Posted by Eeyore on April 21, 2009, 3:28 pm
Tim Jackson wrote:
> Eeyore wrote:
> > Tim Jackson wrote:
> >> Eeyore wrote:
> >>> Malcom \"Mal\" Reynolds wrote:
> >>>
> >>>> Was watching a "green" show that had a
> >>>> Trombe wall and started thinking that if
> >>>> properly modified, a design might
> >>>> include little niches where you could
> >>>> plant sun tolerant plants. This could
> >>>> serve two purposes. The plants might
> >>>> help absorb heat and they would probably
> >>>> help clean indoor air.
> >>>>
> >>>> Are there any serious problems with my
> >>>> thinking? I can see that proper watering
> >>>> and maintenance might present a problem,
> >>>> but not insurmountable.
> >>> I doubt the plants would absorb any more heat than the wall but do take
> >>> a look at this.
> >>> http://en.wikipedia.org/wiki/Low-emissivity
> >> That Wiki looks very suspect to me.
> >>
> >> It states
> >> "The principal mechanism of heat transfer in multilayer glazing is
> >> thermal radiation from warm surfaces to cooler surfaces"
> >>
> >> That is rarely true, unless the gap is evacuated, eg a Thermos flask.
> >>
> >> As I understand it radiant heat transfer between two surfaces at around
> >> ambient temperature is quite a bit smaller than the conductive loss
> >> through the best commercial glazing. Radiant loss to *space*
> >> (effectively a very cold surface) from glazing or any other surface can
> >> be quite significant, but depends on the atmospheric humidity and the
> >> solid-angle of view of the sky from the window. Basically only on a
> >> very cold clear night with a good view of the sky.
> >>
> >> If you compare the published r-value of high-performance glazing to that
> >> of plain glass, the improvement is in the order of at best tens of
> >> percent, not hundreds.
> >
> > You're not familiar with low-E glass ? Since I believe it was invented by
> > Pilkington of the UK I can see that Americans might reject it on a NIH ( not
> > invented here ) basis.
> > http://www.google.com/search?hl=en&rls=en&hs=hdv&q=low-e+glass&btnG=Search
> I'm familiar with it, I've read manufacturer's spec sheets, I understand
> how it works, its uses and abuses. I disagree with the statement I
> quoted as unwarranted on the basis of physics. The statement actually
> has nothing to do with coated glass, it is supposed to be a statement of
> the problem that glass addresses.
Sorry, I don't quite get it. Low-E glass AIUI can do nothing but improve the
performance of a Trombe Wall by maximising solar gain.
Maybe you could simplify it for me ?
Graham
Posted by Tim Jackson on April 21, 2009, 7:59 pm
Eeyore wrote:
>
>
> Sorry, I don't quite get it. Low-E glass AIUI can do nothing but improve the
> performance of a Trombe Wall by maximising solar gain.
>
> Maybe you could simplify it for me ?
>
>
I don't disagree with you. I was concerned that the Wiki you referred
to looked a bit flakey, and was newly edited. However as I mentioned
elsewhere, I have since looked into it in more detail and now understand
where it is coming from.
It's just that there is so much hype around glazing, marketing people
seem to go to some lengths to conceal the facts in case anyone realises
that it is not actually a very good investment.
Tim
Posted by Eeyore on April 21, 2009, 8:48 pm
Tim Jackson wrote:
> Eeyore wrote:
> >
> > Sorry, I don't quite get it. Low-E glass AIUI can do nothing but improve the
> > performance of a Trombe Wall by maximising solar gain.
> >
> > Maybe you could simplify it for me ?
> I don't disagree with you. I was concerned that the Wiki you referred
> to looked a bit flakey, and was newly edited. However as I mentioned
> elsewhere, I have since looked into it in more detail and now understand
> where it is coming from.
> It's just that there is so much hype around glazing, marketing people
> seem to go to some lengths to conceal the facts in case anyone realises
> that it is not actually a very good investment.
I understand you scepticism but low-E glass has been around a long time now.
Graham
Posted by daestrom on April 20, 2009, 9:34 pm
> Eeyore wrote:
>>
>> Malcom \"Mal\" Reynolds wrote:
>>
>>> Was watching a "green" show that had a
>>> Trombe wall and started thinking that if
>>> properly modified, a design might
>>> include little niches where you could
>>> plant sun tolerant plants. This could
>>> serve two purposes. The plants might
>>> help absorb heat and they would probably
>>> help clean indoor air.
>>>
>>> Are there any serious problems with my
>>> thinking? I can see that proper watering
>>> and maintenance might present a problem,
>>> but not insurmountable.
