Jump to content


ebuild is sad to announce its closure - it has become too time and resource intensive to develop, manage and maintain.

However, ebuild will remain on-line in archive mode (ie no posting facilties) for several weeks so that users can use it as an information resource.

U-Values, Thickness, Confusion.


  • Please log in to reply
11 replies to this topic

#1 MarkH

MarkH

    Member

  • Members
  • PipPip
  • 48 posts
  • LocationWest Wales

Posted 14 April 2016 - 11:51 AM

If I build a wall using blocks with a claimed u-value of 0.11W/m2 and a thickness of 200mm, how does it's u-value vary if the wall is, say, only 100mm thick. Why, when 175mm of external insulation is added to a wall built with 0.11W/m2 blocks do the various online calculators give values of >0.11? Why doesn't a wall built of 0.11W/m2 material have a u-value of 0.11W/m2?

I'm missing something here I know...

#2 SteamyTea

SteamyTea

    Advanced Member

  • Members
  • PipPipPipPip
  • 4,322 posts
  • LocationCornwall

Posted 14 April 2016 - 12:13 PM

It is an exponential law, it is why the first inch of insulation seems to save twice as much as the second inch.
Also you have to look at the units. Insulation is measured in watts, not joules, so you have to be careful as what you want to answer i.e. how powerful a heater, or how much energy a year.

#3 Alphonsox

Alphonsox

    Advanced Member

  • Members
  • PipPipPipPip
  • 1,495 posts
  • LocationCounty Down, NI and Forest of Dean, England

Posted 14 April 2016 - 12:27 PM

U-value is Thermal Transmittance (U) and describes the amount of energy lost through a m2 of a particular thickness of material. I find using thermal resistance "R" a more useful way to visualize things where R=1/U.
Stack different materials together and you get the overall thermal resistance by adding the individual R values. To get the overall U-value take the reciprocal of the overall R value.

In you example work out the R values of the block and insulation, add them together and divide into one to get the overall U-value.

Edited by joiner, 17 April 2016 - 05:30 AM.
Typo: "by one" changed to "into one" for reciprocal.


#4 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 14 April 2016 - 12:29 PM

View PostMarkH, on 14 April 2016 - 11:51 AM, said:

If I build a wall using blocks with a claimed u-value of 0.11W/m2 and a thickness of 200mm, how does it's u-value vary if the wall is, say, only 100mm thick. Why, when 175mm of external insulation is added to a wall built with 0.11W/m2 blocks do the various online calculators give values of >0.11? Why doesn't a wall built of 0.11W/m2 material have a u-value of 0.11W/m2?

I'm missing something here I know...

First of all, the online calculators aren't always accurate; some are OK, some are less so.

To get an accurate answer it is best to forget about the quoted U value altogether, and look at the specified lambda value instead. This is a fixed property of the material and doesn't change with thickness. You can use the material lambda value to calculate the thermal resistance, the R value, for any thickness of any material quite easily.

For example, say you have a bit of PIR foam with a lambda value of 0.022 W/m.K and you want to know the R value of a 175mm thick sheet of it. The calculation is to just divide the thickness (in metres) by the lambda value. In this example, 0.175 / 0.022 = 7.955

You can then add all the R values up for a wall, roof, or floor to get the total thermal resistance (R value) and convert that to the U value by taking it's reciprocal. For the above example of just a sheet of PIR foam, then the U value for an R value of 7.955 = 1 / 7.955 = 0.126 W/m2.K

Edited by jsharris, 14 April 2016 - 12:29 PM.
cross-posted with the above


#5 ADLIan

ADLIan

    Member

  • Members
  • PipPip
  • 19 posts

Posted 14 April 2016 - 12:51 PM

OP is confusing lambda (thermal conductivity, W/mK) with U-value (thermal transmittance, W/m2K) - note 'm' in units or 'm2' (not sure how to superscript here).

The block referred to is probably Celcon Solar or similar with lambda value of 0.11 W/mK. For approx U-value follow instructions from JSH above but remember for Building Regs the calcuation is much more complex and takes account of repeating thermal bridges ( eg mortar beds with lightweight block, studs in timber frame etc), mechanical fixings (eg wall ties in cavity construction) and gaps in the insulation layer.

