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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorPingy
    • CommentTimeNov 5th 2007
     
    To highly insulate the agricultural building that I'm converting I'm considering building a block inner wall to create a 250mm cavity and filling it with mineral wool. Is this the best way forward to achieve high insulation and still have internal thermal mass? Has anyone already done this?
    • CommentAuthortony
    • CommentTimeNov 5th 2007
     
    Great plan
    • CommentAuthorchuckey
    • CommentTimeNov 6th 2007
     
    I am interested in how you are intending to stop this mass of glass fibre from settling down into the lowest part of the cavity, or are you going to "punch it down" hard, so its compressed fully?
    With this amount of insulation, intersticular condensation is certain. I think you must use a vertical DPC on the inside of the blocks and small bleed holes in the lower edges of the outside skin. A point I have been been putting forward is that if you use a paint type vertical DPC and plaster over it, almost any fixing device will pierce it. It is far better to put the vertical DPC on the cavity face of the inner skin, so you can fix mirrors or power points with fixings (almost) as long as your blocks are thick (100 mm?)
    Another practical problem are the cavity ties, one, they are thermal bridges, two, Can you get them this long?
    The joist ends that go into the new block inner skin need rubber boots (to keep the aiir tightness of the construction.
    I wish you the best of luck.
    Frank
  1.  
    If timber frame cavity batts were used (140 + 90mm?) there shouldn't be a problem with sagging, but maintaining the integrity of the membrane on inside of the cavity while fixing purpose made wall ties seems tricky! I think I'd risk the tiny piercings on the inside, painting it on though after any chasings etc for services have been cut. Good idea though if space available, as the usual timber frame inner wall is more responsive, but has little thermal storage.
  2.  
    We've got a 250 cavity in one of our buildings at work which, along with dense concrete blocks and deep floor slabs, gives us a thermal mass bldg with a very low heat-load.
    • CommentAuthortony
    • CommentTimeNov 7th 2007
     
    Chuckey -- Sorry I disagree with the damp membrane -- never put an impervious later on the cold side of the insulation.

    Further I do not think that there will be any interstitial condensation in the cavity insulation.

    If it were mine I would use fibreglass batts which would not shrink move or settle.
    • CommentAuthorPingy
    • CommentTimeNov 7th 2007
     
    Thanks for the feedback everyone.

    Tony - I think you may have misread Chuckey's post. He did say install a damp proof membrane on the cavity face of the inner skin, therefore it would be on the warm side of the insulation.

    I was planning on using mineral wool cavity batts which should be self supporting. I agree with Charlie Duke that installing a damp proof membrane on the cavity face of the inner skin sounds very tricky, particularly as you've got wall ties to contend with. I take your point though Chuckey about having small bleed holes through the outside skin to allow any moisture to escape. Good idea about the rubber boots on joist ends.

    I've done a quick condensation risk analysis using BuildDesk's free sofware and it does show condensation forming on the inside face of the external skin but considers it acceptable as it would not be permanent. My model was very simplistic (230mm brick outer/ 250mm mineral wool batts / 100mm concrete block inner.

    With regard to cavity ties I know it's possible to get them for new build (Denmark use 300mm cavities) but I'm not sure how I go about getting ones that can be fixed to an existing wall. Has anyone got any ideas? Surely with this level of insulation the cold bridging of the cavity ties would be minimal?

    Is it really necessary for a damp proof membrane if the inner skin is dense concrete blocks with wet plaster coat? If it is necessary then what are the paint on types made of?
    • CommentAuthorjon
    • CommentTimeNov 7th 2007
     
    If you have an existing structural skin, why do you need cavity ties? Is the internal pressure within the building too high for the internal skin to work by itself?

    Cavity ties exist for structural reasons. If there is no structural reason, consider asking your Engineer to not introduce ties!
  3.  
    Is the building characterful or good to look at from the outside? I presume so, because if not you might be better externally insulating the existing structure. A timber clad finish (or even a sheet material like Onduline) would not look out of place if it is an agricultural building to start with. You would keep more internal floor area that way. Just a thought... maybe there would be planning issues?
    • CommentAuthortony
    • CommentTimeNov 7th 2007
     
    Simple cavity ties can be almost anything -- off cuts of barbed wire, helical stainless steel thingies, polypropylene is my favorite as no thermal bridging.
    • CommentAuthorjon
    • CommentTimeNov 8th 2007
     
    >>Simple cavity ties can be almost anything -- off cuts of barbed wire, helical stainless steel thingies, polypropylene is my favorite as no thermal bridging.<<

    Eek! Off cuts of barbed wire.

    Yes they can providing they can transfer the load through the skins (if built as a cavity wall to Code or if just following the simplified requirements of Building Regulations).. but if you introduce something that may have a significantly lower lifespan than the wall you may be raising the annualised (whole life) embodied content by needing replacements later (together with the extra cost this involves).

