Sorry, I've spent the day gettingon with my life so it'll have to be #302

Tom, I didn't say the internal layers had NO effect, just that they don't have MUCH effect. Certainly not enough to justify the pro-multifoilists claims or the cost.

Biff, don't wriggle - you said each successive layer has exponentially less effect, therefore negligible; I say each layer has near enough equal effect; the temp drop is distributed nearly linearly through the stack of layers. Of course diminishing returns from adding more layers, in similar way to adding more conventional insulation thickness, but no more diminishing than that.

Mike, no, you put the hemp *in* the coffee.

Tom, we can argue about whether the rate of diminishing return is linear or exponential but the real issue is:
Are the internal layers of foil worth the money when you buy Tri Iso 10 or would you be better off with foil backed bubble wrap that has all the draught proofing advantages but little of the cost disadvantage.?

So let's get this straight. You think the inner layers of foil are doing a good enough job to justify the price of Tri Iso 10.

Ah well.

Hi again folks,
I have been away from this topic for a while because it has made me very tetchy in the past, for which I apologise.

However on reading the latest couple of pages I note Tom's rather inaccurate claim that a "proof" of his from a thread on the old forum remains unchallenged: http://www.greenbuildingforum.co.uk/forum/index2.php?DATEIN=tpc_tuzncnnui_1170338947&showpage=1
Funcrusher already debunked the maths in an algabraic way, but I would also like to challenge the logical processes.

Tom stated the following:
<< Between all pairs of foils, Q is the same; Paul in Montreal says: "Each foil will be in equilibrium if the amount of energy it radiates is exactly equal to that which it receives - from both sides".>>

Paul's statement is correct but Tom has assumed that because at equilibrium the amount of heat radiated and received by each individual foil is the same, that the amount of heat transmitted from one foil to another will be the same. This is an incorrect assumption.
The foil facing the heat source must receive and radiate more energy than the second foil or it would have no heat shielding characteristic at all (ie no insulative value). The insulative property is achieved by the high reflectivity of the shiny surface. If this first layer absorbs only 5% of the incident energy and then when it reaches a steady state re-emits this 5% again (as 2.5% from each side), then we can see that the second foil will receive only 2.5% of the amount of energy being received by the first foil (on its heat source side). Interestingly when the system reaches equilibrium, the first foil will receive a proportion of the 2.5% re-emitted back from the second foil making the temperature gradient a little steeper still.
When the heat source is applied, the first foil will heat it up to a temperature where it will be re-emitting at a rate of 5% that of the source. With consecutive steps of similar attenuation percentages of the Quantity of Heat (Q) being transferred, how can a linear temperature drop across a system of foils be a realistic expectation?

As I made clear at the time, all of the foregoing was in relation to an idealised multifoil in a vacuum where radiant energy is the only means of heat transfer. The formula I supplied to calculate the heat transfer between two folis cannot be used directly for a system of foils. The maths for this will be considerably more complex.

Tom’s “proof” starts with the false assumption that equal amounts of energy are entering and leaving all of the foils and then by wrongly assuming equal temperature difference, “prooves” the first incorrect assumption !?!?
Sorry Tom, but this is not anywhere close to a scientific proof.

I've been watching you..........

My memory of that argument is that the foils would act as tom said if they were matt black, but if silver then as martian says. The best argument that i can get for those inside foils is that they are protected from dirt and corrosion.
I love the bit where Tom tenaciously works out a plausable reason for the internal foils then has Mitten "finally confirm" that this is the case. In other words Mitten has no idea why they are there cos they've just transposed the idea from space use without thinking how the science changes for domestic use; then Tom comes along with his idea and Mitten says " oh, er, yeah, that's it, erm, well done for working it out without me telling you - of course i've known this all along but couldnt tell anyone for commercial reasons (phew that gets me of a hook, i'll use that one cos i can't find a better one)". I like Tom's story of how radiation works on a micro scale through insulation, and can see that coating the bubbles with silver would reduce radiative flow (this is the theory behind Thermilate microspheres, though dont know if it works in practise). However, i dont think an internal layer of foil would do the same. Those etheric soldiers would cross a thousand lakes happily until they reach the foil and get turned back to the other shore of a single lake, whence they turn into conductive soldiers and most walk straight through. The overall resistance caused by an internal foil would thus be minimal. Probably.
Assuming i am wrong and the foil does turn back radiative transfer internally, i cannot see that the temperature wave effect would be very significant. If internal temps were a constant 20C and external temp went up from 0C to 10C and back again, there would only be a temperatue wave as described by tom for half the day, ie when going from 10 to 0. The rest of the day when it is warming up there is no wave but still cold enough to need insulating against. Or is there?

