# Calculating cost to heat house using R-values

1. Nov 12, 2012

### nerdy_hottie

1. The problem statement, all variables and given/known data

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) 2. Relevant equations P=(1/R)*A*/\T 3. The attempt at a solution P=(1/Rfront wall, back wall)/\T*(Afront wall+Aback wall)+(1/Rroof)*/\T*Aroof =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

2. Nov 12, 2012

### haruspex

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