**1**

# U-Values, Thickness, Confusion.

### #1

Posted 14 April 2016 - 11:51 AM

I'm missing something here I know...

### #2

Posted 14 April 2016 - 12:13 PM

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

Posted 14 April 2016 - 12:27 PM

^{2}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

Posted 14 April 2016 - 12:29 PM

MarkH, on 14 April 2016 - 11:51 AM, said:

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/m

^{2}.K

**Edited by jsharris, 14 April 2016 - 12:29 PM.**

cross-posted with the above

### #5

Posted 14 April 2016 - 12:51 PM

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

Posted 14 April 2016 - 09:03 PM

ADLIan, on 14 April 2016 - 12:51 PM, said:

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

Posted 14 April 2016 - 09:13 PM

### #9

Posted 15 April 2016 - 07:31 AM

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

Posted 15 April 2016 - 10:13 AM

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

Posted 15 April 2016 - 10:40 AM

MarkH, on 14 April 2016 - 09:31 PM, said:

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

### #12

Posted 16 April 2016 - 10:57 PM

gravelld, on 15 April 2016 - 10:40 AM, said:

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...