Ta mate

Posted By: biffvernonthe radiator may be up at 50 degrees and more

And I'm at 37 degrees (core), say 32 skin surface (does anyone know?), a differential of 13-18 degrees, in same league as UK building calcs 0-21 degrees, therefore directly comparable.

Johan, you're right that gross radiant emission from a (black) body, disregarding any return radiation from surrounding bodies, is proportional to 4th power of its absolute temp. But in practice, it's the nett radiant transfer between two bodies of only slightly dissimilar absolute temp (one emitting slightly more than the other), that we sense and buildings experience. Applying the 4th power law, www.greenbuildingforum.co.uk/forum/index2.php?DATEIN=tpc_tuzncnnui_1170338947 shows that the radiant transfer between two such bodies is, surprisingly simply, 'near enough' proportional to the simple first power of the temp difference between them, regardless of mean absolute temp of the system (the delta-t of radiator output calcs). With the important proviso that this 'near enough' approximation only holds good within a narrow range of absolute temp, e.g. 273K to 294K as experienced by UK buildings.

Posted By: johanHow do the manufacturers derive a k/R-vaule for multifoil?

That's the very key question, to which hangs a long and ornate tale. The answer is unexpected and revolutionary, not at all self-evident as your rhetorical tone implies. 17 European companies manufacture multifoils and, having just now reached agreement on protocol, the testing houses of 7 European countries are begining construction of between 13 and 20 very expensive test rigs. US, Canada, Australia have agreed to sit back and watch, for the time being. Multifoils will be tested against conventional insulants, Cellotex etc as well as mineral wool. Each rig has dozens of thermocouples recording at frequent time intervals, the 'like real life' dynamically varying test cycles will run over months, or a year. Under investigation will be actual energy loss, and old-fashioned slide-rule-era concepts like R-value won't get a look in. The huge data sets will be number-crunched to reveal the hidden patterns and the key determinant criteria, chaos-theory-style. Each insulant's performance will be shown to vary considerably, depending on the prevailing conditions. The days of single R-value rating for a given insulant are numbered; in future the expected heat-retention performance of any given insulant will be modelled, with local weather data, plugging in the key operative criteria that they're about to start deriving empirically. There will be big winners and embarassing big losers.

Some very impressive and interesting science going on here. However, getting back to basics the Building Reg killed TRADA certificate for Actis was based on comparitive tests between MF and mineral wool over a winter period. During cold damp winters would the insulation value of the wool be severely reduced by additional conduction caused by damp air within the wool, thereby allowing another insulating product to match it for performance even if, technically, it might be inferior. In nice dry laboratory hot-box conditions the wool may perform quite differently.
I have not seen any TRADA certificate type tests between MF and more airtight Kingspan type products reported; perhaps this would not produce the results wanted by MF manufacturers? Whatever results the old or new tests indicate obviously it is the performance in the field (or attic) that really counts and perhaps simple comparative tests may be a better way forward.

Posted By: Charlie Duketests between MF and more airtight Kingspan type products

The rough equivalence seems to be 30 multifoil gives whole-season heat loss equivalent to 200-250 min wool equivalent to 120 PUR. But all of them at an insulation value about 40% of what you'd expect from the min wool or PUR published U-values - in other words pathetic!

Yes, nom 30, why not?

Posted By: fostertom

Posted By: johanHow do the manufacturers derive a k/R-vaule for multifoil?

That's the very key question, to which hangs a long and ornate tale. The answer is unexpected and revolutionary, not at all self-evident as your rhetorical tone implies. 17 European companies manufacture multifoils and, having just now reached agreement on protocol, the testing houses of 7 European countries are begining construction of between 13 and 20 very expensive test rigs. US, Canada, Australia have agreed to sit back and watch, for the time being. Multifoils will be tested against conventional insulants, Cellotex etc as well as mineral wool. Each rig has dozens of thermocouples recording at frequent time intervals, the 'like real life' dynamically varying test cycles will run over months, or a year. Under investigation will be actual energy loss, and old-fashioned slide-rule-era concepts like R-value won't get a look in. The huge data sets will be number-crunched to reveal the hidden patterns and the key determinant criteria, chaos-theory-style. Each insulant's performance will be shown to vary considerably, depending on the prevailing conditions. The days of single R-value rating for a given insulant are numbered; in future the expected heat-retention performance of any given insulant will be modelled, with local weather data, plugging in the key operative criteria that they're about to start deriving empirically. There will be big winners and embarassing big losers.

That's a long reply Tom! You are missing the point though. The multifoil manufacturers are quoting u-values in their documentation. How did they come up with them? Per definition it should be impossible as the u-value relates directly to the k/R-value which per definition only applies to conduction of heat and not radiation.

