Saturday, March 10, 2012

All the Framing is Backwards!

We've had people go by our site and wonder why we were doing things backwards.  Normally, one builds the concrete walls and places a wood floor system on top of it.  We didn't.

The wooden house frame rests on the footings - not on the concrete walls.  Why?  One thing we're discovering about radically energy efficient buildings is the structure!

When the structure is not designed for insulation, it gets very hard to achieve thermal-bridge-free construction.  Placing the first wood (or steel, or whatever) floor frames on top of the concrete walls means there is a strong connection to the concrete (a heat conductor, not an insulator).  Unless the concrete is on the warm side of insulation, this is a significant thermal issue.  In our design, the structural concrete is all on the cold side of insulation.  (This means the outside shell of the building is hard.  When one thinks about a building lasting 25 years, having rigid foam on the outside might seem OK.  But what if we want  it to last 150 years or more?  Well, I don't know if that will happen with this house, but in case it does, it seems a good idea to have the outside shell be hard and durable.  One loses some potential to have thermal mass inside, but that can be achieved in other ways).  Getting back to the wood frame, we therefore have thermal separation between the inner frame of the house (which is the structural frame) all the way from the footings, up to the roof.  Therefore, framing starts in the basement, not on top of the concrete walls.  This might seem like a radical departure from conventional practice. But so far, in our project, we've found no real problem, and we are framing the 2nd floor walls now.

There are some considerations to handle, however.  First, we start with wooden walls, not floors.  The reason for this is that if we started with floors, the insulations inside them would get all wet.  So we wait for the roof to be on and the building closed up before building the basement floors.  This also reduces the natural settling of the building - pretty much all of the shrinkage of framing lumber happens in the floor frames.  Although we built the wide footings very level, we also shimmed the walls so they are not in contact with the concrete and any water can drain from under the walls, into the space between the footings, and finally to the sump pit.  

The walls against the concrete are not sheathed.  Again, this is so we can insulate the space behind the frames after the building is closed in.  We borrow shear strength from the concrete walls to stabilize these open frames.  The photo below shows a wide 3/4" plywood top plate (placed over the upper top plate) which reaches to the concrete and laterally anchors the walls to the concrete with steel brackets.  The 1/2" female Zamac anchors ($1 ea) were placed in the forms during the wall pour, but could probably be drilled in afterwards.  The concrete crew didn't really pay attention to the location of these anchors so you'll see that some of the steel brackets connect to the underside of the plywood, and some to the upper side.

1 comment:

  1. Hi - I teach a Passive Solar Design course at Sheridan College and was wondering if I could ask you a few questions and possibly visit your project site?