WAC 51-11-1004
Section 1004: Floors over unconditioned
space.
1004.1 General: Tables 10-3, 10-4 and 10-4a list
heat-loss coefficients for floors over unconditioned spaces in
units of Btu/h • ft2 • °F.
They are derived from procedures listed in RS-1, listed
in Chapter 7, assuming an average outdoor temperature of 45°F,
an average indoor temperature of 65°F, and a crawlspace area
of 1350 ft2 and 100 ft of perimeter. The crawlspace is assumed
to be 2.5 feet high, with 24 inches below grade and 6 inches
above grade.
1004.2 Crawlspace Description: Four configurations are
considered: Vented crawlspace, unvented crawlspace, heated
plenum crawlspace and exposed floor.
Vented crawlspaces: Assumed to have 3.0 air-changes per
hour, with at least 1.0 ft2 of net-free ventilation in the
foundation for every three hundred ft2 of crawlspace floor
area. The crawlspace is not actively heated.
Floors over unheated areas, such as garages, may only use
those values which have R-0 perimeter insulation.
Unvented crawlspaces: Assumed to have 1.5 air changes
per hour, with less than 1.0 ft2 of net-free ventilation in the
foundation for every three hundred ft2 of crawlspace floor
area. The crawlspace is not actively heated. Floors over
unheated basements may only use those values which have R-0
perimeter insulation.
Heated-plenum crawlspaces: Assumed to have 0.25
air-changes per hour, with no foundation vents. Heated supply
air from central furnace is blown into a crawlspace and
allowed to enter the living space unducted via holes cut into
the floor.
Enclosed floors: Assumes no buffer space, and a covering
of one-half inch of T1-11 on the exterior of the cavity
exposed to the outside air or rigid insulation below a
concrete floor, such as over parking garages.
1004.3 Construction Description: Floors are assumed to
be either joisted floors framed on sixteen inch centers, or
post and beam on four by eight foot squares. Insulation is
assumed to be installed under the subflooring between the
joists or beams with no space between the insulation and the
subfloor. Insulation is assumed to be uncompressed. Exposed
floors also include concrete with continuous rigid insulation
assumed.
Perimeter insulation is assumed to extend from the top of
the rim joist to the crawlspace floor and then inward along
the ground (on top of the ground cover) for at least
twenty-four inches.
Floor coverings are assumed to be light carpet with
rubber pad.
TABLE 10-3
DEFAULT U-FACTORS FOR FLOORS OVER VENTED
CRAWLSPACE OR
UNHEATED BASEMENT
| Nominal R-value |
U-factor |
| Floor |
Perimeter |
Post & Beam |
Joists |
| 0 |
0
11
19
30 |
|
0.112
0.100
0.098
0.093 |
0.134
0.116
0.114
0.107 |
| 11 |
0
11 |
|
0.052
0.048 |
0.056
0.052 |
| 19 |
0
11 |
|
0.038
0.036 |
0.041
0.038 |
| 22 |
0
11 |
|
0.034
0.033 |
0.037
0.035 |
| 25 |
0
11 |
|
0.032
0.031 |
0.034
0.033 |
| 30 |
0
11 |
|
0.028
0.027 |
0.029
0.028 |
| 38 |
0
11 |
|
0.024
0.024 |
0.025
0.024 |
TABLE 10-4
DEFAULT U-FACTORS FOR FLOORS OVER HEATED
PLENUM CRAWLSPACES
Nominal R-value
Perimeter |
U-factor |
| 11 |
0.085 |
| 19 |
0.075 |
| 30 |
0.069 |
TABLE 10-4A
EXPOSED FLOOR
Nominal
R-value |
U-factor |
| Concrete |
Wood Joist |
Metal Joist |
| R-11 |
0.077 |
0.088 |
0.14 |
| R-15 |
0.059 |
0.076 |
0.12 |
| R-19 |
0.048 |
0.062 |
0.11 |
| R-21 |
0.043 |
0.057 |
0.11 |
| R-25 |
0.037 |
0.051 |
0.10 |
| R-30 |
0.031 |
0.040 |
0.09 |
| R-38 |
0.025 |
0.034 |
0.08 |
| Note: |
Crawlspaces used as heated plenums have approximately 30% higher heat-loss rate than unvented crawlspaces with the
same assumed ACH. Default U-values in Table 10-4 reflect this higher rate of heat loss. |
[Statutory Authority: RCW 19.27A.020, 19.27A.045. 04-01-106,
§ 51-11-1004, filed 12/17/03, effective 7/1/04. Statutory
Authority: RCW 19.27A.025, 19.27A.045. 01-03-010, §
51-11-1004, filed 1/5/01, effective 7/1/01; 98-03-003, §
51-11-1004, filed 1/8/98, effective 7/1/98. Statutory
Authority: RCW 19.27A.020 and 1990 c 2. 91-01-112, §
51-11-1004, filed 12/19/90, effective 7/1/91.]