Externally rendering and insulating a single-skin wall

Discussion in 'Builders' Talk' started by Elwyn, May 8, 2006.

  1. Elwyn

    Elwyn New Member

    I am considering exernally insulating (thermally) a single-skin kitchen wall and applying an external roughcast render to match the rest of the house.

    To do this I will need stainless steel expanded metal lathing and special fixtures to support the metal work.

    Is is difficult to render (presumably with a lime based render) onto expanded metal lathing - has anyone had experience of such work?
     
  2. Have never seen anything as heavy as roughcast go onto external insulation batts, usually they are glued and screwed (Heavy gauge) and meshed, then lightweight finishes applied. If you can secure the ex met then there is no reason why you can not give it a good scratch coat then harl it.(rough cast)
    Would it not be more practical to insulate internally with shelterboard.
     
  3. skinny d

    skinny d New Member

    I can't comment on the external insulation part but heres my experience of rendering expanded metal lath.

    I've a dormer window which is a 1/2" ply clad stud frame, I wrapped it with tyvek then vertically battened it with 1x2" roofing battens. I then re-wrapped it in tyvek (stops battens rotting and prevents render pushing through, also had lots of scraps left over from reroofing) and used galv staples to fix the mesh to the battens.

    Applying the first coat (3:1 sand:cement with Rendamix) was a nightmare, pushing the render on too hard just pushed it through the mesh (tyvek didn't stop it enough). As I'm definately not a pro plasterer I resorted to using a trowel to dab on small amounts of render at a time using very little pressure. The first coat took me ages and then some.

    The next two coats (4:1 and 5:1) went on a treat ast the first coat had bunged up all the holes and made the mesh stiff.
     
  4. Elwyn

    Elwyn New Member

    In older 1930s houses it was common to have a single-skin for the kitchen area so that the pantry area was kept relatively cold - particularly during the summer since there were no fridges.

    These kitchen areas can be improved by externally insulation, with either Cellotex or Kingspan, and then using metal lathing to render a good cement/lime roughcast to match the existing property.

    The biggest problem with a single-skin external wall is condensation which most people confuse with rising damp.

    By doing it this way you have got the equivalent of a double-skin wall which is well insulated.
     
  5. mudhut

    mudhut New Member

    robbo, any tips on laying first coat on lath? skinnyd had a job on there, possibly having a zero suction background would warrant a super stiff mix??

    i only ask as i have a big lath job coming up :)
     
  6. skinny d

    skinny d New Member

    Hi mudhut,if the mesh is directly onto a solid background then my problem shouldn't occur, I had a 1" airgap behind due to the battens which allowed the render to slop through.

    The reason I put the 2nd layer of tyvek over the battens was to prevent this but it just wasn't taut enough. I go this idea from the TRADA timber house book which said something along the lines of "if 1" battens are used, use paper-backed mesh to prevent bridging of airgap, if 2" battens are used then just use normal mesh".

    When I asked my local merchant for paper backed mesh they looked at me blankly (it happens a lot). I have seen some mesh/paper used on a timber frame on Grand designs. The mesh was fairly sparse (maybe 2cm pitch) compared to the mesh my merchant supplied (9mm pitch) but was somehow bonded to the paper. The render was then applied and stuck to the paper rather than the mesh, which appeared to be there for strength and support after it had gone off.

    Hope this helps, Dave

    Ps It was definately my poor trowel technique that caused the render to push through, a better man than me would have less trouble, but I would hazard an opinion that rendering mesh is more difficult than rendering a brick wall.
     
  7. Hi Mud hut. If I am rendering the cheeks of a dormer then I use the two coat plastic stop and angle beads after first tacking a layer of regular bitumous type reinforced roofing felt to the ply board with galvanised clout nails or staples. After setting the stop and angle beads I apply the ex met directly to the felted dormer cheeks again with galvo studs then give it a good scratch coat, this goes on with ease, then after a couple of days float render it as per normal with waterproofer in the mix,...and the scratch (optional).
    If it is solid wall I fix sheets of ex met with washered steel (masonary) nails, then ccratch and float.
     
  8. No need for a stiff mix mudhut if you have no counter battens leaving a void between mesh and felted ply..Whats the job you have to do mate?
     
  9. mudhut

    mudhut New Member

    looks like i just hijacked a thread, sorry elwin!

    thanks robbo and skinnyd for the informative responses, the "job" is my extension, but as i am still in the ground it will probably be late autumn before i get round to the rendering - or early 2012!

    it will be 150x50mm studwork with, i think, 12mm or so osb sheathing. after that i am not yet sure on the detail but imagine builders paper, counter batons and then EML (this is currently in the hands of my "designer")

    the paper backed lath sounds ideal, but may be pricey if it is little used in the uk. failing that an extra layer of builders paper, nice and tight, under the lath may help?

    what would you expect the make up to be? no extra insulation is needed and the inner skin of studwork will have vapour controll.
     
  10. skinny d

    skinny d New Member

    Hi, from what I can remember the TRADA book recommended wrapping the OSB sheathing in a breather membrane then battening it. The battens should then have dpm strip galv nailed to the tops to prevent the battens getting damp from the render in the mesh. Top and bottom the battened air gap should be ventilated and protected with bug mesh (easily done with a bell cast bead at bottom and fascia board at top). Then (using builders paper or not) the mesh it put on with the main "grain" of the mesh horizontal and the strips of the mesh facing inwards and downwards (if that makes sense). Getting the paper and mesh drum tight is pretty important otherwise the render slops through and the flexing of the mesh shrugs the render off.

    But hopefully someone with a lifetimes (rather than 1 dormers) experience will add more to this. And sorry Elwyn for the hijack.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice