Recently I designed and built my home in Shirley, Massachusetts. The design goal was to build a net-zero-energy house. However, it had to be comfortable to live in, easy to build with low-skilled labor, and very affordable.
I had to keep it simple because I had my family for laborers, and they do not have construction skills. The home also had to be comfortable to live in, with adequate daylighting and easy circulation. The budget demanded that the house be kept small, but several design tricks involving lines of sight were used to make the space feel much larger than it actually is.
Most of my research showed that net-zero-energy houses were too expensive. However, becoming net-zero was one of the parameters for the house that I refused to compromise on.
No combustion appliances
In January 2012 my wife and I purchased a one-acre lot in Shirley. The lot had a crumbling house and a halfway decent garage on it. The newest thing on the site was a code-approved septic system installed in 2008. The garage proved to be a good storage shed during construction and is still serving that purpose today.
Over the past few years I had been developing a plan for the eventual home. The method of construction that made the most sense to me for net-zero was a simple wood-framed home with high levels of insulation. I did several heat-loss calculations and compared different heating systems to the cost of installing them, the cost of maintaining them, and the fuel cost.
In the end I decided that the least expensive way to go was to not purchase a fuel-burning heating system at all. I saved tens of thousands of dollars by making this choice. I do not have a chimney, boiler, air handler, carbon monoxide detectors, gas line, gas meter, fire proofing around a boiler, combustion appliance zone venting and testing, distribution systems such as hydronic baseboards or circulators, a gas permit, or the associated labor to install all these systems.
Saving this money allowed me to afford the extra insulation and solar panels.
Reusing the existing crawl space foundation
Early in the design we were confronted with the town’s bylaws about road frontage and constraints on the house. In the end we decided to build on top of the existing foundation. This made the project a renovation and not new construction.
With the help of friends and family, we tore down the existing house which had two bedrooms, an oil boiler, and a propane range. The bulkhead lead to a 5-foot tall basement area just large enough to house the oil boiler. Then it stepped up to a crawl space with dirt floors and no clearance to the wood joists. The foundation itself only went 8 inches above grade, and was falling apart.
We kept the existing block foundation by repointing the mortar. The bulkhead was demo’d and filled with compacted soil excavated from inside the foundation. Because the crawl space was going to be heavily insulated and air sealed inside the building envelope, I paid particular attention to preventing water intrusion. We installed a perimeter drain to daylight with careful use of gravel and filter fabric.
The old foundation wasn’t level or square
As with any project there are unexpected issues that arise. One of these situations was the water main from the street. During the home inspection, we saw that the main was plastic and believed it had been replaced and we would not have to put money into it. However, after demolishing the house we found that the soil was wet around the pipe. It turns out that the plastic was tied to a cast-iron pipe just below the surface, and the cast-iron was rusted through.
I dug out the old pipe and installed a 2-inch sleeve underground, to just beyond the foundation, and buried it with a marker. That way I could proceed with the house and install a water line later.
The foundation is only 20 feet by 36 feet. However, its diagonals were off by 3 inches, and it was out of level by 9 inches. I decided to level the foundation and square the framing. This worked out well. Groton Engineering out of Groton, Massachusetts, did a foundation inspection and explained how to proceed. I built a water level and checked each block individually. As the existing foundation sloped I would cut a block and mortar it in place. By doing this procedure I created a level foundation. Then I added two courses of block on top of that.
The final course was a U-shaped block to allow for a horizontal piece of rebar. This is known as a “bond beam.” I placed a vertical piece of rebar in every other core and wire-tied it to the horizontal length. All rebar was #4. Then I made jigs to hold the 12-inch anchor bolts in place. I placed the anchor bolts before the pour so that I could wire-tie them to the horizontal piece. This also eliminated the air pocket that normally forms by inserting the bolts second. I also poured reinforced footers for posts down the middle of the house.
When I ordered the concrete I bought one truckload. This was enough to fill the entire block wall, and pour a 3-inch non-structural slab. Although the slab was not necessary, it cost the same to buy one truckload of concrete it did to buy only half, so the slab was logical. It also provides some protection from burrowing rodents that might try to find their way in.
Making sure that the air barrier is continuous
Most projects I’ve been a part of think about the air sealing separately from the build. This approach is a mistake, since it doesn’t lead to the best air-sealing details. With my home I integrated the air-sealing details into the building process. Inside the crawl space I installed a 16-mil reinforced vapor barrier with double-taped seams. The vapor barrier is mechanically fastened to the top edge of the block wall with plastic clips that are tapped into holes in the concrete.
Then I took a roll of Grace Ice & Water Shield (a brand of rubberized asphalt peel-and-stick membrane), ripped it lengthwise, and capped the foundation. This created my capillary break at the sill and gave me 3 inches of membrane on both sides of the wall which I used to attach my air barrier to. This layer of Ice & Water Shield capped over the vapor barrier on the interior of the crawl space wall, covering the plastic clips (see Image #2, below).
