The Buildings
Envelope
The Buildings Envelope

R Values are the thermal resistance of a
building product.
R values are given to certain
materials to evaluate there ability to
resist the flow of heat or cold.
R factors can be added to find an
assemblies overall thermal resistance.
The Buildings Envelope
R factors can be added
One more time
R factors can be added
The Buildings Envelope

A U factor is a thermal coefficient and is
the reciprocal of an R factor.
U=1/R
U values are given to certain
materials to evaluate there ability to
resist the flow of heat or cold.
U factors can not be added.
The Buildings Envelope
U factors can not be added
One more time
U factors can not be added
The Buildings Envelope

U – Coefficient is defined as the number
of Btuh that pass through 1 square foot
of wall, floor roof and etc. under actual
conditions when the actual conditions at
the inside and out side air temperature
is 1 degree F under a steady state of
heat flow.
1 / R =U
The Buildings Envelope
Q = A*U*TD (Q = heat gain, A = area of
building component, TD = temperature
difference).
 Q = BTU h


We’ll be using this formula very soon!
The Buildings Envelope

A typical U value for a ceiling component
with a total thermal resistance of R-30
would be calculated as follow: U=1/30,
or U=.033. This U value is then used to
calculate the heat flow through the
ceiling component using the heat
transfer formula: Q = A*U*TD (Q = heat
gain, A = area of building component,
TD = temperature difference).
The Buildings Envelope
Q = A*U*TD (Q = heat gain, A = area of building
component, TD = temperature difference).
Example: calculate the total heat gain through a 10 x 10
bedroom ceiling with a thermal resistance of R-30
and the temperature difference of 55 degrees
between the interior conditioned zone (75 degrees)
and the vented attic (130 degrees).
Formula used is Q = A*U*TD, total heat gain through
the ceiling is Q = 100*.033*55,
Q = 181.5 BTUH (British Thermal Units per Hour).
The Buildings Envelope

k Values are the thermal conductivity of
a building product.
k values are given to certain
materials to evaluate there ability to
resist the flow of heat or cold per
inch.
The Buildings Envelope
k values is the Heat flow through
homogeneous solids.
Conductivity or conductance is
designated as k values and is defined
as the number of Btuh that flow through
one square foot of material one inch
thick when the temperature drops
through the material under conditions
of steady heat flow.
The Buildings Envelope

k Values can be calculated by dividing the
thickness of a particular material in inches (or
meters) by the K value.
(’R’ = d / K)

This slide is impressing your date only.

In physics, thermal conductivity, k, is the
property of a material that indicates its ability
to conduct heat. It appears primarily in
Fourier's Law for heat conduction
The Buildings Envelope

C values
C values is referred to as a basic
rating for material and is referred to
as conductivity of homogeneous
material for thickness other than one
inch. All other conditions remain the
same.
The Buildings Envelope

a = Air-Space Conductance and is
effected by the position and by the
emissivity of E of the surfaces.
The Buildings Envelope

f = Film or Surface Conductance
Coefficient. This is the rate of heat
flow in Btuh through 1 square foot of
surface due to the motion of air
against the surface, for 1 degree
difference in temperature.
The Buildings Envelope
The Buildings Envelope










5/8 GWB
V.B. 4 mil poly
2x6 studs
R-19 Batt Insulation
Weather Barrier
(Tyvek)
5/8 Exterior GWB
4 Brick
Air Space
Inside Air Film
Outside Air Space
The Buildings Envelope
 Wall
Cavity








R value
5/8 GWB
V.B. 4 mil poly
2x6 studs
R-19 Batt Insulation
Weather Barrier (Tyvek)
5/8 Exterior GWB
4 Brick
Air Space
 Inside Air Film *
 Outside Air Space **
.56 (textbook pp 33)
------ (Negligible)
Cavity (not needed)
19.00
------ (Negligible)
.56 (textbook pp 33)
1.11 (Handout)
1.18 (table 2-4 pp 34)
.68 (table 2-5 pp 34)
.17 (table 2-5 pp 34)
Total 23.26
* Still air - Vertical
** 15 mph winter
The Buildings Envelope
 At








the Studs
R value
5/8 GWB
V.B. 4 mil poly
2x6 studs ***
R-19 Batt Insulation
Weather Barrier (Tyvek)
5/8 Exterior GWB
4 Brick
Air Space
 Inside Air Film
 Outside Air Space
.56 (textbook pp 33)
------ (Negligible)
5.6
At Studs
------ (Negligible)
.56 (textbook pp 33)
1.11 (Handout)
1.18 (table 2-4 pp 34)
.68 (table 2-5 pp 34)
.17 (table 2-5 pp 34)
Total 9.86
*** Handout – Softwood – Douglas fir = 1/k = 1.06-.99 aver. = 1.025 x 5.5”= 5.6
The Buildings Envelope

Using the formula
Q = A*U*TD
R value for Cavity is 23.26
R value for Studs is 9.86
U = 1/R = 1/23.26 = .043
U = 1/R = 1/9.86 = .10
The wall is 10’-0” high and 30’-0” long
10 x 30 = 300 square feet
Studs 16” on center
(30’-0” / 1.333 (16”) = 23 studs + 1 = 24 studs)
24 studs x (.125 (1- ½”) x 10’) = 30 square feet
The Buildings Envelope

Using the formula
Q = A*U*TD
TD Temperature Deference – Spokane
-15 degrees Winter
68 degrees indoor
Difference = 83 degrees
The Buildings Envelope

Using the formula
Q = A*U*TD
Cavity
1,071 Btuh = 300 x .043 x 83
Studs
249 Btuh = 30 x .10 x 83
The Buildings Envelope

Using the formula
Q = A*U*TD
However we need to remove the studs.
300 – 30 = 270 sf
Ratio is 30 / 300 = .10 or 10%
studs to cavity
The Buildings Envelope

Windows or Fenestration
Q = A*U*TD
4 x 6 window = 24 sf
double glazed window =
2.08 R factor
or
.481 U factor
The Buildings Envelope

Using the formula
958 = 24*.481* 83
The Buildings Envelope

Using the formula
Q = A*U*TD
Cavity
878 Btuh = 246 x .043 x 83
Studs
249 Btuh = 30 x .10 x 83
Window
958 Btuh = 24 x .481 x 83
The Buildings Envelope

Using the formula
Q = A*U*TD
Cavity
878 Btuh = 246 x .043 x 83
Studs
249 Btuh = 30 x .10 x 83
Window
958 Btuh = 24 x .481 x 83
South Wall House
Sf of Area R value
Cavities
140
21.25
Studs & plates38
9.12
Headers
80
22.88
Windows
286
2.08
U value Temp. Ch.
0.047059
76
0.109649
76
0.043706
76
0.480769
76
BTU h loss
500.71
316.67
265.73
10450
Total 11533
E & W Walls of House
Sf of Area R value U value Temp. Ch.
Cavities
240
21.25 0.047059
76
Door (2ea.) 40
10.45 0.095694
76
Studs & plates
41.78
9.12 0.109649
76
BTU h loss
858.35
290.91
348.17
Total 1497.4
The Buildings Envelope
 Amount
of KW = (BTUh x
Annual Degree Days) / (diff in T
x 3,413 BTUh x 100%)
 For
electrical systems
 Annual degree days in Spokane
is 6655 Heating days x 24 hrs per day
That’s Real Big!