looks good. How's the floor finished over the plate? How/where is the pultruded fibreglass angle bolted to the plate? Do they offer a T section? If you have load bearing EPS or XPS fitting tight below the plate, do you need the angle?

    • CommentAuthortony
    • CommentTimeFeb 4th 2011
    I would be nervous of the long term durability of the fibreglass, how thick is it and is it reinforced and with what?
    • CommentAuthorwookey
    • CommentTimeFeb 6th 2011
    The floor is finished over the plate however you finish the rest of the floor. Tiles in my case. The idea would be to have it finished more or less flush with the screed, although in the above diagrm there is about 10-15mm to pack with something to get the internal floor to be more or less level with the back of the cill.

    No, none of them seem to offer T-sections - I sections, channel and angle. I guess it doesn't really matter. For bolting them together I was thinking stainless countersunk bolts (so the surface remains flush), one either end of the strengthening member, about 20mm in from the cavity edges.

    As you say, with XPS in the cavity below cut off at the right height, that should mean that the plate alone is plenty rigid enough.

    Tony - they normally use this stuff for external channeling covers - e.g by the sides of railways and plant areas generally. I don't think there is much danger of it not being durable enough. 6,8 and 10mm thickness options. I would give you a link to racolin's site but it's so totally broken that I can't.
    Look at page 7 in the catalogue link I gave, or here: http://www.gridflooring.com/solid_plate_fibreglass_grating.html
    Good find. I'm thinking of a variation on this for use with a power floated cast in-situ suspended concrete slab.

    The outer leaf brickwork & XPS perimeter insulation at thresholds would be set 8mm lower than the rest of the foundation walls. The DPM would be wrapped over the top & an 8mm grit top GRP plate fixed to the outer leaf with sleeved anchors & countersunk slot head bolts. The plate projecting 100mm over the perimeter insulation with cast-in sockets attached by countersunk slot head bolts.

    The slab would then be cast flush with the top face of the plate with the cast-in sockets providing the fixing at the inner leaf end. Once the slab has gone off the plate would be removed, glue/screw fixed forming the 4th side of an 18mm birch plywood box & re-fitted.

    Thus the GRP plate provides the formwork for its own recess during slab casting, protects the insulation/DPM during construction, forms the 4th side of the airtight box, protects the vulnerable bottom edge of the plywood, mechanically secures the DPM to the outer leaf & provides a sill for sealing to the door frame. While not changing floor levels, reducing the perimeter insulation thickness at the threshold position or limiting the door frame options.

    In my case, I think 8mm will be more than strong enough because the door's weight will be transferred via straps & the plywood box to the inner leaf. And the plate will be fixed on all 4 sides & fully supported by outer leaf, XPS perimeter insulation & slab.

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