The sill plate was pressure-treated per code; however with the Ice & Water Shield I don’t think this was necessary. Then I added a second kiln-dried plate.
The sill plates on three sides of the house were 2x8s, and on one side was a 2×12. (This is because I had to create a short overhang to create a square house.) If you look down the wall, the sill plate is square, but the foundation is wavy. I verified that everything was square with strings and batter-boards at first, and then double-checked by simply pulling diagonals. The simple rectangular footprint makes squaring easy.
Avoiding thermal bridges
I built a central girder to support the floor joists at mid-span. This is common on most homes. However, I did make a change to help with the insulation. Instead of setting the ends of the girder into beam pockets in the foundation, I cut the beam back by 1 foot to stop that thermal bridge. In order to resist lateral forces on the girder I also turned my last joist into perpendicular joist blocks set into joist hangers. This also provided easy access to the full rim joist for insulating later. The joist hangers were some additional cost, but I saved on the labor of creating beam pockets and shimming the girder.
The joists were 2x10s set 16 inches on center. They only have a 9 foot clear span when all was said and done, but I purchased 20 footers so it spanned continuously from one side of the house to the other. Obviously the joists are larger than needed per code. However, this detail creates a much stiffer floor that I know will never cause an issue with flooring later on, even if I install tile. To me, building the best floor for an additional $250 in material cost, and no additional labor, was an easy choice to make. The platform came out perfectly level and solid.
Double stud walls
Building the walls was relatively easy. I built double stud walls, with the interior and exterior studs offset from one another. The rough cavity is 11.25 inches deep. This was done because I needed a 2×12 in the great room.
A double stud wall is built to the inside, so it eats floor square footage. In my case I could not enlarge the foundation, so I was limited to how deep I could make the walls before the rooms became too small. After working through dozens of floor plans, the 11.25 inch thickness was all I could achieve. However, if I could have enlarged the foundation I would have preferred 18-inch-thick walls.
Sudbury Lumber did a takeoff from my floor plan and sent me all the lumber for the house on one truck. The two long exterior walls are framed with 2x6s, 16 inch o.c., and are 10 feet tall. The gable exterior walls are framed with 2x4s, and all interior walls are 2×4. The exterior wall is structural and picks up the roof trusses. Each stud lines up with the joist and roof truss.
There are two lofts inside the house. The loft floors are framed with 2x10s resting on the interior 8-foot walls, but they also grab the exterior studs to resist outward deflection of the roof (see Image #3, below).
The roof trusses are scissor trusses built and delivered by Quick Build Truss Company. The scissor trusses allowed a very deep cavity for insulation. The average depth of insulation is 25 inches, but tapers from bottom to top. The outside pitch is 12:12 and the inside is 11:12. (See Image #4, below.)
The steep angle allowed a lot of room in the lofts. The trusses were fastened to the top plates with Simpson ties that resist lateral forces better than a twist strap. Each truss lines up with a stud, 16 inches o.c.
The exterior sheathing is covered with peel-and-stick
Exterior sheathing is ½-inch plywood. Instead of using a common water-resistive barrier (WRB) like asphalt felt or Tyvek, I used Grace Ice & Water Shield (see Image #5, below). This product sticks very well to plywood. In doing so, it air seals every seam and every puncture from nails and screws. I wrapped it around the house starting at the bottom and working my way up the walls and over the roof.
Directly above each window and door I left some paper backing on the back side of the peel-and-stick membrane so that I could lap window flashing up under the higher course. The bottom edge of the Ice & Water Shield was attached to the membrane that was on top of the foundation. This meant that the air barrier was unbroken from inside the crawl space to outside the house and up over the roof.
A particularly nice aspect of installing the Ice & Water Shield on the outside is the ability to easily inspect the air barrier. All I have to do as project manager is walk once around the house. So long as I don’t see any plywood, I know my air barrier is complete.
An exterior vapor barrier
Ice & Water Shield is a vapor barrier, so you should be careful about attaching it to the outside of the home in the same manner as I did because there is a potential for condensation inside the building. Several parts of the system make me confident this will work. First, in every situation I’ve read about or seen of a structural failure due to moisture, the failure was associated with reverse flashing or air leakage. I don’t have either of these.
Also, during the winter I can control the relative humidity (RH) inside the home with my ventilation system (an HRV). I kept my living environment at 40% RH, which is much higher than most homes in the winter but low enough not to have moisture issues.
I also chose the best insulation for this system, which is dense-packed cellulose. Cellulose manages the moisture in a wall cavity and did not allow any moisture to reach the exterior sheathing all winter.
