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Green Building Bible, Fourth Edition
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    • CommentAuthorgyrogear
    • CommentTimeFeb 6th 2018
     
    Posted By: Kenny_MI am still not clear on whether vapour open and breathable are the same thing


    https://www.mouldedfoams.com/eps-frequently-asked-questions.htm

    "EPS is said to breathe. What does this mean?"

    "The breathability characteristic of EPS refers to its ability to allow any absorbed moisture to escape when conditions change. It therefore reduces any tendency towards the formation of vapour dams.

    In applications where high humidity and temperature differentials are likely, a water vapour barrier such as plastic sheeting should be used. The vapour barrier is best installed on the warm side of the structural component, with the insulation as near as possible to the cold side".
    ==========

    Posted By: Kenny_MI am still not sure about the logic of having the gable wall heavily insulated while the north facing front is completely uninsulated.


    well, you are still saving on energy-losses through the gable ! Total energy losses will be lower !

    Better to insulate half a roof, than no roof at all !
    (because energy losses are expressed as watts of passage (by number of square meters of envelope) (by temperature difference).

    so if the outside temp falls by two degrees, inside heat will tend to move outwards, but the bigger the shield, the less energy will move...

    gg
    • CommentAuthordelprado
    • CommentTimeFeb 6th 2018
     
    breathable has no meaning so ignore it.

    In my mind there are two ways to build a building

    1. Judicious use of VCL internally, backed up with lime or similar based products to help any moisture get to the outside (the VCL merely slows this down, rather than blocking)

    2. Taken from the building principles of yesteryear, do nothing to interrupt the passage of moisture. I think this is more idiot proof, and relying on proper ventilation to ensure there isn't excessive internal moisture. In my case I am relying on wet plaster for air tightness, and a MHVR for ventilation.

    Air tightness is much more important - because its such an easy path to, say, a joist end to rot it with moist air. I have no data but my assumption is that the water carrying through a wall is fairly low as its limited to diffusion.

    Re the comment by diathonite being bad to work with, etc. I dont think it matters so much more the base coat - but I am sure with all that cork in it doesnt trowel as well. But I think its a moot point because their finish plaster is just a normal NHL top coat, I think, plus you could always use what you want here.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 6th 2018
     
    Posted By: Kenny_Mby "breathable EWI" do you mean EPS or a natural insulation like the Diathonite/wood fibre etc?
    Both.
    Posted By: Kenny_MI am still not clear on whether vapour open and breathable are the same thing ...
    Anything that's water vapour permeable is also often called 'vapour open' or 'breathable' (in declining order of precision!).
    Posted By: Kenny_M... or if EPS is breathable in the way that is normally meant when talking about lime mortar/wood fibre etc?
    Yes. But all these materials have different levels of water vapour permeability - more permeable means more vapour open, more breathable. They vary from highly to moderately permeable, but all those mentioned are permeable enough.

    CO = Conservation Officer.

    If I were you, in this case I'd just strip the cement render and repoint to a high standard (which won't be cheap) with lime mortar. Anything else will be a half-measure with gaps, until you can impliment an insulation strategy that works for the whole house (even if you do it by stages). You'll have to face the CO anyway at that stage. Meanwile, tho you may have given away the lever that the wall's already rendered, at least it will be sound, dry and therefore less lossy that at present.
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2018 edited
     
    An oft-quoted reference about breathability, what it means and how it affects things is Neil May's Breathability article, which I think has been quoted on here before but perhaps bears repetition.

    http://www.sustainablebuildingresource.co.uk/fileserve/fileserve/1575/4426eed166fdc96b93c00c7eb841e08388706c39/

    There's an awful lot to absorb in it :bigsmile:

    Edit: note that this is quite a long article, not the short one sometimes seen.
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    Thanks for the comments folks, lots to think about.

    I was intrigued by the "EPS as vapour permeable as wood" thing and did a search on this. I found a PDF with this very title, published by the EPS manufacturing industry. They were using the vapour diffusion resistance coefficient μ value to show that EPS had similar breathability to wood. However, looking at their own figures pasted below show that EPS is also about as vapour permeable as some forms of concrete.

    Examples for μ-values:
    Air ............................... μ = 1
    EPS ............................. μ = 50 - 60
    Wood (spruce) ............ μ ≈ 54
    Concrete ..................... μ = 50 - 100
    Glass ........................... μ = 10.000 (I presume this means glass wool)
    PE-foil (0,1 mm) .......... μ = 65.000

    Comparing that to that quoted for example:
    Diathonite Evolution = 4
    Diffutherm wood fibre boards = 5

    If these figures are correct then "EPS as vapour permeable as wood" could have as easily have been "EPS as vapour permeable as concrete" although that probably wouldn’t have sounded as good in their marketing.

    It seems from the above figures that EPS is not particularly vapour permeable, in comparison to the products listed above. I suppose the real question is though, whether this matters. My understanding of how old stone buildings work, is that vapour enters the stone mainly from rain and this breathes out through the lime mortar. However, if there is a less permeable material applied to the exterior then provided the wall is dry to begin with then it shouldn’t need to breathe to the same extent. Dampness will also rise from the ground, as a house the age of mine won't have had a damp proof course, and escape through the stone and mortar, but as long as the base of the wall isn't covered this should also evaporate out.
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    gryro - Regarding the insulation covering part of the house. I get that, notwithstanding the concerns I have already raised about creating a situation where there will be one much colder wall facing north, insulating part of the exterior will reduce the energy loss, but for me its more of a case which measure gives me the biggest bang for the cash I have available to me as I only have limited funds. If I had unlimited cash, and planning consent, I would insulate the entire 3 walls tomorrow.

    Insulating one wall at a cost of something in the region of £5k might reduce my energy consumption by say 5%, but as heat loss is proportional to the temperature difference between the surfaces, and most of the rooms that border that wall are kept fairly cool, I doubt that it would be more than 5%. If I spend the same money on a condensing boiler I would reduce my energy usage by around 30%. The same £5k again would get me new windows and CWI in the extension, which as its where we spend most of our time, and its heated at a higher temperature, would also be a bigger gain in energy consumption.

    Someone mentioned in a previous post that I would be paying for not getting EWI but that is simply not the case. At current energy costs I wont even make back the cost of the replacement boiler until it is near due for replacement.

    I think my strategy needs to be to put the money I do have into improving the modern extension, as insulating this type of construction is probably more well understood, and its the main living area so its where there is most to be gained.
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2018 edited
     
    Posted By: Kenny_MMy understanding of how old stone buildings work, is that vapour enters the stone mainly from rain and this breathes out through the lime mortar. However, if there is a less permeable material applied to the exterior then provided the wall is dry to begin with then it shouldn’t need to breathe to the same extent.

    I suggest reading Neil May's article.
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    Thanks djh, for the article, and the pun...

    I have only had a quick scan as I am supposed to be working, but the opening gambit about breathability is quite good. I know I keep using the term breathability, but I am aware that this in practice means vapour permeability, as opposed to air.

    Interestingly, pages 21-22 describe one of the main issues that I am concerned about which is the inability of polystyrene to draw the rising moisture from the walls and pass it to the outside. May suggests that polystyrene insulation prevents this vapour transfer, while wood fibre draws it out and transfers it to the outside.

    It's easy to forget that the old part of my house will almost certainly not have a damp proof course so there will be moisture rising from the ground that needs to be dissipated.
  1.  
    Posted By: Kenny_MI think my strategy needs to be to put the money I do have into improving the modern extension, as insulating this type of construction is probably more well understood, and its the main living area so its where there is most to be gained.

    That's sound thinking. But if the gable wall needs something doing to it then give the choice between removing the cement render and re-pointing, hoping to get the CO to agree to insulating later or removing the render and replacing it with EWI - I would go the EWI route. I suspect there ma not be much to choose between each on cost. Of course if the wall can be left alone for a time then it makes sense to do the big gains first.

    Posted By: Kenny_MIt's easy to forget that the old part of my house will almost certainly not have a damp proof course so there will be moisture rising from the ground that needs to be dissipated.

    The stone house that I insulated has no damp course and rising damp has never been a problem except on one internal wall which has tiled floors both sides. The makeup of the walls are 50cm thick with stone facings internal and external with rubble infill. The mortar, if you can call it that, is sandy earth with some lime in the facing skins with sandy earth (no lime) filling in with the rubble. The basalt stone breaks drill bits and anything put into the mortar quickly falls out. It is virtually impossible to hang anything on the walls.
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    Thanks Peter. I think one of the problems with old houses is that the materials in use are so variable. You are talking about stone and I am talking about stone, but your basalt is breaking drill bits and my sandstone is soft and almost crumbly to touch. I am sure that these two types of stone have very different vapour permeabilities, as even different types of sandstone have wildly varying properties, and even the ground they are built into have different properties and moisture content.
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2018
     
    Posted By: Kenny_MI am sure that these two types of stone have very different vapour permeabilities

    For 'rising damp', it's mainly the capillary properties associated with liquid water that matter rather than the vapour permeability.
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    Posted By: djhFor 'rising damp', it's mainly the capillary properties associated with liquid water that matter rather than the vapour permeability.