The interesting thing with multifoil though is to look at which countries uses it. France, UK, Spain, etc. I find it hard to believe it's a coincidence that all countries where it gets truley cold in the winter like the Scandinavian countries, Canada, Germany, etc. all have discard it...

Tom, all you need to do is prove Planck's law is flawed and I'll believe you...
As long as there is no scientific evidence that it does work it's hard to believe. From biff's (and others) experiments it looks like it doesn't work particulary well for insulation of buildings.

I'd imagine if you use IR heaters instead of radiators multifoil would work fantastic!

No Tom, I understand what you're saying but it doesn't answer the question I asked.

Posted By: JohanTom, a question for you. How do the manufacturers derive a k/R-vaule for multifoil? Per definition it should be impossible...

Oh I see, sorry. Answer is, I don't know, must be some 'equivalent to' thing. It's certainly a much better figure than the useless R-value that everyone gets when testing multifoils 'by the book' in the current hotbox test. Multifoils don't pretend to perform well on conduction/convection alone, with radiation-within-the-pores carefully minimised by the way the test's set up.

Under carefully created steady-state conditions, heat transfer through the body of an insulant happens overwhelmingly by conduction/convection within-the-pores. Multifoils perform pathetically at this, but conventional insulations have been developed over decades to do well in this artificial test environment, without any check, until now, about how that corresponds to real life.

Under dynamically varying temperature conditions, like real life, the reverse is true. Then heat transfer through the body of an insulant happens overwhelmingly by radiation within-the-pores. Multifoils perform very well at this, but conventional insulations might as well have been custom-designed to facilitate, not resist radiant transfer. The swift radiant transfer within-the-pores continually evens-out the local temp differentials that drive conduction/convection, before the latter can get started.

Anyone with the patience and stamina left for this topic may find this interesting.

At Interbuild last week both YBS & Actis stands expected MF to be allowed to be used at an R value of about 5 within the year, based on the tests mentioned above which are being commenced this comimng winter. The countries where the tests are being conducted range from Finland to Spain so there should be some extremes to record. We may see earlier than expected then if the claims for MF are correct, but still have to wait for the EEC to accept the methodology before building regs can change.

Charlie, so the MF guys before commencing these extraordinary "never been done before " tests are already predicting and expecting to be allowed to quote R5 (coincidentally the equivalent to 200mm mineral wool with a lambda of 0.04W/mK). Haven't we all been here before?

Tom
Air leakage through building envelopes is the major issue as far as energy (heat and cool) conservation, insulation is the secondary issue. Thickness of insulation compensates for air leakage in a very expensive way. If multifoil can produce an 'airtight' sealed building envelope I believe it could be more effective than the 400mm of air permeable insulation required for the PassivHaus standard. The problem is maintaining the desired level of airtightness beyond the first season. Air leakage through building envelopes has been very good for insulation manufactures. It is in their interest to push the energy wasting nonsense of ‘breathing wall’. It is their best kept secret.

Cheers

Steve

Posted By: steveleighAir leakage .... is the major issue .... , insulation is the secondary issue

That only becomes true once insulation has reached a high standard - higher than current Bldg Regs require, but perfectly attainable nowadays, no point in going much further. It's 'insulation - problem solved - on to the next priority'.

Posted By: steveleighThickness of insulation compensates for air leakage

Airtightness is no kind of alternative to insulation (or insulation/thermal massiveness - massiveness can be a fair substitute for insulation, subject to getting the various factors right).

Posted By: steveleighThe problem is maintaining the desired level of airtightness beyond the first season

Just a matter of detailing - primarily not relying on sticky tape at all.

Posted By: steveleighenergy wasting nonsense of ‘breathing wall

'Breathing wall' means transparent to water vapour, says nothing either way about airtightness. A 'breathing wall' generally aims to be water vapour transparent but airtight.

Steve,
I have been given a quantity of industrial cold store panels 6m x 1m x 100mm they are plastic coated sheet steel some filled with rock-wool some with polystyrene bonded to the steel. In the cold store they were the walls and roof , with the joints (large tongue and groove) sealed with mastic. internal temp was a constant 5'c external was open to UK climate. When removed there was no sign of water ingress.
If i installed them on top of the purl ins, sealed them all around and covered them with pan tiles (for looks/UV protection) then in theory the dew point would be within the insulant, I just don't understand how vapour can condense in that scenario ? any comments
tom

>with an MVHR all water molecules within the envelope can be easily managed

That's it, it has to be managed. It doesn't just look after itself. That may be appropriate for industrial buildings like cold stores but it does not a healthy home make.

>The alternative is for everybody to live in a naturally vented home built with natural materials and kitted out with natural fixtures and fittings.
Is that a problem?