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General: Would somebody be able to check my logic on U-values?

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    • CommentAuthoraa44
    • CommentTimeMar 11th 2010
     
    I am trying to choose between two building methods. One will give me a U-value for my walls of 0.12. The other one will give me a U-value of 0.15. Am I right in thinking that this means that the higher one will lose 0.03 watts per square metre for a one centigrade difference?

    I live in Shetland where the mean annual temperature is about 7 C. Assuming that I want my house to be an average of 18 C inside, this implies an average temperature difference of 11 degrees. So the additional heat loss from the higher U-value would be 11 x 0.03 = 0.33 watts per square metre per hour.

    If so, would I be right in assuming that this equates to 24 x 365 x 0.33 = 2.89 kw per year per square metre.

    Assuming that I am using a heat pump that is costing me, say, 10p per kWh, then this would be an additional annual heating cost of 29p per square metre of wall per year.

    I appreciate that there are loads of other factors involved but is my thinking roughly right? The quote for the method with the lower U-value is about £15 per square metre more than the other one so for a saving of 29p per year I don't think that it's worth it.
    • CommentAuthorMike George
    • CommentTimeMar 11th 2010 edited
     
    Yes, you are correct in your thinking and your calcs. Have a look here http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=5333&page=10. Chart plotting the law of diminishing returns - Page 10 onwards
    • CommentAuthortony
    • CommentTimeMar 11th 2010
     
    But how long will the building be there? and how much will energy costs rise

    only comparing two methods is not so great -- I think you can always get a lower price and a lower U value

    With zero inflation you are saying the house is only worth 50 yrs but with any kind or reasonable energy cost inflation you could be wishing you had done it better after only 10 years

    Also how much would it cost to upgrade as a retro fit

    I built to near 0.1 for my walls.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 12th 2010 edited
     
    Andy

    Internal temp of 18C, you are a hardy lot up there!

    You need to have a look at the climate regime for the Shetlands and work out the Degree Days. These are a measure of how much and how long you will be heating (why you get very large numbers). You may be better off making your own chart up for your needs, you should be able to find some local weather station data to put into it.

    Posted By: aa44If so, would I be right in assuming that this equates to 24 x 365 x 0.33 = 2.89 kw per year per square metre.


    Think this should be kWhy-1m-2 (kiloWatt hour per year pre square metre of wall surface area) at delta 11C.

    I would you along with Tony and try and get a lower value if you can, and what are the values for your windows, doors, floors and roof, these surface areas will make up the majority of your exposed surface area.
    Have you considered your air changes per hour?

    As you are up in the frozen north you have high winds and lots of rain (same as here) as well, these will adversely effect your energy use.

    Posted By: aa44I appreciate that there are loads of other factors involved but is my thinking roughly right? The quote for the method with the lower U-value is about £15 per square metre more than the other one so for a saving of 29p per year I don't think that it's worth it.


    Think you have really hit the nail on the head here, the price difference breaks even at about 52 years (at today's costs) but as you are only talking about just the walls and not the whole house I think you have probably made the right choice (unless you can get the price even lower).
    There is always a lot of speculation about future energy prices but as our energy mix is changing towards nuclear/renewables (and possibly clean coal) and that we will have wage inflation (been very low the last decade) my view is that unless you can afford the best (totally off grid) it is not worth worrying about in the long term as it just becomes a guessing game.
    • CommentAuthorRobinB
    • CommentTimeMar 12th 2010
     
    Would there be any logic in putting more insulation on the windward or sunless sides, and less elsewhere?
    • CommentAuthoraa44
    • CommentTimeMar 12th 2010
     
    Thanks very much for your comments. If energy prices go up that much then I'll stick a bigger wind turbine up!
    • CommentAuthoraa44
    • CommentTimeMar 12th 2010
     
    @ RobinB - I'm not sure what would be best. The North side is the most sheltered but the winter gales come from the south-west.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 12th 2010
     
    Posted By: aa44The North side is the most sheltered but the winter gales come from the south-west.


    Would have to do a full site survey for that one and find the relationship between windspeed, rainfall, solar gain and temperature. Just had a great idea for a project for someone.



    Posted By: aa44I'll stick a bigger wind turbine up!


    That's the spirit, bigger is best. Don't you have a few very large ones going up there at the moment?
    • CommentAuthorGavin_A
    • CommentTimeMar 12th 2010 edited
     
    You've not allowed for the wind chill factor, which is likely to be pretty significant unless you're in a particularly sheltered spot, as the nasa estimates for shetland in general give around 9m/s average wind speed through the winter months.

