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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    •  
      CommentAuthorfostertom
    • CommentTimeOct 27th 2010
     
    Posted By: WatchItyou would need more than, equiv 80mm diam hole
    I think I'd sooner be on the lee side of a curtain of glassfibre roll, than a brick wall with 80 diam holes @ 1m c/cs. Anyway, neither could be called significantly airtight.
    • CommentAuthorMike George
    • CommentTimeOct 27th 2010 edited
     
    Posted By: WatchitBut Mike, I see that as the point in using a steady state test method. Yes granted the induced air flow may or may not be similar to typical conditions in situ, but at least it is giving a level playing field on which ther performances are based. Otherwise you would have a different air flow for each product tested.


    Hi Watchit,

    The point I am trying to make is that until the NPL report, hot plate testing took no account of air infiltration, meaning that any u-value derived did not [and does not] either. This is still the case I believe so while the heat loss attributable to conduction may well be tested on a level playing field, any losses due to air infiltration are not.

    So assuming that multifoil has less ventilation losses than say, mineral wool [or indeed poorly fitted mass insulants], then the real life combined conduction and ventilation losses may be very different from what the steady state derived u-value indicates.

    This criticism of hot plates resulted in the method used in MAT8, although as I indicated above, the methodology is not detailed enough to be verified as being indicative of real life conditions, in fact according to the BBA referred to above, it is not. Furthermore, because of the lack of detail, any such test cannot be duplicated - therefore it is flawed.

    A final point I would make is the restrictions imposed by the Authors of MAT8. It is expressly forbidden to quote even a single sentence. Why is that I wonder? It certainly makes it very difficult to even pass comment on the report, never mind make an official response.
    • CommentAuthorbiffvernon
    • CommentTimeOct 28th 2010 edited
     
    Posted By: fostertomand you in particular Biff have consistently stonewalled alternative interpretations of physics that have been offered. 'Stonewalled' means not disputed - just ignored.


    No, I've never been keen on alternative interpretations of physics. Best ignored. The standard model works just fine for everyday purposes. Cold fusion? Zero point energy? Of course one has to keep an open mind. Science is a work in progress.

    Seriously though, this whole debate should be a trivial matter to settle from a theoretical consideration. I'm not a physicist but even my limited understanding shows that while the outer foils can contribute a significant insulation effect the internal foils suffer rapidly from diminishing returns making multifoils an economic rip-off. And, as I've said many times and nobody has demonstrated me wrong, yet the point still gets repeated, the size of air gap next to the mulitfoil has no bearing on the performance of the multifoil itself. The foil only 'knows' about the first half light wavelength.

    I've just acquired a set of merino wool vest and long-johns (base layer, the marketing folk call them these days). This material could be a serious rival for aerogel and pretty green if you like the look of New Zealand farming.
    • CommentAuthorMike George
    • CommentTimeOct 28th 2010 edited
     
    Biff,

    Just to say the size of the airgap does indeed make a difference - at least according to manufacturers hot box results. It seems 20-25mm is around the optimum and R-values of up to 1.9m2K/W have been demonstrated. Take away the airgap and the R-vale drops. Don't ask me to explain why this is the case because I cannot.

    Edit:

    Thinking more about this. Why would the manufacturers [and indeed BBA Certification] insist on airgaps if they make no difference to hot box tested performance? To do so would undermine their own marketing position relating to headroom savings in loft applications.
    •  
      CommentAuthorfostertom
    • CommentTimeOct 28th 2010
     
    Posted By: biffvernonthe size of air gap next to the mulitfoil has no bearing on the performance of the multifoil itself. The foil only 'knows' about the first half light wavelength.
    That is true as far as I'm concerned - no dispute - in fact my boffin colleague is currently exploring the theoretical possibility (not in a MF context) that even that gap is unnecessary.
    Posted By: biffvernonwhile the outer foils can contribute a significant insulation effect the internal foils suffer rapidly from diminishing returns
    That is the point on which you've 'stonewalled' - actually I now recall, it's nothing to do with interpretations of laws of physics!

    Is see that if you go 5% transmitted of 5% transmitted of 5% transmitted you get into diminishing returns and yes, only the outermost layers would have any significant effect. But in saying that you've ignored the 95% reflected from layer 2, which doesn't whistle off into space but is 95% re-reflected back when it hits layer 1 and is returned for another go at layer 2. At that point 5% of that 95% that was previously rejected, does get through layer 2 - and so on multi times over. As far as I can see, that does mean that the total loss from layers 1 to 6 is in fact evenly distributed between the layers, so each does play a full part.

