Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

Buy individually or both books together. Delivery is free!


widget @ surfing-waves.com




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




    • CommentAuthorEd Davies
    • CommentTimeApr 1st 2013
     
    This otherwise quite sensible article says:

    Designers tend to settle for the simple metric of the U-value of the window, the heat transfer coefficient of the whole system including conduction, convection and radiation. It represents the heat flow in watts per hour through each square metre of the window for a 10°C temperature difference between the indoor and outdoor air temperature.

    What?

    I really expect and hope Prof. Roaf did not write that. I bet she's more than a tad fed up with whoever mangled her article in this way.
    • CommentAuthorTimber
    • CommentTimeApr 1st 2013
     
    Ha, whoops!

    There was an advert on the front of the latest TTJ (timber trade journal) for Caberdeck flooring with a massive clanger in it! I think - as you suggest, these things get mangled by an editor who doesn't know the technical aspects and thinks that what they have re-written has the same meaning.
  1.  
    Ooh dear! Yes, didn't notice this! And I usually do notice the proof-reading errors!
    •  
      CommentAuthorfostertom
    • CommentTimeApr 2nd 2013 edited
     
    I assume it's
    Posted By: Ed Daviesheat flow in watts per hour through each square metre of the window for a 10°C temperature difference
    that the tickle - but what about
    Posted By: Ed Daviesthe U-value of the window, the heat transfer coefficient of the whole system including conduction, convection and radiation
    AFAIK, that's another kind of clanger, that may well have come from Sue Roaf, or almost anyone.

    Rolling radiation loss into Uw or Ug figures is seriously guesstimatey. Radiation loss is greatly de-coupled from conductive/convective loss.
    The latter is proportional to difference between internal and external air temp,
    whereas the former is proportional to the difference between the fourth powers of the abs temps of internal bodies (not air) and external scenery, sky etc (not intervening air).
    Very different, and barely correlated.
    • CommentAuthorEd Davies
    • CommentTimeApr 2nd 2013 edited
     
    Good point Tom. Same question could be asked for U-values of more opaque parts of the building fabric. We know that it's better to put more insulation in roofs because they can be exposed to radiation losses to clear skies but do we take that into account with HDD calculations, etc?
    • CommentAuthorEd Davies
    • CommentTimeApr 2nd 2013 edited
     
    On second thoughts, from the point of view of outgoing long-wave radiation I don't think building elements other than windows are that much more opaque as glass is pretty much opaque at those wavelengths too. Warm body in room emits photon with wavelength around 10 µm which strikes glass window. Photon is absorbed¹. Energy gets out and walks in the form of conduction through the glass. On the other side it might well be radiated away again in another 10 µm photon or moved by convection/conduction.

    So from the U-value point of view I'm not so sure that windows are special - for any part of the building some account needs to be taken for radiation to/from temperatures other than those of the air on each side. Hmmm…

    ¹ or reflected - one of the points of low-e glass coatings.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 2nd 2013 edited
     
    My guess is that radiation loss, and therefore this kind of uncertainty about it, is far less significant for opaque fabric (walls, roof etc) than for transparent. And particularly important because in low-energy buildings, esp ones that take account of 'passive solar' (e.g. PH), the whole concept of solar gain thro south windows, falling on heavy internal 'storage' elements (floor, walls etc) depends on such assumptions about re-radiation loss, as soon as the sun goes in.

    For opaque fabric, radiation loss (if any) is from internal room/objects to the internal wall/roof surface, then from external wall surface to scenery/sky etc. The temp gradient from inside room/object to scenery/sky is divided into three, in which the middle segment (thro the wall) greatly dominates, leaving v little gradient to drive radiation in the first and last segments.

    Whereas for transparent fabric, the full temp gradient from internal room/object to scenery/sky etc is in full force, driving the radiation loss. It's true that the glass, and its coatings, do obstruct that radiation loss, by being relatively transparent to hot (high frequency) incoming solar radiation, and relatively opaque (a lot or a little) to tepid (low frequency) outgoing radiation from room/objects.
    However that doesn't make it same as opaque fabric - the full temp gradient applies, albeit the resultant radiant heat flow encounters resistance at the glass line.

