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. 

What is Passive House Comfort?

Superior Comfort is achieved Passively

Passive House is just as much about comfort as it is about energy efficiency. But what is COMFORT?

Passive Houses are ultra comfortable because:

1) They are ALWAYS Draft Free!

2) They are ALWAYS Quiet!

3) They are ALWAYS Let in Lots of Sun Light!

4) They are ALWAYS Warm!

5) They ALWAYS have Fresh Filtered Air!

6) Oh - they are ALWAYS Low Energy!

So lets talk "Comfort" for a minute... While writing this post I did an internet search for images of "Comfort" and after seeing photo's of Mattresses, Bra's and Lasagna and having a good laugh and some internal comments about our society I decided I had to narrow down my search to "Home Comfort".

There were 2 common themes:
Fireplaces, coffee, blankets and snow out the windows

Fireplaces are not exactly Passive House Comfort

So this is a little closer
White, bright, couches and smiles

Now - guess the season... You never know in a Passive House!

So, there is more to comfort than temperature

As mentioned above, the things that effect your comfort include:

• Drafts - either near a heating or cooling duct, or near a window. In the insulation industry, drafts are the most common comfort complaint.
• When the furnace kicks on you get a blast of cold air before it becomes hot. If your furnace is not sized properly you get this event more often than you should.

• You typically feel this if you have inefficient glass (single or double pane) and warm heated air. The warm air will cool off when it hits the window and drop (since cold air is heavier than warm air). The dropping of air is the draft you feel. 

• There are the most obvious drafts you feel that are coming directly from the outside. Through electrical outlets, under the baseboard, under doors, around windows, into the basement, etc. 

• Drafts near chimneys are very common as well. Neither the glass doors nor the flu create a perfect seal. Cold air will drop and be drawn into the house.  

Not in a Passive House. There are no air leaks, there is no furnace, the windows are super insulated and there is no chimney. Drafts are an aspect of comfort that often get overlooked.

• Noise - Have you ever heard a train blow it's horn, or your neighbor's dog bark at dawn on a Saturday or an airplane fly over, or those construction vehicles work near your house? We have all experienced this in a house before. But since we have just learned to live with it we don't realize how annoying these things can be. 

Thanks Floris from 475 HP Building Supply! 

Outside sound is not a problem in a Passive House (as you can hear from the short video from above). There is also no problems with noise from inside either. All of the mechanical equipment in the house is tested to operate almost silently. There is no Jet Engine noise from the bath fans (there are no bath fans), there is no swoosh of water from an upstairs toilet flush in the downstairs walls (from insulated drain pipes), there is no rumble followed by a winding noise from the furnace kicking on (there is no furnace). We have lived in noisy houses all out life and have learned to live with it without knowing we could do something about it. Passive Houses are quiet!

• Daylighting

Another important aspect of Passive House is proper orientation. There is typically a lot of south facing glass. However when you utilize the Passive House windows they can be arranged on any side of the house. While we do utilize some of the passive solar techniques, they are not vital to the success of a passive house. In the Rochester Passive House - EVERY room has a window in it. In fact the 2 long walls of the house face East / West. We did what we could to maximize the southern window exposure and were smart about window size everywhere else. We even added a transom window in the 2nd floor laundry room to give that room some natural light.  

• Surface temperatures (Not just heated floors)

The number one comment we get from clients looking for a high performance house is..."We have to have radiant floor heating". So comfort is obviously important and our clients have done the research and found that radiant heating systems are more comfortable than forced air systems.

The radiant floor heating typically will over heat a passive house. 

Passive House Design utilizes some Passive Solar techniques to help heat the house in the winter - with proper shading to ensure cooling in the summer. 

What exactly is radiant heat transfer? Well, without getting into Planck's Law, Stefan-Boltzmann Law and blackbody emissive power... It is essentially any materials ability to give off or receive infrared energy to other objects within a line of site of each other.

Huh??? 

So in a typical room you are losing your Radiant Energy (body heat) to all of the cold surfaces around you (making you feel cold). Some of the more common surfaces are, the studs (or thermal bridges), the window surfaces, the cold tile floor, the cold granite countertop etc. All of them add up and pull your radiant energy to make you feel cool even when the air temperature is warm.

The idea behind radiant floor heating is warm feet plus a large warm surface in the house will offset all of the cold surfaces to make you comfortable.

