ASHRAE-90.1-2022- - Page 274

Extracted Content

Figure A9.4.6.3 Geometry of single cavity layer or double-layer wall.

The integral represents the combined R-value of the air space and insulation over the region 0 < x < Xa . Because the thermal conductivity of air is independent of the thickness, Equation A9.4-25 can be simplified using the air space mean thickness ( Ya ) to produce Equation A9.4-26:

 ------------- m -  dx  K 

R

= ---- Ya - + -----1  xa

---- KYa - a + ----- X 1 a  x 0 a  Y ------------- mK – y

xa  Y ------------- m - y -  dx + --------------- Ym

0  K  K  Ym 

(A9.4-26)

However, if the air space is characterized by convection instead of conduction then the term Ya / ka can be replaced by the R-value for convection (R-0.92 h·ft [2] ·°F/Btu for walls ). Adding the inside and outside layers is expressed in Equation A9.4-27:

RBP = R 1 BP + R 2 BP

(A9.4-27)

Add the air film resistances at the exterior ( RAT ) and interior ( RAB ), which are defined as Equation A9.4-28:

RAB

1 = -------hAB

where hAB is the air film heat transfer coefficient at the exterior in Btu/h·ft [2] ·°F.

(A9.4-28)

(A9.4-29)

RAT

1 = ------ - hAT

where hAT is the air film heat transfer coefficient at the interior in Btu/h·ft [2] ·°F.

The sum of the R-values for the insulation and air films beyond the girt are expressed in Equation A9.4-30.

RBP + air = RBP + RAB + RAT

(A9.4-30)

272 ANSI/ASHRAE/IES Standard 90.1-2022 (I-P)