Calculating cost to heat house using R-values

[nerdy_hottie]

Homework Statement

Consider a flat roofed rectangular house that is 10.0 feet high with 39.0 feet long sidewalls and 20.0 feet long front and back walls.

We are given that:
Wall: (R Value =0.20 plaster)+(R Value=10.00 insulation)+(R Value=0.60 plywood)+(R Value=0.50 cedar shingles).
Roof: Same as walls except R Value=31.00 insulation.
All R values are in units of ft2*oF*hr/Btu

If average winter indoor and outdoor temperatures are 69.0oF and 18.0oF respectively and the furnace produces 105 Btu/gal of oil at $4/gal, what is the monthly heating cost in dollars? (Use 30 days/month and Do not enter units)

Homework Equations

P=(1/R)*A*/\T

The Attempt at a Solution

P=(1/R_{front wall, back wall})/\T*(A_{front wall}+A_{back wall})+(1/R_roof)*/\T*A_roof
=3894.4 Btu/hr
multiply by 24 hours, then by 30 days=2803968 Btu
Divide by 10^5 Btu/gal
multiply by $4/gal
=112.16

 

[haruspex]

=3894.4 Btu/hr

I think you've counted only one wall in each direction instead of a pair.