>>
>> I doubt the plants would absorb any more heat than the wall but do take
>> a look at this.
>> http://en.wikipedia.org/wiki/Low-emissivity
>>
>> Graham
>>
>>
> That Wiki looks very suspect to me.
> It states
> "The principal mechanism of heat transfer in multilayer glazing is thermal
> radiation from warm surfaces to cooler surfaces"
> That is rarely true, unless the gap is evacuated, eg a Thermos flask.
> As I understand it radiant heat transfer between two surfaces at around
> ambient temperature is quite a bit smaller than the conductive loss
> through the best commercial glazing. Radiant loss to *space* (effectively
> a very cold surface) from glazing or any other surface can be quite
> significant, but depends on the atmospheric humidity and the solid-angle
> of view of the sky from the window. Basically only on a very cold clear
> night with a good view of the sky.
Actually the statement is correct with regard to window glazing. The glass
itself conducts heat, but the most any gas between the panes is an excellent
insulator. The only 'trick' is having the spacing just right. To much
distance between the panes and the gas inside will start to convect between
the cold pane, down and up the warm pane. With a small enough gap this
convection is greatly reduced because the downward motion along the cold
pane interferes with the upward motion along the warm pane and the
convection currents never really develop.
Well designed windows you have convection from the room warming the inner
pane, radiant transfer from the inner to the outer pane and convection
currents on the outside cooling the outer pane. Low-E coatings minimize the
middle step.
daestrom
Posted by Jim Wilkins on April 20, 2009, 10:14 pm
> Well designed windows you have convection from the room warming the inner
> pane, radiant transfer from the inner to the outer pane and convection
> currents on the outside cooling the outer pane. Low-E coatings minimize the
> middle step.
> daestrom
When I built my multilayer window inserts in ~1981 the suggested
spacing was 20mm or 3/4". The two added layers of polyester film add
enough insulation to the original 1970 windows that the IR temperature
of black tape on the film is only 2 - 3F lower than the same tape on
the wall nearby, when outdoor temp is well below freezing. There's
some variation across the walls and on upwind and downwind sides of
the house so I don't have more precise measurements.
Pyrex is naturally low-E and takes forever to cool after blowing
labware out of it. It's dangerous because it doesn't radiate enough
heat to warn the hand even when it's nearly red hot.
Jim Wilkins
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> > Tim Jackson wrote:
> >> Eeyore wrote:
> >>> Malcom \"Mal\" Reynolds wrote:
> >>>
> >>>> Was watching a "green" show that had a
> >>>> Trombe wall and started thinking that if
> >>>> properly modified, a design might
> >>>> include little niches where you could
> >>>> plant sun tolerant plants. This could
> >>>> serve two purposes. The plants might
> >>>> help absorb heat and they would probably
> >>>> help clean indoor air.
> >>>>
> >>>> Are there any serious problems with my
> >>>> thinking? I can see that proper watering
> >>>> and maintenance might present a problem,
> >>>> but not insurmountable.
> >>> I doubt the plants would absorb any more heat than the wall but do take
> >>> a look at this.
> >>> http://en.wikipedia.org/wiki/Low-emissivity
> >> That Wiki looks very suspect to me.
> >>
> >> It states
> >> "The principal mechanism of heat transfer in multilayer glazing is
> >> thermal radiation from warm surfaces to cooler surfaces"
> >>
> >> That is rarely true, unless the gap is evacuated, eg a Thermos flask.
> >>
> >> As I understand it radiant heat transfer between two surfaces at around
> >> ambient temperature is quite a bit smaller than the conductive loss
> >> through the best commercial glazing. Radiant loss to *space*
> >> (effectively a very cold surface) from glazing or any other surface can
> >> be quite significant, but depends on the atmospheric humidity and the
> >> solid-angle of view of the sky from the window. Basically only on a
> >> very cold clear night with a good view of the sky.
> >>
> >> If you compare the published r-value of high-performance glazing to that
> >> of plain glass, the improvement is in the order of at best tens of
> >> percent, not hundreds.
> >
> > You're not familiar with low-E glass ? Since I believe it was invented by
> > Pilkington of the UK I can see that Americans might reject it on a NIH ( not
> > invented here ) basis.
> > http://www.google.com/search?hl=en&rls=en&hs=hdv&q=low-e+glass&btnG=Search
> I'm familiar with it, I've read manufacturer's spec sheets, I understand
> how it works, its uses and abuses. I disagree with the statement I
> quoted as unwarranted on the basis of physics. The statement actually
> has nothing to do with coated glass, it is supposed to be a statement of
> the problem that glass addresses.