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. 

Blower Door / Window Door Tests

Passive House Airtightness Achieved!

We ran a few different tests today to validate our airtightness numbers:

Our Field Measured Volume is 29356 CUFT

We did a Window Blower Door Test with the Duct Blaster

Here are photo's from our "Depressurization Test"

52 CFM50 - 0.106 ACH50

Here are photo's from our "Pressurization Test"

65 CFM50 - 0.133 ACH50

We also ordered the "D Ring" for the blower door to get an accurate Blower Door Number 

I should note - the tolerance for the D-Ring is:

125 CFM - 30 CFM

Here are photo's from our "Depressurization Test"

 

49 CFM50 - 0.100 ACH50

Here are photo's from our "Pressurization Test"

52 CFM50 - 0.106 ACH50

To average the results we are at 50.5 CFM50 

A few notes from the tests:

• The blower door pressurization test was done in the protected breezeway door vs the duct blaster pressurization test that was done on the open back yard. 
• We did a blower door pressure test up to 90 Pascals to see when/if the Studor Vents (AAV's) opening would effect our results - there was no noticeable difference:
  
• The Blower Door Frame was extended fully to accommodate the width of our door. The seal was probably not perfect
• All of our plumbing is not installed yet - we will do another test once it is.
• We were having trouble syncing our computer to the manometer to complete a multi-point test, once we do that we will post those results!

From what I could find the tightest house in the world is in Alaska - 0.05 ACH50 - we are 25 CFM50 away from that.

Stay Tuned!

Blower Door Test #3

3rd Blower Door Test

With all windows and doors installed, all wall penetrations complete and sealed and drywall up it was time for us to do another blower door test. This time we had to design / create a perfectly sealed "blower window" to test the house. 

In previous tests the basement windows were not installed, and I could simply install a small piece of rigid foam, foam it in place and tape the duct blaster to the foamboard

Now we needed to find a way to test the house, and create the same (or better) seal to the house between the fan and the house:

So we cut a piece of leftover ZIP board, and cut a hole in it to seal the duct blaster to it

We added a handle, airtight gasket from 475 building supply, and some VELCRO strips to mount the manometer to.

We installed a compression weatherstrip around the perimeter to seal nicely to the window frame

We removed an operable sash from one of the ZOLA windows. We chose our window wisely.

1. Window small enough our scrap of ZIP would fit over the window
2. Window size is repeated more than once in the house (so we can test in different locations)
3. Window that is repeated on different sides of the house (East and South in our case). 

All of these situations allow us to test the house in multiple locations on multiple floors in both situations to ensure a more complete and comprehensive airtightness

We installed the Blower Window to the outside of the window frame for the depressureization test. This will force the blower door pressure onto the frame, pushing it tight to the frame to compress the weatherstrip. The handle was installed to help pull the window tight as necessary

We did add a pull pole attached with a small clamp to make it more hands free.

Now we are ready for the test!

That's right! 52 CFM50. 

Passive House Volume: 29356 CUFT - We verified this in the field

RESNET / BPI Volume: 38698 CUFT

This puts the Rochester Passive House at:

0.11 ACH50 - Passive House

0.085 ACH50 - RESNET!

In the next week or so we will be doing a pressurization test with a very similar technique. 

Stay Tuned!

1st Blower Door Test

The Initial Blower Door Test

Airtightness is of a building is related the the overall comfort of a building. The tighter the house, the less drafty it is and the more comfortable it is. In this Blower Door test we will be verifying the tightness of the primary air barrier, and sealing any holes we may have. It is much easier to seal them now, than after the insulation is in and the drywall is up.

The blower door is a diagnostic tool used to measure how tight (or leaky) a structure is. You should see a blower door done when you have an energy audit done on your home or if you own a Certified Energy Star, LEED or NAHB Green Homes you have had this test performed on your house (probably before you moved in). 

A few acronyms before I move on:

ACH50 - Air Changes Per Hour at 50 Pascals - With a blower door running at 50 Pascals (forcing a 20-25 mph wind on all 6 sides of the house at once). The number time time you will entirely replace the air inside the house in 1 hour

CFM50 - Cubic Feet Per minute at 50 Pascals - With a blower door running at 50 Pascals (forcing a 20-25 mph wind on all 6 sides of the house at once). The number of cubic feet of air moving though the blower door fan (and thus the amount of air being drawn into the house) in 1 minute

There are limits to each of the above programs, 

 - NYS Building Code is 7 ACH50 - or - Visual Inspection (99% of homes is a visual inspection)

 - LEED and NAHB Green are on a sliding scale (the tighter you are, the more points you get - and you may not need any of those points to achieve the certification) - so just because you have a Green Home, or a LEED home does not make it a Tight Home

 - EPA's Energy Star (in our area of the country) is 4 ACH50

However none of the is even close to the Passive House Standard. 

To meet Passive House Standards the house has to be 0.6 ACH50 or lower.

The blower door test will give you a CFM50 (cubic feet per minute at 50 Pascal's pressure difference between the inside and outside). Then the volume of air in the house must be calculated.

Calculating the Volume:

RESNET, BPI and ASTM standards calculates the Volume fairly similarly:

Measure the house from the outside, Measure the height from the basement slab to the upper ceiling and boom you have your volume. Pretty Easy, pretty straight forward

Passive House does things a little different. The measurements are all taken from the inside (since walls are so thick) and excludes the area's for interior walls. 

How much of an impact does this have on our house??

RESNET, BPI and ASTM standards has a volume of:                                      38,698 CUFT. 

Passive House Standard has a volume of:                                                         27,299 CUFT.

That is almost a 30% difference! 

This impacts the calculation quite a bit since:

So the Passive House Standard is even tighter than you'd expect!

OK - So enough "Technical Information" Lets see house we did!

Test Results:

We need to be under 273 CFM50 at the end of the build, our Goal for this test is 150 CFM50.

Here is our prep:
We sealed off the:

Front Door

Large Basement Window

Plumbing Penetrations

Small Basement Window

We chose to run the test out of the small basement window using the "Minneapolis Duct Blaster"

 

Here is our test result for today:

83 CFM50

0.129 ACH50 - per RESNET Standards

 0.182 ACH50 - per Passive House Standards

We determined the test to be a HUGE Success. We also did a little bit of air sealing and brought the test down to 73 CFM50. 

With the initial Blower Door test complete we can now continue installing all of the windows - my next post will discuss the installation procedure