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Energy Assessment: U value calculator that copes with Multifoil????

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  1.  
    • CommentAuthorNoyers
    • CommentTimeFeb 16th 2009
     
    The two "U value calulators" most often quoted here do not seem to cope with multifoil

    http://www.vesma.com/tutorial/uvalue01/uvalue01.htm
    http://www.celotex.co.uk/Other-Resources/U-value-Calculator

    Anyone know a solution? i am interested in calculating heat loss through the following construction: -
    600mm Stone (rendered both sides)
    25mm- 38mm air gap
    Multifoil
    25mm - 38mm air gap
    20mm Polystyrene
    12mm Plasterboard
    All joints sealed and vermin proof, (the stone wall will just have to breathe through the outer skin - I am installing (sealed) inspection sites so I will be able to tell you in due course, using an endoscope, whether there really is mould growth between the wall and the multifoil).
    PS External insulation impossible for several reasons.
    • CommentAuthorTimber
    • CommentTimeFeb 16th 2009
     
    Why not just use rigid?

    you could fit 100+ mm of rigid in that space!

    Timber
    • CommentAuthorNoyers
    • CommentTimeFeb 16th 2009
     
    Well, Timber, not quite so simple; walls are old stone, neither true nor straight so I would need a frame to mount the rigid insulation - the "out of true" dimensions are so great that dot & dab cannot be used but I take your point. The other minor problem is a source of reasonably priced rigid foam in SW France, here they have glass wool and multifoil.
    Off topic: they also claim that pointing the stone work them applying a coat of "hydrofuge" to seal the outer and the inner surfaces means you don't need to insulate stone walls! Not at all sure I follow the science behind this line of thinking. It also seems that these walls do not then need to "breathe" or perhaps the "hydrofuge" is a wonderfully clever waterproof but permeable to vapour - who knows?
    Noyers
    • CommentAuthorsinnerboy
    • CommentTimeFeb 16th 2009
     
    this calculator allows you to alter lambda values

    http://www.xtratherm.com/software/software.php
    • CommentAuthormark_s
    • CommentTimeFeb 16th 2009
     
    how thick are the walls?

    How much roof overhang?

    I'm thinking that if you are going to render the outside anyway, then you could look at external insulation - needs permission I think under french rules because it alters the appearance. Then use the solid walls as thermal store?

    certainly parge coat the inside with lime mortar whatever you do - just stop drafts
    • CommentAuthorNoyers
    • CommentTimeFeb 16th 2009
     
    Mark_S: Yes. sorry, should have said outside wall "pointed" not rendered; inner wall will have parge coat of lime mortar.
    House in conservation area so altering outside appearance not possible; roof overhang 150-200mm (walls curved), plus upper edge of wall finishes with a "genoise" (? spelling) so altering outside a nightmare anyway.

    Using walls as thermal store is fine idea but for the small matter of their incredible mass. Assuming walls average 600mm thick and density of limestone is 2600 kg/cu.m then just for this tiny old farmhouse which doesn't even have standing headroom over much of 2nd floor, the mass of above ground external walls is over 140 tonnes!!There are substantial internal walls too! If I had a figure for the specific heat of limestone I'd be willing to bet that the energy required to raise these walls from say 5C (today) to 20C would be enough to make any energy supplied smile.
  7.  
    You'd need around 536kWh to raise your structure by 15C - the specific heat capacity of limestone is 920 J/Kg/K Of course, you wouldn't be raising all 140 tonnes by 15C as there would be a thermal gradient from inside to outside. If we assume it's linear then perhaps you'd need just 268kWh to make your walls a comfortable temperature. You'd need a ten bar electric fire (10kW) blasting away for over a day to do this :) But what we didn't count was the thermal mass of the inside of your house - this is also a lot higher than most people think as, for example, wood has a higher specific heat capacity than limestone, even when allowing for its lower density (so the heat capacity per unit volume is greater).

    Paul in Montreal
    • CommentAuthorCWatters
    • CommentTimeFeb 17th 2009
     
    Isn't there a paper somewhere from the BRE that gives a measured figure for the U-Value of multifoil with air gaps?

    If you can calculate a U-Value for the rest just take the reciprical to get the thermal resistance, add up all the thermal resistances and take the reciprocal again to get back to a U-Value.
    • CommentAuthorNoyers
    • CommentTimeFeb 18th 2009
     
    C Watters: Well I have looked and can't find a resistance figure for multifoil.
    If you know any further details of the BRE paper I'd be interested.

    On a practical note I would strongly advise against using multifoil on wooden battens inside a stone wall - it's rather like building a new timber frame house but inside a wonky stone one, the labour element involved in making it airtight and vermin proof, is horrendous. It would certainly be a prohibitively expensive way to insulate if one had to pay for the labour. My advice would be the hemecrete (or equivalent) route; even as a fairly incompetent plasterer I would guess the hemwcrete type solution to be at least 10x faster than the method I am using.
    • CommentAuthorNoyers
    • CommentTimeFeb 18th 2009
     
    Paul in Montreal: Thanks for the specific heat of limestone figure. I have been meaning to calculate the mass of this stone ruin for some time, your calculation is interesting, I will go and get a bigger back of an envelope and recalculate the mass with less "guesstimation" and will include floors and ceilings.
    Not sure I follow your calculation:
    - Is assuming a linear temperature gradient through the stone valid?
    - Have you allowed for the U value of limestone (what did you use?)
    - Did you assume 5C outside, 20C inside?

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