Not every new home built has the budget to be high performance. That doesn’t mean some energy efficiency measures and decisions based on sound building science can’t be added to a design to build a better house. I was recently involved in a project that had a tight budget, we were able to squeeze an additional 3% in to accommodate a few upgrades that I felt would slightly reduce the operating costs, create more comfort, and help the durability of the structure. This build happened during the summer/fall of 2020, right at the time of the lumber price increases, luckily most of the materials had been ordered before the increases.
The home is constructed in Northern Minnesota, climate zone 7. It’s a slab on grade, three-bedroom, two bath, 1,550 square feet ranch style. There is an attached, conditioned garage with an additional 720 square feet. The main source of heat is in-slab hot water served by a natural gas boiler, which also supplies the domestic hot water for the home. A mini-split air source heat pump supplies heating during the spring and fall and cooling during the summer. There is also a sealed combustion gas fireplace sized large enough to heat the entire home should there be a failure of the boiler. A heat recovery ventilator (HRV) supplies the home with fresh air and is used for spot ventilation using bump switches in the bathrooms which increase the fan speed of the HRV when the bathroom is in use.
During the initial planning, I conducted a simple energy model of the home using the Department of Energy’s BEopt 2.8. I was looking at two different upgrades, the first was the efficiency improvement in moving from 3 ACH50 to less than 1 ACH50 in the building tightness. The second was to see how adding enough exterior insulation to eliminate the need for a class I or class II interior vapor control layer would affect the performance. Because this home was being built in climate zone 7, the exterior insulation requirement would need to be R-15 over a 2 x 6 wall. BEopt suggested a savings of 2.4 MM BTU or 2.4 million BTUs per year moving from 3 to 1 ACH50 air tightness. An additional reduction of 4.2 MM BTU could be realized by adding R-15 exterior insulation, at a substantial cost to the project. Based on this information, we decided to only make improvements in the air tightness of the home, the goal was to be under 1 ACH50. (A quick side note, the State of Minnesota has modified the energy code and eliminated the exterior insulation requirement for climate zone 7. Our only requirement is R-21 cavity insulation.)
I also had a third-party consultant perform a heat loss calculation for the home, the results suggested a heating load for the home at 30,000 BTU and an additional load of 13,000 BTU for the attached garage.
The builder and I choose to use a frost protected shallow foundation. This type of foundation system is common in my market, but we used a couple of different products over what I have used in the past. The first was a manufactured insulation product designed for frost protected shallow foundations called Mono Slab. It’s an EPS foam wedge form designed to retain the thickened concrete footing and meet the insulation requirements for the frost protection. The form installation went quickly even though there were several inside/outside corners and an elevation change that dropped the garage slab seven inches from the house slab.
The second change over what I have done in the past was the use of closed cell spray foam (CCSF) under the slab. We typically use EPS, and in the distant past, XPS foam to meet the insulation requirement for under the slab but wanted to try the CCSF to better air seal the slab and speed the process. The decision to use CCSF was based on a recently conducted blower door test on a new home which failed. Part of the reason for the blower door test failure was the polyethylene sheeting and insulation used under the slab was not properly sealed to the penetrations through the concrete slab. I suspect air was moving down the passive radon pipe into the rock bed for the radon system during the negative pressure blower door test, then working its way around the plumbing and other penetrations in the concrete and moving into the home. The closed cell spray foam should provide a better air seal. The spray foam contractor had 1.5-2 inches of CCSF installed under both the house and garage slabs in four hours. A substantial savings in time, but a slightly higher overall cost for this detail over using EPS or XPS foam.
After the CCSF installation was complete, the heating contractor installed the hot water heat tubing, and the concrete contractor finished the reinforcement for the concrete slab. The slab was then poured.
