I am an architect. I have spent the last five years thinking about, sketching, drafting, changing, overanalyzing, second guessing, and fretting about the house that my partner Catherine and I would someday build on our rural land in northern Minnesota. This is probably not unlike the experience of many non-architect dreamers, but the difference is that I am armed with AutoCAD and REMDesign energy modeling software to analyze every conceivable scenario.
I also like building science. The firm that I work for, Wagner Zaun Architecture, encourages me to learn and think hard about many aspects of building performance, durability, cost effectiveness, and efficiency, and how they all relate to design and aesthetics. This, as many of you will attest, leads to more questions… and fear. If I push the thermal performance, will I compost my walls? The energy model says 12,000 BTU/h is my peak load: Is that really enough? Will the builder laugh at my “brilliant” flashing detail that took me three hours to draw?
Despite my background, I am burdened with the same issues that all other dreamers are. I have a not-quite-big-enough budget. I want a house that is not too big and not too small. I want an energy-efficient house with passive solar features that doesn’t look like the awkward love-child of the Saskatchewan Conservation House and an Earthship. I want a house that is beautiful and durable. I want a house that is environmentally sensitive and responsible. I want a house that enters into a conversation with our land, not one that holds dominion over it. I want a house that I will love.
A single-story house with a slab-on-grade foundation
On October 12, 2015, the rubber hit the road and we started construction. I can’t endlessly tweak (fuss over) plans and details any longer. The bank owns my soul, and the excavator is on site making a house-shaped scar on our previously pristine site.
This installment is an explanation of the concepts employed in the design of our dream house, with reflection on some of the choices and struggles. This will be followed with future ruminations on progress, lessons learned, and concessions made in the face of reality.
The “final” design is a one-story, slab-on-grade house (see Images #2 and #3, below). It is a two-bedroom, 1 1/2-bath home with an additional small office for farm operations. The main house is 1,668 gross square feet (1,575 conditioned square feet) with an additional 250 square foot screened porch.
As I said, I am a building science nerd, so I’ll cut to the quick. The stats for the house are as follows:
- Climate Zone 7, with 9,500 HDD and 250 CDD
- A dead-flat site with good solar exposure
- Under slab insulation: a continuous horizontal layer (8 inches) of Type IX EPS (R-35)
- Walls: 2×6 framed structural cavity wall with R-21 dense-packed cellulose or fiberglass batts, with the structural Zip sheathing serving as an air barrier, and 7 3/8-inch EPS nailbase panels on the exterior (R-29) for a total of R-50
- Ceiling: Engineered roof trusses with 20-inch-deep energy heel and 24 inches of blown cellulose (R-80)
- Windows: Duxton Fiberglass windows with 1 3/8-inch triple-pane IGUs with argon gas fill and Cardinal 180 coatings on surfaces #2 and #5; center-of-glass specs: U-factor = 0.13; SHGC = 0.56, VT = 70%
- Airtightness target: 1 ach50 or less
The design goal for the house was to take full advantage of the good solar exposure and to reduce the mechanical load to a level where a minisplit heat pump would take care of the majority of my heating needs. Although we have very few cooling degree days, the notion of air conditioning and humidity control is appealing to me, which also prompted the desire to use a minisplit.
I wanted to have an all-electric house so that I could eventually offset the majority, if not all of my consumption with photovoltaic (PV) panels. Aside from a propane cooktop (one of the few requests from my very patient partner), which will be fed from a 100-gallon propane bottle, the house will be entirely electric. Although our region’s current electrical grid supply is very coal-dependent and propane is cheaper, I rationalized that I am reducing my load to a point that I can easily offset it with PV later, and I won’t be reliant on a propane delivery truck. Also, I am more afraid of gas than electricity, which may not be totally rational, but I am calling the shots.
Heating and cooling provided by a ducted minisplit
The mechanicals are as follows:
- Heating/cooling: Ducted Fujitsu 12RLFCD minisplit heat pump, single zone; backup consists of 6 electric radiant cove panels with individual thermostatic controls, totaling 3,450 watts; a small wood stove in the center of the house
- Domestic hot water: 50 gallon electric-resistance Marathon storage tank water heater
- Electric clothes dryer with through-the-wall exhaust
- Electric oven with a propane cooktop and a range hood rated at less than 300 cfm; makeup air unit with 1000-watt electric resistance heater tied to the range hood and clothes dryer
- Venmar E15 HRV, exhausting bathrooms and near kitchen, and supplying into the minisplit supply ducts.
I intend to monitor the electrical usage on the main house feed along with the individual radiant cove circuits to determine how often they kick on and under what conditions. We also have 30 acres of poplar woods to feed the woodstove. Poplar isn’t great firewood, but it is free and I’ll have more than enough deadfall to keep us warm after the energy apocalypse.
A frost-protected shallow foundation
The first item I grappled with was the foundation. I initially wanted a frost-depth stem wall (which is 5’-0” below grade here) just because it “felt right.” However, in the face of incremental cost increases and above-grade wall changes discussed below, I eventually arrived at a frost-protected shallow foundation.
Another factor influencing the choice was the very high water table on our site. I have dug many post holes on the site that fill up with water three feet down.