    This simple point that I've made, you've simply stonewalled - you don't indicate that you've read it, that you understand my point, whether you agree/disagree, or why. This allows you to maintain unchallenged this insight that you've been promoting for what - 4yrs?

    It's still true that another kind of diminishing returns still applies - same as adding sequential layers of 50 EPS - later layers have diminishing effect on the total - but that's not the kind that you were claiming.
    •  
      CommentAuthordjh
    • CommentTimeOct 28th 2010
     
    Paul in Montreal: "It may be old, but the problems that Tom claims are being ignored are right there in the research back then"

    Ah, I hadn't realized you were posting it purely as a response to Tom's resurrection from the grave of his dynamical inadequacy obsession. Please accept my apologies!
    • CommentAuthormarktime
    • CommentTimeOct 28th 2010
     
    Mike, isn't that what you'd expect: with the air gap contributing to the thermal resistance the latter would increase with width. The peak effect may be some other factor coming into play as the air gap is increased.
  1.  
    Yes, agree, in theory [still] air has a very good thermal conductivity, allthough once convective currents come into play the resistance diminishes. For some reason others [Biff and Tom?] discount the thermal value of an air gap in this context, though, I assume, accept it say a double glazed unit scenario
    •  
      CommentAuthorfostertom
    • CommentTimeOct 28th 2010
     
    Posted By: djhhis dynamical inadequacy obsession
    I'm proud of my obsession - keeping it alive because something tells me there is something there.
    Posted By: Mike Georgeothers [Biff and Tom?] discount the thermal value of an air gap
    Not at all - I just don't want to rely on it in 'why multifoils work'.
    • CommentAuthorbiffvernon
    • CommentTimeOct 29th 2010
     
    Posted By: Mike GeorgeYes, agree, in theory [still] air has a very good thermal conductivity, allthough once convective currents come into play the resistance diminishes. For some reason others [Biff and Tom?] discount the thermal value of an air gap in this context, though, I assume, accept it say a double glazed unit scenario


    Of course air is a good insulator. The point is it doesn't become a better insulator because it's next to a bit of foil. And you can't say multifoil is great when it's the air outside the multifoil that is providing the benefit.
    • CommentAuthorMike George
    • CommentTimeOct 29th 2010 edited
     
    Posted By: biffvernonAnd, as I've said many times and nobody has demonstrated me wrong, yet the point still gets repeated, the size of air gap next to the mulitfoil has no bearing on the performance of the multifoil itself. The foil only 'knows' about the first half light wavelength.


    Posted By: biffvernonOf course air is a good insulator. The point is it doesn't become a better insulator because it's next to a bit of foil. And you can't say multifoil is great when it's the air outside the multifoil that is providing the benefit.


    So to clarify then, you accept that the airgap next to the foil is reducing the heat lost from the conditioned space, but you think the size of the airgap makes no difference?

    Also if you follow your logic in the latter quote, then you also cannot say double glazed windows are great when its the air outside the glass that is providing the benefit. Better start a thread about double glazing snake oil sellers:bigsmile:
    • CommentAuthorCWatters
    • CommentTimeOct 29th 2010 edited
     
    Sounds like you are slightly at cross purposes.

    The size of the recommended unventilated gap between multifoil and plasterboard makes no difference to the performance of the multifoil _itself_ but obviously does effect the performance of the overall solution. The size of gap doesn't effect the heat radaited off the foil (the half wavelength argumant) but obviously does effect the amount of heat conducted through the air gap (the air is a good insulator argument).

    Pehaps also worth adding that the recommended gaps for Triso-super 10 aren't 25mm. That's the size of the battens they recommend either side. The foil is nominally 30mm thick where uncompressed so the air gap could be 25-15=10mm. Page 3..

    http://www.insulation-actis.com/documentations/12pdf3.pdf
  2.  
    Thanks Colin,

    The foil I was referring to isnt tri-iso, its a competitor. And according to the manufacturers [Euroform] hot box test results [which model not just the multifoil but the airgap and adjacent plasterboard etc] the size of the air gap affects the results. They have found the optimum to be around 25mm and this is reflected in their BBA documentation, which is currently being updated as a result of recent product changes and test results
    •  
      CommentAuthorfostertom
    • CommentTimeOct 29th 2010 edited
     
    Posted By: CWatters25mm. That's the size of the battens they recommend either side. The foil is nominally 30mm thick where uncompressed so the air gap could be 25-15=10mm
    No - the air gap will unavoidably be 25-30 = -5mm - the MF will be compressed 5mm and in intimate contact with both adjoining surfaces. In other words the usual batten recommendations are rubbish.

