WAC 51-11-1007   Section 1007 Ceilings.  

     1007.1 General: Table 10-7 lists heat-loss coefficients for the opaque portion of exterior ceilings below vented attics, vaulted ceilings, and roof decks in units of Btu/h•ft2•°F of ceiling.

     They are derived from procedures listed in Standard RS-1, listed in Chapter 7. Ceiling U-factors are modified for the buffering effect of the attic, assuming an indoor temperature of 65° F and an outdoor temperature of 45°F.

     Metal Framed Ceilings: The nominal R-values in Table 10-5A - EFFECTIVE R-VALUES FOR METAL FRAMING AND CAVITY ONLY may be used for purposes of calculating metal framed ceiling section U-factors in lieu of the ASHRAE zone calculation method as provided in Chapter 25 of Standard RS-1.

     1007.2 Component Description: The four types of ceilings are characterized as follows:

     Ceilings Below a Vented Attic: Attic insulation is assumed to be blown-in, loose-fill fiberglass with a K-value of 2.6 hr • ft2 • °F/Btu per inch. Full bag count for specified R-value is assumed in all cases. Ceiling dimensions for flat ceiling calculations are forty-five by thirty feet, with a gabled roof having a 4/12 pitch. The attic is assumed to vent naturally at the rate of three air changes per hour through soffit and ridge vents. A void fraction of 0.002 is assumed for all attics with insulation baffles. Standard-framed, unbaffled attics assume a void fraction of 0.008.

     Attic framing is either standard or advanced. Standard framing assumes tapering of insulation depth around the perimeter with resultant decrease in thermal resistance. An increased R-value is assumed in the center of the ceiling due to the effect of piling leftover insulation. Advanced framing assumes full and even depth of insulation extending to the outside edge of exterior walls. Advanced framing does not change from the default value.

     U-factors for flat ceilings below vented attics with standard framing may be modified with the following table:


  U-Factor for

Standard Framing
Roof Pitch R-30 R-38
4/12   .036 .031
5/12   .035 .030
6/12   .034 .029
7/12   .034 .029
8/12   .034 .028
9/12   .034 .028
10/12   .033 .028
11/12   .033 .027
12/12   .033 .027

     Vented scissors truss attics assume a ceiling pitch of 2/12 with a roof pitch of either 4/12 or 5/12. Unbaffled standard framed scissors truss attics are assumed to have a void fraction of 0.016.

     Vaulted Ceilings: Insulation is assumed to be fiberglass batts installed in roof joist cavities. In the vented case, at least 1.5-inches between the top of the batts and the underside of the roof sheathing is left open for ventilation in each cavity. A ventilation rate of 3.0 air changes per hour is assumed. In the unvented or dense pack case, the ceiling cavity is assumed to be fully packed with insulation, leaving no space for ventilation.

     Roof Decks: Rigid insulation is applied to the top of roof decking with no space left for ventilation. Roofing materials are attached directly on top of the insulation. Framing members are often left exposed on the interior side.

     Metal Truss Framing: Overall system tested values for the roof/ceiling Uo for metal framed truss assemblies from approved laboratories shall be used, when such data is acceptable to the building official.

     Alternatively, the Uo for roof/ceiling assemblies using metal truss framing may be obtained from Tables 10-7A, 10-7B, 10-7C, 10-7D and 10-7E.


TABLE 10-7

DEFAULT U-FACTORS FOR CEILINGS
Ceilings Below Vented Attics

  Standard Frame Advanced Frame
Flat Ceiling Baffled  
R-19 0.049 0.047
R-30 0.036 0.032
R-38 0.031 0.026
R-49 0.027 0.020
R-60 0.025 0.017
Scissors Truss    
R-30 (4/12 roof pitch) 0.043 0.031
R-38 (4/12 roof pitch) 0.040 0.025
R-49 (4/12 roof pitch) 0.038 0.020
R-30 (5/12 roof pitch) 0.039 0.032
R-38 (5/12 roof pitch) 0.035 0.026
R-49 (5/12 roof pitch) 0.032 0.020
Vaulted Ceilings    
  16" O.C. 24" O.C.
Vented    
R-19 2x10 joist 0.049 0.048
R-30 2x12 joist 0.034 0.033
R-38 2x14 joist 0.027 0.027
Unvented    
R-30 2x10 joist 0.034 0.033
R-38 2x12 joist 0.029 0.027
R-21 + R-21 2x12 joist 0.026 0.025
Roof Deck
  4x Beams, 48" O.C.
R-12.5 2" Rigid insulation 0.064
R-21.9 3.5" Rigid insulation 0.040
R-37.5 6" Rigid insulation 0.025
R-50 8" Rigid insulation 0.019


