Thermal mass

Have been going back and forth trying to decide about thermal mass.  I see it commonly in insulated concrete form construction   My thinking is along those lines to smooth out indoor temperature over the 24 hour day.  So am looking at high mass for doing passive solar and low mass for building with double stud construction with high actual insulation value in the floor, wall and ceilings.  With this in mind am not just looking at concrete to add mass on the floor but doing something that I have not heard of others trying.

My idea is to take the inner wall frame that is actually holding up the roof and putting all of the insulation out board of this framing.. hence double stud.  On the east and west wall I am looking to build shelves into the 2 x 6 wall studs.  on these shelves put water bottles (2 liter bottles) from floor to ceiling.  Initially do not plan to cover the bottles so I can adjust to get to the desired temperature range thru a full year.  Once I know ABOUT what is correct then go ahead and panel the walls with full thickness wood paneling with a vent at both top and bottom to allow air flow both to allow for moisture that naturally flows thru the walls and to allow for the air to reach the thermal mass.

I have done a little with double stud walls, and exterior rigid insulation.  Exterior rigid insulation removes the thermal bridging and can be retrofit to an existing house, but is pricey and not the most permie options for materials.  Double stud is really expensive if you aren't very careful about your design.  Only 2x4, no 2x6, not needed for cavity space. Do 19.2 or 24 oc studs instead of 16, minimum you need for the sheeting on that side of the wall.  Make sure to frame the window and door openings bigger to account for the bucks.

Some things I have been looking at is that the cost of the framing studs will likely add about $500 extra in materials.  Going with the wider spacing is ok but I kind of would like walls that dont let the sheet rock have waves.  (this might induce me to use 5/8 rock)  The other thing that most people have abandoned is how we  used to keep the frames from racking.  Before the mid 1940s there just was nothing that we would call sheathing.. NO Plywood NO OSB.  So we let in wood diagonally into the frame.  The real question other than to keep a frame from racking ... do we really need to apply a layer of sheathing.. 7/16 osb actually does not have a lot of holding power but because it has a lot of nails every 6  to 12 inches it works.  A Simpson tie strap in an X at the corners will accomplish exactly the same thing.  Putting an in board air barrier and and outboard Water Resistive Barrier (WRB) that is also an air barrier will allow for a huge perm rating.  Meaning water vapor can very easily allow the wall to dry from both sides.  We see a lot of houses being built with Zip system or Tyvek to do the WRB.    

Our please has 2x6 @ 24" in the exterior and 2x4 @ 16" on interior. Wall depth is 16" with 3" closed foam and the rest dense packed cellulose. The ICF foundation links up to the wall foam. Studs don't line up much. Windows are fastened to the interior of the 2x6.

Thermal mass; 1st floor is 5" crete, 2nd floor is hardwood. Each room is a different species since they are leftovers from a local mill. I looked up the thermal mass by species and choose that goes where based on that. Interior window sills are deep and long. I did 3" crete counter top type slabs for each.

A key part that was overlooked was the interior designer understanding the thermal dynamics. Once furniture, rugs, and other got in alot of the direct solar gain was lost.

steve pailet wrote: The real question other than to keep a frame from racking ... do we really need to apply a layer of sheathing..  

Nope.

Sheathing is needed at the corners and that's it.  Our (stupid) house in Wisconsin was built that way, with 1" extruded foam sloppily placed down and then vinyl siding over that.  No one shared my California-raised concerns about this (earthquakes and energy use codes...).  If you build in diagonal bracing then you might not need OSB/plywood sheating at all - but of course this may all depend on how much you care about inspectors and local building codes!

"High performance framing" uses 24" oc framing with very carefully considered window and door placement to minimize thermal bridging without double studs.  The best combination of cost * effectiveness seems to be a 2x6 wall with 2" of external insulation (for my area ... your mileage will vary).

You're paying careful attention to vapor and air movement - that's good!  I'll add that you can, with care, design a wall without the interior plastic barrier.  Drywall done well with appropriate paint will give you a perm rating of 1.0 or better.  Electrical runs and outlets seem to be the major hazard here ... as well as a happy homeowner who thinks a wall is just for making holes in.