Double Wall Construction

Double Wall Construction

Wheatland Passive House is well on its way both under construction and though the certification process. We are in line to meet all of the PHI requirements. I figured it was a good time to go into some details on our 4th go around at double wall construction, and what we have learned in the process.

I have dealt with double wall construction on previous projects, but I it was well before my Passive House training. 

While I was with Airtight Services we did a wide range of consulting and along with insulation services, but cellulose is the specialty. They can dry blow, dense pack, and damp spray cellulose. The preferred method for us insulating the double wall system is to install a fabric on the back side of the interior wall and insulate the outer wall and empty cavity with dry blown dense packed cellulose. Samples can be cored and weighed to ensure correct density. Once all of the rough in mechanicals are in, the remaining interior wall can be damp sprayed (and any interior walls can be damp sprayed for sounds deadening) to complete the insulation package. This is ideal because it maintains a protected air barrier with few penetrations allowing drying potential in either direction depending on interior and exterior finishes.

"Original Double Wall - July 2010"

This project didn't start off as planned. 

As you can see, there are a few problems right off the bat.

1) No fabric was installed between the walls for us to dry blow the cellulose. The wall is too thick to install damp spray cellulose in the entire wall - It would NEVER dry.

2) There was also quite a few area's where plumbing was running between walls. So we'd have to work the fabric around the obstacles. 

3) The studs were lined up, so you couldn't physically get a stapler between them to staple up a netting after the fact.

We were forced to work the netting in between the walls in small sections. There were some area's (above and below windows) that were too small to work in, so we had to face staple the netting. 

This was the common wall between the house and the garage - it came out pretty nice...

But this is what most of it looked like

It was tight enough for us to insulate the wall though

It worked out okay because of the fact we were actually pushing the netting against the framing, so we didn't need to bunch of staples - except on the ends.

We learned a lot on this project - what to do, what not to do. A few years later we were fortunate enough to go to the Passive House Tradesmen course in NYC 

Airtight Services Gang 2014

After learning some great new techniques, some material science, and terms like "Floppy Bit" we were ready to try the double wall again.

"Double Wall 2 - July 2014"

For this project we were hired to work with the framer to install the air barrier as they were framing the walls. The house is a single story slab on grade house with exterior load bearing walls. The builder got the entire project weathered in for us.

We started by taping the OSB ceiling with some 3M tape. The framer did hand the OSB upside down - the smooth side would have been much better to tape to.

The details got tricky at the gable ends

There was a small OSB gusset to tie the inner and outer wall together. We did have to seal off the  OSB to the truss with some spray foam to prevent blowing cellulose into the attic while trying to achieve out 3.5 lb/ft3 density. 

The framer framed the interior walls on the ground and we installed the primary air barrier (Intello)

You can see we tucked the Intello under the bottom plate of the wall. We sealed the Intello to the slab with Contega HF - or more technically known as "Green Goo"

Things got a little tricky at plumbing penetrations

We installed spacer blocks at the top the wall to maintain plumb

Once all of the interior walls were in, it was a matter of sealing the Intello to the OSB ceiling air barrier with some vana tape - and then install our furring strips on the ceiling to create out service cavity for the ceiling

At the ends of the different wall sections we pulled the Intello to the inside and taped the 2 ends of the wall together. 

 This project was also our 1st try at a Zehnder install. We were able to fit all of the tubing into soffits and dropped ceilings.

After the rest of the rough electric, plumbing and ventilation we were ready to insulate with dense pack cellulose. We really liked the fact the cellulose pushed the Intello tight to the wall.

You can see there was definitely some bulging with the dense packed cellulose. Another good reason to place it on the exterior side of the interior wall. For those of you totally against damp spray cellulose because of moisture concerns - at this point you probably could just use rock wool, or another batt product to finish off the service cavity. 

I did mention the gable ends were tricky. We did make the drywaller mad - for some reason no one wants to make the drywaller mad. 

We had to face staple to upper portion of the gable end, and sheet rock right over it. To this day the drywall is still there, with no cracks, and no one will ever know the difference.

I was not there when they were hanging the drywall over the Intello on the exterior gable end, but I am sure they lived to tell about it. 

The house tested out at - well we had to use the duct leakage tester to do it. We used a scrap of Intello as the shroud.

108 CFM50 is roughly 0.3 ACH50 - This project was definitely a success. There is an article in the JLC on this project - https://www.jlconline.com/how-to/framing/scissors-trusses-and-home-performance_o

"Double Wall 3 - December 2016"

This project - Rochester Passive House - was our 1st go at a 2 story double wall, in a certified passive house where we needed to account for thermal bridging between floors. 

I have written at length in the blog about this house, my house. So in the effort of keeping your attention - if you want to read about the Rochester Passive House double wall check out this post:

https://rochesterpassivehouse.blogspot.com/2015/12/wall-assembly-2.html

We decided to move the load bearing to the interior wall, and balloon frame the outer wall to ensure continuous insulation at the band joist. It was obviously successful as we tested out at under 0.1 ACH50

BUT.... What about vinyl siding.

"Double Wall 4 - April 2019"

Wheatland Passive House was designed much like Rochester Passive House. We didn't want to reinvent the wheel, and reuse as many thermal bridge calculations as we could. The biggest difference is - vinyl siding vs Smart or Hardie Siding. Vinyl siding needs a sturdy backer to maintain warranty. The 1.5" air space needs to go! 

So what if we moved the shear strength of the wall to the exterior wall, but kept the load bearing strength to the inner wall...

We would end up with something like this:

So with this assembly we connected the inner and outer walls at the 2nd floor sub floor. This does a few things for us.

1) We are able to maintain our air barrier and moisture control layer at the exterior side of the interior wall. 

2) We transfer the sheath strength of the exterior wall to the interior wall at the 2nd floor sub floor, at all window openings and again at the top of the 2nd floor wall. 

3) We eliminate the extra lumber needed to fur out the outer wall to create the air gap for our WRB

4) We can insulate the outer cavity from the inside, and not off of a lift - so the insulator is happy

5) The home owner saves a little money on the upfront cost of siding

So now, lets take a look at the difference in the Thermal Bridge Calculation between Rochester Passive House Band Joist and Wheatland Passive House Band Joist. Does the 3/4" sub floor thermal bridge impact the overall energy model?

Rochester Passive House Band Joist Thermal Bridge Model

Wheatland Passive House Band Joist Thermal Bridge Model

Rochester Passive House Thermal Bridge Psi = 0.001 Btu/(h-ft-F)

Wheatland Passive House Thermal Bridge Psi = 0.007 Btu/(h-ft-F)

Over the exterior perimeter of the house (166 linear feet) this thermal bridge only impacts the 

Heating Demand 0.05 kBTU/ft2-yr

Or roughly $0.11 per heating season.

Overall I am a big fan of the double wall, and I am sure new details will emerge as different situations arise. 

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!