    A nom 50mm batten, prob actually 45 or 47, stands a chance of avoiding too much surface contact, but still rubbish.

    I'd use 2 x nom 38 cross battening, prob actually 2 x 35. That pretty well guarantees that contact will only be with the criss-cross of battening, and keeps it pulled away from the adjoining surfaces. But still fairly rubbish.

    What was under developoment, until industry conspiracy destroyed the MF market, was stand-off brackets (a bit like cav wall ties that restrain partial cav fill) that wd keep the MF taut and centred in the cavity, compressed/in contact only at the clips - and the clips wd be avail with a variety of endings to accept various board facings, and at their other end a variety of endings to mount to various backings. This wd make MF's claimed insulative value avail for fitting within a trad 50mm masonry wall cavity.

    Ah, what might have been.
    •  
      CommentAuthordjh
    • CommentTimeOct 29th 2010
     
    Posted By: fostertom
    Posted By: djhhis dynamical inadequacy obsession
    I'm proud of my obsession - keeping it alive because something tells me there is something there.

    Good for you; keep it up, Tom :bigsmile:
  3.  
    Posted By: fostertomWhat was under developoment, until industry conspiracy destroyed the MF market, was stand-off brackets (a bit like cav wall ties that restrain partial cav fill) that wd keep the MF taut and centred in the cavity, compressed/in contact only at the clips - and the clips wd be avail with a variety of endings to accept various board facings, and at their other end a variety of endings to mount to various backings. This wd make MF's claimed insulative value avail for fitting within a trad 50mm masonry wall cavity.


    I thought you espoused the virtues of EWI for new builds? Surely the cavity wall is not suitable for passivhaus-level insulation. And for a retrofit, I can't see how you'd get the MF in there in the 1st place, spacers or not. BTW, such brackets are available for wrapping ductwork in foil-faced "double-bubble" type insulation.

    Dunno why everyone is getting so bent out of shape on air-gaps when you all have mentioned the correct spacing necessary in double and triple glazing for optimal performance. MF with airspaces is conceptually identical to low-E coated double/triple glazing. If that doesn't have magical insulative properties (and it doesn't nor does anyone claim that it does) then neither does MF. Of course, the foil facings do reduce some radiative transfer and I posted some technical papers earlier which solve the energy flow equations for such configurations (and were used to correlate against actual measurements in real test buildings over several months). The configuration of the airspaces is quite important here as convection effects make a difference.

    Paul in Montreal.
    • CommentAuthorMike George
    • CommentTimeOct 29th 2010 edited
     
    So we can all agree that the size of the air gap does make a difference? its how much of a difference that is moot [this being why the manufacturers have started to use hot box testing to optomise]
    •  
      CommentAuthorfostertom
    • CommentTimeOct 29th 2010
     
    Posted By: Paul in MontrealSurely the cavity wall is not suitable for passivhaus-level insulation
    True, it's not, but with MF's claimed values, and no doubt further improved ditto under development, then trad 50 cavity or perhaps 100 might make a comeback, which wd delight a lot of people.
    Posted By: Paul in Montrealfor a retrofit, I can't see how you'd get the MF in there in the 1st place
    Quite - I wasn't suggesting that.
  4.  
    Posted By: Mike George
    Posted By: biffvernonOf course air is a good insulator. The point is it doesn't become a better insulator because it's next to a bit of foil. And you can't say multifoil is great when it's the air outside the multifoil that is providing the benefit.


    So to clarify then, you accept that the airgap next to the foil is reducing the heat lost from the conditioned space, but you think the size of the airgap makes no difference?


    Sometimes I despair. What aspect of the fact that still air is a good insulator do you not understand, Mike?
  5.  
    I don't understand your response to the very simple question I asked, and which you have highlighted. A simple yes or no will suffice. Or perhaps you are stonewalling as Tom suggests?
    •  
      CommentAuthordjh
    • CommentTimeNov 1st 2010
     
    Posted By: Paul in MontrealSurely the cavity wall is not suitable for passivhaus-level insulation.

    Depends what you mean by suited. It wouldn't be my first choice, but it works for this certified passivhaus:

    http://www.greenbuildingstore.co.uk/page--denby-dale-passivhaus.html
    •  
      CommentAuthorfostertom
    • CommentTimeNov 1st 2010
     
    Posted By: Mike GeorgeOr perhaps you are stonewalling as Tom suggests?
    To me, stonewalling means no response/no acknowledgement at all - not mere diversionary/slipperiness.
  6.  
    :clap:
  7.  
    Posted By: Mike GeorgeI don't understand your response to the very simple question I asked, and which you have highlighted. A simple yes or no will suffice. Or perhaps you are stonewalling as Tom suggests?


