Calculating U value for walls

I'm having trouble getting the definitive way of calculating the U value of a wall. Do I need to take the reciprocal of all of the seperate Rvalues of the wall materials ie.

1/R1+R2+R2

Also How do I calculate the R value for a give thickness of a material taken that I know the U-value.

I don't want to use software, I want to know the formula so I understand what is going on.

The u value is the reciprocal of the sum of the resistances.

Resistance is the product of the thickness in m and the resistivity or the thickness in m divided by the lambda value.

Eg 50mm of celotex(0.02)gives a resistance of 0.05 /0.02 = 2.5 and 50 mm of rockwool(0.04) gives a resistance of 0.05/0.04 = 1.25

therfore u value is 1/ (2.5+1.25)=0.26 wcm2.

Dont forget bto add surface resistances and allow for cold bridging.

I am trying to calculate the U value of an exposed gable wall which is constructed in solid brickwork, the wall is 400mm thick with 12.5mm plaster finish. Can you help

Paul in Montreal posted on 03-11-06

Hot2000 gives the following figures for your wall structure:

400mm 1920kg/m^3 red fired brick RSI=0.45 12.5mm 1680kg/m^3 gypsum plaster RSI=0.02

Composite RSI for the wall strucutre including air layers RSI=0.63

U is the reciprocal of RSI and so U=1.58

In other words, pretty dismal!

I assumed medium density brick and dense plaster. Using light plaster gives RSI=0.62 for the composite strucutre which is U=1.61

In comparison, a 2x6 construction timber frame filled with batts, 19mm polyisocyanurate board etc. etc. gives a composite RSI=3.92 which is U=0.25 - about 6x better (or equivalent to brick that is over 9feet thick!

Hope this helps,