Heat Loss Through Wall: Thermal Resistance Calculation

[Math Jeans]

Homework Statement

The wall of a house consists of three layers: A wooden outer wall with an R factor of 1.0, a 3 inch layer of fiberglass with an R factor of 11, and a gypsium-board inner wall with an R factor of 0.33. (All R factors are in h*ft^2*F/BTU). When the temperature is 68 degrees Fahrenheit inside, and -4.0 outside, what is the rate of heat loss through an 8ft by 15ft section of this wall.

Homework Equations

The Attempt at a Solution

I was a little confused as to what they means by "R factor", and basically tried to add of the R factors as the regular resistance, and calculate the heat loss through:

rate of heat flow= (delta)T/R_eq

However (not suprisingly), I was wrong. I know I am missing something with this problem, but I cannot figure it out.

 

[blochwave]

From what I've read, you just sum them, so Rwood+Rfiberglass+Rgypsum=Rtotal and that's what you use.

Make sure you take into account the surface area that was given, a quick check of your units in the equation you end up using should show an extra ft^2 that shouldn't be there

 

[Math Jeans]

So are you saying that the R Factor is the resistance per square foot?

 

[blochwave]

Well the R-factor's units wouldn't make that make sense.

If you look at your equation, which is correct but not quite finished, you have said

rate of heat flow = deltaT/R

rate of heat flow is power. R is in units of temp*area/power, deltaT is temp

so you've said rate of heat flow = power/area, which isn't true

What that equation gives you is power per unit area. Now you need to multiply by the actual area to find the net power

 

[Math Jeans]

Ok. I'll try again. So I need to take the original answer I got (basically taking the R-factors and treating them like the actual resistance of each part of the wall), and multiplying that answer by the area?