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General: R value to U value conversion

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  1.  
    • CommentAuthorBFG
    • CommentTimeNov 18th 2009
     
    Hi all,

    Quick question and hopefully a quick answer. I did a quick forum search, and couldn't find what I was looking for.

    How do you convert R values to U values? Insulation rolls are quoting an R value of 3.75, so how much do I need to put in the ceiling to get a U value of 0.2 or better?

    Many thanks,

    Rob
  2.  
    Rob

    If there are no repeating thermal bridges then the U value is approximately given by the reciprocal of the sum of the R values. If you have timber joists or rafters bridging the insulation then the R value needs to be corrected for the bridging. If you're interested, I have a spreadsheet which covers a ceiling below loft situation.

    David
  3.  
    Posted By: davidfreeboroughU value is approximately given by the reciprocal of the sum of the R values.
    It's not approximate, it's exact.

    Just to note that there is a difference between North American R values and European ones (sometimes called RSI). If you have RSI, then the U value is just the reciprocal (and vice versa if you want to go from U to R). To convert a US R value to RSI, divide by 5.678; to convert an RSI value to North American R, multiply by 5.678.

    OK, some examples - insulation rolls of R=3.75 sounds like it's an RSI value. In US R values, this would be R21 - which sounds about right for 5" thick fibreglass. To convert that to U would give 1/3.75 = 0.26 which is about a factor of two away from passivhaus levels.

    Thermal bridging calculations are the same whether you work in U or R as it's the final composite value that takes all the construction details into account that matters. For example, a standard 2x6 construction timber frame wall with nominal R29 (US values) batt insulation installed would probably work out closer to R22 once the bridging of the studs is taken into account. This would be 1/(22/5.678) = 0.25 in U values - i.e. not that good.

    Paul in Montreal.
    • CommentAuthorBFG
    • CommentTimeNov 18th 2009
     
    Not sure I'm at the level where a spreadsheet is going to be much good to me. The main thing I was after is whether a bigger R value is better or worse - as it's the inverse, I guess bigger is better to reduce the U value.

    Would it be possible (or advisable) to put as much insulation as I can fit between the joists, then go over the joists with some kind of insulating board, and then loft boards on top of that? Or are the joists not really going to impair the insulation that much?

    Thanks,

    Rob
  5.  
    Paul

    I said approxinately because you need to add the surface resistance.

    Rob

    If you put 100mm Celotex Kingspan between the ceiling joists then (depending on the timber) the effective thermal resistance can be reduced from 4.35 to 2.45. If you're using an insulation with a lower thermal resistance (closer to timber) then the difference isn't so marked.

    David
  6.  
    Posted By: davidfreeboroughI said approxinately because you need to add the surface resistance.
    That has nothing to do with the conversion between R<->U as you have to take this into account anyway.

    Paul in Montreal.
  7.  
    Paul

    I understand R value to be a property of a material, excluding surface resistances, and U value to be a property of a construction, including surface resistances. So going from one to the other you need to add surface resistances.

    David
  8.  
    Posted By: davidfreeboroughI understand R value to be a property of a material, excluding surface resistances, and U value to be a property of a construction, including surface resistances. So going from one to the other you need to add surface resistances.
    You can use either unit for either: materials have a U value and the final construction does - it doesn't make any difference which you use. I find R values easier as you're adding simple numbers Rsurface+Rlayer1...+Rlayer_n+Rsurface = Rconstruction. With U values you're adding the reciprocals. You can still add 1/Usurface too.

    Paul in Montreal.
    • CommentAuthorCWatters
    • CommentTimeNov 18th 2009 edited
     
    Posted By: davidfreeboroughPaul

    I understand R value to be a property of a material, excluding surface resistances, and U value to be a property of a construction, including surface resistances. So going from one to the other you need to add surface resistances.

    David


    Not allways! Different people use R and U Value to mean different things. Also depends on how that countries regulations require it to be measured.

    The Thermal Resistance is the basic physical property of the material. In SI units it's the resistance of a square meter slab that's one meter thick (without surface effects). The units are in mK/W. Some people call this the R-Value. I believe the regulations in the USA require surface effects as well as actual thickness (obviously) to be taken into account when you quote an R-Value in the context of building insulation. Metric R-Values will be in m^2*K/W.

    The reciprical of Thermal Resistance is the Thermal Conductance and the units are W/mK. We tend not to call this the U-Value though. We tend to reserve U-Value for either a) The thermal conductance of the actual thickness or b) the U-Value in the Building Regs which refers to the whole wall. The units are in W/m^2*K.

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