Homeowner’s Energy Workbook – Part 7 – The Skeleton

You need to get familiar with the skeleton of your house.  Most of our houses are framed with studs, commonly known as two by fours or two by sixes.  These are pieces of wood that have been milled from trees that are nominally 2 inches by 4 inches on a side.  (The milled pieces.  Not the trees!)  They used to actually be 2 inches by 4 inches, but with the fancy milling that goes on in saw mills, they are now one and a half inches by three and half inches or one and half inches by five and half inches.  This is important because there is now less space in the wall for insulation and other stuff.

Balloon Framining

Balloon Framing

Up until the late 1940’s the wall studs ran all the way from the foundation wall up to the attic framing.  This is known as balloon framing.  The studs had to be really long to go that far.  It’s handy for running wires in wall cavities, but it forms a chimney up the outside walls of the house.  The rising air (convection) can flow all the way up from the basement to the attic!  And with open floor cavities, it can even flow under the floors and up the inside walls.  This can make the inside of the house really uncomfortable.

Both because we had harvested most of the easily accessible, tall trees an because it is more convenient, since that time we have been using a platform framing technique.  Each level gets constructed individually.  The deck of the second floor serves as the platform on which to build the walls.  This technique makes the stud cavities in the outside walls shorter, limiting the convective loops.

If you start in the basement (assuming you have a basement), the Rim joist is at the top of the basement wall, connecting all of the first floor floor joists.  On top of that is the stud wall, reaching up to the Band joist that runs around the middle of the house, between the floors, connecting all the second floor floor joists together.  Then the second floor stud walls reach up to the ceiling rafters (unless you have a third floor).  The ceiling rafters form the ceiling plane, the top, inside surface of the house.  The roof is commonly framed with Trusses these days that are often built off site and trucked in.  In many cases the roof is framed with individual two by lumber on site.


Thermal Envelope

The Building Envelope is a term for all of the materials that separate the inside space from the outside space.  It includes the walls, windows, doors, floors, and roof.  Part of the building envelope is the Thermal Envelope or Thermal Barrier.  The space inside that is what is conditioned – heated and cooled.  It is the conductive heat barrier or blanket keeping the occupants comfortable.

Aligned with the Thermal Envelope is the Pressure Envelope or Pressure Barrier.  This is the convective heat/cool barrier.  The key to a comfortable living space is for the Thermal Barrier and the Pressure Barrier to be working together, to be aligned.  This is a really important issue that we will get into in more detail later.

I want to go back to that “house as a system” idea.  The floor, walls, ceilings, roof windows, doors are all part of the system. Except in “paradise climates” a house missing the windows would obviously not keep the occupants very comfortable.  If the walls were missing, that wouldn’t be much of a house either.  These are the extreme, and gratefully uncommon, conditions.

Let’s consider a house heated with fuel oil on a cold winter day.  If the oil runs out, the furnace can’t burn to produce heat.  The house cools.  If it gets cold enough, the water in the pipes starts to freeze.  As the ice in the pipes expands, it cracks the pipes.  When the outside temperature warms, the water melts and shoots out of the pipes, getting the walls wet.  The wet walls are a perfect breeding ground for mold growth.  And so on, and so on.

Again, this is a somewhat extreme system connection.  There are millions of much more subtle connections that are going on all the time in houses.  Many of them don’t mean much on a human time scale, but as houses get more efficient, the interactions, the systems become increasingly more interrelated, and the potential problems are amplified.  So we always want to ask before we start changing something, “What else is going to change if I do this?”

One of the things I have found living in an older house, is that one remodeling project seems to lead to a dozen more.  Changing a sink may require replacing the vanity, the pipes, the drain in the floor, the color of the bathroom walls, the floor covering, the towel racks, the lighting, and on and on.  The system unfolds before my eyes and a Saturday project becomes a Fall project!


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