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    • CommentAuthornick1c
    • CommentTimeDec 11th 2018
     
    We have (finally) got permission to build a cantilevered, reverse level, house overlooking the Atlantic near Lands End on a narrow (8' wide in places) lane which has overhead power lines along it.
    Aside from the obvious access issues the thing which is bothering me most is getting the wall build up right. We currently plan to have a stick built timber I-beam frame with a minimal amount of steel to support the cantilevers, filled with blown cellulose insulation and covered with a combination of timber and render sitting on a passive slab. It seems to me that the obvious location for the air tightness layer is on the outside using OSB as this makes getting round the cantilevers easy. However an energy consultant has voiced some concern about this, saying that the ideal is to have increasing vapour permeability as you move out through the wall to avoid condensation problems. Landsend weather, the local station, measures humidity and it rarely seems to drop below 90%, often sitting at over 95% for days at a time. What would people suggest as the best build up to avoid condensation problems in the future?
    • CommentAuthortony
    • CommentTimeDec 11th 2018 edited
     
    Sack them then, you can have vapour open air tightness layer! (The OSB could be a problem in terms of not being vapour permeable enough too).

    When you say humidity 95% are you talking absolute humidity, relative humidity or what?

    I would avoid organic materials for both the structure and the insulation, I would also go for a robust fully fail safe design philosophy.
    • CommentAuthornick1c
    • CommentTimeDec 11th 2018
     
    Their concern was to do with having a gradient of permeability, with it increasing as you move out through the wall, they weren't saying that OSB was impermeable.
    I assume that it is RH, it just says 'humidity' on the site.

    http://www.landsendweather.info
    • CommentAuthorgoodevans
    • CommentTimeDec 12th 2018
     
    Nick - have a look at this thread also:
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15945&page=1#Item_1

    Timber I beams are not cheap and because they are designed to be deep the insulation is usually located in between and it usually means that the outside flange gets cold and needs to be well ventilated to prevent condensation. You also have steel involved. Have a look at arranging most of the insulation to be outside the structural elements of the build as per Foster Toms suggestion in the above thread. This way the structural elements stay warm and dry.
    • CommentAuthornick1c
    • CommentTimeDec 12th 2018
     
    I know I-beams aren't the cheapest, but they are light, straight, strong & give a low timber fraction for insulation. I had hoped that getting the frame pre-cut offsite, delivered to a farm shed nearby that has artic access & then shuttled in (mainly by me) would result in lower labour costs to offset the I-beams. I get the point about the outer flange though. Is EPS a better bet than wood fibre board? It doesn't have to be the absolute thinnest wall possible.
    •  
      CommentAuthordjh
    • CommentTimeDec 12th 2018 edited
     
    Posted By: nick1cTheir concern was to do with having a gradient of permeability, with it increasing as you move out through the wall

    That's one reasonable way to design things, but it's not essential.

    What energy/thermal performance level are you aiming at?

    Cantilevers and the associated steel can be tricky. Insulation over the outside is definitely a good idea but details need careful design. Do you have any drawings you'd be willing to post?

    Blown cellulose between the I-beams (or dual studs) is a good choice. EPS is a pretty good choice for an external layer of insulation, since it is breathable, doesn't rot and is fairly cheap. It's also easy to cut to shape. Woodfibre is similar, but can rot and is more expensive. It is more natural and has less embodied energy though, so the decision is a matter for personal preference.
    • CommentAuthorgoodevans
    • CommentTimeDec 12th 2018
     
    So far as insulation is concerned...

    I have modelled the various ways of preventing repeating thermal bridges with rafters at 600mm centers- I looked at I beams, hit and miss schemes etc. But I found that if you are using wood as the structual element it makes very little difference over using standard joists.

    I seriously looked at I beams for my roof - the beams were pricey - but because the beams are slender they require more care with bracing, blocking (filling in the flange area with wood), storage, cutting and specialist connectors. In the end I was pleased I went the traditional route - It is more forgiving, and easier for the builders to either get right first time - or to fix after.

    In the end I used roof grade timber (TR26) everywhere on the roof - a fair amount of it in all - 220mm deep attic truss rafters, osb3 T&G4 sarking layer (air tight layer) and 120mm of false rafters on top. With insulation between the rafters - it does mean that 9% of the roof has 340mm of wood thermal bridge. But even that bit has a U value of .44 the other 91% of the roof has a U value of .12.
    • CommentAuthornick1c
    • CommentTimeDec 12th 2018
     
    Hopefully this gives an idea of what we hope to achieve. Our original intention had been to try and met Passivhaus standard, but because of the cantilever and the view being to the North we were advised this was not possible. It would be good to get as close as possible without it becoming financially prohibitive.
      Screen Shot 2018-12-12 at 15.50.33.png
    •  
      CommentAuthorfostertom
    • CommentTimeDec 12th 2018
     
    Posted By: nick1csaying that the ideal is to have increasing vapour permeability as you move out through the wall to avoid condensation problems
    That's an old rule of thumb, rightly rarely mentioned these days, based on simplistic understanding of vapour movement, which isn't like either compressible gas, or liquid, or conducted heat, or electromagnetic flows. My playing with WUFI satisfied me that it has no validity. One famous failure, which ruined an up and coming eco architect firm, was due IMO to relying on that formula, in the days before WUFI simulation became common.