    I don't know what affect that will have exactly, but I'd think (at a guess) it could easily increase your annual heatloss by anywhere between 25-50% over the pure degree day calculations. (eta - actually, 50%'s probably way too high for just wind chill alone as opposed to including increased drafts etc)

    If you're going for either ground source, or particularly air sour heat pumps, it's also worth considering that the increased heat loss will have the most effect when it's coldest, at the exact same time as the heat pump's efficiency is at it's lowest, resulting in either a cold house, or increased heat pump installation costs if you have to increase the size to cope, and higher energy bills due to the reduced efficiency, particularly if you end up using a resistive electric heating backup more often than you otherwise would have done.
  10.  
    aa44. You are roughly correct. For the purposes of comparison, you are thinking on the correct lines.

    Dont forget air infiltration. This can have a big impact on actual running costs. Do you different construction solutions have differences in the membrane detail? Anthing with an un-interupted membrane will have a benificial effect on the building infiltration by reducing air ingress
    • CommentAuthorCWatters
    • CommentTimeMar 12th 2010 edited
     
    Posted By: aa44I am trying to choose between two building methods. One will give me a U-value for my walls of 0.12. The other one will give me a U-value of 0.15. Am I right in thinking that this means that the higher one will lose 0.03 watts per square metre for a one centigrade difference?


    Which would be easier to get right? I reckon a lot of good intentions are spoilt by poor work when the instulation is fitted badly. The one that's easier to get right with fewer air gaps might work out better even if the theoretical u-value is worse.
    • CommentAuthortony
    • CommentTimeMar 12th 2010
     
    and there is thermal bypass to consider. ease of alteration, longevity, real world vs theoretical U values and loads of other variables.
    • CommentAuthoraa44
    • CommentTimeMar 12th 2010 edited
     
    They are actually both ICF methods from the same supplier. One has 100 mm insulation either side of the concrete (U 0.15) and the other has 50mm on the inside and then 200 mm on the outside (U 0.12). This second one uses two panels on the outside and a fancier tie and costs about £15 per square metre more.

    I had been looking at whether I could reduce the cost by building a blockwork cavity wall with something like 200 mm of Kingspan in it but the building regs won't allow a masonry building of any height in the proposed location (towards the top of a slope by the coast in the windiest part of the country) so it has to be timber frame, SIPS or ICF. ICF appeals because of it's excellent structural rigidity and airtightness. I'm in a timber frame house now and it sounds like somebody is playing the drums on the wall when the wind gets up.

    @SteamyTea - The windfarm plans up here have really divided the community. The plan is for something like 100 machines, each about 150m tall and there is no doubt that they would completely dominate the landscape. But that's a different debate!
    •  
      CommentAuthordjh
    • CommentTimeMar 12th 2010
     
    Posted By: aa44building regs won't allow a masonry building of any height in the proposed location (towards the top of a slope by the coast in the windiest part of the country) so it has to be timber frame, SIPS or ICF

    I don't understand that. What's the problem with masonry? How can the weather distingusih between ICF and masonry with EWI for example?
    • CommentAuthoraa44
    • CommentTimeMar 12th 2010
     
    I believe that it is because a masonry build is, to a large extent, dependent on the strength of the mortar joints to stop the walls blowing over, whereas a poured concrete wall (potentially with reinforing in it, does not have the same weak points. It's a bit like comparing a sheet of plywood with a sheet of corrugated cardboard.
    •  
      CommentAuthordjh
    • CommentTimeMar 12th 2010
     
    I believe that it is because a masonry build is, to a large extent, dependent on the strength of the mortar joints to stop the walls blowing over

    So a masonry wall with thin-joint adhesive-based mortar might be alright? (I don't know the relative strengths, I'm just guessing)

    It's a bit like comparing a sheet of plywood with a sheet of corrugated cardboard.

    Sorry, I don't understand that analogy. Sheets of wood glued together versus sheets of cardboard glued together with air gaps? There was a cardboard structure at Ecobuild that was strong enough to walk on :) The chap said it could conceivably be turned into an external wall construction, though that wasn't the idea.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 12th 2010
     
    Posted By: aa44poured concrete


    Don't let Biff see this.

    Only 150m tall, I would be against them, they should be going for much larger ones, a few 5MW ones are what is really needed :bigsmile:
    • CommentAuthormike7
    • CommentTimeMar 12th 2010
     
    Posted By: aa44 The quote for the method with the lower U-value is about £15 per square metre more than the other one so for a saving of 29p per year I don't think that it's worth it.


