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Essential for Newbies: Are all U-values created equal?

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  1.  
    • CommentAuthorRobinB
    • CommentTimeApr 28th 2009
     
    Is an American U-value say the same as a European or British one?

    I found this definition which sounds quite different to the one in the Newbies glossary on this forum ( very useful glossary, by the way)

    "U-value (coefficient of heat transmission) - The rate of heat transmission through 1 square foot of building envelope for 1 degree Fahrenheit difference in temperature between indoors and outdoors."

    Or do square feet and degrees fahrenheit magically just happen to tally with square metres and degrees c. ?
    thanks!
  2.  
    I've posted this before, but it's worth reposting. The US uses Imperial units and tends to use R values rather than U values. There are simple conversion equations to go between the different forms.

    See http://en.wikipedia.org/wiki/R-value_(insulation)

    Square feet and degrees F definitely do not magically tally with square metres and degrees C!

    Paul in Montreal.
    • CommentAuthorCWatters
    • CommentTimeApr 28th 2009
     
    What he said. Best to stick to using U-Values quoted in Metric units (eg W/M^2/C or in words "Watts per square meter per degree Centigrade"). If you need a conversion have a go and then post for someone to check it.

    It's worth mentioning that on some US web sites they talk about the thermal resistance of a wall being R where R=1/U (in the right units). On other sites they use R to refer to the thermal resistance of a material "per foot of thickness". So if you see something specified as "foot" rather than "square foot" beware the thickness may need to be included.

    In some cases I've also seen R values for a material adjusted by some seemingly arbirary factor in an attempt to correct for the surface finish of the material - this may mean that doubling the thickness may double the thermal resistance but does not double the R value a manufacturer quotes for compliance with regulations. I may have that totally wrong though!
    • CommentAuthorRobinB
    • CommentTimeApr 28th 2009
     
    This was the web-page that confused me, written for US market, it says U value is BTU/hr.ftsquared.degrees F and the U values they quoted were tiny. So I guess the moral is don't compare U values unless you know what U is standing for? At least they kindly quoted their method for calculating the U value.

    http://www.durisol.net/documents/ThermalPerformance.pdf

    Seems R values also open to differnet calculation methods to! thanks Paul and CWatter for helpful comments
  5.  
    Here's a page of conversion values: http://www.city.leduc.ab.ca/Leduc/1024/City_Departments/Planning/Codes_and_Inspections/Conversion_Factors.asp

    1 RSI = 5.678 R (USA)

    So a European U value is 5.678 times larger than the equivalent US one.

    E.g. In the US, a wall that is R30 is pretty well insulated. R30 = 5.28RSI

    5.28 RSI = 0.18 U

    So, in your docment, a U value in US units of 0.0441 is equal to 0.25 in Euripean U units.

    Personally, I prefer R values as bigger=better. The problem with U values is that the better they are, the closer to zero. It's hard to tell at a glance how much better U=0.10 than U=0.15 whereas in US R values 56.7 sounds a lot better than 37.8 - though in all cases diminishing returns rapidly set in. In other words, wall that's R40 will not save twice as much money as one that is R20 - it will incrementally save half of what was saved by the R20 to begin with.

    Hope this all helps :)

    Paul in Montreal
    • CommentAuthorchuckey
    • CommentTimeApr 28th 2009
     
    In the 1970s we, in the UK, also used U values based on BthUs/sq. ft/ deg F. Its a shame they did not call the metric U, Um (no thats the name!).
    Frank
    • CommentAuthorjerseyman
    • CommentTimeApr 28th 2009
     
    Sort of hijacking this thread but the definition of u-values has always puzzled me a bit viz:

    "a measure of heat transmission through a building part or a given thickness of insulating material, expressed as (W/m2K) that will flow in 1 hour through 1 square metre of the structure or material from air to air with a temperature diff of 1°C."

    The bit that puzzles me is "1 square metre of the structure or material ". These seem to be two different measures.

    Thermocellit block 300 mm thick are quoted as having a u-value of 0.36 W/m²K

    Aerogel is quoted as 0.013W/mK

    The former I assume is the U-value of 300mm thick Thermocellit and 1000 mm of Aerogel, but just looking at the raw figures how would you know this.

    (I am working on the basis that the only stupid question is the one you don't ask)
    • CommentAuthormike7
    • CommentTimeApr 28th 2009
     
    jerseyman - the confusion is here:- the scientific measure of conductivity of the material is in W/m2 per unit of temperature gradient in degrees across one whole metre, or K/m. One of the 'm's cancels out, leaving you with W/mK.

    The u-value is a practical measure which takes account of the thickness of the material being used to give the loss per m2 per degK of temperature difference across that actual thickness of wall or whatever.

    That works out to the u-value being the conductivity divided by the thickness in metres. Not surprising people get confused. Me too.
    • CommentAuthorCWatters
    • CommentTimeApr 29th 2009
     
    Posted By: jerseyman

    Thermocellit block 300 mm thick are quoted as having a u-value of 0.36 W/m²K
    Aerogel is quoted as 0.013W/mK

    The former I assume is the U-value of 300mm thick Thermocellit and 1000 mm of Aerogel, but just looking at the raw figures how would you know this.


    What he said.

    It's exactly what I was on about above. The heat lost through a sheet of material is proportional to the area (m²) and temperature difference (K) but inversly proportional to thickness (1/m). So the units are really Wm²K/m. It would be a lot easier if that's what people wrove but it's conventional for the m to cancel leaving WmK.

