Notes:
1. The air space in the loft area is divided between pitched and ceiling
components, ie. Ra=0 180 2=0090 m^2 K/W.- The U-value is calculated perpendicular to the insulation, therefore the
pitched component resistance is adjusted by multiplying by the cosine of
the pitch angle, ie. 0819. - Proportional area of bridging parts (rafters and joists) is 50 400 =
0 125 or 125%. - With an air space resistance value (R1) of 0120 m^2 K/W between tiles and
felt, the resistance of the tiling may be ignored.
Thermal resistance (R) of the pitched component:
Raftered part Non-raftered part
Rso= 0„045 Rso= 0„045
R 1 = 0„120 R 1 = 0„120
R 2 = 0„004 R 2 = 0„004
R 3 = 0„714 Ra = 0. 090
Ra = 0. 090 0„259¾87„5%
0„973¾12„5% = 0„122 = 0„227
Total resistance of pitched components =
(0„122+0„227)¾0„819 = 0„286 m^2 K/W
Thermal resistance (R) of the ceiling component:
Joisted part Fully insulated part
Rsi= 0„104 Rsi= 0„104
R 6 = 0„081 R 6 = 0„081
R 5 = 0„714 R 4 = 5„000 (200mm)
R 4 = 2„500 (100 mm) Ra= 0. 090
Ra= 0. 090 5„275¾87„5%
3„489¾12„5% = 0„436 = 4„615
Total resistance of ceiling components = 0„436+4„615
= 5„051m^2 K/W.
U=^1 R= 0 : (^2861) + 5 : 051 =0:187 W=m^2 K
Thermal Insulation, Calculating U-Values---4