Barndominium Part 2-Foundation and Shell

*This post originally appeared as two articles on the Green Building Advisor website.*

The bardominium, part storage facility, part shop, part man-cave with a home inside.  Working on this type of project has had its difficulties.  We have taken the framing crew outside their comfort zone, though they are well versed with post and frame structures, they were not familiar with many of the materials I was asking them to work with.  I was able to have them change some of their sequencing, but I lost of few of the battles.   On top of those difficulties, the project started about a month later than expected forcing crews working during sub-zero temps, that’s, 0°F.  Progress at times has been slow.  The foundation portion earlier in the fall went well, though I think I would change a couple of the decisions we made.  More on that in a bit.

The Foundation.

The foundation of the barnodminium project is a frost protected shallow foundation which includes cast-in-place post brackets to secure the wall framing to the foundation.  We have a 30-inch by 24-inch reinforced thickened edge footing that is thermally broken from the ground.  We used three inches of closed cell spray foam, roughly R-20, for the thermal isolation, both under the footing and under the five-inch slab.  You can read more about using CCSF sub-slab here.  Why under the footing?  Two reasons, the climate zone and heating source.  Living in a heating dominated climate can result in a moderate difference between the ground temperature and desired temperature of the living space.

The heating system for this structure includes hot water in-floor heat.  We want more heat to end up in the conditioned portion of the home and less moving into the earth.  The heating fuel source also affected our decision, a geothermal ground source heat pump will be supplying the heat for the slab, these systems operate at a lower temperature than fossil fuel boiler systems.  Another reason, to get as much heat as possible up to the living space.

Something interesting I discovered after planning the spray foam details, CCSF at one inch will stop approximately 90% of the conductive heat flow, at two inches, this reduction increases to 94% and at three inches, 96%.  To get to 98% we would need six inches of closed cell spray foam.  At 11.5 inches, 98.9%.  This data comes from Demilec and was based on a delta tee of 40°F, roughly what we expect the difference to be between the living space of the barndominium and the wintertime ground temps.  Definitely diminishing returns.  That being said, you still need to apply at least code minimum amounts of R-value to achieve the proper insulation levels for an assembly.  I’ve seen people in my climate try to install too little, especially in ceilings and walls only to have it become a problem.

There were a few reasons we chose to use closed cell spray foam as the insulation choice under the slab.  Probably the biggest reason, speed of installation.  The concrete foundation work was performed late in our construction season.  It’s important that temperatures are above a certain level to achieve the desired performance and durability of the product, getting the work done as quickly as possible was the biggest factor.  Other reasons, its ability to air seal all the penetrations through the slab and also act as the vapor control layer.  The product that was chosen is one that uses HFO as the blowing agent, I believe it was the best choice based on the conditions listed above.

As I said, this is a frost protected shallow foundation.  These systems require both under slab insulation along with slab edge (where we see the biggest delta tee in the foundation assembly in my climate) and an insulation wing to prevent frost from driving under the slab.  This is the first project I’ve been involved with that included using closed cell spray foam for both these outside locations, three inches was sprayed on both the concrete edge and wing.  The concrete edge spray foam is an area where I would make a change.  I think I would go with one of the rigid sheet insulations instead.  We ended up with some damage to the CCSF at the concrete edge from a scissor lift driving too close to the building.  We now have to remove and replace the CCSF in several areas where the damage occurred.  Another reason is it’s a little uneven in spots.  This insulation needs to be protected from not only physical damage, but also sunlight.  We may be trimming the insulation back in a few areas, so our metal flashing cap protects the product correctly.

If we hadn’t decided to use the CCSF as our sub-slab insulation, what would have been our choice?  EPS or GPS foam in a type IX density would have been my next pick.  This would have been a cheaper option but the time to install and then add the sub-slab vapor retarder would have taken much longer.  As a side note, something I have noticed in my market is the location of the sub-slab vapor retarder, most contractors I’ve worked with put the poly under the foam insulation.  This traps a lot of the moisture from the concrete pour in the insulation layer where it can take years to dry.  Dr. Joseph Lstiburek has been telling us for years the poly needs to be on the topside of the insulation.

