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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorwoodgnome
    • CommentTimeJun 25th 2014
    I will have my single storey living room roof abutting a 100 block/200 full fill cavity/100 block wall.
    One side of the roof will be exposed to severe driving rain from the SW at times.
    Render finish to outerleaf.
    Any ideas for stepped trays or something better?
    The standard trays ( i found a couple of old ones for a 100mm cavity) will prevent maintaining full depth insulation
    Not sure if i can get 200mm cavity trays anyway. What are the alternatives?
    Mate reckons less insulation is better than a wet living room wall..
    • CommentAuthorArchmoco
    • CommentTimeJun 25th 2014
    Speak to your building control officer, the external render would be the waterproofing, and with full fill cavity insulation (pumped bead?) where is the water going to go? Similarly I alway query the point of weep holes in a block render building, to me this is more of a hangover from brickwork construction, as brick is not waterproof. However if it was my house I would get something made up, or see if you can incorporate a rainscreen to help protect the wall. Most companies will make you a bespoke unit, if you are not using a pumped insulation work out how you are going to get insulation into all the gaps
    • CommentAuthorwoodgnome
    • CommentTimeJun 25th 2014
    Insulation is Isover CWS 32 ( 2x100 mm thick batts)
    Can you elaborate on what you mean by a raindscreen?

    • CommentAuthorArchmoco
    • CommentTimeJun 25th 2014 edited
    Rainscreen, batten the blockwork and clad with timber. Any thing to keep the rain directly off the wall.

    I have made an edit as the slump comment is not quite correct. The batt insulation is not bad but you need to be careful with the install. I used the same insulation on my gables ends but at the end of the slopes, where the batt steps and around the windows it was packed with standard roll. It was only completed before Christmas but a thermal camera image has shown the slumping I'll post an image tomorrow. I'm a bit disappointed but at least I've PIR rigid on the outside under a rainscreen. In the future I'm sticking with rigid board or pumped bead
    • CommentAuthortony
    • CommentTimeJun 25th 2014
    I wouldn't think that you would need trays with render. Soakers are great with cove flashing.

    What is the roof covering, type of tiles?
    • CommentAuthorMike George
    • CommentTimeJun 25th 2014 edited
    Cavity trays are one of the things in cavity walls which are essential where there is high exposure.

    No render is waterproof (at least not over any length of time) - and in extreme conditions the rainwater will track around the cheaper alternative of lead flashing

    I have seen many extensions where water shows on the head of openings cut through cavity walls.
    • CommentAuthorwoodgnome
    • CommentTimeJun 25th 2014
    Tony roof will be natural slate.
    Mike.. What are your thoughts on the problems with insulation where the trays are?
    I don't like full fill insulation and it should not be used in high exposure areas. The drive for more and more insulation has meant that some have lost sight of priorities in building.

    Cavity trays are easy to make yourself - even stepped ones as they are just folded lead - no welding necessary - though there can be a significant amount of waste (which is why they are often bespoke made and welded as this minimises the waste)

    If you use cavity trays in your proposed construction then the insulation will be sat in the tray and prone to saturation - this may or may not be a problem depending upon the insulation you use but the water present will negate the insulation properties to some extent - for air entrained insulations this will be significant.

    Many will disagree with me as they prioritize more and more insulation - forgetting (or ignoring) that there is a law of diminishing,
    returns - your mate not being one of them :)
    • CommentAuthortony
    • CommentTimeJun 25th 2014
    Soakers, no flashing but a cover to the top of the soakers 100mm up the wall above the slates and diamond cut in angled uphill. Cover with stop or Belcast bead and render to it.

    How does rain penetrate render?
    • CommentAuthorArchmoco
    • CommentTimeJun 25th 2014
    Mike ,
    Generally agree but I've no issue with full-fill EPS, and used it successfully on many a job- Springvale platinum Eco bead (pumped version). I personally believe in maximising insulation, but pointless if there is bad detailing and poor airtightness.
    Posted By: tony

    How does rain penetrate render?