    What's more, generally, those internal room/objects (incl the room's heat-source surfaces) that can 'see' the outdoors, thro the glass, are warmer than room air temp,
    while those internal room/objects (incl the room's heat-loser surfaces such as external walls) that can't 'see' the outdoors, thro the glass, are cooler than room air temp.
    So the room/objects that emit the radiation-loss are generally, by definition, warmer than would be expected by simply assuming that radiation loss is proportional to room air temp (same as conductive/convective loss is). Thus radiation loss is generally under estimated.

    Similarly, when the weather is critical, the scenery/sky is generally colder than external air temp.
    So the scenery/sky that 'pulls' the radiation-loss is generally, by definition, colder than would be expected by simply assuming that radiation loss is proportional to external air temp (same as conductive/convective loss is). Thus radiation loss is again generally under estimated.
    • CommentAuthortony
    • CommentTimeApr 2nd 2013
     
    http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=1258&page=1#Item_2

    I did try to talk about this a good while age, may be we weren't ready then, but are we now?
    •  
      CommentAuthorfostertom
    • CommentTimeApr 2nd 2013 edited
     
    I think so.
    Presumably, in hot box type tests, the internal surfaces of the box on the hot side will equilibriate at about equal to hot-side air temp, likewise on the cold side.
    Then they measure the actual heat transfer from hot side to cold side, at given maintained delta-t, without distinguishing between conductive, convective or radiant modes of transfer.
    That must give an over-optimistic heat transfer coefficient U, because as I've suggested above, in real life
    the delta-t between object temps, as affecting radiation will be different from
    the delta-t between air temps, as affecting conduction/convection.

    In warming weather, the radiant delta-t between object temps may actually be smaller than
    the conductive/convective delta-t between air temps,
    but when it matters, in weather that's getting colder, and/or when the sun goes in,
    the radiant delta-t may be v much greater than the conductive/convective one, on which hot box testing is based.

    So when it matters, radiation loss thro windows may be much higher than anyone has suspected.
    That has great bearing on the typical assumption in passive solar design, that heat stored into heavy masses inside south windows, actually stays there - maybe it's re-radiated away much quicker than expected.
    • CommentAuthorEd Davies
    • CommentTimeApr 3rd 2013
     
    As I said above, I think glass is pretty opaque to long-wave IR so I don't think there's a lot that's special about windows in this respect. E.g., a Uw 1.2 W/m²·K window and a U 1.2 W/m²·K wall will both be affected by the radiant temperatures of humans, gerbils, heating devices, cold skies, etc, in the same way.

    However, in the real world windows tend to much poorer insulators than the surrounding walls. This means that with the window the resistance provided by the interface between the air and the glass on each side is a larger proportion of the total insulation. It's this resistance that radiation effects bypass so windows will be disproportionately affected.

    On the outside the interface resistance is usually taken to be small, because at least some wind is assumed, so the relative effect will be smaller.

    What I wonder is how much difference this actually makes. I've no idea how much energy is really transferred by radiation at typical temperatures.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 3rd 2013 edited
     
    Posted By: Ed DaviesOn the outside the interface resistance is usually taken to be small, because at least some wind is assumed
    That's another, comparable approximation that we accept trustingly. How long ago were these assumptions and approximations established, and do they still hold good in the new regime of ultra-low U values? Are all our so-precise calcs (esp in e.g. PHPP) in fact way out?
    Posted By: Ed DaviesI've no idea how much energy is really transferred by radiation at typical temperatures
    We ought to know, shouldn't we.
  2.  
    Posted By: fostertomAre all our so-precise calcs (esp in e.g. PHPP) in fact way out?


    No idea about PHPP but many studies I and other have conducted using Tas demonstrate that dynamic results bear little relation to steady state u-value calcs...
    •  
      CommentAuthorfostertom
    • CommentTimeApr 3rd 2013
     
    Yeah, that's another thing again.