There is a way that foil (not necessarily multi) would work that has been implied but i dont think explicitly made (or i glazed over at that bit, so sorry for any repetition). Thermal comfort can be maintained at lower air temperatures if the mean radiant temperature is higher. Thus a foil that increases the apparent radiant temperature with a radiant human (or radiator) in it (by reflecting back that radiation), comfort can be maintained without inreasing the u-value of the walls or the temperature of the walls or - most importantly for energy consumption - air temp. This is the 'magic' of foil - nothing increases (much) in temperature except the human (or other radiating object). Which is presumably why the hotbox isnt relevant here. There needs to be a hot object test (i think one was described earlier) cos an object like a human can keep warm in low air temps if radiant temp is high.
Mind you, this effect may not be very relevant cos who wants to live in a buck rogers style silver room? If you put a board infront of it then the foil will be reflecting back the radiant temp of the board which would be a combination of the (hot) radiant temp from human/heater and the (cool) air temp, so conductive losses would be more significant and the human wouldnt benefit from its/heater's own heat reflected straight back.
So the question i am asking myself is whether it is worth putting down mylar in between my joists (with polysyrene spacers) to reflect heat back in (i am too skint to use multifoil even if, as i doubt, it works much better that single layer of mylar topped with rockwool), and/or lay mylar on top to prevent infra red emission, or whether the whole business wouldnt be worth the effort and just use rockwool covered in plastic to prevent wind losses. Or whether to paint with Thermilate for the buck-rogers effect without the silver decor.
i've got a radiant thermometer so might do some in-situ tests. Too tired to describe them now tho.
Thanks team.

Multifoil rules OK

<blockquote><cite>Posted By: mrswhitecat</cite>Cut to the chase fellas. I've come to this thread at least 18 months too late and whilst I enjoyed Saint's prose on the 1st page, I can't quite face 11 pages of what seems to be a lot of lively debate. Have any conclusions been drawn yet? Is anybody capable of presenting a critical appreciation of the arguments for and against?</blockquote>

Not me . Going to be REALLY BAD here and not enter the debate per se .

As long as this appears daily

http://www.planningportal.gov.uk/uploads/br/multi-foil-insulation_july2005.pdf

and as long as paragraph 3.10.2 of

http://www.bre.co.uk/filelibrary/rpts/uvalue/BR_443_(2006_Edition).pdf

remains un altered

- I don't see the point in using multi foils .

The installation instructions for multifoils typically require a 25mm air gap either side so perhaps they should be compared with 50mm of regular insulation like Celotex. The data here..

http://www.planningportal.gov.uk/uploads/br/multi-foil-insulation_july2005.pdf

says.. "Using the average U-value for the four measurements, i.e. 0.49 W/m²K, and working backwards through the U-value
calculation indicates an R-value for TRI-ISO SUPER 9? (inclusive of the airspaces on either side of it) of 1.72 m²K/W."

By comparison the data for 50mm of GA3000 Celotex (admitedly from the makers web site) says the R-Value is 2.15 m²K/W.

To me this says that if you are pushed for space then 50mm Celotex is actually better than a correctly installed foil with 25mm air gap either side....or at least it appears that way based on the above. Obviously I'm not quite comparing like with like. The newer foils might also be better.

sinnerboy - liked the pdf, which I have sent to the archi. Should scare him that I am reading his sort of stuff and focus his attention when I quizz him (yet again) about the insulation.

If the airspace on either side of a multifoil is said to be important, I'd take that as an admission that the internal foils are not important. After all, the internal foils don't 'know' whether or not there is an external airspace.

(Oh isn't it a joy to see the revival of a long dead thread. Thought the beast had finally been laid to rest, but no, it just keeps popping up.)

Posted By: SaintCWatters,
Interesting comparison in your latest posting.
Yes multifoils are totally reliant on airspaces to provide any sort of reasonable insulation performance.
However when you say there has to be a 25mm airspace either side of the foil and so that compares with 50mm Celotex you're omitting the thickness of the multifoil itself. So an installed foil solution would be 25+30+25 i.e 80mm thick.

