In article , Hugo
Nebula abuse@localhost.? writes
On Sat, 30 Sep 2006 23:29:09 +0100, a particular chimpanzee named Tony
Bryer randomly hit the keyboard and produced:
On Sat, 30 Sep 2006 22:38:05 +0100 Hugo Nebula wrote :
I was sceptical too, but it appears to be correct. The R-value of
Alreflex (the nearest equivalent I could think of) with no cavity
either side is 0.15m2K/W; the R-value with cavity both sides is 1.56,
so the R-value with one side exposed must be 0.93. Multiply that
figure by the conductivity of polystyrene (0.037W/mK) and the
equivalent thickness is 34mm so they're not far off.
Note that the following BRE document casts doubt on the TRADA figures
being quoted for this sort of insulation
http://www.planningportal.gov.uk/upl...n_july2005.pdf
That's for multi-foil insulation which tries to claim R-values
equivalent to 200mm of rockwool. Alreflex is bubble-wrap sprayed
silver. The R-value includes a value for the material as well as
increased resistances of the air either side as it has a higher
emissivity (or is it lower?, it's late & I can't remember). The
values I quoted were taken from the BBA certificate for it, so if you
can't believe them, who can you believe?!
Although the BRE report refers to multilayer I believe that the failings
pointed out apply to all products that (over) rely on reduction in radiation for
their 'enhanced' properties.
The easy flaws to spot a
1. They rely on a carefully controlled air gap to meet their stated
performance.
2. The gap must be still air or heat will lost by conduction to the moving air
(forced convection if you like), so no draughts and no ventilated roof or
cavity.
3. The gap has an optimum width, too great and there is room to create a
convection loop giving more heat loss.
4. The reduced radiation relies on clean reflective outer surface, get it dirty
or dusty and there goes the radiation benefit resulting in more heat loss.
5. The reflective surface may lose its shine with age resulting in more heat
loss.
To stand a chance of this product getting close to its design claims you
have to install it in an unrealistically perfect environment. In contrast,
trapped air systems (foams, rockwool or whatever) can achieve high
performance with a lot less attention to detail.
--
fred
Plusnet - I hope you like vanilla