    Rain penetrates render primarily by absorption. If BS Standard mixes are used then the mortar is both liquid and vapour open. Areas of high exposure having 3 coats achieving a greater thickness and hence further for the water to penetrate before it reaches the substrate.

    Harling and Dashing the best finishes as they have a greater surface area than flat render - facilitating more rapid evaporation.

    In periods of prolonged rain however, even 3 coats may not be sufficient to stop water tracking through the porus substrate and onto the inner face of the external cavity leaf. The water then running down the inner face until it hits a cavity tray or head of a doorway without a tray above ---- where it will show.

    Where inappropriate (cement rich)mixes are used, the effect on the render will be to make it harder and less amenable to expansion and contraction - causing map cracking - water then enters via the cracks and becomes trapped as the richer - more liquid (and vapour) impermeable render will not facilitate evaporation to the outside air.

    Hope that helps :)
    Posted By: ArchmocoMike ,
    Generally agree but I've no issue with full-fill EPS, and used it successfully on many a job- Springvale platinum Eco bead (pumped version). I personally believe in maximising insulation, but pointless if there is bad detailing and poor airtightness.

    Mostly agree, but what happens to the water in a cavity filled with beads when it gets to a cavity tray? Or where there isn't one?
    • CommentAuthorwoodgnome
    • CommentTimeJun 26th 2014 edited
    So, there's either use cavity trays and loose some insulation or omit the trays and risk water penetration and possibly a wet living room wall.
    Had a look and can get trays for 200mm cavity.
    Would a silicon render offer more protecion over sand cement lime mix?
    Having been very particular with insulation fitting and door and window detailing it seems daft to loose insulation in the walls.
    I'm still trying to get my head around the relative vapour/liquid performance of silicone/acrylic renders.

    I've been told all kinds of things by different reps...each one selling their own product and rubbishing the competition :(

    So I don't know the answer to that one sorry...
    • CommentAuthortony
    • CommentTimeJun 26th 2014
    I reckon that as soon as any render is painted wet water cant get through it in sufficient quantities to cause problems I find it difficult to see how water could get through on a regular basis even on unfinished render or lime render.
    Have to agree to disagree :). Painting render makes the problem worse. I have seen many many houses with solid walls which have water ingress through rich and/or painted render. The paint expands differentially to the render and cracks along the same fault lines. Water then gets in. Specialist (so called breathable) paints are claimed to be different...I don't know enough about them to put an argument one way or the other.

    Lime render partly works according to the absorption / evaporation principle I refer to above
    • CommentAuthorwoodgnome
    • CommentTimeJun 28th 2014
    I am going to have a look at a 200mm cavity tray to see what can be done.
    I may end up putting a rainscreen on part of the wall to eaves level. Just need to decide what it will look like. I take both Tony's and Mikes comments on board and i dont think i can risk omitting the trays without added protection of the render. Seems its a detail that doesnt get mentioned when discussing full fill insulation etc. Of course if i knew what i know now, the design would be different anyway.
    Posted By: woodgnomeThe standard trays ( i found a couple of old ones for a 100mm cavity) will prevent maintaining full depth insulation
    Semi-rigid pre-formed cavity trays don't need to go all the way across the cavity. There's no reason why you couldn't use a 100mm cavity tray in a 200mm cavity, especially if the upstand were supported by a piece of rigid insulation or high density mineral wool between it & the inner leaf.

    • CommentAuthorborpin
    • CommentTimeJun 30th 2014
    Posted By: woodgnomeAny ideas for stepped trays or something better?
    Have you actually spoken to any manufacturers? cavitytrays.co.uk for instance?
    • CommentAuthorwoodgnome
    • CommentTimeJul 1st 2014
    Spoke to Cavity Tray people. They knew what i wanted and can make a 103 mm wide tray and i can fit the inner upstand part vertical or diagonal across first 100mm cavity slab. The recommend diagonal and cut insulation to angle.
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