Combined Method (Wall)
This method considers the upper and lower thermal resistance (R)
limits of an element of structure. The average of these is
reciprocated to provide the U-value.
Formula for upper and lower resistances = (F^1
x...Rx)
Where: Fx= Fractional area of a section
Rx= Total thermal resistance of a section
Using the wall example from the previous page:
Upper limit of resistance (R) through section containing blocks †
(Rso, 0„055)+(brkwk, 0„122)+(ins, 2„631)+(blocks, 0„555)+(plstr,
0„081)+(Rsi, 0„123) = 3„567 m^2 K/W
Fractional area of section (F) = 93„43% or 0„934
Upper limit of resistance (R) through section containing mortar †
(Rso0„055)+(brkwk, 0„122) +(ins, 2„631)+(mortar, 0„114)+ (plstr,
0„081)+(Rsi, 0„123) = 3„126 m^2 K/W
Fractional area of section (F) = 6„57% or 0„066
The upper limit of resistance =
1
(0„943...3„567)+(0„066...3„126)= 3„533m(^2) K=W
Lower limit of resistance (R) is obtained by summating the
resistance of all the layers †
(Rso, 0„055) + (brkwk, 0„122) + (ins, 2„631) + (bridged layer,
(^1) ... [0„934... 0„555] + [0„066 ... 0„114] = 0„442) + (plstr, 0„081) +
(Rsi, 0„123) = 3„454 m^2 K/W
Total resistance (R) of wall is the average of upper and lower
limits = (3„533+3„454)...2 = 3„493 m^2 K/W
U-value =R^1 =3„493^1 = 0„286 W=m^2 K
Note: Both proportional area and combined method calculations
require an addition of 0„020 W/m^2 K to the calculated U-value. This
is for vertical twist type wall ties in the wide cavity. See page 338
and note 2 on page 471.
Thermal Insulation, Calculating U-Values---3