    How do dynamic results compare with U value based calcs using degree-days i.e. averaged over a 'typical' month (assuming the 'typical' month that's described in the degree-days figures is derived from the same date-range as the 'typical' month that's described in the weather data fed into a Tas calc).
    •  
      CommentAuthorfostertom
    • CommentTimeApr 3rd 2013
     
    Posted By: Ed DaviesI think glass is pretty opaque to long-wave IR
    Not so sure - AFAIK it's just a %age opaque. We scorned the old hard-coat on Pilks K-glass (remember that?) because it was so weak in blocking IR compared to the European soft-coats, which themselves came in different grades of effectiveness.

    Do we have info on the %age of IR that's blocked by different coatings? Remember the coatings have no relevance to the conductive/convective transmission thro multi-pane units - they're purely about radiant transmission.

    Isn't it true that even a top grade coating only blocks - I dunno - 65% of the IR? and more typical soft coats perhaps 50%?
    If so, then it is true that
    Posted By: fostertomthat doesn't make it (transparent fabric) same as opaque fabric - the full temp gradient applies, albeit the resultant radiant heat flow encounters resistance at the glass line
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 3rd 2013
     
    As most windows are vertical, and most houses are surrounded by a other houses, isn't the temperature difference just the difference between the inside of the house and the outside of the house it is looking at rather than the black cold space?
    So you are probably only talking about 10 to 20°C for a few days rather than a couple of hundred °C.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 3rd 2013
     
    I agree, that's the order of it - not deep space but the temp of particles in the atmosphere - but that still can be way below zero, hence ground frosts even when the ground-level air is above zero. Low-mass things like tree leaves (as well as grass on the ground) also get very cold and so constitute a large-area and strong 'pull' to radiation thro glass from the interior.
    • CommentAuthortony
    • CommentTimeApr 3rd 2013
     
    If the window can see the sky....
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 3rd 2013
     
    At a 20 K temp difference the losses are 0.0083 W/m^2
    40 K it is 0.13 W/m^2

    I think, so pretty minor
    •  
      CommentAuthorfostertom
    • CommentTimeApr 4th 2013
     
    These are strictly radiation losses?
    • CommentAuthortony
    • CommentTimeApr 4th 2013
     
    it is a small loss but a huge difference in percentage terms and it could be 200K for a window with a view of the horizon.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 4th 2013
     
    Posted By: tony200K for a window with a view of the horizon.
    I am not so sure it is that high when looking at the horizon, there is a lot of warm air and particles before you reach cold space.
    I think that if it was a serious effect then we would be getting windows icing up in air conditioned buildings in the West Indies.
    • CommentAuthorEd Davies
    • CommentTimeApr 4th 2013
     
    Posted By: SteamyTeaI am not so sure it is that high when looking at the horizon, there is a lot of warm air and particles before you reach cold space.
    Indeed. From pointing an IR thermometer upwards on a clear night I think the effective radiant temperature of the sky then is around -40 °C or so. Difficult to be sure as those thermometers aren't brilliant at measuring temperatures much lower than themselves but seems about right.

    Romans making ice cream in the desert, etc.
    • CommentAuthorSeret
    • CommentTimeApr 4th 2013
     
    Posted By: fostertomNot so sure - AFAIK it's just a %age opaque.


    That's correct, about 2% of long wave IR will pass through glass.

    Posted By: SteamyTeaAt a 20 K temp difference the losses are 0.0083 W/m^2
    40 K it is 0.13 W/m^2

    I think, so pretty minor


    That's for a black body, in reality the emissivity will mean it's even lower. For example a cotton sofa at ΔT of 20K will emit about 0.007Wm-2. Low-e windows can have an emissivity of something like 0.05 IIRC, so you'd be looking at 0.0005Wm-2. There's a reason we don't really bother going out of our way to separately model radiative heat loss in the standard models for buildings. It's just not a big deal at the temperatures involved.