Once construction started, we discovered that creating the foundation forms with foam is fairly time-consuming (see Image #1 at the top of the page), and for future projects I want to look into a flat structural slab sitting in a simpler tub of foam. They might require a thicker slab with more steel, but the labor savings might be better than the additional cost to form a thickened-edge slab.
Nailbase panels on the exterior side of the 2×6 walls
The main item that was the subject of the most consideration on this house was the wall assembly (see Image #4, below). Since wall assemblies are often the hottest debate topic here, let’s start with that.
For the above-grade walls, I knew I wanted at least an R-45 assembly to meet my energy goals. I also initially wanted stone exterior cladding. As a result, the original plan utilized Quad-Loc ICF wall construction which I arrived at for a number of reasons. I wasn’t comfortable putting a masonry cladding over a high-R wood-framed wall, and I liked the security of a relatively storm-proof and rot proof ICF wall system.
The pricing for the stone work came in astronomically high, as did the “Plan B” brick option. This reality, combined with the facts that nobody in my area was familiar with Quad-Loc forms, and the nearest supplier was 200 miles away, started to chip away at the ICF wall strategy.
Ultimately, the cost of the concrete for the walls plus the nagging global-warming-potential guilt put the final nail in the ICF coffin. I begrudgingly decided to opt for a wood-framed wall with wood siding over a furred rainscreen.
Our design firm has done many double-stud walls with dense-packed cellulose. My builder of choice was also familiar with double-stud wall construction and had proved to be meticulous at detailing an interior vapor retarder/air barrier, achieving impressive blower door results well below my target.
However I have been increasingly concerned about the potential for moisture problems with double-stud walls, and more and more interested in continuous exterior foam strategies. I had numerous conversations with other nerdy types about the merits of using the structural sheathing as the air barrier in the assembly and concluded that it was the best strategy. Just like everybody else hoping to grace the world with the “perfect wall” solution, I was off to the drawing board (again).
Our office has done a few projects with single-stud walls wrapped in multiple layers of rigid polyisocyanurate foam, and we got quite a bit of feedback from our builders that the assemblies were time-consuming and a bit fussy. Lots of cutting and fitting, long cap-nails required to hold the water-resistive barrier (WRB) in place, and the need for fairly substantial furring to meet the nail holding requirements of most sidings. Regardless, I likely would have pursued this route with my own house, rationalizing that the form was simple and thus it wouldn’t cost much — but then I stumbled onto nailbase panels.
Nailbase panels are a product offered by many SIP manufacturers, and they are available in numerous specified thicknesses of EPS foam with a layer of 7/16” OSB bonded to one side only. Usually marketed as a retrofit product, we got to thinking about their potential for new construction.
Since we are essentially eliminating the structural component from the SIP, we amuse ourselves in the office by calling them IPs. I never personally cared much for SIP construction because the reliance on glue to hold the whole works together leaves me feeling uneasy. Also, the effort to run electrical and other services in the exterior walls seemed overly complicated. In contrast, the IPs come in 4’ x 8’ sheets, with notches on every panel edge for an OSB spline to connect adjoining panels. There is no internal or panel edge framing. You simply cut them to size, apply sealant to the edges and screw them to the framed structural wall.
The following are some things I like about the IP wall:
- The ability to cut the IPs to size on site allows for easy changes and adjustments that SIPs don’t.
- Attaching them to the outside of a framed structural wall makes framing the house walls easy, lets me utilize my exterior structural sheathing as the assembly’s air barrier.
- Keeps the structural sheathing on the warm side of the wall (unlike double-stud walls).
- Leaves the interior wall cavity open for whatever you want to stuff into it.
- With the structural sheathing as the air barrier on the exterior side of the wall cavity I also don’t need to mess with airtight boxes or excessive sealants at mechanical and electrical penetrations at the interior.
- The premium grade OSB that SIP manufacturers use makes furring and/or siding attachment simple.
A few of my reservations about the IPs are:
- Uses a lot of foam
- They make a mess when cutting with power tools.
With the 8-inch nailbase panel you get an R-29. That alone would meet code in our climate even if I chose to leave the framed wall cavity empty. I chose to fill the cavities with dense-packed cellulose because I wanted the additional R-value and the cellulose installer will be on site to do the attic insulation anyway. I may also decide to insulate the cavities with fiberglass batts myself, and the consequences of a “do-it-yourself” quality job will be minimized by the continuous exterior insulation.
With my sheathing nestled within the wall’s thermal boundary, the ratio of exterior to interior insulation is 1.38 (29/21), which is safely above the 0.7 minimum that the IECC calls for to allow a Class III vapor retarder to be used. Thus, my interior vapor retarder will be two layers of standard latex paint on the interior side of all exterior walls. I considered, for a time, that maybe I should still install a polyamide (“smart”) vapor retarder on the warm side, but at the end of the day I convinced myself that it isn’t necessary.
I am the kind of person who might cut into my walls in a few years to see how the sheathing looks. When I do, I’ll send in the report. For now, we are going full speed ahead to enclose the structure before the deep Minnesota snow falls. I have already made a few concessions to keep the builder happy, so look forward to those in the next installment.
Elden Lindamood is an architect with Wagner Zaun Architecture in Duluth, Minnesota.