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Hi Tim,

What you say is interesting. When you say warm roof, do you mean that the mf is sandwiched between the rafters and the tiles?
Care to give more details of the type of building, heating system and how the mf was installed?

dimengineer, Don't mind Biff - he's already been told what the internal layers do but doesn't want to acknowledge. I'd be very interested in your take on it. Here's my most recent effort, from http://www.aecb.net/forum/index.php?topic=1649.0
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AFAIK, the main point is that the ratio of conductive to convective to radiant modes of heat transfer at micro scale across the void spaces within any insulation, differ markedly, depending on whether the micro scale temp gradient across a void space (and it surrounding solid matrix) is steady-state, or dynamically varying.

In steady-state conditions, conduction and convection greatly outweigh radiant transfer across the void. In dynamically varying conditions, radiant transfer dominates over conductive and convective.

Some thought and visualisation will reveal why - it's to do with the instantaneous nature of radiant transfer versus the time-lag nature of conductive and convective.

That difference doesn't matter under long-established steady-state temp gradient conditions, when 'beginning' and 'end' of the transfer are long gone/still to come.

However under constantly varying temp gradient conditions, when every moment is a new 'beginning', all modes - conduction, convection and radiation - start instantly. But of the three, only radiant gets received instantly; conduction and convection get received after a time-lag. As a result, radiant gets the new variant portion of the temp gradient filled-in long before conduction and convection get there. So conduction and convection are always beginning, but soon petering out as the temp gradient disappears; the temp gradient disappears because it's infilled by the radiant, with little 'help' from the conduction and convection.

So, under steady-state temp conditions e.g. as laboriously and unnaturally established in the hot-box, conductive and convective modes of transfer prevail, across the voids within an insulant. Conventional insulations are good at resisting that; multifoils are as poor at that as you'd expect.

Under real-life dynamically-varying temp conditions, the radiant mode of transfer prevails, across the voids within an insulant. Conventional insulations offer little defence against that; their internal surfaces, being average mud-colour, have average reflectivity. Whereas an insulant that majors on resisting radiant transfer, such as multifoils, will do splendidly under such conditions.

It's not that multifoils perform as well, under real-life dynamically-varying conditions, as other insulations are expected to. It's that conventional insulations in real-life fail to fulfil the expectations of their steady-state hot-box tested k-value ratings, by a factor of 1:1.5 to 1:2, or worse. Whereas multifoils do much better.

Well, that's my version - I'd appreciate comment.

So also with most multifoils, which are peppered with stitch holes - on a recent rough calc giving open-area equiv to a 81mm sq hole per m2. Some multifoils are imperforate, being spot-welded through - or else connected only along the edge, which may be cut off once cross-battened into place.

Posted By: dimengineer I'm a chartered chemical engineer and have a degree in this kind of stuff (heat transfer etc), so I reckon multifoils work pretty well.

Its also the consensus at work, amongst the other engineers & architects I work with.

Tim

I wouldn't touch them with a barge pole. The UK authorities did pull the plug on multifoil radiant barriers, when used on their own.

I note that a US Court fined a multifoil supplier over $100,000 for making misleadingclaims about his product. Full story at:


The British Advertising Standards Authority got Actis, a French company, claiming their reflective foil insulation is 'Equivalent to 200mm of traditional Rockwoool insulation'. A complaint has been upheld after ASA went to independent technical experts.

The judgment can be seen at:

Think hard before you buy.

The ruling:

Actis Insulation Ltd
Unit 1
Cornbrash Park
Bumpers Farm Industrial Estate
Chippenham
Wiltshire
SN14 6RA

Date: 31st May 2006
Media: Brochure

Sector: Household

Public Complaint From: Gloucestershire

Complaint:

Objection to a brochure for roof insulation. The brochure stated "TRI-ISO
SUPER 9 Insulation for roofs ... Thermally equivalent to 200 mm of mineral
wool when installed in a roof situation, as certified by the European
certifying body, BM TRADA CERTIFICATION (following real building trials,
certification n°0101) ... THERMAL EFFICIENCY equivalent to 200 mm of
mineral wood RT = 5* ... *in situ measured values." The complainant
challenged:

1. the claim "Thermally equivalent to 200 mm of mineral wool" and

2. the quoted thermal resistance "RT = 5".

