Merge pull request openenergymonitor#32 from danbates2/patch-3

Update fabricheatloss.md

Author: glynhudson

view/sustainable-energy/building-energy-model/fabricheatloss.md

@@ -1,40 +1,66 @@
 ## Building fabric heat loss, thermal conductivity and U-values
 
-Building fabric heat loss is heat loss through building elements such as:
+Building fabric heat loss is heat loss through building elements such as, walls, windows, floors, roofs, doors and more.
 
-- Walls
-- Floor
-- Loft/roof
-- Doors and windows
-- Thermal bridges
+Building elements are composed of materials, and a material's thermal conductivity is called a k-value. The units are in W/m.K (Watts per meter-Kelvin). Kelvin as a unit has nothing to do with the naming of k-value. 
 
-Heat loss is calculated using the physics equation for heat conduction:
+A fundamental physics notation to know is that Watts is the amount of energy per unit time.
+Watts is quals to Joules per second. W = J/s.
 
-    Heat Loss = k x A x (TINTERNAL - TEXTERNAL) / l
+Heat loss for an area, such as a wall, is calculated using U-values. The U-value is the k-value multiplied by thickness, depth, d. The units are in W/m^2.K (Watts per meter-squared-Kelvin).
+Multiple material layer k-values and thicknesses are simply summed together to make a building element U-value.
+
+Example:
+
+    A wall is made up of bales of hay with a layer of 50mm wood-fibre board on each side.
+    k-value of hay bales = 0.07W/m.K
+    k-value of wood-fibre board = 0.04W/m.K
+    thickness of bales = 400mm
+    thickness of wood-fibre board (total) = 100mm
+    
+    (0.07 * 0.4) + (0.04 * 0.1) = 0.032 W/m^2.K
+
+    The wall is 10 metres long and 2.6m high.
+
+    0.032 * 10 * 6 = 1.92 W/K
+
+This means for every Kelvin, or degrees C, temperature difference, 1.344 Watts of thermal energy will be transferred.
+
+A standard equation for heat conduction, which will give us the amount of heat transfer for a given temperature difference, is given here:
+
+    Heat Loss = k x A x (Ti - Te) / d
 
     k = thermal conductivity of the element material
-    A = cross sectional area of the element
-    l = element thickness
+    A = surface area of the element
+    Ti = internal temperature
+    Te = external temperature
+    d = element thickness or depth
+
+    The straw bale wall is part of a building with an internal target temperature of 20degC.
+    The temperature outside is -10degC. So the temperature difference Ti-Te is 30degC.
+    1.344 * 30 = 40.32 Watts are being lost through the wall.
 
-(HyperPhysics: [Heat conduction](http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html#c2))
 
-**Example:**
+Heat transfer has a direction. In UK houses we're normally concerned with minimising heat loss, like above. The way we set up the equation means a positive result would mean *heat lost*, but in a scenario where the temperature difference is negative, this could mean a negative result, and could represent heat flowing into our building, maybe cooling is required instead of heating.
 
-Imagine a house that is a hollow cube of uniform material, no windows, no openings, no draughts, just a simple hollow cube.
+Temperature difference is sometimes written as ΔT (delta T).
 
-Lets say this cube house is made of nothing but mineral insulation 100mm thick, with internal dimensions: 7m wide, 7m long and 7m high.
+***Another Example:***
 
-Our cube house is situated in a climate with no wind or solar gain just a stable 12C outside air temperature year round.
+Imagine an object that is a hollow cube of uniform material, no windows, no openings, no draughts, just a simple hollow cube.
+
+Lets say this cube object-house is made of nothing but mineral insulation 100mm thick, with internal dimensions: 7m wide, 7m long and 7m high.
+
+Our cube house is situated in a climate with no wind or solar gain just a stable 12C outside air temperature all year.
 
 How much energy would it take to keep this hypothetical house at a stable 21C?
 
 ![Cube house](files/cube.jpg)
 
-As we heat the house, heat will flow from the hotter internal air through the walls to the colder external air via conduction and so the equation that we need is the fundamental physics equation for heat conduction.
+As we heat the house, heat will move from the hotter internal air through the walls to the colder external air via conduction and so the equation that we need is the fundamental physics equation for heat conduction.
 
-    H = (kA / l) x (Tinternal – Texternal)
+    H = (kA / l) x (Ti – Te)
 
-
 See the great hyperphysics site for more on the [heat conduction equation](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html) and everything else physics.
 
 The [Wikipedia table on material thermal conductivity](http://en.wikipedia.org/wiki/List_of_thermal_conductivities) tells us that mineral insulation has a thermal conductivity of 0.04 W/mK. We can take the area of the material to be the internal area of our cube house (imagine folding the cube house out so that we just have this one dimensional wall of area A and thickness l), there is of course a difference between the internal area and the external area of our cube house but lets come back to that one later and take the internal area for now which is: