Insulation and Heat Loss Information
Energy Efficient Houses usually cluster their glass on the south side, minimize exterior skin area, put most of the insulation in the ceiling, keep air leaks to a minimum and use appliance and body heat as major heat sources.
We measure the thermal efficiency of wall, roof, and floor sections by calculating the resistance of these assemblies to the transfer of heat.
Basically, heat always moves from warm to cold. If the interior design temperature is 68 degrees and the outside temperature is 38 degrees the temperature difference of 30 degrees causes the heat to move toward where it is cold.
The resistance of a wall assembly to transfer that heat is calculated by determining the total resistance of heat transfer in the wall: that is, the resistance of the exterior air film plus the resistance of the siding plus the resistance of the building paper plus the resistance of the sheathing plus the resistance of the cavity insulation plus the resistance of the air barrier and finally the resistance of the interior wall surface. When we add all of these together we get a thermal resistance number (the U-Value) for the wall. For most general calculations we can take the reciprocal of the R-value to get a useable U-Value.
So, if the R-Value of the wall insulation is R-21, its U-Value is the reciprocal of 21 (or 1 divided by 21) which is 0.0476 on my calculator.
R 21 = U-Value of 0.0476
R 30 = U-Value of 0.0333
R-49 = U-value of 0.0204
To calculate heat loss of a building we have to break the house down into surface areas and then determine how much heat moves through those surfaces.
In our house the major surfaces to calculate are:
First floor area= ________ sf
Second floor area=_________sf
Gross wall area [East wall area + North wall area + West wall area + South wall area] = _____________sf
Net wall area (total wall area minus total window area + total door area)
Roof area= ____________sf
If you have these areas and u-values you can then begin an actual heat loss calculation like the one above.
The calculator above is for a colder temperature than ours. It assumes an outside temperature of 17. We will want to change that. What do you think the average outside heating temperature would be for our area? How can you find out?