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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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    • CommentAuthorFieldfare
    • CommentTimeJan 24th 2019
    Help! we are just doing 1st fix of our barn conversion. Things are going well but we are not 100% sure how to treat the internal gable wall in the picture (5m x 2m with a pitch to about 4m in a room of 10mx 5m) It is a 2ft rubble filled wall rendered on the outside with gappy lime pointing that all needs re-doing (lots of hairline cracks and bits falling off- air is seeping through). The simple and commonsense approach to create a warm house is to create a stud wall with 50mm insulation, membrane and gypsum board it up (U-Value of 1??). The alternative is to re-point in lime and leave exposed (U-Value of 2.5??). It might look good but are we just risking having a cold room which is going to be a pain and expense to heat? We already have a 5M triple-glazing of a U-value of 1. Also will it be expensive to properly point? Does anyone have any thoughts on how much it might cost (approx. 14m2 of smallish stones prob. 200M linear of pointing?). Thanks in advance for any thoughts.
      barn wall.jpg
    Can you put external insulation on the wall? I have done this on stone rubble walls with good results and no problems using the standard EPS and thin film render.

    Do you plan to render the internal surface? This would be recommended to stop the air flow through the wall.
    • CommentAuthortony
    • CommentTimeJan 25th 2019
    In all cases purge the inside , lime is best, then same u value as other walls, or 200mm insulation, vapour barrier linked hermetically to adjoining air tightness barriers and then plasterboard. Crucial to not leave any air paths to outside
    • CommentAuthorjfb
    • CommentTimeJan 25th 2019
    What is the barn for? Are you living in it permanently, renting it, holiday letting it?
    For any of these and especially if it will be your home why would you not want to insulate this wall? Building control would have something to say about it as you can't make it into a dwelling and not do so.
    If it were mine and external wall insulation isn't an option I would render the inside with lime (that is hydraulic not hydrated) , then put on put woodfibre boards and finish with another lime render on the boards. I have done that very successfully on a similar property I live in with 60mm thick boards (in theory doesn't quite reach required insulation values for building regs but you want to be careful about interstitial condensation and the real world results have been very good). I prefer this breathable buildup to the vapour control layer and studs/insulation approach for an old wall like that. But it is maybe a little more expensive/less common. Not that hard in reality though.
    Whatever you do it is going to cost some money! But you absolutely need to get some insulation in.
    AIUI the way a rubble wall works is quite different from a modern cavity wall. Moisture (from driving rain and condensing moist air from the inside) soaks into the wall and accumulates, especially in the the rubble fill. On dry days, the wall drys out by the moisture permeating out through the lime pointing. All those little cracks and draughts in the pointing are important for the drying, the wall must not be made airtight on both sides, it needs to breathe.

    External insulation changes this by stopping the rain and condensation getting in, allowing vapour to escape, but stopping the air movement through cracks from drying the wall.

    We had good results insulating internally with a ventilation gap between the insulation and the stone. However one section had been cement rendered externally by a previous owner and wool insulation installed hard against the inside of the stone, the wool was found to be wet as the wall could not breathe inwards or out.
    • CommentAuthorgyrogear
    • CommentTimeJan 25th 2019
    +1 for Willie

    Also (? more important ?), what is the nature of that floor slab ?

    • CommentTimeJan 25th 2019
    Posted By: WillInAberdeenAll those little cracks and draughts in the pointing are important for the drying, the wall must not be made airtight on both sides, it needs to breathe
    Not quite - 'breathing' emphatically doesn't rely on cracks, draughts, air-leakiness.

    Breathing is permeation of water vapour (gas, not liquid) through solid material. That solid material can (should) be completely airtight and permeation continues just the same.

    Some solid materials, like strong cement mortar and renders, are not vapour permeable so shouldn't be used.
    Use a solid material that is airtight but vapour permeable, such as lime mortar and render.

    If your outer face has cracks, draughts, it will prob be permeable to harmless water vapour, whatever it's made of. But it will actively soak up liquid water, likely faster inward than outward during a wet period. Bad idea.

    Make the wall airtight, vapour permeable.
    • CommentAuthorFieldfare
    • CommentTimeJan 25th 2019
    Hi all- thanks for your comments.To give a bit more info. this is going to be a 1st floor lounge of our house and the floor is going to be timber on top of ply that you can see now. We want it to be warm and cosy. We were toying with the idea of keeping it uncovered purely for aesthetic reasons. I understand that an uninsulated, rubble-filled wall will have a U-value of about 2.5W/m2? Adding 50mm of Celotex or similar insulation (with proper vapour barrier) will reduce that to just 0.4W/m2. Is that sort of correct? From an initial scan of U-Values I can't quite get my head around what the heat loss difference between the two is. An insulated wall will lose 6.25 times as much heat than an insulated one?
    • CommentAuthorgyrogear
    • CommentTimeJan 26th 2019 edited
    Posted By: FieldfareI understand that an uninsulated, rubble-filled wall will have a U-value of about 2.5W/m2? Adding 50mm of Celotex or similar insulation (with proper vapour barrier) will reduce that to just 0.4W/m2.

    It is probably more like U = 1.5.

    Meaning that theoretically the insulated wall you quote would have a heat-loss characteristic around four times better than the uninsulated stone wall.

    You might read this :

    (although the referred source seems to have permeated out...)

    and this :

    Fieldfare - Based on your wall size given above and assuming the u value given by Gyrogear of 1.5 this will give you a heat loss of about 450w with a 20 deg. diffidence between inside and out. This means a heat loss of, in round numbers, 11kWh per day.

    Basically the wall as it is (and assuming you can mitigate the through air flow) is worse than your windows! The wall and especially the corners is where the mould will grow. Apart from that the wall will be uncomfortable to sit near as you will feel the cold pulling the heat.

    If you want to keep the wall as a feature then (apart from the work of making the pointing look uniform and decent I would suggest that external wall insulation (EWI) with EPS is a must. Otherwise I would still suggest EWI as it will be much better for the wall structure to be within the heated envelope than outside.
    Remember that to comply with the strict 'letter of the law' re Building Regs you have to achieve a U value of 0.3W/m2K if the wall constitutes more than 50% of the external wall area in that room, unless one of the 'get-out clauses' applies. Are all the walls external? If so, then that wall probably isn't anywhere near 50% of the external wall area in that room.

    Historic Scotland have done a lot of research, including in-situ U value testing IIRC, to show that rubble-filled stone walls often perform better than is commonly supposed, but although the U value may not be as high (poor) as had been thought, with a wall in poor condition you may have significant ventilation heat-losses. You can't really mix up ventilation and fabric heat losses, but the effect of ventilation heat loss can be to make the wall (or other building element) perform worse (sometimes far worse) than the U value would suggest.
    • CommentAuthorphiledge
    • CommentTimeJan 26th 2019
    No experience and very limited knowledge of original stone walling, but if the build up is basically 2 walls with rubble infill, what would stop you taking down the inner "leaf" removing the rubble and replacing with insulation then rebuilding the inner stone leaf with screw in ties to the outer leaf. Retains the feature stone wall without loss of floor space.
    • CommentAuthorjfb
    • CommentTimeJan 26th 2019
    Too destructive of integrity of outer wall. Much simpler to insulate internally or externally (though yes you lose aesthetic character on one side)
    • CommentAuthorgyrogear
    • CommentTimeJan 27th 2019
    Posted By: jfbthough yes you lose aesthetic character on one side

    If necessary, a part of the wall could be left untreated, in a "memory-performance trade-off", constituting a sort of "picture on the past", showing the original state of the wall for the esthete, the curious or the nostalgic etc. With varnish protection nonetheless...

    I did same on a previous house we had, where the extension was added onto a beautiful granite gable wall, part of which we left visibule.

    gg, if you are doing that, how about framing it out and adding a high-spec double-or triple-glazed unit ?
    • CommentAuthorgyrogear
    • CommentTimeJan 27th 2019
    Er apologies, Nick, I was referring to the *INSIDE* face of the wall (:-

    But yes, U R right if the O.P. wants a Trombe Wall ! Brilliant idea, in fact !

    ''Er apologies, Nick, I was referring to the *INSIDE* face of the wall (:-''

    So was I.

    I was thinking IWI to the majority of the wall, with a 'viewing panel' as you suggested, but 'tweaking' your suggestion (with hi-spec glazing over the (internally) exposed stone) to improve the U value and mitigate the thermal bridge. Put a 3G pane in there and you could almost halve the heat-loss when comparing the glazed value with the un-insulated wall U value.
    • CommentAuthorgyrogear
    • CommentTimeJan 27th 2019 edited
    I get you this time -- a very wythe idea !

    • CommentTimeJan 27th 2019
    We have a single-glazed window in the external wall of our house. The only single-glazed window in the house! It's actually inside the conservatory (well, sun room but SWMBO calls it a conservatory) and its purpose is to show the straw inside the wall. It's a substitute for the lime render over the rest of the wall, so thermal performance is not too important.

    But yes, a double or triple glazed window over part of a stone wall would be a feature in itself.
    • CommentAuthorgyrogear
    • CommentTimeJan 27th 2019 edited
    sounds nice !

    reminds me of that song, "Grass is Glass, I Want my Gable Back..."
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