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 Installation

With all of the engineering and design that goes into a house like this, why would window installation be any different? EVERYTHING in a passive house is modeled, examined, and optimized thermodynamically. For now we will be talking about the thermodynamic analysis of the Passive House Windows and how this translates to the positioning of the windows within the wall system.

A typical new construction window comes with a nailing flange that is part of the frame of the window. This requires the window to be nailed directly to the wall, resulting in the window being flush with the outer edge of the exterior wall.

Passive House windows don't come with the nailing flange, this allows flexibility in location of installation within the wall system. The fact is, in a thick wall (12 inches or more, further details on wall thickness will be addressed in a later post) you typically don't want to install the window on the outside of your wall. You want to install the window in the middle of the assembly, or more specifically, as close to the middle of the R-Value of the wall as possible  (if your wall assembly includes more than one type of insulation, this may not by the exact middle).

This detail is from a 475 Blog Post that goes into quite a bit of detail regarding installation of air barriers to windows to achieve Passive House Level air tightness.

So in an effort to not bore your with thermodynamic details - here is why we want to keep the window in the middle of the wall:

It is more efficient - this simple detail can reduce your heating demand by as much as 22%

It is more comfortable - it helps keep all of the surface temperatures of the house within 6 degrees of each other, eliminating all of the cold drafts

It is more healthy - With such consistent temperatures, there is little opportunity for condensation, which means there is little opportunity for mold growth.

I found a fantastic page on Therm Analysis. The photo below shows an example window thermal analysis.

Thanks to ECO Houses of Vermont for the above detail!

Basically a Therm Analysis identifies the temperature gradient though a solid material. The rainbow colors shows how heat is flowing though the material. Looking at the photo's above; The image to the left has the window installed to the outside of the wall assembly. The isotherms are disrupted by the location of the window. This will result in colder spots where the frame of the window meets the wall.

The center image shows the window installed in the middle of the wall assembly. The isotherms seem to be pretty aligned.

The far right image also shows the window installed in the middle of the wall. In this case the window frame has also been wrapped in insulation. This added insulation further decreases the disruption of the isotherms. However the ability to add the extra insulation is a function of the size of the window frame.

Pretty amazing how just moving the window to the middle of the wall has a measurable impact on energy efficiency, and surface temperature two key elements for Passive House efficiency and comfort.

In my next post I am going to get into a little bit more on Therm analysis and Thermal Bridges.