Wall Assembly 1

Double Wall Construction

Maybe I am biased to double wall construction because the company I work for Airtight Services is a cellulose insulation company; but it is still probably the most cost effective way to achieve the R-values necessary for Passive House.

Based on our energy model we needed to achieve about an R-54 wall. There were a couple options:

• 2x6 wall with 6" of polyisocyanurate rigid foam to the outside
• There are some more difficult window installation details when your window is cantilevered over nothing but foam.
• Some long screws are need to attached a furring strip over the rigid foam (9" long) and those can add considerable cost to the project - upwards of $0.75 each
• The labels on rigid foam insulation scared me a little bit too - 15 YEAR LIMITED THERMAL WARRANTY
• Polyisocyanurate has the WORST embodied energy of the good types of insulation meaning it takes the most amount of energy to make relative to how much it will save

This graph was posted on Green Building Advisor. A quick cross check with other sources confirmed these numbers.

• ICF with similar amounts of rigid foamboard
• With similar complications of the above 2x6 wall, the added cost of the concrete and pumper truck
• ICF with framed interior wall
• We will be doing this for the basement/foundation
• The overall wall thickness is thicker than a double wall construction (24" for R-54 ICF & Framed vs 16" for R-54 Double Framed)
• This becomes more important in our area since the tax assessor measures the square footage of the house from the outside - thick walls take up quite a bit of area
• Double Wall Construction
• Again I work for a cellulose company
• Framing is more complicated than a standard framed house
• Common building materials, so finding a crew is not as difficult
• When you use the correct moisture management materials the wall will last a LONG time

Here is our wall assembly detail

The make up of the wall assembly from inside to out is:
1) 2x4 wall, 16" OC - Structural
2) Zip System: Primary Air Barrier (and temporary weather barrier before curtain wall is assembled)
3) 7" "airspace" - to be filled with cellulose
4) 2x4 cladding wall 16 OC - Basically a balloon framed 20' tall wall.
5) Solitex Mento Plus vapor open weather resistant barrier (WRB)
6) 2x4 horizontal furring strip - to help hold Mento on the wall and provide the air space for a rain screen to help siding dry.
7) Siding - type still to be determined - LP Smart Side is our 1st choice budget depending.

Due to the multiple layers of the wall, and order of assembly I will only be discussing the Interior Wall assembly in this post, I will discuss the outer wall assembly, continuation of the air barrier to the 2nd floor ceiling, blower door testing, window installation and many other details in future posts.

For step one we totally assembled the structural walls, with window openings covered with the Zip System. This will allow us to conduct the essential blower door test prior to insulation to test our primary air barrier for leaks.

 Raising of the 1st Floor Walls

Homemade wall jacks allowed us to build the entire 45 foot wall in one section and raise with a crew of 3 guys. It is basically 2 come along winch's and some 16' 2x4's hinged to the deck.

 Wall half way up

Wall totally up

Due to the size of the south windows on the 1st floor we elected to leave them uncovered for day lighting into the house and material loading. We will seal them up for the blower door test.

Because the windows are so large, and the shading is essential in the summer time we didn't have enough room between the 2nd floor deck and the top of the rough opening for headers. So we raised the headers into the band board of the 2nd floor deck, and hung the floor joists from the headers with hangers (much like an recessed beam). This is a good way to incorporate advanced framing techniques into the build. Everywhere there is wood, there is not insulation - so instead of both headers and rim board, we have just headers. Now we can have insulation where the header would normally be.

With the 2nd floor deck on we framed up the 2nd floor exterior walls much like the 1st floor walls

We overhung the bottom sheet of Zip System on the 2nd floor wall to cover the 2nd floor band board and tie into the 1st floor walls.

Taping and rolling each seam is vital for good adhesion between the Zip Tape and Zip Board. 

(View from the Outside)

(View from the Inside)

We overhung a layer of Zip board on top of the 2nd floor walls for a few reasons:

• More specifically, the GREEN side of the Zip is the air barrier, and we need an edge to tape the wall Zip to the ceiling Zip.
• The overhang will allow us to tie together the interior wall to the exterior wall

All of the structural walls are installed and all of the seams taped (except for the front door and the 1st floor south windows)

Front (West)

Side (South)

Back (East) 

Side (North) - Breezeway and Garage will be on this side

Once we have the roof on, and the zip system installed on the 2nd floor ceiling we can cover up the window opening and run our 1st blower door! 

I am not sure what the next post will be, there are a few things that will be happening at once - Breezeway and Garage Foundation, Roof with Zip installation on the ceiling, Outer Wall... So Stay Tuned!