Passive Solar Design for New Mexico

Class III

Review of previous classes:

Heat Transfer in materials

Heat flow at a particular point in a material = Qat a point = A k dT/dx, where k is the conductivity, A is the area, and dT/dx is the slope of the temperature profile at the point in the material.

Steady state heat flow through a slab of material is given by Qat a point = A DT/R, where R is the "R-value", or "thermal resistance" of the material. This formula is obtained by assuming a linear (constant slope) temperature profile in the material. The value of R is related to the conductivity and thickness (Dx) of the material in the following way: R = Dx / k. Note that the larger the thickness, the larger R is. Note that the larger the conductivity, the smaller R is.

Material R-Value
single pane of glass 1
double pane of glass 2
concrete (per inch) .1
fiberglass batt (per inch) 3.17
urethane foam (per inch) 6.0
wood (per inch) 1.0
Drywall .9
inside/outside air films .68/.21




To calculate the total R-value of a wall, add up the R values of each layer, including the R-values for the air films on each side.

To calculate the total heat loss, add all of the heat fluxes together.

To calculate the "Building Load Coefficient" (BLC), divide the total heat loss rate by DT. Note that this is only a property of the building.

The number of degree days (DDy) in a year is the total annual sum of the daily average temperature difference DT = 65 oF - Toutside, average, over all the days for which Toutside, average is less than 65 oF.

The total heat loss over the year is then Qyearly = DDy  x BLC x 24 hours