    Accepted, but it is still likely to be different. No two types of stone are going to be exactly stone, are hard stones not less likely to be porous than softer stones?
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 6th 2018
     
    Posted By: Kenny_Mpages 21-22 describe one of the main issues that I am concerned about which is the inability of polystyrene to draw the rising moisture from the walls and pass it to the outside. May suggests that polystyrene insulation prevents this vapour transfer, while wood fibre draws it out and transfers it to the outside.
    Can anyone imagine what the great May is on about here? Reads like sales-pitch pseudo science to me (like super-efficient on-peak electric radiators).

    Water vapour is harmless until it condenses as liquid water - or until something obstructs existing or rising liquid water content from evaporating to vapour.

    There is a plottable gradient of water vapour resistances (or resitivities) through the thickness of a wall, which marginally affects the local concentration of water vapour, but the biggest determinant of concentration (hence local proximity to dew point) is the total vapour resistance of the wall. EPS does add to that, but its contriubution is usually diluted/dwarfed by other materials/layers in the make-up. It doesn't make much difference whether its contribution occurs inboard or outboard in the sandwich.

    The old rule of thumb 'outboard resistance (resistivity?) should be one third (some said one fifth) of the inboard resistance (resistivity?)' is discredited, replaced by WUFI calc. (My guess is that its application led to the interstitial condensation disaster at the pioneering near-PH Primary School at Dartington, now demolished and being rebuilt with insurance money, reputation of promising young Architectural practice ruined).
    •  
      CommentAuthordjh
    • CommentTimeFeb 6th 2018
     
    May isn't talking about vapour transport, he's talking about liquid water, and specifically about the hygroscopic properties of woodfibre. As regards vapour, he's very careful to actually blame the render whilst making it appear he's critical of the EPS, if you read carefully. (IMHO, anyway).
    • CommentAuthorKenny_M
    • CommentTimeFeb 6th 2018
     
    Tom - Surely its not unreasonable to suggest that water is drawn up from the ground through stone walls? Is this not the whole purpose of the damp proof course, to prevent that capillary action in porous materials, by placing an impermeable barrier between the foundation walls and the ground floor?

    If it is accepted that the damp proof course is performing a function, then what happens in houses that don't have a damp proof course? Presumably the water will either have to evaporate out of the walls as vapour or build up to saturation point. Anything that provides a resistance to the transfer of vapour will surely increase the likelyhood of the walls becoming saturated? As EPS has a vapour resistance of something like 10 times that of say wood fibre, and infinitely more than having no additional cladding on the walls, then surely its not unreasonable to suggest that it could impede vapour transfer, cause the walls to saturate, and seek to exit the wall somewhere less desirable.

    There are some areas in the old part of my house where I believe I am seeing this effect. Varnished wood floors over the top of solid stone floors, paint and cement over the sandstone on the outside, modern impermeable paints over everything on the inside. The water in the stone saturates and starts to rot the skirting boards and door frames from the inside. None of the above would likely be a problem in a modern house with a damp proof course, but if there is no damp proof course the water has to exit the walls somewhere.
    •  
      CommentAuthorfostertom
    • CommentTimeFeb 7th 2018 edited
     
    Absolutely, capillary action can draw liquid water up a wall - in your case anyway. Coating the wall with impermeables, as in your last para, will certainly prevent drying of the masonry - but those impermeables are a v far cry from the very or moderately permeable insulations etc we're talking about.

    In a breatheable wall, such drying is hopefully continuous, but can include seasonal fluctuation. As long as it more or less re-dries every year, that's all you can ask of a damp wall, and is fine. That seasonal re-drying usually relies on drying inward as much as outward - which designs with inboard vapour barrier obstruct. The danger comes when the wall doesn't re-dry enough and liquid accumulates year on year.

    Looked at in that seasonal re-drying light, vapour permeability doesn't have to be ultra, just a long way from impermeable.

    I love hygroscopicity but May doesn't explain how it would help in this kind of case.
    • CommentAuthorgoodevans
    • CommentTimeFeb 9th 2018 edited
     
    Posted By: Kenny_MGlass ........................... μ = 10.000 (I presume this means glass wool)
    PE-foil (0,1 mm) .......... μ = 65.000
    sorry a couple of days late - the glass and foil numbers are 10,000 and 65,000 I think - in parts of europe the period is used for the thousands separator.
  2.  
    Posted By: goodevansthe glass and foil numbers are 10,000 and 65,000 I think - in parts of europe the period is used for the thousands separator.

    Yup - over here we use the period for the thousands separator and comma for the decimal separator (as does a lot of Europe). It can play havoc with, say, data imported into something like Excel.
    • CommentAuthorKenny_M
    • CommentTimeFeb 9th 2018
     
    Ahhhh, now it makes sense.

    Thanks guys, now that you mention it I have seen that notation before in European docs. I was baffled looking at that and thinking that those numbers couldn't be possible, hence the reason why I presumed they meant glass wool!
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