      Table 10-7A

Steel Truss1 Framed Ceiling UO		        
Cavity           Truss Span (ft)          
R-value 12 14 16 18 20 22 24 26 28 30 32 34 36
19 0.1075 0.0991 0.0928 0.0878 0.0839 0.0807 0.0780 0.0757 0.0737 0.0720 0.0706 0.0693 0.0681
30 0.0907 0.0823 0.0760 0.0710 0.0671 0.0638 0.0612 0.0589 0.0569 0.0552 0.0538 0.0525 0.0513
38 0.0844 0.0759 0.0696 0.0647 0.0607 0.0575 0.0548 0.0525 0.0506 0.0489 0.0474 0.0461 0.0449
49 0.0789 0.0704 0.0641 0.0592 0.0552 0.0520 0.0493 0.0470 0.0451 0.0434 0.0419 0.0406 0.0395


    Table 10-7B

Steel Truss1 Framed Ceiling UO with R-3 Sheathing2		      
Cavity           Truss Span (ft)          
R-value 12 14 16 18 20 22 24 26 28 30 32 34 36
19 0.0809 0.0763 0.0728 0.0701 0.0679 0.0661 0.0647 0.0634 0.0623 0.0614 0.0606 0.0599 0.0592
30 0.0641 0.0595 0.0560 0.0533 0.0511 0.0493 0.0478 0.0466 0.0455 0.0446 0.0438 0.0431 0.0424
38 0.0577 0.0531 0.0496 0.0469 0.0447 0.0430 0.0415 0.0402 0.0392 0.0382 0.0374 0.0367 0.0361
49 0.0523 0.0476 0.0441 0.0414 0.0393 0.0375 0.0360 0.0348 0.0337 0.0328 0.0319 0.0312 0.0306


    Table 10-7C

Steel Truss1 Framed Ceiling UO with R-5 Sheathing2		      
Cavity           Truss Span (ft)          
R-value 12 14 16 18 20 22 24 26 28 30 32 34 36
19 0.0732 0.0697 0.0670 0.0649 0.0633 0.0619 0.0608 0.0598 0.0590 0.0583 0.0577 0.0571 0.0567
30 0.0564 0.0529 0.0502 0.0481 0.0465 0.0451 0.0440 0.0430 0.0422 0.0415 0.0409 0.0403 0.0399
38 0.0501 0.0465 0.0438 0.0418 0.0401 0.0388 0.0376 0.0367 0.0359 0.0351 0.0345 0.0340 0.0335
49 0.0446 0.0410 0.0384 0.0363 0.0346 0.0333 0.0322 0.0312 0.0304 0.0297 0.0291 0.0285 0.0280


    Table 10-7D

Steel Truss1 Framed Ceiling UO with R-10 Sheathing2		    
Cavity           Truss Span (ft)          
R-value 12 14 16 18 20 22 24 26 28 30 32 34 36
19 0.0626 0.0606 0.0590 0.0578 0.0569 0.0561 0.0555 0.0549 0.0545 0.0541 0.0537 0.0534 0.0531
30 0.0458 0.0437 0.0422 0.0410 0.0401 0.0393 0.0387 0.0381 0.0377 0.0373 0.0369 0.0366 0.0363
38 0.0394 0.0374 0.0359 0.0347 0.0337 0.0330 0.0323 0.0318 0.0313 0.0309 0.0305 0.0302 0.0299
49 0.0339 0.0319 0.0304 0.0292 0.0283 0.0275 0.0268 0.0263 0.0258 0.0254 0.0251 0.0247 0.0245



    Table 10-7E

Steel Truss1 Framed Ceiling UO with R-15 Sheathing2		    
Cavity           Truss Span (ft)          
R-value 12 14 16 18 20 22 24 26 28 30 32 34 36
19 0.0561 0.0550 0.0541 0.0535 0.0530 0.0526 0.0522 0.0519 0.0517 0.0515 0.0513 0.0511 0.0509
30 0.0393 0.0382 0.0373 0.0367 0.0362 0.0358 0.0354 0.0351 0.0349 0.0347 0.0345 0.0343 0.0341
38 0.0329 0.0318 0.0310 0.0303 0.0298 0.0294 0.0291 0.0288 0.0285 0.0283 0.0281 0.0279 0.0278
49 0.0274 0.0263 0.0255 0.0249 0.0244 0.0239 0.0236 0.0233 0.0230 0.0228 0.0226 0.0225 0.0223

1 - Assembly values based on 24 inch on center truss spacing; 11 Truss member connections penetrating insulation (4 at the eaves, 7 in the interior space); 1/2 inch drywall ceiling; all truss members are 2x4 "C" channels with a solid web.
2 - Ceiling sheathing installed between bottom chord and drywall.




[Statutory Authority: RCW 19.27A.020, 19.27A.045. 04-01-106, § 51-11-1007, filed 12/17/03, effective 7/1/04. Statutory Authority: RCW 19.27A.025, 19.27A.045. 01-03-010, § 51-11-1007, filed 1/5/01, effective 7/1/01; 98-03-003, § 51-11-1007, filed 1/8/98, effective 7/1/98. Statutory Authority: RCW 19.27A.020 and 1990 c 2. 91-01-112, § 51-11-1007, filed 12/19/90, effective 7/1/91.]