I believe the foil is "pinched" where the two layers of 25mm battens cross (It's why they say you should have one set vertical, one set horizontal, nailed together through the foil). So the gap would be more like 25+5+25. I ignored the 5. If that's not how it's done then yes it's even worse.

This may have been covered on this thread, I have read most but I certainly could have missed it.

I have recently finished some training on residential energy efficiency based on an Australian scheme that will be introduced to NZ. The Aussies like there radiant barriers and talked about an aspect that I was not familiar with, this explained the reasons for rating radiant barriers in different directions (normally up and down).

A single radiant barrier works well in a downwards direction because the air in contact with the cold side (underneath) is held in place by the barrier even when heated and expanded.

A single radiant barrier doesn’t work very well in the upwards direction (& has a much lower R-value rating) because the air in contact with the cold side (on top of barrier) rises quickly when heated through stratification which then creates a lot of air movement.

So the idea of using wading between layers of radiant barrier for loft conversions makes a lot of sense if it traps air against and above radiant barriers.

From http://www.greenbuildingforum.co.uk/newforum, about heat transfer between triple glazing panes:

Posted By: Stephen TConvection and conduction vary according to the temperature difference, so yes they are 'independant of temperature range'
Radiation however, varies with the temperature differnce raise to the fourth power

Posted By: fostertomNot exactly - radiant transfer is the difference between the radiation emitted by the first source, proportional to the fourth power of its abs temp; and the radiation emitted by the second source, proportional to the fourth power of its abs temp. Radiant transfer is proportional to the difference between two fourth powers, not the the fourth power of the difference. And over the narrow operating band say 253K to 313K, that works out close enough to proportional to the first power of the difference, just like conduction and convection.

Posted By: Stephen TWent back and checked my textbooks and i think you are correct

Posted By: fostertomThanks Stephen T. This was the same mistake made by people 'proving' why multifoil insulation couldn't work, on this forum

It doesn't matter what the proportion is at source or within the room - that has no bearing on the proportion within the body of the insulation. As soon as heat moves through the surface of the insulation and on through it, the proportion of the different types of transmission will vary continually, depending on the microscale features that the heat flow encounters along its way.

For example, looking micro-scale at heat travelling through the solid material between the bubbles of a foamed insulant, that wd be entirely conductive. When that heat flow reaches the 'near shore' of a bubble, how does the heat flow reach the 'far shore' before continuing on its pure-conductive way though the next bit of solid material?

As the heat flow reaches the 'near shore' and warms it up, there's a temp differential across the void space, because the 'far shore' hasn't been warmed up yet. Heat flow converts from pure conductive, to a mixture of conductive, convective and radiant.

Drawn by the temp differential, conduction continues by the long route along the solid-material 'shores', round to the opposite 'far shore' - a relatively slow process.

Convection begins, direct across the void - also a process that takes time before convected heat starts arriving at the far shore.

Radiation begins, direct across the void - and heat starts to be received at the far shore instantly, at a rate dependent on the emissivity of the 'near shore' surface and the 'receptivity' (same thing, actually) of the 'far shore' surface. The mud-coloured solid material that surrounds the bubbles of a typical insulant is a good average emitter, like nearly all building materials short of polished chrome or optical black.

So whenever a heat flow encounters any void (bubble) space within a typical insulant, a goodly proportion of its transmission across or around to the far side of the bubble will be by radiation.

Now the important bit - because radiant heat starts to be received by the 'far shore' instantaneously, the 'far shore' gets considerably warmed up by the radiant component, long before the conductive and radiant components arrive at the 'far shore'. If fact, as the radiation warms up the far shore, the temp differential disappears, so the 'draw' that drives all three modes ceases - maybe before any at all of the conductive or convective heat actually gets there.

That would mean that whenever a heat flow encounters any void (bubble) space within a typical insulant, not just 'a goodly proportion' but nearly all of the transfer across or around to the far side of the bubble will be by radiation. So any insulant that sets out to resist radiation within itself would be surprisingly effective - and largely independent of the thickness of the insulation slab. Such an insulant would be ordinary foam insulation, but with the interior surfaces of its bubbles chrome plated instead of mud-coloured. Or multifoil insulation. Or mineral fibre with every strand chrome plated. Which of these sounds worth Sheffield Insulations paying Ă‚ÂŁ8m to buy into?