    Offcuts of barbed form the same function providing the euler length is right (ie the wire is thick enough) as galv ties but have a tendency to retain water because they are usually made of a twisted pair: This aids decay. Also galvanised is not the way the industry is going and you might have difficulty getting this through many regulators.

    Interstitial wall (rather then edge restraint) ties are only there, from a historical perspective, to reduce costs.
    • CommentAuthorPingy
    • CommentTimeNov 9th 2007
     
    I see your point Jon about possibly not needing cavity ties. I'll talk to my structural engineer.

    Has anyone got any suggestions as to what I could use for the inner skin as an alternative to concrete blocks? I was wondering whether compressed earth blocks or unfired bricks were viable alternatives.
    • CommentAuthorjon
    • CommentTimeNov 10th 2007 edited
     
    Just going off-the-wall for a moment: Why were you using an inner skin at all?

    An alternative could be just insulation fixed (glued) internally to your external skin and then facing direct onto the insulation (with light mesh strengthening pinned to the insulation and coated with plaster skin). Insulation can generally carry between 1 and 4 kPa so is a strong material in compression: (the deformation is the problem as it has a low modulus). Perhaps surprisingly, insulation 250-300 thick can be made to work as a semi-structural material! (particularly the grades known as 'hard': (though in this case glue can't be used as the fixing due to the differential movement) Wet finishes also aid with reducing air exchange rates. It can't be made to work as a structural material without a significant amount of work done on fire protection.

    This is likely to have a low relative embodied cost because you're doing away entirely with your inner wall and just leaving what would have been there anyway (insulation and skim). Also likely to be thinner. Needs careful consideration of vapour proofing (might not be required): Depends what your outer wall is made of and what the edge details are. You would need professional advice as you have to be careful of not creating thermal bridges (cold spots that attract consendation) and also to bve careful that you're not creating a situation that accelerated the decay of the external skin or those parts of the internal structure that are fixed to it.

    You will need professional advice if you're considering any of the above. It's difficult to tell without seeing the building and proposals.
    • CommentAuthorPingy
    • CommentTimeNov 10th 2007
     
    The reason I'm looking at building an inner skin is to create thermal mass within the insulated envelope. Of course in an ideal world the best way of insulating the building would be externally. Sadly we're not allowed to do this.

    The building is an 'L' shape with a courtyard. The main living area will be in the south facing leg of the 'L' and the bedrooms will be in the west facing leg. My concern with dry-lining / internal timber framing is the likely overheating in summer.

    Open to suggestions!
    • CommentAuthortony
    • CommentTimeNov 10th 2007
     
    Thermal mass and the solid feel are the way to.
    • CommentAuthorjon
    • CommentTimeNov 11th 2007
     
    Thermal Mass can be useful to balance daily load, particularly the first 30-50mm. After 50mm you'ld be looking at longer periods.

    If it's likely to overheat then do you have too many windows? Consider using brise-soleil or other canopies over windows particularly concentrating on the South and West facing windows.

    Thermal mass would keep the rooms that might overheat in that overheated state at the time you want to out the kids to bed on the west facing bedrooms..

    Be careful that you don't introduce something for the right reason that turns out to be a liability later: Particularly if you or your family needs cold air at night to sleep.
    • CommentAuthorjon
    • CommentTimeNov 11th 2007
     
    Also, If you are introducing a non-structural internal skin, you might consider incorporating internal thermal shutters in lieu of curtains/blinds
  4.  
    I spotted some items in Wickes the other day which looked as though they could be adapted for long wall ties. They are galvanised metal strips designed for floor joist strutting, predrilled at ends so one end can be bent at 90 deg and screwed to inside of outer leaf. At 450mm ish long and 35p each seem OK.
    The reason for needing ties is that an unsupported wall has defined max length & heights in Building Regs; even normal cavity walls cannot be more than 9m long without a buttressing wall.
  5.  
    So, Charlie, the building regs allow you build an un-buttressed cavity wall 9m long, presumably with 2 skins of 100mm block or brickwork. If you built the same length un-buttressed wall in a single skin of solid blockwork, what thickness of blockwork would be required? Do the regs give this information.

