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. 

2017 Utility Data

2017 Energy Data

To meet International Passive House (PHI) Standards the house must be built to the following performance criteria:

• 4.75 kBTU/sf/yr - Heating Demand (or 3.17 BTU/hr/sf Heat Load)
• 5.39 kBTU/sf/yr - Cooling Demand
• 38.0 kBTU/sf/yr - Primary Energy (2.6 Source Energy Factor)
• Thermal Bridge Free
• Airtightness below 0.6 ACH50

To meet Passive House US (PHIUS) Standards the house must be built to the following performance criteria - this is based on Climate Zone 5 (Rochester International Airport)

• 6.0 kBTU/sf/yr - Heating Demand
• 2.2 kBTU/sf/yr - Cooling Demand
• 6200 kWh/per person/yr - Primary Energy (3.16 Source Energy Factor)
• Thermal Bridge Free
• Airtightness below 0.05 CFM50/sf envelope area

* Keep in mind the way both standards measure sqft is different
** Watch our for UNITS! kBTU and kWh

To keep this post relatively short, we will only focus on the 1st 3. I suppose you'll just have to trust me that we are still below the airtightness and we are thermal bridge free.

I have compiled all of the energy data from 2017. Here is a quick break down of the different components I have been metering over the past 12 months:

Over the past year, my total electric usage - according to my bill was 8129 kWh - $1050.63

Here is the Breakdown, based on my meter readings:

*** I estimated the electric use for the water heater or ventilation system for January 2017 because I didn't install my meters on those components until Feb 1, 2017

Here are some comments on the utility data breakdown:

• My largest single expense was the $17/month Service Charge
• My Average monthly bill is $87.55
• The 10 year average HDD for Rochester, NY is 6265
• 2017 had 5766 Heating Degree Days - so it was a "warm" winter.

Okay this is all great, but low utility bills are objective. After all, this barn used 100% less energy than me last year:

(This is also a case when a single pane window is more efficient than a wall)

This is where comfort comes into play. Now here is Western NY I have done plenty of Energy Audits and people will go to some pretty extreme lengths to keep their heating bills as low as possible. I have heard the following quotes more than once:

"We turn the thermostat back to 55 F at night"

"We only set the thermostat to 64 F during the day"

"We don't heat the upstairs"

"We don't heat the basement"

"We don't heat the extra bedrooms"

"We burned 9 cords of wood last winter"

I've heard them all.

Well, obviously we have done none of that, and we have kept the whole house warm (or cool) all year.

I downloaded the weather data from my NETATMO weather station (Its a really cool device that can give you more data than you'll ever care to analyze).

I took a look at every day last year and took the high and the low temperatures - both indoor and outdoor. Then I averaged the high temperatures over the course of each month and I averaged the low temperatures for each month and made this graph:

The Blue Line is the Outdoor Average Maximum temperature for each month

The Red Line is the Outdoor Average Minimum temperature for each month

The Green Line is the Indoor Average Maximum temperature for each month

The Purple Line is the Indoor Average Minimum temperature for each month.

Basically the Outdoor Temperature was between the Blue and Red Line most of if not all of the year.
On the same note, the Indoor Temperature was between the Green and Purple line most of the year.

Here is some interesting notes:

• We maintained the indoor temperature between 70-72 degrees for most of the year (Close to 80% of the time)
• The highest recorded indoor temperature was 79 F - this is because we had a 72 F day on Feb 24th. I was NOT going to turn on the A/C is February. 
• The lowest recorded indoor temperature was 64 F - this was because I left too many windows open overnight to achieve some passive cooling in September.
• There have been a few times it has been colder than 30 F outside and we have had to open windows to cool off. This does mess with the Relative Humidity a little bit. On Christmas Day 2017 we had to open windows to cool down the house with company over. We were at 75 F inside.
• In the summer the heat pumps only ran in the "dehumidify" mode (where they try to maintain 60% RH), and we only turned them to A/C with a set temperature when we were gone for a week for vacation.
• I will be doing a ERV vs. HRV post in the future so I don't want to go into too much detail, but we have swapped to the ERV core for the winter to help maintain RH. From January-March we struggled to keep the RH around 30-35%. So far this winter we are in the 40-45% range - even with the bomb cyclone dropping temps here to around 0 F for a few days. 
• For 80 % of the year we are in the 40-60% RH. The other 20% is due to opening windows for passive cooling in the shoulder months - when it gets down to 55 F at night and 90% RH. Then heats up to 80 F during the day at 60% RH. I am not good enough with Excel to show you a graph of that. 

Okay, Back to the Utility Data:

Now to take a closer look at how we used our Energy compared to the PHI and PHIUS Standard: The number on top of the Bar Graph is the Passive House Limit. The number in the blue is the 2017 energy use for RPH.

One quick note on the primary energy from PHIUS. It is calculated as 6200 kWh per person per year divided by 3.16 (losses in the grid). The number of "persons" is based on number of bedrooms +1 (just like RESNET). We have 3 bedrooms and an office without a closet. For purposes of this calculation included the office as a bedroom. Amazingly, that closet matters! We currently have 5 people living in the house, so I figured it was okay. I know neither standard is perfect but seems silly that the difference between certification and not is whether or not you build a closet in a room.

Thanks for taking the time to read, and if I messed up any of my calculation I'm sure someone will tell me. 

I'll be working on a post for HRV's VS ERV's and my experience with both shortly.