Codes Section: 3.1, 7.1 (Ed 11)

Adjudication:

Actis Insulation Ltd (Actis) said they had stopped advertising TRI-ISO
SUPER 9 because it had been replaced with their new product TRI-ISO SUPER
10. They said the efficiency of their products was demonstrated by their
track record in the market. Actis said they had commissioned BM TRADA
Certification Ltd (BM TRADA) to test, assess and report on the TRI-ISO
Super 9 product. They provided us with a copy of the BM TRADA
Certification and Report dated August 1997 and said that it substantiated
their claims. Actis explained that TRI-ISO Super 9 was different from
traditional bulk insulation because it was a multi-foil product that used
layers of reflective foils spaced with synthetic wadding and foams. They
said the product required less space than traditional bulk insulation and,
therefore, internal insulation cavities could be made smaller and internal
useable spaces could be enlarged without compromising efficiency of
insulation. Actis argued that traditional methods of testing were not
appropriate for their product because traditional methods measured thermal
efficiency mainly by conduction and did not take into account the
influences of convection, radiation and change of phase. They said their
product combined various energy transfers of radiation, conduction,
convection and change of state rather than just conduction. Actis also
argued that traditional methods of testing did not allow representation of
the real behaviour of building materials once used on site. They pointed
out that BM TRADA had used "in situ" testing involving a real external
environment with variations in temperature, humidity, etc. rather than the
traditional methods of laboratory testing. Actis maintained that the BM
TRADA Certification demonstrated the thermal efficiency of their product
and provided proof of their claims.

1. Complaint upheld

The ASA obtained expert advice. We understood that BM TRADA had tested
TRI-ISO SUPER 9 and the mineral wool in two separate roof installations.
However, we noted that BM TRADA had not used the standard industry methods
of testing and that the report provided by Actis did not include
sufficient detail to support their own methods of testing.

We acknowledged that BM TRADA Certification was a leading multi-sector
certification body accredited by the United Kingdom Accreditation Service.
We considered that the BM TRADA report did not provide enough detail to
support their methodology instead of the methodology employed by the
internationally recognised ISO industry standards. We concluded Actis had
not substantiated the claim. We noted the ad was no longer appearing but
told Actis not to repeat the claim in future advertising until they were
able to provide sufficient substantiation.

2. Complaint upheld
We understood that RT was a symbol of total thermal resistance and
typically had the standard unit of measurement of m²K/W. We noted that the
claim "RT=5" was not qualified by any recognised units of measurement e.g.
m²K/W and a small footnote stated only "in situ measured values" without
further explanation. Because the value of 5 was not qualified by any
recognised units of measurement, we considered the claim "RT=5" was
ambiguous and should be qualified in future. However, we noted that the BM
TRADA report did specify an overall resistance (RT) of 5.0m²K/W derived
from the in situ testing. We understood that the in situ measured values
did not meet with ISO recognised international standards for determining
declared and design thermal values for building materials and products.

We considered that the BM TRADA report did not include sufficient detail
to demonstrate the validity or robustness of their testing methodology
instead of the methodology employed by ISO standards. We concluded that
the report did not substantiate the claim " RT=5". We told Actis to remove
the claim until they were able to provide sufficient substantiation.

The brochure breached CAP Code clauses 3.1 (Substantiation) and 7.1
(Truthfulness).

Further to the above:

The UK authorities pull the plug on multifoils, when used on their own. The Multifoil Council) made pleaded to the Office of the Deputy Prime Minister when the last round of Part L (energy aspect) of the building regs was being assessed and had a reprieve to 01/01/2007 to give them time to show that multifoils worked as claimed. This reprieve has been rescinded early because of convincing evidence that the multifoil claims are exaggerated. Multifoils will only be permissible if they can pass hot box tests, which as far as I know they have never have. All the local authorities and the NHBC and such bodies have all been told to no longer accept multifoils. I am not sure if this is still the case.

Posted By: Mike George...several pages back...

What a thread!

Skywalker,

Thanks for the input. Indeed, i am sure that these issues have been talked about before, but it is difficult to sift out the pertinant info i such a large thread.

And as such i am also sure i am added to the guff, but i hoped that some of these questions would bring up important points about the whole MF argument.

As for my opinions, I am not clever enough to figure all the radiation/convection/conduction agruement, but i do know about some of the short falls in traditional constructions.

Timber

OK, Where shall I start. Firstly, I'm not going to try and explain in precise detail exactly how multifoils work, bacause a) I dont know in sufficient detail and b) more pertinently, I have been following this thread for a good while and it does seem that the "antis" will not accept any explanation - Fostertom has written at length an awful lot of sensible and scientifically correct stuff, to be met with barrages of abuse and "we dont believe you". I have no doubt that any explanation I try will be met with the same jeers.