    As for radiation to the sky (space being what, 3K or somthing?), as ST points out the atmosphere isn't particularly transparent, especially at low angles. If it wasn't so we wouldn't have seasons, and in fact life on Earth wouldn't be possible. The insulating effect of the atmosphere stops the Earth radiating away all the heat it gets from the sun and raises the average temperature from what it's supposed to be (about -15º) to a nice comfy 15º. Which kind of makes the idea of a "habitable zone" for exoplanets a bit moot, as technically not even Earth is in the sun's habitable zone. But I digress...
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 4th 2013
     
    Yes I have pointed mine at the sky and got readings from about 20 to -195°C. Don't trust any of them as it is really out of the distance range.

    Now did the the Romans use just temperature or did they use evaporation cooling, or even a chemical process. Often wondered what they did for us.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 4th 2013
     
    Posted By: SeretThat's for a black body
    I used 0.92 as that is what is claimed for glass. But that is close to 1.
    Would a cotton sofa at 20K shatter when sat on :wink:

    Was a bit about 'habitable zones' in New Scientist and they have extended the zones now because water vapour plays a crucial role in climate, also explains, to some extent, Earth early climate.
    •  
      CommentAuthorfostertom
    • CommentTimeApr 4th 2013 edited
     
    Posted By: Seretabout 2% of long wave IR will pass through glass
    Sounds a bit too general - if that's true of ordinary glass, why bother with coatings?
    Posted By: SeretThere's a reason we don't really bother going out of our way to separately model radiative heat loss in the standard models for buildings. It's just not a big deal at the temperatures involved
    If so, then again, why do the coatings on glass (vs none, or low-grade ones) make such a big difference to overall heat transmission?
    •  
      CommentAuthorfostertom
    • CommentTimeApr 4th 2013 edited
     
    Posted By: SteamyTeaWould a cotton sofa at 20K shatter when sat on :wink:
    He actually said
    Posted By: Seretcotton sofa at ΔT of 20K will emit about 0.007Wm-2

    I'm suggesting that at times when these considerations really matter - i.e. when outside air temp is falling, or it's a cold clear-sky night, or just when the sun goes in, the delta-t as affecting radiation will be a lot more than 20K delta-t. It will be between the sofa at 20C, and
    Posted By: Ed Daviesthe effective radiant temperature of the sky then is around -40 °C or so
    - so 60K delta-t, or heading that way.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 4th 2013
     
    Just pointed the IR Thermometer to the slightly hazy North Sky and it is showing -39, horizontal the lowest I can get is -2, air temp is 3.4°C.

    The reason for coating glass is that it can allow, to a certain extent to filter just one wavelength. If you remember I did some tests a few years back that shows that the coated glass works for trapping in air at a higher temperature.
    • CommentAuthorSeret
    • CommentTimeApr 4th 2013 edited
     
    Posted By: fostertom
    Posted By: Seretabout 2% of long wave IR will pass through glass
    Sounds a bit too general - if that's true of ordinary glass, why bother with coatings?


    You're confusing emissivity with transmittance. Low-e coatings are to stop the window itself radiating, not to stop IR radiated from other sources. Ordinary float glass is perfectly good at blocking long wave IR, which is why it works for greenhouses, but it's emissivity is above 0.9, so it's an excellent radiator. Hence low-e coatings, they cut radiative heat loss from the windows themselves enormously.

    I've just realised I did my maths wrong anyway, it's Thot4-Tcold4 not ΔT4.

    So the sofa at 20°C, if it were taken outdoors into freezing temps would radiate at about 75Wm-2, but through the window of your living room it would only manage 1.5Wm-2. Reflection and the low-e coating on the window would mean that the window would itself radiate bugger all of the other 73.5W.

    If the sofa were radiating to a -40°C sky through the window it would manage about 3.8Wm-2.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 4th 2013 edited
     
    Posted By: SeretThot4-Tcold4not ΔT4.
    I just used an online calculator, so not sure how it worked it out. Both are small losses really, and then people pull curtains over windows and turn up the heating.
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press