    If you are told the WmK value for a given material you have to divide the WmK figure by the thickness in meters to get the U-Value in Wm²K.
    • CommentAuthorRobinB
    • CommentTimeApr 29th 2009
     
    thanks guys! It is as confusing as I thought but much clearer now.
  11.  
    This is why I prefer R values - you just add them up and bigger is better. No messing around with reciprocals and values less than 1 that are hard to visualize.

    Paul in Montreal
  12.  
    if a canadian window has a u value of 0.24 what would that be in the uk is there any easy way to convert
  13.  
    Posted By: jolly-green-giantif a canadian window has a u value of 0.24 what would that be in the uk is there any easy way to convert

    Would be easier to read if you put in some punctuation your comments :)

    "However, European insulation values are given in metric U-factors (Resistance Systeme Internationale, ie, RSI). As such, the calculations for thermal transfer and building envelope volumes use metric instead of Imperial units for thickness (centimeters instead of inches), weight (kilos instead of pounds), volume (cubic meters instead of cubic feet) and temperature (centigrade or Celsius, instead of Fahrenheit).

    Metric U-factors are defined as Watts per square meter per degree Celsius. To convert inch-pound Imperial U-values to metric U-factors, multiply by 5.678. To convert metric U-factors to Imperial inch-pound U-factors, divide by 5.678. To convert Imperial inch-pound R-values to metric R-values, multiply by 0.1761. One inch = 2.54 cms. One (RSI) U-factor coefficient = 5.678; one R-value = .1761 RSI. "

    http://housebuildingspain.com/page3/page13/page13.html

    If the R is in imperial, than your window of 0.24 would have a British U value of 1.36272

    Hope this helps,

    Paul.
  14.  
    ok paul lets smoke the peace pipe lol
    Well thats a turn up inline windows quote 0.24 u value wich i thought was great but i can get windows in uk at 0.7 u value so that knocks the socks off the canadian ones or am i missing something:bigsmile:
  15.  
    i was lead to belive the u vale in canada was calculated in a differnt manner as well the temp range is different any thoughts
    • CommentAuthorStephen T
    • CommentTimeNov 9th 2009
     
    When it comes to windows, Uwindow is not same on both sides of the Atlantic. It is very hard to accurately compare window U's for North American Windows and those for European Windows. I'm not even sure it's the right question, but i'll drift into that later.

    Being Canadian i'm more familiar with North American approaches, but am learning more and more about how Europe calculates Uwindow.

    In North America Uwindow is calculated at a -20C outdoor temperature. As I understand it, in Europe Uwindow is calculated at a 0C outdoor temperature. This has no effect on frame components. Their conductivity does not vary with small changes in temperature.

    It does have an effect on gas properties and therefore affects Uglass. The result is the optimum spacing in North America is 12.7 mm (1/2”) , while it's 16mm in Europe. On a related front 3mm glass is permitted in North America, while 4mm glass is the minimum in Europe. So bringing a typical triple glazed North American window to Europe means the airspace is likely 11mm – even more sub-optimal. The work around is to use krypton instead of argon. When you do this North American windows would have equivalent Uglass's to their more widely spaced European counterparts – providing they both had the same type of low-e.

    Another challenge is the different sizes used for calculating Uwindow. The sizes for calculating Uwindow in North American are as follows:
    Outswing Casement: 600mmx1500mm
    Inswing Tilt & Turn: 900mmx1500mm
    Fixed: 1200mmx1500mm
    Europe uses 1290mm x1470mm. As I understand it, BFRC divides this into a 645-645x1470 Casement-Fixed an the PH people simulate the whole thing as 1 Tilt & Turn. The differences in sizes are important because good glass insulates better than good frames. Even if temperature corrected, accurate comparisons require like sizes.

    In the end the best way to compare windows is to do some sort of whole building simulation. The advantage of the PHPP is that it asks for Uframe and Uglass and calculates Uwindow for each window based on its size. The Canadian HOT2000 program has a detailed window mode that does the same thing.

    The advantage of this more detailed approach is that besides accounting for the effect of Uwindow, it also accounts for the effect of G (SHGC in North America). This is where I start to sound like a broken record, but ….. windows are not walls they are not only a source of heat loss but also a source of heat gain. Even in relatively cloudy UK a superinsulated house will cost less to heat with good windows than with no windows at all.

    When i've looked at this, the very best North American windows produced slightly lower space heating loads than a typical PH windows, despite the fact the typical PH window insulates better. Where North American windows shine, so to speak, is that their slimmer frames permit higher solar gains, and that matters, even in the cloudy UK.

    Hope that helps
    • CommentAuthorstephendv
    • CommentTimeNov 9th 2009
     
    Posted By: Stephen T
    When i've looked at this, the very best North American windows produced slightly lower space heating loads than a typical PH windows, despite the fact the typical PH window insulates better.


    Amen brother. I've found the same thing comparing triple glazed windows with lower U values and also lower G values, versus DG units with worse U values but better G values. Granted our planned build is in Northern Spain (winter sun, -7C at coldest) and is designed with big south facing windows and smaller north facing one, the PHPP says:
    - DG units, Uframe = 1.4, Uglass = 1.1, G=63%: Total heating demand = 16.9kWh/m2
    - 3G units: Uframe = 1.4, Uglass = 0.7, G=50%: Total heating demand = 18.6kWh/m2

    Best of both worlds: DG for south, 3G for everything else: 16.7kWh/m2 so in our climate not a huge advantage to going 3G, even for just the north facing windows.

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