Ian

#6 MarkH

MarkH

    Member

  • Members
  • PipPip
  • 48 posts
  • LocationWest Wales

Posted 14 April 2016 - 09:03 PM

View PostADLIan, on 14 April 2016 - 12:51 PM, said:

OP is confusing lambda (thermal conductivity, W/mK) with U-value (thermal transmittance, W/m2K

Yes, I think I was.

Thanks everyone who's replied above, much appreciated. I'm scrambling to learn so much new stuff this was neglected and I was going along with the throwaway use of 'u-value' a lot of the manufacturers seems to employ. I suspected the online calculators weren't all that great and in light of the above it'd probably be a good idea to run through some calculations myself and get a grip on it. But, any recommended calculators?

So essentially R-value is derived from Lambda and u-value from R, right?

Puts my basic wall makeup at a shade over 0.10 W/m2k u-value therefore (not taking into account thin mortar, gaps etc).

Again, thanks.

Edited by MarkH, 14 April 2016 - 09:10 PM.


#7 jsharris

jsharris

    Please ignore all posts by me, some are erroneous

  • Member Blogger
  • PipPipPipPip
  • 11,461 posts
  • LocationWiltshire/Dorset border

Posted 14 April 2016 - 09:13 PM

Yes, that's right. you often have to dig around to find the lambda, as some manufacturers hide it away, but once you have it you can work out the R values for each thickness of each material, then add up all the R values to get a single number, then divide this number into one and you get the centre panel U value, excluding cold bridges etc, which is a whole new exercise in arithmetic fun........................

#8 MarkH

MarkH

    Member

  • Members
  • PipPip
  • 48 posts
  • LocationWest Wales

Posted 14 April 2016 - 09:31 PM

View Postjsharris, on 14 April 2016 - 09:13 PM, said:

....excluding cold bridges etc, which is a whole new exercise in arithmetic fun........................

Not sure I'll be going too far down that rabbit hole... Probably.

#9 ADLIan

ADLIan

    Member

  • Members
  • PipPip
  • 19 posts

Posted 15 April 2016 - 07:31 AM

MarkH

If it's a new build dwelling (or other building subject to Part L1A/L2A) this is a 'bridge' you will have to cross at some point. Thankfully there are lots of set values for the thermal bridges at juncions and around openings, known as the psi-value, depending upon the exact detail. These are used to estimate the additional heat loss from these non repeating thermal bridges, the Y-value.

Enjoy the maths.

#10 temp

temp

    Advanced Member

  • Moderators
  • 10,200 posts

Posted 15 April 2016 - 10:13 AM

Lambda (thermal conductivity, W/mK) tells you how much heat (in Watts) escapes through a cubic metre of the material for every degree of temperature drop across it.

Most walls aren't a metre thick so to take that into account you divide by the thickness of the wall.

Most walls have an area bigger than one square metre so to take that into account multiply by the area of the wall.

Edited by joiner, 17 April 2016 - 05:32 AM.
Typo: "meter" to "metre"


#11 gravelld

gravelld

    Regular Member

  • Members
  • PipPipPip
  • 655 posts

Posted 15 April 2016 - 10:40 AM

View PostMarkH, on 14 April 2016 - 09:31 PM, said:

Not sure I'll be going too far down that rabbit hole... Probably.
If you do have U=0.1 for your walls, that's a high performance wall. It might be worth looking at the psi because (1) the importance increases as your U decreases and you might be on the verge of building a bloody good house and you don't want to waste that and (2) it can actually have bad effects if, e.g. you end up with interstitial condensation.

The point is: be aware, measure, model, and try to understand. You're building a proper house, don't leave it to chance.

#12 MarkH

MarkH

    Member

  • Members
  • PipPip
  • 48 posts
  • LocationWest Wales

Posted 16 April 2016 - 10:57 PM

View Postgravelld, on 15 April 2016 - 10:40 AM, said:

If you do have U=0.1 for your walls, that's a high performance wall. It might be worth looking at the psi because (1) the importance increases as your U decreases and you might be on the verge of building a bloody good house and you don't want to waste that and (2) it can actually have bad effects if, e.g. you end up with interstitial condensation.

The point is: be aware, measure, model, and try to understand. You're building a proper house, don't leave it to chance.

Thanks. Good advice that I will follow. Now to google Psi...