    Crackers! Of course still air is a good insulator. You know that, I know that, every man and his dog know that. The thicker the air layer (provided you can keep it still by adding something woolly or foamy) the more insulation you get.

    And it has nothing whatsoever to do with how a piece of aluminium foil behaves. To claim that multifoil is good so long as it has an air layer next to it is like saying a shirt is really good for keeping you warm so long as you wear a woolly jumper on top of it. Daft.
    • CommentAuthorMike George
    • CommentTimeNov 1st 2010 edited
     
    OK Biff, lets be serious for a moment. I'm not sure you are following what I am saying, so I'll try again.

    Firstly lets draw a distinction

    A hot plate measures heatloss through a single layer of material
    A hot box measures heatloss through a multitude of layers of different materials, foil, wool, air whatever.

    In this way the contribution an air layer makes to the overall thermal resistance can be measured holistically, and hence the u-value can be derived.

    What I am saying, is that there is an optimum thickness of air between a multifoil and the plasterboard [on the warm side] and that this has been demonstrated by a hot box test. This width is around 25mm.

    I said this in response to your comment:

    Posted By: biffvernonAnd, as I've said many times and nobody has demonstrated me wrong, yet the point still gets repeated, the size of air gap next to the mulitfoil has no bearing on the performance of the multifoil itself. The foil only 'knows' about the first half light wavelength.


    Now, I'm not saying that the multifoil knows anything, but the fact remains that the size of the gap does affect the overall performance of the OVERALL construction. Whether this is purely down to the stillness of the air, [and lets face it its not the same as a sealed double glazed unit]or some other factors I know not

    Would you agree with that?
  8.  
    Posted By: Mike GeorgeWhether this is purely down to the stillness of the air, [and lets face it its not the same as a sealed double glazed unit]or some other factors I know not
    How is it any different from a sealed double glazing unit (except for the lack of argon). Even the low-E coating on the glass is essentially the same as the out foil layer on the multifoil?

    I've already posted links to papers that model such cavities with foil-faced insulation. They physics is well understood and correlates well with actual test enclosures. There is no mystery here!

    Paul in Montrela.
  9.  
    Paul, my point was in response to Biff's insinuation that the size of the air gap makes no difference.

    I have not read your links, sorry. I will endeavor to do so if I get time. Perhaps you can tell me if the studies you mention encapsulated the air adjacent to the foil in the same way that a factory sealed double glazed unit does?
  10.  
    Posted By: Mike GeorgePerhaps you can tell me if the studies you mention encapsulated the air adjacent to the foil in the same way that a factory sealed double glazed unit does?
    In a similar way - they treated the cavities formed between the drywall and adjacent insulation as entirely surrounded by wood strapping and essentially sealed to moisture transport. They did model convection in the cavities (and, of course, found a difference between cavities that had horizontal straps versus vertical, for the same overall cavity volume. The vertical spaces were worse, of course, because the convection had more vertical space to circulate. They also modelled the radiant barrier effect of any foil facings on the insulation and did models for both faced and unfaced insulation, finding that the foil facing does improve performance by around 10% on average.

    Posted By: Mike GeorgePaul, my point was in response to Biff's insinuation that the size of the air gap makes no difference.
    Biff's insinuation, if he'll pardon me, is that the airgap makes no difference to the radiant barrier effect of the foil, so long as there is a gap. Of course, for convection, it does make a difference, as is the case with double/triple glazing, though the thickness of the gas gap does depend somewhat on the gas used.

    Paul in Montreal.
  11.  
    http://www.eng-tips.com/threadminder.cfm?pid=391

    I think these guys would put paid to alot of these questions very quickly... :)
    • CommentAuthorMike George
    • CommentTimeNov 2nd 2010 edited
     
    Thanks Paul,

    Being pedantic, similar is not the same. It is why I said:

    'Now, I'm not saying that the multifoil knows anything, but the fact remains that the size of the gap does affect the overall performance of the OVERALL construction. Whether this is purely down to the stillness of the air, [and lets face it its not the same as a sealed double glazed unit]or some other factors I know not'

    Now, since my question ws directed specifically to Biff, can I respectfully ask that you allow him to carry his own debate.
   
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