Framing and Air Sealing
I believe air sealing starts at framing. Some of the methods I choose are different than what you might see at the typical new home build. I prefer to use a double bottom plate. This allows us to use a sill seal and multiple beads of acoustical caulking (a Steven Baczek and Jake Bruton detail) and secure the first bottom plate to the concrete without having the rest of the wall attached. Part of the difficulty in dealing with acoustical caulk is it’s messy. It seems to end up everywhere, especially when you try to slide a wall into place. I found it’s easier to attach the first bottom plate using a few small concrete anchors with the air sealing detail in place, add an additional bead of caulking to the top side of the bottom plate, then stand the wall with he second bottom plate on top of the first. We then used a long concrete anchor that is drilled through both plates and secured to the slab. This home has nine-foot ceilings so adding a second bottom plate puts us 1 ½ inches over on the typical height. This is corrected during the roof framing where we add 2 x 4 ceiling strapping. You can read more about this detail in a past GBA article, Cold-Climate Attic Air Sealing. The home was designed so that all bearing points are on the exterior walls. This way the interior walls could be build after the roof is in place, simplifying any interior air sealing details and allows a continuous class I or II vapor control layer, which is required in this climate unless enough exterior insulation is installed.
Once the wall framing is complete, the wall sheeting is installed. My first choice would have been using a product that has an integrated water resistive barrier (WRB) such as Zip. Budget constraints limited us to standard OSB and a mechanically attached WRB. We taped the seams of the OSB using a couple different tapes. I have used 3M’s 8067 acrylic tape with success in the past but wanted to try Siga’s Wigluv on a project. I was given a few rolls to sample. Both tapes installed exactly as expected with a very tight bond to the OSB. One advantage of the Siga tape, it has a permeance to allow moisture to move through.
Another detail I like to use is to add a second air sealing layer and water protection to the bottom of the OSB where it contacts the foundation insulation. The product I chose was Prosoco’s FastFlash. This gives us a belt and suspenders air sealing detail at the bottom plate with the added benefit of protected the edge of the OSB from exterior water. This is completed before addition of a metal z-flashing and the mechanically attached WRB.
Because the OSB is the main wall air control layer, to provide continuity to the assembly, this layer needed to be extended to the interior ceiling. This was accomplished using a 12-inch-wide piece of 3M 8067 tape. The tape is connected to the OSB, extends over the top of the top plate with a small portion, about two inches or so extending to the inside of the framing where is can be sealed to the ceiling air control layer later. Installing 12-inch-wide tape nine feet off the floor with a little wind can be challenging, but after a few feet of practice, the process isn’t too difficult.
After the top plate taping is complete, the trusses can then be installed. Because this is a slab on grade structure with some exhaust and ventilation ducts that need to exit the building, we choose to use a special kind of truss over the mechanical room that would allow the ducts and other mechanical systems to remain inside the conditioned space. The truss is called a plenum truss, it contains a notch to raise the ceiling height in a section of the truss. Ducts and exhaust venting can be installed inside the conditioned space and the ceiling can remain flat throughout the home. The ceiling air control layer follows the notch in the truss and notch is filled in with framing later. I’ve used this detail in the past with good results.
Once the trusses and roofing are installed and the structure is dried in, I installed the ceiling air control layer and sealed it to the 3M 8067 tape that was extended from the OSB wall air control layer. I chose to use a reinforced polyethylene sheet for the ceiling air/vapor control because I’m not worried about the potential to dry to the inside from a vented attic space. If the ceiling is air-tight and doesn’t contain a room that is cooled more than normal, such as a chilled wine room or walk-in cooler or freezer using the building’s ceiling, there should be little risk of condensation forming in the attic, at least in my climate. The access to the attic is in the garage, so there were only a few penetrations through the poly, two plumbing vents, a radon vent and a return and supply duct for the HRV unit that could not be kept in the plenum truss. All were sealed and no leaks were detected during the initial blower door test.
Speaking of the initial blower door test, it was conducted before the mechanical rough-in and window install. This allowed us to test the integrity of the exterior air control layer before any punched openings were made. The house tested at .55 ACH50, 140 CFM of leakage. Our goal for the home was 1 ACH50 or less, I was pleased with the results.
