Roof and wall are analyzed in the same way. In winter the heat loss is simple transmission based on the inside and outside temperature, and U-value of composite structure;

Q(winter) = U*A*(Ti-To)

Ti = Inside air temperature

To = Outside air temperature

In summer the solar radiation affects the outside surface of wall and roof. The absorbed radiation increases the temperature of the outside surface to a value that is greater than outside air temperature. This outside surface temperature is called Sol-air temperature. It depends on the properties of wall and roof structure, outside surface material and color, and solar radiation intensity component perpendicular to the outside surface. The solar radiation amount depends on the orientation of the surface, solar altitude angle, and solar azimuth angle.

An approximate equation for the sol-air temperature (Ts) of the outside surface of a given wall or roof is:

Ts = To + A * (Id + Is) / Hso

A = absorption coefficient of outside surface

Id = direct solar radiation on surface

Is = scatter or diffuse solar radiation

Hso = outside wall or roof surface film resistance

The heat gain through a wall or roof in summer is:

Q(summer) = U*A*(Ts - Ti)

Q must be adjusted for thermal lag and time delay. OR

Q(summer) = U*A*(CLTD) using the CLTD method

Wall CLTD selection

1. Use ASHRAE table 8.5, page 8.26 (page 26 note) to determine (a) principal mass of the composite wall, and (b) ASHRAE code number of the wall construction.
2. Determine placement of the principal mass with respect to insulation and the outdoors. This can be (a) inside, and (b) outside.
3. Calculate the overall resistance (R-value) of the composite wall.
4. Determine secondary material category of the composite wall. Category examples are (a.) stucco (b) steel or lightweight siding, and (c) face brick
5. Use ASHRAE table 8.6A, 8.6B or 8.6C (page 27,28,29) to determine the wall type no.
6. Determine the latitude of the building location.
7. Select the appropriate ASHRAE table from 8.3A (24^oN), 8.3B (36^oN) or 8.3C (48^oN) based on latitude (page 16-24 note).
8. Go to table for given wall type number.
9. Determine CLTD for (a) wall number and (b) hour of the day
10. Correct CLTD for (a.) indoor temperature = 78^oF (b) outdoor mean temperature = 85^oF. (The CLTD tables are based on this condition. A correction is required for other conditions)
11. CLTD (corrected) = CLTD (table) + (78-Tr) + (Tm-85).

Roof CLTD selection

1. From ASHRAE table 8.5, page 8.26 (page 26 note) determine (a) principal mass of the composite roof, and (b) ASHRAE code number of this mass.
2. Determine placement of the principal mass with respect to insulation and the outdoors. This can be (a) inside (b) outside (c) evenly placed or no insulation.
3. Calculate the overall resistance (R-value) of the composite roof.
4. Determine presence or absence of a suspended ceiling.
5. From ASHRAE table 8.2 page 8.25 (page 25 note) determine the roof type no.

12. Determine the latitude of the building location.
13. Select the appropriate ASHRAE table from 8.2A (24^oN), 8.2B (36^oN) or 8.2C (48^oN) based on latitude (page 8.15,15 note).
14. Go to table for given roof type number.
15. Determine CLTD for (a) roof number and (b) hour of the day
16. Correct CLTD for (a.) indoor temperature = 78^oF (b) outdoor mean temperature = 85^oF. (The CLTD tables are based on this condition. A correction is required for other conditions)
17. CLTD (corrected) = CLTD (table) + (78-Tr) + (Tm-85).

ROOF AND WALL HEAT GAIN (CLTD METHOD)

In CLTD/SCL/CLF method the heat gain through wall and roof is

Q = U*A*(CLTD)

Q = Sensible Heat Gain through Wall or Roof

A = Surface Area of Wall or Roof

U = Overall U-Value for composite Wall or Roof

CLTD=Cooling load temperature difference from ASHRAE table for a given

1. Latitude
2. Wall or roof type
3. Wall or roof exposure orientation
4. Hour of day

ASHRAE tables are for latitude 24^oN, 36^oN or 48^oN, which cover U.S.A and Canada, for 16 predefined wall types and 14 predefined roof types.

NOTES: Wall CLTD values in the table are based on

• Dark wall surface outside
• Indoor room temperature = 78^oF (Troom).
• Outdoor maximum temperature = 95^oF (Tmax)
• Average outdoor temperature = 85^oF (Tavg)
• Daily range = 21^oF
• Outside surface film resistance = 0.333 hr.ft².^oF / Btu
• Inside surface film resistance = 0.685 hr.ft².^oF / Btu

Tavg = Tmax - ( Daily range / 2)

Tmax and daily range are available from ASHRAE weather data for cooling loads design calculation.

ADJUSTMENT TO TABLE DATA (CLTD Corrected)

CLTD Corrected = CLTD table + (78-Troom) + (Tavg-85)

Example: Wall Type = 9; Tavg = 90^oF; Troom = 70^oF; Medium Construction; 9 PM; West Wall

CLTD Corrected = CLTD table + (78-70) + (90-85) = 47 + 8 + 5 = 60

EXAMPLE: ROOF

NOTES: Roof CLTD values in the table are based on

• Dark roof surface outside
• Indoor room temperature = 78^oF (Troom).
• Outdoor maximum temperature = 95^oF (Tmax)
• Average outdoor temperature = 85^oF (Tavg)
• Daily range = 21^oF

Outside surface film resistance = 0.333 hr.ft².^oF / Btu

Inside surface film resistance = 0.685 hr.ft².^oF / Btu

Tavg = Tmax - ( Daily range / 2)

Tmax and daily range are available from ASHRAE weather data for cooling loads design calculation.

ADJUSTMENT TO TABLE DATA (CLTD Corrected)

CLTD Corrected = CLTD table + (78-Troom) + (Tavg-85)

Example: Roof Type = 5; Tavg = 90^oF; Troom = 70^oF;
At 5:00 PM

CLTD Corrected = CLTD table + (78-70) + (90-85) = 71 + 8 + 5 = 84^oF

EXAMPLE: WALL AND ROOF