    So I think you're basically right with your wall buildup - see what I'd recommend, in
    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=15945&page=1#Item_15

    Those architect drawings look like Jacob Down's, St Ives?

    Cantilever doesn't rule out PH, just means a sub-optimal form factor (ratio of external surface to useful floor area, which actually looks quite good) so you have to insulate more - but then it's quite large, which in itself improves form factor (i.e. easier than any small house). North facing - unless severely shaded from SE to SW sky, that should also be overcome with design ingenuity. It may be lack of the latter which makes PH 'not possible'.
    •  
      CommentAuthordjh
    • CommentTimeDec 12th 2018
     
    What Tom said. How much insulation are you planning under the floor and in the roof?

    Thanks for the pictures. The small scale forced by this forum makes them very difficult to read unfortunately :( I'm not quite clear where the cantilever is?

    Personally, I'm very nervous of airtightness barriers on the outside. You need to plan every detail carefully in advance. Ground floor to walls; roof to walls; penetrations for downpipes, wiring and plumbing etc and for everything you ever want to hang off the wall in the future.
    • CommentAuthornick1c
    • CommentTimeDec 13th 2018
     
    The East and West elevations are fully cantilevered, the North isn't but the South is to the East of the store, presumably making the SE corner the most structurally challenging.
    I was thinking of 300mm in the walls & 450mm in the roof.
    From what has been said above it sound like the following build-up would work, but sounds expensive!
    Inside - outside: plasterboard or fermacell - 25mm batten - taped OSB [airtight layer] - frame w. 300mm warm cell - OSB - 50mm EPS - batten - rain screen. I assume that the area under the cantilever would need more insulation due to the steels for support.
    Is this sensible and is there a more economical way of getting it to work?
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018
     
    Nick, If you attach a PDF document instead of a JPEG I think the images are not shrunk to 600 pixels wide.
    • CommentAuthornick1c
    • CommentTimeDec 13th 2018
     
    Is this better?

    If not the planning reference on Cornwall councils site is: PA 18/07233
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018 edited
     
    Nick,

    squinting at the picture it looks like you have approx 1.2m cantilever to the east and a 1.8m cantilever to the west.

    It also looks like the north east corner is cantilevered at the top of the external staircase in both directions. Insulation here is not required - could you consider making the staircase and north part of the balcony self supporting (i.e. a wall/column supporting the corner).

    Has an engineer been commissioned to propose a structural solution yet? How much insulation are you prepared to have under the east and west cantilevers. How much depth is allowed for the structural elements of the first floor/cantilever.

    I don't think you need 300mm thick structural Timber for the walls - you could have simple solid timber 150mm deep - osb outside that as the Air tight layer and 150mm of eps outside that and either battens and rainscreen or acrylic render. You could loose the internal battens and plasterboard direct to the frames.

    With sold timber window fixing is easier and the connection between the cantilevers and the ground floor wall is easier.
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018 edited
     
    yes that's better
    •  
      CommentAuthorfostertom
    • CommentTimeDec 13th 2018
     
    Can practically see your house from our cliff-edge caravan site at Gurland (6 visits last year). Yes, windy, wet, foggy but also as south as you can get, much brilliant weather, and no frost, early spring. Palm tree land!

    Those cantilevers - not excessive, instead of steels, cd be done by supporting the roof off the lower walls by cross walls/columns, let the roof do the cantilevering and hang the upper walls from that. The floor overhangs should be do-able with joists if they don't have to support walls, roof, snow load.

    In that case, easy to encase the whole incl floor cantilever soffits in a 'tea cosy' of a) airtightness and b) external insulation. PH should be easy from the insulation POV, and I'd have thought the site has plenty of southern exposure.

    However, balcony and steps shd be independently supported, not by long cantilever beams puncturing the tea cosy.
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018
     
    I agree with Tom in all aspects.

    The 1st floor balcony door takes out some options for that part of the wall carrying load back to the ground floor walls.

    There are many different approaches - It just depends on your red lines - e.g. cross walls, columns etc.
    • CommentAuthornick1c
    • CommentTimeDec 13th 2018
     
    The cantilevers are 1m to the east and south, 1.5m to the west.
    We would support the staircase with posts.
    The majority of the first floor will be open plan with a vaulted roof. There is a drop to a normal ceiling level to the west for 5-6m and a small pantry in the SE corner. It may be possible to bisect the span of the ridge in the main room with a support for the ridge beam so that it would have the total span divided roughly into thirds.
    You could see us from Gurland point, our view is along the coast to Cape Cornwall & the Brissons.
    •  
      CommentAuthorfostertom
    • CommentTimeDec 13th 2018
     
    Posted By: nick1cYou could see us from Gurland point
    See ya soon!
    • CommentAuthorgoodevans
    • CommentTimeDec 13th 2018 edited
     
    so no roof trusses going north/south - but a ridge beam (gluelam/steel) going east west. I can see support for the ridge on the west side (although the support has no direct path down through the ground floor beyond the west ground floor wall). Assuming another support mid way along the open plan area you will still need another support (pref above the ground floor east wall) to support the end of the beam.

    Perhaps the pantry can be flipped around 90 degrees so that the long side is against the east wall that allows the short wall of the pantry to be directly under the ridge beam and over the ground floor east wall.
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