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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorNeilD
    • CommentTimeMar 23rd 2018
    Hi again,

    My SE has prescribed wind posts due to high wind loads (altitude and proximity to coast). Wall construction is 100/200/100 cavity. It has been suggested in some locations where we have other steels interconnecting to use 100mm box sections as windposts that sit in the inner skin blockwok (medium dense blocks).

    I'm not sure how best to detail the air tight barrier in these locations. Elsewhere it is wet plaster but I am concerned it will crack badly where the masonry meets the windposts if I just mesh over the posts. I'm considering in the area where I have these windposts using a membrane over them lapped under a parging coat on the blockwork. Then dot and dabbing plasterboard over the top. Hopefully this will avoid recurring unsightly cracks and leaks.

    Thoughts? Am I worrying about cracking too much?

    Many thanks
    • CommentAuthortony
    • CommentTimeMar 23rd 2018
    Try and avoid dot and dab. Could the wind post be a T section, top oh the T in the cavity and a 100 web in the blockwork forming a movement joint, then expamet and wet plaster it,
    • CommentAuthorNeilD
    • CommentTimeMar 23rd 2018
    I did float the idea of using a L-section but for two particular posts that have RSJ's running perpendicular to the external wall linking to them he says they would be better as box sections. I don't see why as the blocks should take the compressive loads. I'll have to ask for clarification.

    Surely you can't expamet and plaster over a movement joint? Won't it just crack. I was thinking to tie both sides of the wall (+the external) to the L or T section if I could do it that way.
    • CommentAuthorNeilD
    • CommentTimeMar 23rd 2018
    The worry I have about dot and dabbing over a parge in this case (I would never normally even consider it) is the parge cracking but this being unoticeable due to the plasterboard. Is this why you wouldn't do it here Tony or is just a general loathing of dot and dab?!
    • CommentAuthortony
    • CommentTimeMar 23rd 2018
    Always windy behind dot and dab
    • CommentAuthorNeilD
    • CommentTimeMar 23rd 2018
    Good to know I'd have a wind post there then :wink:
    • CommentAuthorTimSmall
    • CommentTimeMar 24th 2018 edited
    Might be worth checking with a specialist windpost fabricator? "Spine windpost" type might work for you? You could probably extend it 50mm into the cavity without any problem (and a pretty small linear thermal bridge).

    Mortar bed reinforcement, might also be worth looking at, depending on the wall, or even filled hollow concrete blocks (with reinforcement bars) for the particular wall panels which need reinforcment...

    Or even concrete lintels (cast in-situ or otherwise).
    • CommentAuthorrevor
    • CommentTimeMar 25th 2018
    I had windposts in my end gable but they were in the cavity and wall tied to the masonry block on both skins of the cavity wall. The SE specified them as one length and from the foundations up were ca 7M long. I had a grave concern that as we were building the wind would move the posts on block work that had not already "gone off" . We braced the posts very much like you would a yacht mast and it seemed to work. The brickie had not come across windposts done this way before, his previous experience of them was in storey size lengths and flanged, bolted together as construction progressed to that level, so that the leverage effect of the wind on the length did not affect the courses laid. So maybe something to consider if yours are in one piece and are in an exposed area to do as storey length sections. Being within the cavity cold bridging was an issue but the air barrier was not. I note you have a 200 cavity so it looks like you could put the posts in the cavity within the insulation thereby sacrifice a bit of insulation for ease of forming a robust air tight barrier. Is there a reason why they are in the wall and not in the cavity? As an aside my SE stated had the architect made the opening for the bifolds 4 m instead of 4.6 M then we would not have needed wind posts. So by redesign could you get rid of your posts?
    • CommentAuthorNeilD
    • CommentTimeMar 26th 2018 edited
    Thanks Revor and Tim - very helpful.

    I've already looked at redesign options and there isn't really any scope here although in other areas I have done this. I need to get back to the SE to check why he wants them in the inner skin. He has said it is to transfer loads from interconnecting steels but I don't really understand why the blockwork can't do this. This is only for two windposts elsewhere I have specified they sit in the cavity.
    • CommentAuthortony
    • CommentTimeMar 27th 2018
    That sounds like thermal bridging to me
    • CommentAuthorTimSmall
    • CommentTimeMar 27th 2018
    Might be worth taking the SE's requirements for all of the wind posts, and giving them to a specialist windpost fabricator to see if they can come up with a less thermally bridged alternative (and also whether a spine type design would suit for the ones you originally queried about).
    • CommentAuthorbhommels
    • CommentTimeMar 28th 2018
    From my single windpost experience: architect specified a nice elegant U-shaped ancon one, to sit in the cavity of our 100-200-100 block-cavity-block wall. Builder inquired and found them out of stock, on 6 weeks delivery, so instead he went for a rather long piece of steel box section to sit in the cavity, hugging the inner leaf. Masonry anchors embedded in the inner leaf were bolted or screwed into the box section, and I took to the job of sculpting Celotex to surround the windpost as to limit the thermal bridging. Where the post met the roof, restraint straps were used to tie the glulam ridge beam to the windpost. The post was cut to length so the top layer of roof insulation would go over it.

    The advantage of such a wide cavity is that although a beefy 80x80mm (?) box section was used, it left 120mm to be filled with Celotex between the outer leaf and the box section, which, as it is tied to the inner leaf, is part of the warm volume. The U value of 120mm of Celotex is quite close to 200mm of mineral wool, so no big steps in U values either.
    Initially I thought the box section was going to be a disaster, In the end I was happy with the solution although a few more details could have been obsessed over from a thermal bridging point of view.
    • CommentAuthorNeilD
    • CommentTimeApr 2nd 2018
    Thanks for the additional comments. Tony: regarding thermal bridging my plan was to do as bhommels did where posts sit in the cavity. Unless I am missing something I think they will have a negligible thermal impact.
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