In my last posting about the Code Minimum House, I talked about how the foundation was constructed. Next up, the framing. This home is being built with a small budget. To keep costs in line, the framing and insulation packages were kept simple. No fancy sheathing products and a code minimum insulation package. Air sealing is one detail that wouldn’t significantly add costs, a low hanging fruit if you will. The air sealing package is where we decided to take this code minimum home and improve it’s performance.
In the planning stages of this home, I performed modeling to see how adding insulation or improving air sealing would affect the cost to operate the home. We could save around 6% of the heating and cooling costs simply by decreasing the air leakage rate from the code minimum 3 air changes per hour at 50 pascals (3 ACH50) to 1 ACH 50. A few additional sealants and tapes along with some additional labor, not a major cost. We could have saved an additional 7% by adding R-15 in exterior insulation, but the price would have increased the budget to the point where the build would have been scrapped. Of course, this is only taking into account energy cost savings, to some, comfort, noise reduction or just reducing their carbon footprint may be more important than cost savings.
Framing a slab on grade home starts with the walls. I really like a detail we used on last years Concreteless Slab On Grade Home, the double bottom plate. Being on a concrete slab, we used a treated 2 x 6 as the bottom double plate. To air seal the plate, we used a detail I first saw used by architect Steven Baczek and builder Jake Bruton. A bead of acoustical caulking on the concrete, then a foam sill seal was set along with a second bead of acoustical caulking on top of the sill seal. The treated 2 x 6 was then set on top and fastened with a few small concrete screws to hold the plate in place. After the wall section was build, a third bead of acoustical caulking was added to the top of the treated plate and the wall stood up. We then secured both bottom plates to the concrete slab using 7 inch concrete anchor bolts.
The walls are framed using 2 x 6 on a standard 16 inches on center layout. After all the exterior walls were stood, the second top plate was added. Excluding the second bottom plate, this wall design is typical for my area of Minnesota. The entire structure was then sheathed with oriented strand board (OSB). None of the door or window openings were cut out. We left them intact so that I could perform the first blower door test. At this point, all the exterior walls are completed. The home was designed so that no interior walls were needed to support the roof. This was done so that we could install a continuous air control layer on the ceiling.
The roof is constructed using manufactured trusses. The home has a flat ceiling throughout, no cathedral ceilings. Keeping the ceiling simple reduces costs. There is one challenge with a singe level, slab on grade home, where to put any ducts or vents. We are required by Minnesota code to install balanced mechanical ventilation, typically achieved by a heat recovery ventilator (HRV). These units require two vents to the outside. Then there’s the dryer, kitchen ventilation and the exhaust vent for the natural gas heating system. There’s also the line set for the mini-split air source heat pump. To simplify the installation of all this piping and keep it inside the building’s envelope, we had a plenum truss designed into a few of the trusses. The air control layer extends into this plenum area. Kind of hard to see in the photo below, but this is the notch in the trusses where the plenum chase is located. Second picture shows how we lined the space with scrape OSB and plywood to protect the ceiling air/vapor control layer and give the HVAC, plumber and electrical crews a place where they could fasten any materials to the ceiling.
It’s important that the air control layer be continuous and I’m a fan of using the exterior sheathing as part of this air control. We already have the bottom plates of the wall framing sealed to the concrete slab. Next we sealed all the seams of the OSB using an acrylic tape, effectively air sealing all the walls. The plan requires us to extend the wall air control layer to the interior ceiling. We did this with a twelve inch wide split back tape sealed to the OSB, extended up and over the top 2 x 6 plate, and left hanging inside the wall. After all the trusses were set, this tape was sealed to the ceiling air/vapor control layer which is a 20 foot wide, 6 mil reinforced polyethylene sheet. Typically we would be using lots of staples to attach the poly to the ceiling, I decided to use a different method of attachment on this project. We used a pneumatic cap stapler which allowed much fewer connection points, and many less potential tiny little air leaks that can add up during the blower door test.
I have been involved in a few discussions about using polyethylene sheeting as an air/vapor control layer in a heating climate. Most building science experts suggest using materials with a little higher or variable perm rating for the vapor control layer, and I agree with them to a point. I try to use one of the three currently available “smart” vapor control products available in North America for all exterior walls in projects I’m involved with. The Code Minimum House is using two of the three, Siga’s Majrex and Certainteed’s Membrane. The ceiling is the area where I feel comfortable making an exception. I believe there is low risk of a well air sealed, ventilated attic having a moisture issue where interior drying is required. Cathedral or vaulted ceilings may be another story, I may or may not use a smart vapor control layer depending on the ceiling and roof design. I will cover this topic on a future blog post.
After the ceiling air/vapor control layer is installed, we strapped the entire ceiling with 2x lumber. The 2x will help secure the ceiling air control layer during the first couple blower door tests. It also helps protect the poly from damage and give us a space so all ceiling electrical remains inside the building’s conditioned envelope. The last advantage, we end up with a very flat ceiling. We used this detail in last years Concreteless Slab on Grade home. After strapping the ceiling, the interior walls are constructed.
Just like the main air control layer, I like to keep the interior air/vapor control layer continuous. This requires a piece of Membrain be installed on the exterior wall intersect with the interior wall. After the insulating is complete, we can install the rest of the exterior wall Membrain and tape to the pieces behind the interior wall intersects.
Another detail I used last year was the water/air sealing at the foundation foam to wall sheathing connection. We used Prosoco’s Fast Flash to seal this difficult transition. Originally, I was planning on taping this connection but was worried about the ability of the tape to maintain it’s connection long term to the EPS foundation foam. The Fast Flash is a fluid applied membrane which bonds well to most surfaces. We are air sealed already at this critical connection but I like having a back-up to the sill seal and acoustical caulk detail, plus the edge of the OSB is now protected from any water that should find it’s way to this area.
Now that the exterior wall and ceiling are all air sealed with no windows or doors cut out, the last location where air sealing is required is the wall between the attached heated garage and the home. I considered using drywall on the garage, or cold side of this wall assembly as the air barrier but this wall has a lot of electrical work on both sides that needs to be completed. To keep it simple for the electricians, we decided to use Siga’s Majrex. Majrex is the most expensive of the three variable perm, vapor control products, Certainteed’s Membrain, the least costly and is what will be used on the remaining exterior walls. I have now had the opportunity to work with all three smart vapor control products. Last year’s Concreteless Slab on Grade home used Pro Clima’s Intello, I used Membrain on my personal house and Majrex on this project. Majrex would be my choice on all projects if the budget allows. It was the easiest to attach and seems like the most durable (durability is the reason we installed Majrex on this wall instead of Membrain, which is the least durable of the three). It is also designed specifically for cold climates. Read more about smart vapor control layers here.
Okay, we are completely air sealed at the rough framing stage, time for the first blower door door test. We did well, .55 ACH50 on a negative pressure test. I didn’t conduct a positive test at this point but I am planning on a positive pressure test during the next round of testing, which will be after windows are installed and plumbing, electrical and HVAC are complete.
Stay tuned, more to come from the Code Minimum House.