Not Needed In a Passive House... ALL of the surfaces are within 6 degrees of each other - Window glass surface, basement slab, interior walls, tile floors, etc. So the radiant losses are never large enough to effect comfort - if anything they allow a lower air temperature to keep you comfortable.

• Odors

How do you remove odors from your house now?

Open the windows

While this will give you the fresh air you need, it doesn't prevent pollen and dust from coming in as well. I'd also say this doesn't work too well in the Winter.

Turn on our Air Purifier

Cleaning the air is another common option. This doesn't however replenish the levels of Oxygen in the house.  Depending on the size of the purifier they also run off of 100 or more Watts. 

Maybe the most common...

Light a scented candle or use a scented aerosol spray to cover it up. 

 

If you don't think Odor Control is part of Comfort who are you kidding! Every Grocery Store I have even been in carries all of these products. If people could just live with the odors there would be no market for them.

There are problems with just covering up the odors - but I don't need to get into that. There are plenty of articles out there that will do that for me.
Article 1
Article 2
Article 3

There are plenty more, but back to fresh filtered air...

Passive House Utilizes a whole house ventilation system complete with heat recovery. This system runs off of 40-50 Watts (depending on if the ground loop pump is operating)

Fresh Air is drawn in from the outside, filtered and conditioned with the air to air heat exchanger. 

In the hottest and coldest months of the year the incoming fresh air is tempered by a glycol ground loop. In the winter it raises the temperature of the incoming air to near 40 degrees. In the summer it lowers the temperature to near 80 degrees (and dehumidifies the air). 

Now the tempered fresh air is filtered again before it enters an air to air cross counter flow heat exchanger. Here it reclaims about 86% of the heat from the stale stinky air from the house. 

The fresh, filtered, warm air is now distributed to the bedrooms and living spaces while the stale air is removed from the kitchen and bathrooms. Once the heat exchange takes place the stale air is blown outside

Every 3 Hours all of the air in the house is refreshed. 

Passive Houses are ultra comfortable because:

1) They are ALWAYS Draft Free

2) They are ALWAYS Quiet

3) They ALWAYS let in the maximum amount of Natural Light

4) They are ALWAYS Warm

5) They ALWAYS have Fresh Filtered Air

6) Oh - they are ALWAYS Low Energy!

Construction Update #3

Construction Update #3

 Well, A lot has been happening since my last post. We have been pushing the interior and exterior work all at once!

Interior Updates

Drywall

Ceilings hung

Bathroom mold resistant drywall

Window drywall returns with Tearaway strip

Primer and Paint

Amish Kitchen Cabinets

 I should write a whole blog post about the kitchen cabinet experience. Dealing direct with the Amish has been interesting, amazing, frustrating and totally worth it! When we 1st met with them there was no showroom, no photos to see and no limitations on what they could do. All they asked for was the Type of wood (Oak, Maple, Birch, Pine, Cherry, Hickory, Walnut, or anything else we would want), dimensions for the kitchen and the dimension for each cabinet and a photo of the style we wanted and they did the rest. A nearby trim carpenter drove them up to the house and they installed them in 1 day. Once the counter top gets installed they will come back and finish the install (We have an upper cabinet that will sit on the counter top). 

Kitchen Island - 48" x 96"

 Breakfast nook - the upper will hold our microwave

Baking Prep Area - There is a bank of uppers to the right of the window that will sit on the counter top (with a pull out drawer for the mixer).

Interior Doors

Staircase (Upstairs and Basement)

The Main Stairs tread will be stained to match the laminate floor. The risers and stringer will be painted to match the trim. 

The Basement stairs will be painted

Basement Walls framed

You can see the cellulose filling the 8" space behind the basement framed wall.

Basement Windows Installed 

The window install detail is much like the above grade wall detail. We installed the window into the framed wall (to ensure it was in the middle of the R-Value of the basement wall). We wrapped the buck in rigid foam and sealed the window to our air barrier. Let me know if you would like more details on this installation.

Exterior Updates

Front Porch Framed and Roofed

The ledger for the front porch was installed over the rain screen

Siding

Trim to be painted white

Front of House (Faces West)

 North Side of the Garage

Back of house 

South side is awaiting a lift to complete the siding on the south 2nd floor and gable

Along with a few more blower door tests we will be installing trim, floors, final grade and water heater shortly! Stay Tuned!