Thermal & Water Vapor

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 addedOne 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 addedOne 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 .56 (textbook pp 33)V.B. 4 mil poly ------ (Negligible)2x6 studs Cavity (not needed)R-19 Batt Insulation 19.00 Weather Barrier (Tyvek) ------ (Negligible)5/8 Exterior GWB .56 (textbook pp 33)4 Brick 1.11 (Handout)Air Space 1.18 (table 2-4 pp 34)

Inside Air Film * .68 (table 2-5 pp 34)Outside Air Space ** .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 .56 (textbook pp 33)V.B. 4 mil poly ------ (Negligible)2x6 studs *** 5.6 R-19 Batt Insulation At Studs Weather Barrier (Tyvek) ------ (Negligible)5/8 Exterior GWB .56 (textbook pp 33)4 Brick 1.11 (Handout)Air Space 1.18 (table 2-4 pp 34)

Inside Air Film .68 (table 2-5 pp 34)Outside Air Space .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 U = 1/R = 1/23.26 = .043R value for Studs is 9.86 U = 1/R = 1/9.86 = .10

The wall is 10’-0” high and 30’-0” long10 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*TDTD 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*TDHowever 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 sfdouble 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 HouseSf of Area R value U value Temp. Ch. BTU h loss

Cavities 140 21.25 0.047059 76 500.71Studs & plates38 9.12 0.109649 76 316.67Headers 80 22.88 0.043706 76 265.73Windows 286 2.08 0.480769 76 10450

Total 11533

E & W Walls of HouseSf of Area R value U value Temp. Ch. BTU h loss

Cavities 240 21.25 0.047059 76 858.35Door (2ea.) 40 10.45 0.095694 76 290.91Studs & plates41.78 9.12 0.109649 76 348.17

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!