So, anyway, it was more than a loft extension - more a whole new roof. I've a large house, 10m x 8.5m with a small front bay - bedroom & hall. The old roof was taken right off, which comprised the original pitched house roof at the front of the house and the flat roof over the large rear extension. A complete 42 degree pitched roof was put over the whole 10 x 8.5m area, with a couple of rooms in there. It was finished just about 12 months ag now. No problems with buidling control (HIllingdon)

Its a warm roof construction, with the multifoil over the rafters, then counter battens, then breather membrane, then tiling battens, then tiles. The multifoil is Tri-Actis 10

In terms of volume it is about 100m3 of new volume - a nice large "playroom", a study, and loo/shower.

My heating bills have dropped - not massively, but they have dropped. So I've gained 2 new rooms and they have added positive insulation value!

In general, the radiators in the loft do not come on (TRVs), although I have to admit when it was -5 out, they did come on.

So what I can say, is that the effective insulation value of the multifoil is very good. Is it better than 200mm of Celotex or whatever?, I dont know. I cant actually do a controlled test really can I?

In terms of insulation, the architect & I kept the builders honest - lots of stapling - with 14mm staples. All joints stapled onto noggins (they forgeot them). Lots of tape (although I'm unconvinced of its effectiveness), and MOST IMPORTANTLY, getting the detail at the eaves correct - brigning the multifoil down to the wall plate and stapling it, taping at the rafter ends and stapling it, and then stuffing the gap on the inside with mineral wool insulation.

Thank you for the details, just one last question, is there any insulation in the loft floor?

Posted By: dimengineerOK, Where shall I start. Firstly, I'm not going to try and explain in precise detail exactly how multifoils work, bacause a) I dont know in sufficient detail and b) more pertinently, I have been following this thread for a good while and it does seem that the "antis" will not accept any explanation - Fostertom has written at length an awful lot of sensible and scientifically correct stuff, to be met with barrages of abuse and "we dont believe you". I have no doubt that any explanation I try will be met with the same jeers.

I'm disappointed that you're not going to try and explain how multifoils work because, as you may have gathered, I am not convinced by the scientific validity of Fostertom's explanation. I was hoping that a professional engineer might be able to shed further light. Note that my concern is not with airtightness or any of the other possible advantages of multifoils but just with the narrow issue of whether the internal layers of foil provide good value for money in preventing heat loss.

I do hope you are not implying that I am barraging you with abuse and jeers. I do try to reserve that tactic for those who attempt to promote environmentally damaging building materials and methods on a Green Building Forum :)

Posted By: skywalkerTimber
I suspect that the air-tightness/draft exclusion the stuff helps with has more to do with the perceived insulation it provides in the real world than it's thermal properties (and have for some time).
S.

There are cheaper ways to get air-tightness. I have seen this stuff used in cavities - nearly up to the bricks or right on it, rendering the stuff useless against the brick or not.

The only way to test this stuff is have two identical houses one with multi-foil and one with foam/rockwool insulation. Both with the same level of air-tightness. Then have two families with identical living patterns. Then see what the heating bills are.

Celebration? Oh, I see we've hit the half millenium!

Families are never the same so it had better be the dog kennel. But computational moddeling should be easier. It works for complex systems such as nuclear reactor design and global warming so building insulation should be a piece of cake. We just need a computational physisist rather than a builder with a bit of pseudoscience.

Posted By: rogerwhitOr just two tightly-built dog kennels equipped with sensors and identically programmed heat emitters?

I thought that approach was held to be the epitome of contemptible unscientificness, compared with the 'scientific' hotbox route? So there's nothing wrong fundamentally, with the famous Actis/BBA twin-shacks approach to accreditation, just needs more rigour?

<blockquote><cite>Posted By: fostertom</cite>I thought that approach was held to be the epitome of contemptible unscientificness, compared with the 'scientific' hotbox route? So there's nothing wrong fundamentally, with the famous Actis/BBA twin-shacks approach to accreditation, just needs more rigour?</blockquote>

It's the pragmatic approach, especially since no-one can agree on the theory!! If it can be thus shown to work, then it DOES work!! And Tom, such a test would incorporate by default your insistence that dynamic conditions (not steady state) are key.

Posted By: fostertomimported anything!

No, dutch bicycles are ok apparently