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. 

Window and Door Installation

Window Installation

Now that we have created a pretty airtight structure we can begin poking holes in it.

To ensure our next blower door test is as good as the 1st one we have to maintain our air barrier.

Step 1

Cut open the window rough openings

I was very impressed at how well the inner wall and outer wall rough openings lined up! 

Step 2

With the Zip board removed, we used 6" Grace Window flashing to extend the Zip System to the inside face of the inner wall. 

With the air barrier extended into the house we can build the solid window buck. 

Step 3

We chose to use 1/2" zip board (the cut out section of the window) for the 2 sides and top. We used left over sub floor for the sill. I figured the thicker sill material would be a good idea with a heavy window suspended between the 2 walls.

Step 4

Now we need to install our 2" rigid foamboard to insulate the window buck and prevent "short circuiting" around the window frame

Step 5

Install beveled rigid foamboard to window sill to allow for water drainage

Step 6

Wrap window buck foamboard with Solitex Mento. The Solitex Mento will be the connection between the Grace 6" tape and the inside surface of the window.

Step 7

Install stop blocks on the outside of the window opening so the window can be lifted and pushed flush to the stop blocks

Step 8

Prep the window - For fixed pane windows a metal clip is inserted into the UPVC frame. We can then screw the metal clip through the rigid foam into the solid buck.

For operable windows we predrill holes though the UPVC frame. Once the window is in place we can install the screws from the Manufacturer. 

Step 9

Shim window and screw into place. Once in place we removed all of the shims (except for the bottom). 

Step 10

Seal the window in place

We filled the void between the Mento and the window with low expanding foam. Then we taped the Mento to the Window on all 4 sides. 

I hope this helps show that the detail of installing the Passive House Windows is just as important as the window itself. 

The door install is very similar with a little change to the threshold.

Because the low profile threshold of the doors (The bottom of the door is only 3/16" above the bottom of the frame) we cannot install to door directly on the subfloor. We raised the door 1" with a combination of rigid foamboard and pressure treated lumber to maintain a thermal break and still give the stability due to the weight of the door. This way the depth of the flooring will not interfere with the operation of the door. 

Also, since the exterior doors are all in-swing we wanted to allow for full 180 degree swing. We installed the door to the inside of the wall assembly. 

With the windows and doors installed (except for the front door) we can begin the installation of our Rain Screen in conjunction with Rough Plumbing and Rough Electric. Our 2nd Blower Door test will be coming shortly as well!

Floppy Bits

Floppy Bits

During my passive house training I learned a rather important "technical" term used pretty frequently during the construction process - Floppy Bit

A Floppy Bit is simply a section of material that is used to connect air barriers that are installed at different times.

I will be discussing the floppy bit installation connecting the basement wall to the 1st floor wall., but 1st I wanted to review how we came to define the interior surface of the foundation wall as our air barrier.

Before we installed the sub slab insulation and the basement slab we installed a radon barrier and sealed it to the foundation wall with glue and tape  See earlier blog post HERE

Now the next break in the air barrier is at the rim joist / mud sill. 

A Floppy Bit is installed in conjunction with the mud sill to connect the air barrier lines

We used Solitex Mento Plus as our floppy bit. This is a very durable vapor open weather resistant barrier. It will allow drying of the building assembly to the outside while preventing moisture from entering the assembly (much like a check valve for water vapor). 

Here is our assembly sequence:

1) Acoustical sealant bead on concrete

2) Sill Seal Gasket with another bead of acoustical sealant on top

3) Pressure treated mud sill

4) Solitex Mento Plus draped from inside to outside.

5) All corners and seams sealed with tape and Contega HF

6) Installed I-Joists on top of Mento 

7) Installed Rim Board

8) Installed Floor Plywood

9) Raised 1st Floor Exterior Walls

10) Wrapped the Mento up over the rim board 

11) We applied a bead of acoustical sealant between the zip board and the sub floor for good measure

12) Sealed and Taped the Mento to the Zip Wall System

This detail ensures a continuous air barrier between our foundation and exterior walls. 

Next post will be after the completion of exterior walls and possibly in conjunction with Truss installation.