    That's a return of 2 %/yr. I'm currently getting 0.5% on some ISA money - maybe we should talk....
    • CommentAuthoraa44
    • CommentTimeMar 12th 2010
     
    Stick your money at Zopa. I'm getting 6% for minimal risk.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 12th 2010
     
    I am getting 6% at Santander and Barclays for no risk, but that is for another thread I think when we know more about FIT's Financing as I think we are in for high inflation (wage and goods) and the BoE will up interest rates to possibly 6% in the next two years, going to hurt some people that have borrowed 20k at 2%-3% for some renewables.
  21.  
    Just out of interest you have said that the better insulated wall is £15/m2 more but what are the two m2 prices?
  22.  
    Hi!
    Sorry to put a spanner in your works, but the heat loss you have measured is in seconds not hours. You will need to multiply your figure by 3600(s/hr) and divide answer by 1000 to obtain kW

    Therefore additional heat loss = 0.03 W/m2.C.s x (3600s/1hr) x 11C / (1kW/1000W) = 1.188 say 1.2kW/m2.hr (for a 11C temperature differential). Now lets look at a worse case scenario and do the calcs for a larger temperature differential (Td), shall we say for convenience outside temperature (Tao) of -10C.
    Td = Tai - Tao:
    Td = 18 - (-10); Td = 28C (- + - = +) Additional heat loss equals aprox 3kW/m2.hr

    Regards
  23.  
    I'm pretty sure onceupon a time is right and the figures are wrong This goes to show the dangers of not stating units, very tempting to save time and not do so but there are good reasons for these protocols which help to avoid this kind of error.

    U values are stated in W/m2K (Big W and Big K because Watts and Kelvin are named after people called Watt and Kelvin but there was no Mr/Mrs meter)
    One Watt of power is equivalent to one Joule of energy used in 1 second
    One degree Kelvin is the same as one degree centigrade

    So the difference in your walls systems is 0.03 W per m2 of wall per second hence the need to multiply by 3600 to get it into hours and divide by 1000 to get it into kW so you are a factor of 3.6 out
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 16th 2010
     
    Or a Ms. Metre even :devil:
    • CommentAuthortony
    • CommentTimeMar 16th 2010
     
    Can we re do the calcs and show the relative costs please?
    • CommentAuthorbrig001
    • CommentTimeMar 16th 2010
     
    I'm fairly sure this is right:
    In Shetland you have approx. 2700 degree days (http://www.shetland.gov.uk/housing/documents/FINALDRAFTAppendix1AssessmentofFuelPoverty.pdf).
    Over a year, your walls would loose:
    kWh = 0.12 * m2 * 2700 * 24 /1000 = 7.776kWh/m2/yr
    or
    kWh = 0.14 * m2 * 2700 * 24 /1000 = 9.072kWh/m2/yr
    • CommentAuthoraa44
    • CommentTimeMar 16th 2010 edited
     
    @ onceuponatime and mikeinwestcork

    Sorry chaps but I think you are wrong.

    Doing my original calculation (11 degrees difference) =

    0.03 joules per sec x 11 degrees diff. x 3,600 seconds = 1,188 joules per hour

    1,188 joules per hour = 28,512 joules per day = 10,406,880 joules per year

    There are 3,600,000 joules in a kWh so 10,406,880 joules per year = 2.89 kWh per year


    Think about it. How could the difference of 0.03 in U-values mean an extra heating requirement of 3kW per square metre per hour? That would mean that for your temperature difference example for a room with 10 square metres of outside wall you would need the equivalent of an extra thirty 1 kW heaters in that single room if you chose the U 0.15 wall over the U 0.12 wall. Does that sound likely to you?


    Finally, this is taken from the website WhyTimberWindows.co.uk (the fount of all knowledge!)

    "U values are quoted in units of „Wm-2K-1“. The U-value indicates how many Watts of energy are lost per square metre for every 1 degree temperature difference between inside and outside, over the period of one hour."
  28.  
    Posted By: aa44Finally, this is taken from the website WhyTimberWindows.co.uk (the fount of all knowledge!)
    "U values are quoted in units of „Wm-2K-1“. The U-value indicates how many Watts of energy are lost per square metre for every 1 degree temperature difference between inside and outside, over the period of one hour."

    The definition is correct if you replace "energy" with "power" & loose the last 6 words. In other words,

    "U values are quoted in units of "Wm-2K-1". The U-value indicates how many Watts of power are lost per square metre for every 1 degree temperature difference between inside and outside."

    Watts measure power in Joules per second. Hours don't come into it unless you convert your Joules into kilowatt hours.

    David
    • CommentAuthorMike George
    • CommentTimeMar 16th 2010 edited
     
    Okay guys, just in case you haven't followed the link I gave above here's a chart showing the law of diminishing returns for insulation. The calculations are carried out using algorithms in the dynamic thermal simulation software TAS.
      The diminshing returns of wall insulation.jpg
    • CommentAuthorMike George
    • CommentTimeMar 16th 2010 edited
     
    The above chart and table relates specifically to heat losses through a wall via conduction. Ie, exactly what is calculated using the u-value. Note the difference in annual heating costs in the last two rows. Improving the u-value from 0.13W/m2K to 0.10W/m2K reduces the annual demand by 160kWh at a saving of £7.

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