I’m a fan of using multiple layers for the wall air control. In this home, the first layer is the taped OSB, the second is a combination of two smart vapor control products, CertainTeed’s Membrane and Siga’s Majrex. I chose to use Majrex on the wall between the house and garage because of its durability, I could conduct a blower door test with Majrex as the only air control product in this area and not worry about it getting damaged or pulled from the wall. The rest of the exterior walls in the home got CertainTeed’s Membrane that was sealed to both the ceiling poly and to the concrete slab. Does this detail of using multiple air control products work? The second blower door test, which was conducted after the windows and doors were installed, most utilities in place and the insulation completed, came in at .47 ACH50. Had we installed enough exterior insulation to eliminate the need for a class I or II vapor control product, we would have used an air-tight drywall method as the second air control strategy.
There are several penetrations through both the interior and exterior air control layers, vents for the heating system, dryer, and range vents, the HRV unit and the line set for the mini-split air source heat pump along with a few electrical wires and boxes. Most of these penetrations were completed after the second blower door test.
Many of these through penetrations are often difficult to sequence during the construction process. They can require coordination between the HVAC crew, plumber, electrician and whoever is installing the air control products and systems. Luckily on this job, the mechanical subs were all from the same company and I was chosen to ensure the air barrier was continuous. I used pipe gaskets adhered to the OSB for the main air seal. The detail was a little challenging to install but I feel was the best option and a detail I will use again.
Unexpected Event and Outcomes
Overall, the project went as planned. Budget was slightly over estimate, mostly because of several finishing upgrades that the homeowner chose over the allowances that were made during the bidding process. We did have one error made by a subcontractor that I believe had a small affect on the final blower door number, the sub was the flooring contractor of all people. As part of the air sealing detail between the garage and dwelling, the Majrex smart vapor control product was sealed to the floor using Contega HF, which has very good adhesion between the membrane and concrete. The common wall framing is on the lower garage slab making the connection between the Majrex and house slab important. This illustration shows the detail.
The flooring contractor cut the Majrex off at the wall leaving a gap between the framing and concrete floor. The drywalled wall on the garage side of the common wall was air sealed, though not well enough to act as the sole air control product between the garage and home. A lesson learned, be sure to have a conversation with all subs about the importance of the air control products used, even the crews installing the floor coverings.
The final blower door test came in at .75 ACH50, 188 CFM. Under what we had planned but a little over what I had expected based on the first two blower door tests. I’m sure what pushed us a little higher was finishing all the exterior penetrations and that cutting of the Majrex.
I recently had a discussion with the homeowner on how the home performed so far with regards to heating and cooling costs along with if he felt the home was comfortable. There was a polar vortex that moved through the area this past February where the temperatures did not get above 0°F and lows that were as cold as -35°F for almost two weeks. (A delta T of 105 degrees!) We have also been experiencing a very warm late spring for our area, having been in the 90°F several days already this year, normally these temps aren’t seen until late July and August. The homeowner indicated his natural gas bill over the past winter, which includes both the space heating needs and domestic hot water ranged between $75 and $90 per month for all but the polar vortex month, which was $130. These costs include heating the 720 square foot garage to 65°F. The monthly electric cost for the home has been running between $30 and $40 to date. He is expecting to have a slightly higher electric bill during the summer for the operation of the air source heat pump. He has projected total electric, heating, and cooling costs between $1200 and $1500 per year. Overall, the homeowner is very pleased with both the operating costs and comfort of the home.
It is possible to get good performance out of current code minimum requirements. With that said, there are two areas that I would go beyond code when building new, the first is in air sealing. The advantages of a well-sealed home go beyond energy cost savings. Comfort and an improved indoor air quality are a couple of the many benefits. It’s not difficult to achieve a lower blower door number while the home is being built, much harder after the fact. The second, add a balanced mechanical ventilation system. This has been a code requirement in my state of Minnesota for nearly two decades now and in my opinion, should be a requirement throughout the US. Build tight, ventilate right.