ASHRAE Journal - April 2020 - 18

TECHNICAL FEATURE

E = ρgQc p (Ta − Tenv exit )

Tenv exit = Tconcrete + 0.292 (Twall − Tconcrete )
Q = C ( ELA) 2 ∆p / ρ

FIGURE 3 Effect of daily temperature variation on inside basement wall temperatures on a cold (20°F) day.

60
56

(5)
(6)

52
Temperature (°F)

and the energy lost is related
to leakage flow using the temperature entering and exiting the
envelope.

Depth
6 in. Increments

48
44

D

Outside
T = 20°F, ±12°F
(24 Hour Sinusoid)

A

C
40
(7)
B
B
36
E = Energy lost due to envelope
Basement
Soil
C
32
65°F
leakage, Btu/s
D
28
Ta = Temperature of air in basePlywood
24
ment = 65°F
A
20
Twall = Average monthly tempera0
10
20
30
40
50
60
70
80
Insulation Concrete
ture for inner surface of the
Time (Hours)
concrete
Tenv exit = Air temperature exiting the
TABLE 1 Comparison of the equation for daily ambient temperature variation to that from HDD.
envelope,† °F
MONTH
Θ MEAN TEMPERATURE
HEATING NUMBER OF
n day
AVERAGE FROM HEATING
ρ = Density of air in basement, slug/
RADIANS
FROM EQUATION DEGREE DAYS
DAYS (M)
DEGREE DAYS =65-HDD/M
ft3
Jan.
-1
39.5483871
828
31
1
38.29032258
3
Q = Leakage flow, ft /s
Feb. -0.866
41.21690323
678
28 31.33333
40.78571429
Mar.
-0.5
45.77419355
657
31 61.66666
43.80645161
cp = Specific heat of air, 0.24 Btu/
lb-°F
Apr.
0
52
474
30 91.99999
49.2
May
0.5
58.22580645
295
31 122.3333
55.48387097
DP = Stack pressure from equation,
2
lb/ft (= Pa/47.88)
Jun.
0.866
62.78309677
159
30 152.6667
59.7
g = Gravity acceleration, 32.2 ft/s2
Jul.
1
64.4516129
56
31
183
63.19354839
C = Flow coefficient, 0.3 for glass
Aug.
0.866
62.78309677
62
31 213.3333
63
wool loosely filled envelopes
Sep.
0.5
58.22580645
162
30 243.6666
59.6
A = Physical leakage area, ft2
Oct.
0
52
391
31
274
52.38709677
ELA = Equivalent insulation leakage
Nov.
-0.5
45.77419355
633
30 304.3333
43.9
area = 0.353 × A (includes flow in
Dec. -0.866
41.21690323
750
31 334.6666
40.80645161
and out, in series), ft2
After some unit conversions, the monthly variation
Typically, there is a concrete slab floor. Sometimes the
in energy loss for physical leakage area is shown for the
basement floor is just the ground soil, bedrock, or a
Seattle area (Figure 7):
gravel layer and poly over the soil.
The top wall edge seal is above the ground level and
An Envelope Sealing and Ventilation Solution
below the joists supporting the floor above. This creates
The objective is to create a tightly sealed perimeter
a flow blocker near the bottom of the coldest section
wall subfloor insulation envelope and to depressurize
of insulated wall and the top of the underground wall.
this envelope to the outdoors with a continuous duty
This "L"-shaped seal near the top of the wall creates a
blower.
tight air seal and minimizes airflow from outdoors into
Figure 8 illustrates typical basement foundation walls
the envelope system. It also reduces stack pressures
and slabs, and a tightly sealed stud wall, and also a
affecting air leakage into the insulated space above. The
stud wall and subfloor insulation envelope system coninsulation can be batt, foam board or a combination of
structed over top. The basement foundation walls can
both. Foam board insulation with its higher thermal
be poured concrete, concrete block and rubble mound.
resistance is used where thinner walls that meet code
†CFD

models of 7 ft tall wall, 3.5 in. insulation, K=0.27 Btu·in/h·ft2·°F, permeability 3 × 106 Darcy's, at given temperatures, show factor or
0.292 (i.e., simple average factor is 0.5)

18

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