We tested the assembly this winter with the help of Bill Hulstrunk, National Fiber’s technical manager. Together we bored test holes in my drywall and probed the walls for moisture using a Delmhorst moisture meter with extended probes. The conclusion was that the framing was dry and the plywood was dry. The assembly works for the long term.
Double-pane Andersen windows
When I installed the windows and doors I used a polyurethane caulk on the nailing flanges and then Grace Vycor flashing tape on the sides and top. Then I removed the paper backing from the Ice & Water Shield that I had applied as my WRB and lapped it over the top of the Vycor so that 100% of the house is shingle-lapped without exception. Windows are nailed into the wood frame the same as they would be at any home, meaning that no special details were required. This is one of the reasons I’m a fan of the double stud system.
The windows are wood-framed vinyl-clad Andersens. The two south-facing windows are fixed, the four north windows are casements, and the two gable windows are awning. I decided not to install sliders or double-hungs because of the air leakage associated with them.
The windows have a U-factor of 0.29. I would have preferred windows with a lower U-factor, but I bought the windows inexpensively as odd-lots. Prior to purchasing them I ran some heat load calculations. My home is heated with electricity produced by a solar array. I had to purchase all the solar panels to make the electricity for the heat. After my heat-loss calculations, I found that the extra solar panels were less expensive than windows with lower U-factors.
I applied this thinking to all my decisions, and found that the least expensive way to be net-zero is not always the most energy-efficient way.
Roof ventilation channels above the Ice & Water Shield
I believe in long-term durability of a structure instead of only building for the initial sale. Because of this I chose a system for the roof that allows re-roofing in the future without damaging the WRB (meaning the Ice & Water Shield).
The way I did this was by purchasing 16-foot 2x4s. I laid the 2x4s on the flat, on top of the Ice & Water Shield, directly over the roof trusses. I screwed the 2x4s in place at the top and bottom with GRK structural screws, and nailed it along the field. This created a 1.5-inch-deep air space. The soffit and ridge vents are connected to this air space, providing ventilation for the underside of the roof deck (see Image #6, below).
I then nailed down 5/8-inch plywood over the 2x4s, and roofed the house with architectural asphalt shingles. When the time for re-roofing comes, nails can be pulled without leaving holes in the air barrier.
A rainscreen gap behind the siding
I created a rainscreen system for the walls by screwing 1×3 furring strips over the studs. Then I trimmed the windows and corners with Kleer brand PVC trim. I made the window and door trim frames on the ground with pocket hole screws. Then I installed the trim frame over the furring strips using a screw-and-plug system made by FastenMaster. This method held all my trim in place while hiding the screw heads beautifully.
Along the bottom of the walls I stapled on CobraVent, which is a plastic sponge-like material, between the furring strips. This will keep the bugs out of the hollow space. I installed James Hardie fiber-cement siding with galvanized 6d ring shanks. It went on easily.
The top of the rainscreen vents into the soffit space. The framing of the soffit is built out on top of the WRB so that there are no penetrations by rafter tails. The soffit consists of a manufactured PVC material with grooves cut into it. I liked it, but thought that the grooves looked a little large for an insect, so I stapled mosquito netting on the back side prior to installing it.
All of this exterior work was accomplished while working off of pipe staging that I rented locally. This meant that nowhere on the house did I leave a hole due to staging systems or ladders that rely on the house for support. Normally, sealing these holes would have been difficult if I were also trying to maintain the lapping of the WRB.
Keeping penetrations to a minimum
Because the air barrier is entirely outside, I was able to schedule plumbing and electrical work independent of my exterior work schedule.
There were a total of ten penetrations through the WRB: a hose bib, the plumbing vent stack, two outside lights, two outside outlets, one data cable, one electrical main, and two HRV vents. This is a far smaller number of penetrations than I would have had if I had attempted the Airtight Drywall Approach, with hundreds of outlets, switches, and pipes.
The plumber (from Spears Plumbing) took advantage of the crawl space by running all of the PEX hung from the joists. No holes were needed in the joists. The crawl space can never be finished anyway, so this approach worked out. (The distance from the top of the polyiso on the crawl space floor to the bottom of the floor joists is 30 inches.)
The electrician worked inside without worrying about air leakage. All interior work used standard outlet boxes. No time or money was spent air sealing the inside. The electrician mounted the panel upstairs and ran most of the wires upstairs.
Electric space heating
The heating system I installed for the first winter was a standard electric-resistance baseboard unit — a single 6-foot 240-volt heater mounted inside the crawl space. It only cost $80 (including the thermostat); it was my whole heating system. I set it to keep the crawl space about 75 degrees, and that warms the floor and keeps the upstairs at 68 degrees. This worked well all winter.
Later, I installed a ductless minisplit heat pump that uses less electricity to do the same job.