    Having just built a small extension out of externally insulated single skin blockwork, the building inspector seemed unsure about what thickness of blockwork was required in different situations. It seems to me that a cavity wall of 2 skins of 100mm can't be as good structurally as a 200mm solid wall, can it? That would suggest that, as a rule, a single skin solid wall should be able to be built at less than 200mm. Could you for example build a whole house out of an externally insulated single skin of 150mm blocks?
    • CommentAuthortony
    • CommentTimeNov 14th 2007
     
    Chris asked,"Could you for example build a whole house out of an externally insulated single skin of 150mm blocks? " I think yes is the answer.
  6.  
    Chris asked about wall thickness requirements for unbutressed 9m length. Well, it depends............. Does it act as retaining wall, is it parapet wall, is it part of a building and constrained at ceiling/roof level etc. Assuming normal residential building wall minimum thickness FOR SOLID WALL is 1/16 of storey height, so 2.4m ht needs to be 150mm thick. The thickness for cavity walls is calculated by adding together the leaf thicknesses + 10mm, giving 210mm in most cases. Walls 3.5 - 9m high x up to 9m long need to be 190mm thick, so normal cavity wall would be OK, but not 150mm solid. Therefore it seems that cavity is more structurally strong that 150mm solid, and whilst you could build a bungalow from 150's, you MAY not be able to build a house, but the regs are contradictory in places. There are special rules for different situations and small non-residential buildings, annexes and garages. Also the regs give requirements for the worst conditions of wind loading and if a structural engineer can prove the requirement is over the top you can normally go for less. The use of wind posts to reinforce walls or corners can also reduce requirements.
    When looking at INTERNAL walls with no wind loading in 2-storey housing you can build long lengths of 100mm masonry if they are loadbearing, as the load stiffens the wall and ties the top of the wall. In this case Pingy is building free-standing walls and therefore they should be tied back for safety.
    As you see it is not a simple subject.
    • CommentAuthorPingy
    • CommentTimeNov 19th 2007
     
    The walls will typically be approx. 2.5m high but up to 5m at the gable end apex. From what you said Charlie I'll assume cavity ties are required.

    Another question:
    Should I ventilate the cavity through the external wall to allow any moisture to drain/evaporate? If so then how much and where should I introduce it? Bear in mind I'm proposing a full-fill cavity with mineral wool batts plus the existing 9" brick walls have been built with lime mortar. What I don't want to do is cause frost damage to the external walls due to 250mm of insulation. I'm not sure whether the lime mortar will be good at wicking away any moisture on the inside surface of the wall, thereby eliminating the need for ventilation, or make ventilation necessary due to its ability to wick moisture in from the outside surface.:confused:
    • CommentAuthortony
    • CommentTimeNov 19th 2007
     
    I do not think that you will need to ventilate the filled cavity.
    • CommentAuthorPingy
    • CommentTimeNov 19th 2007
     
    Not even some drain holes at the bottom?
    • CommentAuthortony
    • CommentTimeNov 19th 2007
     
    No. -- What are the relative floor levels inside to outside?
    • CommentAuthorPingy
    • CommentTimeNov 19th 2007
     
    I'm sinking the inside floor level in one area to enable another floor to be inserted above. However I'm also planning on reducing the outside ground level in this area to ensure I still have 6" difference between the inside and outside floors.
    • CommentAuthortony
    • CommentTimeNov 19th 2007
     
    It is not normal to drain the cavity but then the cavity normally runs down to foundation level and yours is presumably stopping at concrete floor level?

    I might change my mind and suggest that you do drain it even though I do not think that there will ever be any water in it.

    Cut a slot 15mm deep at floor level and drill a 20 mm hole in every third prep joint. then insert a piece of dpc into the slot, peg it in from underneath and fold it up the wall then silicone it to the floor. After the first course of internal blocks are laid build it in in such a way that insulation can continue to the bottom of the cavity. that is across then up the block then trap it in the mortar under the second course.
    • CommentAuthorPingy
    • CommentTimeNov 19th 2007
     
    Sorry for my ignorance Tony but I'm having problems visualising what you've described. Is the 15mm deep slot cut horizontally into the inside face of the existing wall? What is a prep joint? Are the 20mm holes the drain holes to the outside? How do you peg the dpc? What's the idea of the silicone?

    This is where a picture says a thousand words!
    • CommentAuthortony
    • CommentTimeNov 19th 2007
     
    The idea is to catch any (unlikely) water that may get into the cavity and make very sure that it cant get in to the inner skin. So we put a piece of damp membrane ( 450 wide dpc) down the outside face of the bottom block, across the floor and tuck it into the outer wall as low as possible. then seal under it with silicone to stop any water soaking back towards the inside. The last step, the silicone, being impossible has to be done first.

    20mm holes go down hill from as low inside as possible pretty much where the cut is. A perp is a vertical mortar joint between two bricks. the slot is cut across the wall at floor level horizontally, ideally slightly downhill towards outside.

    If you still cant get it i ll send you a drawing, There is a concrete floor inside the walls yes?
  7.  
    Hi,

    Just found this by accident - Zedfabric super-insulation 300mm wall tie. Looks like what you are after. Google for Zedfactory and look under products - Zedfabric
    Cheers
   
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