Another option for the sub-slab insulation could have been the new XPS foam with the HFO/HFC blended blowing agent, though my understanding of the global warming potential of that product is it’s twice that of the closed cell spray foam with the HFO blowing agent we used.

Using one of the rigid foams as our sub-slab insulation choice would have saved costs over the CCSF, even with the labor to install and the additional cost of a sub-slab poly.  Again, the decision was made for CCSF mainly because of the construction schedule.

Wall Framing and the Water Resistive Barrier.

Post and frame construction has many different details over conventional framing techniques.  Number one, the posts on this project are six-foot on center with no top or bottom plates.  The posts resist lateral movement by both their connection to the concrete slab, using a cast in place metal bracket, and by the horizontal 2x wall girts fastened directly to the posts.  In addition, there are diagonal framing members in the corners to also resist racking.  The roof trusses, which are also six-foot on center in the main shop area of the structure sit in a notch cut into each of the posts.  They are tied together by 2x purlins fastened on the topside of each of the trusses.  Neither the wall nor roof have any continuous sheathing, this complicates the water, air and thermal control layers.

Creating a continuous water control layer, then protecting that layer from the thousands of holes that would be punched thought the WRB when the cladding is installed required some thought.  What is typical on a post and frame structure is to have the WRB attached directly to outside of the wall girts.  This leaves the possibility to have any water that should end up behind the cladding be held in tension where it can leak through the water control layer and damage the wall girts or end up inside the wall cavity.  And then there’s all those holes from attaching the claddings that are also potential trouble spots.  There are a few WRB’s I considered; I was looking for something durable with a good family of tapes that I felt would work best in my very cold climate.  What I ultimately decided took the framing crew far outside how they normally work.  Siga’s Majvest was directly attached to the poles, six feet on center.  There were difficulties, the 3-meter wide (nearly 10 feet) Majvest size, 18 feet in the air, and in freezing temperatures made me an unpopular person.  The Majvest comes in two sizes, 1.5 meter or 3 meters, I chose the 3 meter to eliminate one tape seam.  All the seams were taped using Siga’s Wigluv which has a listed low application temperature of 14°F.  There were several times that taping had to be delayed until temperatures rose, sometimes that delay was several days.  Something that we discovered was cold temperature application of the tape had a big effect on how the tape adhered.  Because the Majvest was spanning several feet without a solid backing, the WRB would move slightly during windy days.  Combine that with the tape not having its usual tenacious bond because of the cold temperature application, the tape would release in some areas forcing us to re-tape.  These difficulties resulted in me needing a few questions answered, so I performed a backyard tape test.  You can read all about that testing here.  What are my takeaways in discovering these taping issues, avoid taping in cold temperatures when there is no continuous sheathing present.  You cannot pull the WRB tight enough to eliminate movement caused by the wind.  I don’t believe we would have had any issues if the build was during warm weather.

The next wall challenge was the horizontal wall girts.  Horizontal rainscreen framing is not the direction to properly move water down an exterior assembly.  Because of the vertical pole barn style corrugated metal sheets being used as the cladding on the majority of this project, and the need to have the wall girts directly tied to the poles for the proper structural requirements, we did not have the option to install a vertical strap.

We needed to maintain some sort of space between the 2x horizontal wall girt framing and the Majvest.  We installed strips of Benjamin Obdyke Slicker to the backside of the wall girts.  This gave us a small space for water to move down the WRB, to a metal flashing at the bottom of the exterior wall assembly where it could be kicked outside the foundation.  Down, out and away.

The window installation was the next big challenge.  My original plan and the framer’s execution of their process did not mesh.  This resulted in some on-the-fly problem solving that included a visit from Siga.  The next post is all about the windows.

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