Eight tons of cellulose insulation
Except for some polyisocyanurate which I installed in the crawl space, all insulation is dense-packed cellulose. I wanted National Fiber’s brand cellulose because of its all-borate treatment, fiberization, and tight quality control. I called the manufacturer to track down a preferred installer for my area and ended up hiring Dolphin Insulation out of Littleton, Massachusetts. Chris, the owner, was easy to work with and excited about the project. In fact, his whole crew was happy to be on site working on such an efficient home.
They stretched Insul-Web material across the face of all the studs and trusses. This is a light netting material that keeps the insulation in place during the installation process (see Image #7, below).
After Chris made a call we ended up having several representatives from National Fiber coming out to the house, excited to see such a deep installation of dense-packed cellulose. The wall thickness was not that uncommon, but the roof installation (at over 2 feet of depth) was. There is just over 16,000 pounds of cellulose in the house (see Image #8, below).
Inside the crawl space I laid out a double layer of polyiso rigid board insulation on the floor and a single layer of polyiso on the walls. Then I built a non-structural 2×4 stud wall to the inside. This was also bibbed by Dolphin Insulation so that the whole cavity of the wall and up into the rim joist was dense-packed. This procedure eliminated any cold corners.
After the cellulose installation was complete, and bag count confirmed, crews rolled the walls flat to the stud face in preparation for wallboard. I strapped the ceiling at 16 inches o.c.; it was a little challenging to push the strapping in place against the packed cellulose. I should have performed the strapping before the dense-pack but after the Insul-web. However, it all worked out in the end.
High-gloss paint reflects light
Wallboard and plaster were installed. I chose plaster because I wanted a smooth ceiling surface.
Then I had high-gloss paint installed on the ceilings. I was cautioned against a high-gloss because the paint can show imperfections in the plaster. In order to make the surface as perfect as possible, my painter applied five coats and sanded between each coat. This came out great (see Image #9, below).
The reason I did this was because I have a low ratio of glass area to floor square footage. I was concerned about daylighting in the home. The high-gloss of the ceiling bounces the light dramatically.
The strategy worked; there are no dark areas in the home. It cost me an extra $500 for the paint and labor to accomplish this. However, I saved money by purchasing fewer windows.
A heat-recovery ventilator provides fresh air
I knew that this house would be airtight, so in order to ensure enough fresh air I installed an HRV made by Venmar. The unit has 4-inch metal ports on the sides. I attached rubber couplings and ran 4-inch PVC as my ductwork. PVC makes a perfect airtight seal at joints, because the couplings and pipe are glued together.
There are two short runs that lead outside, a single supply duct to the bedroom, and single exhaust duct from the bathroom. There is a button on the bathroom wall that steps up the fan speed to exhaust moisture after a shower. This system creates nice cross-flow in the house, and I’m pleased.
A solar electric system makes the house net-zero
I had a 4.8-kw photovoltaic (PV) array with micro-inverters installed on the south-facing roof of the house. The PV array connects to the electric meter on the outside of the house. This prevented more penetrations for wires.
It’s very, very tight
I conducted several blower-door tests; the final one was scheduled for after the drywall. A regular blower door could not be used on my house because it does not read below 100 cfm. Instead I built a plywood panel to hold a Duct Blaster fan in a window and taped the edges. I used a manometer in the normal manner for checking the whole house. With Ring 3 on the Duct Blaster I could pull an accurate number for the house. It was 22 [email protected] Pa. This translates into 0.09 ach50.
While I conducted this test, the crawl space trap door was open and the outside HRV vents were open. I did not tape them off because the HRV has internal gaskets that close when off. To put this in some perspective, the 2009 IECC requires no more than 7 ach50. The new 2012 standard is less than or equal to 3 ach50. The Passivhaus standard, which is the most stringent in the world, is 0.6 ach50.
The bottom line: comfort
The end product is a bright and comfortable home. The spaces work well with one another. There is a mudroom when you first walk in with a bench to take your shoes off. The kitchen has plenty of countertop space. The main room has high ceilings, providing a sense that the house has more space then there actually is. There are two lofts which we have not used for anything but storage so far.
During the first winter, I purchased my electricity because my solar panels were not yet installed. There is no fireplace or wood stove, and I was just using the one electric baseboard unit for heat. My stove, oven, and water heater are all electric. There is no propane, oil, wood, or wood pellets on site. Electricity is the only energy source entering the house besides the direct passive gain.
My PV panels have recently been brought online. I’m monitoring the usage and am on schedule to be net-zero in a year. Now I have no heating or energy bills.
[Editor’s note: Readers who want to emulate David Posluszny’s efficiency achievements should be cautious about imitating his decision to install a vapor barrier on the exterior side of his walls and roof. This approach violates the recommendations of most building scientists. For more information on this issue, see Dense-Packed Cellulose and a Wrong-Side Vapor Barrier.]
David Posluszny is an energy specialist at Dolphin Insulation in Littleton, Massachusetts.