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    •  
      CommentAuthorJustin
    • CommentTimeMar 21st 2010
     
    Hi,
    I have 100mm cavity with 60mm K7 (25 min air gap). Roof is full fill warm (175mm PU) between and under rafters. I have no ceiling, internal surface is underside of pitch (30 degrees).

    Detail on manufacturers sites (Kingspan for example) is my main reference - which may be right or wrong. The latter indicates quite clearly in their application data that one should use cavity closers at top of a cavity wall when K7 board is used. My roof pitch is shallow, and consequently the internal and external wall skins both stop at the same level beneath the wall plate. My builders are adamant that they never put top cavity closers at a wall top these days, and that it's adequate (and better) to jam rockwool into the gap to seal the wall top, and interface it to the PU in the roof outside the wallplate. I could not convince them otherwise because I I can't strongly argue strongly against what they have now done, - which is fill the interface with rockwool.

    The eaves are unventilated, so in principle, a closer at the wall top isn't to stop birds/insects mice etc. Why is this specified in Kinkspan K7 data? Is it a hangover from the days when we have ventilated roofs, or is it important to fully prevent air movement for the thermal perfomance of a PU insulated wall?
    •  
      CommentAuthorted
    • CommentTimeMar 21st 2010
     
    I always thought sealing of the cavity at the wall plate was required by Part B fire regs - which mentions sealing of hidden voids. The BCO responsible for our extension insisted on it.
    •  
      CommentAuthorJustin
    • CommentTimeMar 21st 2010 edited
     
    Thanks Ted, i'll ask Building Inspector that when I see him (possibly tomorrow).
  1.  
    Hi Justin. There is definitely a requirement to seal the cavity at wall plate and up the verges of any gable ends. As Ted says this is to prevent fire spreading via the cavity, which can have a significant draft [think stack effect]

    There are many ways to do this and some just turn the top block 'on flat'. Other simple solutions include using slate or other similarly flat [fire retardent] material. Rockwool and others make fire socks for this purposehttp://www.arcbuildingsolutions.co.uk/documents/CavityStopSocksPDF.pdf
  2.  
    Posted By: JustinThe eaves are unventilated, so in principle, a closer at the wall top isn't to stop birds/insects mice etc. Why is this specified in Kinkspan K7 data? Is it a hangover from the days when we have ventilated roofs, or is it important to fully prevent air movement for the thermal perfomance of a PU insulated wall?

    I've seen fibre cement board, plywood or DPC roll used for this job, which implies that it isn't solely for fire protection. With a conventional masonry cavity wall & trussed rafter cold roof, the reasons given were to prevent nesting insects gaining access & prevent moist air from the wall cavity coming into contact with the cold roof structure.

    This junction is critical thermally. So there's alot to be said for avoiding the cold bridging caused by fibre cement board, masonry or timber. A tightly packed Rockwool cavity sock (Rockwool in polyethylene bag) is not a bad compromise.

    David
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 23rd 2010
     
    Makes sense to me to make sure that fire cannot spread and that what ever is used to cap the top does not transmit heat. Sure there must already be solutions to this already.
    • CommentAuthorsquowse
    • CommentTimeMar 23rd 2010
     
    rockwool would not seal against (cold) air ingress no matter how tight it is jammed in surely?
    • CommentAuthorsydthebeat
    • CommentTimeMar 23rd 2010
     
    does anybody put any credence into the requirement for a cavity closer to spread the roof loading onto both masonry leaves??


    in 'storey and a half' dwelling no cavity closer is a dangerous detail
  3.  
    Posted By: squowserockwool would not seal against (cold) air ingress no matter how tight it is jammed in surely?

    With partial fill cavity insulation there is the risk of fire propagating via the cavities & this needs to be treated as a priority. However, I am planning to use full fill mineral wool cavity insulation, so I'm more concerned with cold bridging & air ingress. I plan to close the cavity at the eaves with 9mm OSB3 on a bed of flexible silicone sealant to inner & outer leafs.
    Posted By: sydthebeatdoes anybody put any credence into the requirement for a cavity closer to spread the roof loading onto both masonry leaves?? in 'storey and a half' dwelling no cavity closer is a dangerous detail

    This depends upon the structural design, but it's not normally required. If required it's performed by cavity ties or the roof structure itself. Using masonry would cause serious cold bridging & should be avoided.

    I plan to use 400mm deep Masonite I beam rafters fully-filled with mineral wool insulation. The lower flange will sit on a bevelled bearing plate to top of 9mm OSB3 cavity closer & inner leaf. The upper flange fits to the rear face of a bevelled bearing plate to top of 9mm OSB3 cavity closer & outer leaf.

    The cavity closer will prevent air movement from cavity to roof, while tying together the two leaves, the bearing plates & the two rafter flanges. A woodfibre sarking board supports the breathable roofing membrane & allows the laps to be sealed with tape. The bevelled bearing plate to the top of the outer leaf will prevent cold air penetrating the junction between the outer leaf & the sarking board.

    David
    • CommentAuthorsydthebeat
    • CommentTimeMar 25th 2010 edited
     
    Posted By: davidfreeborough
    The cavity closer will prevent air movement from cavity to roof, while tying together the two leaves, the bearing plates & the two rafter flanges.


    sorry, how would 9mm of osb board tie two masonry leaves together??
    any tensile force will simply rip the osb apart. ?!?

    in your description above, it sounds like the I beam itself is required to tie the two leaves together at the top of the cavity.
  4.  
    Posted By: sydthebeatsorry, how would 9mm of osb board tie two masonry leaves together?? any tensile force will simply rip the osb apart. ?!?

    in your description above, it sounds like the I beam itself is required to tie the two leaves together at the top of the cavity.

    We tie masonry leaves together mainly for stability, there shouldn't be much load involved. 9mm OSB is at least as good as a row of steel cavity ties at 900mm centres & is often used as the web in I beam joists. However, it's main structural purpose here is to tie together the I beam's upper & lower flanges where they meet the wall, preventing spreading forces from weakening the rafter itself & ensuring all loads are carried vertically into the wall.

    David
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