In 2006 my wife, my two small kids and I packed our stuff and moved from Maine to my native Denmark for a year. Before leaving for Denmark I was thinking about transitioning from building traditional 2×6 stick-built homes to building green (whatever that might be). While living in Denmark I heard about the Passivhaus standard, and realized that this is the holy grail of green construction (at least if you think green is mostly about energy, which in my mind is where it’s at).

Fired up and ready to get into this new Passivhaus stuff, I returned to Maine the following year and started up my small design and construction company again. But right then the housing market crashed, and it took me a while to finally land a Passivhaus project.

That was probably not all bad, since I had no experience building energy-efficient homes or anything that smelled like green building (beside common sense). It gave me plenty of time to research the Passivhaus standard, to take the Passive House Certified Consultant training through PHIUS, and to try to find the best way to build a passive house.

Being builder, designer, and engineer on most of our projects, I found myself with my hands full, but also with the advantage of not having to try to get the builder, the designer, and the engineer on the same page — as long as I can keep my own head somewhat straight.

I have now built two passive houses in the last year and a half. The owners of the two homes come from different backgrounds. One couple are treehuggers (used here in the best way possible) and flatlanders (the Maine word for people from outside Maine), while the other couple are “real” Mainers. One couple’s main motivation is cutting their carbon footprint and promoting the Passivhaus standard, while the other couple simply sees the benefit in paying a tiny bit more for their home in order to have very small utility bills. You can guess who’s who.

The Newry Passive House

The Newry house is situated in a field that is bordered by a small creek. The lot is flat. Tall oak trees provide eastern and western shade which helps keep the house cool in hot weather. But when the cold weather arrives, the trees are bare and the eastern and western windows get plenty of sun.

The owners came to me with a Cape design and a layout they liked. I tweaked the design based on what my PHPP calculations told me. It was hard to get accurate weather data, but we got improved data right before we started construction. Based on this weather data, we added 2 inches of EPS below grade at the last minute, and also added 2 inches to the exterior wall thickness. The heating load is calculated at 4.55 Kbtu/(s.f. • year).

Fourteen inches of rigid foam underneath the slab

The foundation consists of frost walls and a slab. We installed 14 inches of horizontal EPS below grade and separated the slab from the frost walls with 6 inches of vertical EPS.

The wall system is a double-stud wall. We first framed the exterior 2×6 wall which carries the load of the roof. The interior walls carry the load of the second floor. This arrangement eliminates thermal bridging through the floor joists.

What’s the best tape for OSB?

The OSB air barrier is located on the interior side of the inner wall. All of the OSB seams were primed with Perm-A-Barrier (a Grace product) and taped with either Gryce Vycor tape or Zip System tape.

The upstairs ceiling was the first place we installed the OSB air barrier. We nailed 1/2-inch OSB to the bottom of the rafters and trusses. We primed the edges and taped the seams with Grace Vycor tape.

After about a month, we noticed that the Grace Vycor tape was starting to peel off. We tried to use an old iron to see if warming the tape would help, but it remained loose. We finally ended up screwing strips of OSB over the tape to keep it in place. Ironing the ceiling made for some funny conversations, but I wouldn’t recommend the practice. After this fiasco, we switched to Zip System tape. (At our next job, we used Vana tape from 475 Building supply; more on that later.)

On the inside of the air barrier we built a wiring chase out of horizontal 2x2s. Our original idea was to insulate these cavities, and then install drywall, but we ended up skipping the insulation, for two reasons. Peeling fiberglass batts in half didn’t seem practical, and blowing the wiring chase with cellulose would have been too expensive. The cost associated with using cellulose insulation is partly due to the cost of installing the fabric that keeps the cellulose in place. So insulating a 2-inch-deep cavity is definitely not one-sixth the price of insulating a 12-inch-deep cavity.

All electric penetrations through the air barrier were brought through PVC pipe. First the PVC pipe was caulked and taped to the air barrier (belt and suspenders); then the wire was brought through the pipe, and finally spray foam was used to fill the pipe, making a tight and long-lasting seal. If you just run your wire through the air barrier and seal it with caulk or tape, the penetration can easily leak, especially if someone tugs on the wire. If you use a PVC sleeve, you can easily replace the wire if you ever need to, and then you can seal the hole again.

Vapor-permeable sheathing

Getting the fiberboard that we used for exterior sheathing turned out to take almost six weeks. We ended up framing the entire house without any sheathing. That is not something I would want to do again, but it was kind of cool to see the house framed like that. (See Image #2, below.)

The house has two shed dormers. We framed the dormer roofs with trusses and used 16-inch TJIs for the rest of the roof (the 12/12 pitch). We built the TJIs “down” to be 24 inches deep by nailing a 2×4 that extended 8 inches below the TJI and then simply nailing a long 2×4 at the bottom, parallel to the TJI. Since the truss heels are only 12 inches deep, we added a small angled section to the ceilings where the ceiling and exterior walls meet. It eliminated a thermal bridge, but added a ton of work for our drywall crew.

The attic over the dormers is filled with a 42-inch-thick layer of cellulose. Insane, you might say. But the cellulose was easy to add and it made the numbers work. We added a catwalk (a 2×12 plank) 48 inches above the bottom of the joists so you can still move around the attic. The attic access is through an outside hatch located in the gable wall.

Glazing mix-up

We used Intus windows and doors. Somewhere along the way, our order got mixed up. We thought we that our south widows would have a solar heat gain coefficient (SHGC) of 0.62, but when the windows arrived we found that they all had a SHGC of 0.5. This didn’t help; the lower SHGC resulted in a big hit for the PHPP calculations. On the positive side, however, I’m sure that the low-solar-gain windows help the house stay fairly cool during the summer.

We installed the operable windows by screwing through the jambs, and we installed the fixed windows with brackets. We taped the exterior seam between each window and the sheathing with vapor-open Contega EXO tape, and then we “overinsulated” the exterior of the window frames with 1 ½ inch of XPS. We could have covered more of the frame, but decided to leave some of the frame visible for aesthetics. On the inside we used vapor-retarding Contega SL tape to seal the face of the window frame. (Watch out if you do this; you don’t want the tape to protrude past the drywall line.)

The thermal bridge value for the window installation (one of the PHPP inputs) was 0.011 btu/ft/hr/F. The color of the PVC windows and the imitation wood looked better than I expected. The colored vinyl and the imitation wood added about 30% to the cost of the windows.

An HRV from Europe

Having installed a Zehnder HRV on a previous project, we decided to use the same HRV for the Newry house. The flexible duct system, the integrated silencers, and the manifold design make this HRV a breeze to install. The owners decided to go ahead and spend the extra money for the Comfofond ground-loop option. The Comfofond is connected to 400 feet of 1-inch tubing 6 feet below grade. The tubing is filled with glycol; the Comfofond pumps the glycol through a small heat exchanger located in the incoming air duct of the HRV.

The data so far suggest that the ground loop is amazingly effective. We didn’t get the temperature sensors installed in the ventilation system until February of this year, but when the outdoor temperature was 15°F, the incoming air was warmed to 38°F by the ground loop. And in early June, when the outdoor temperature was 94°F, the ground loop cooled the incoming air to below 60°F.

Wood stove woes

The house was designed to be heated by a small wood stove. We chose the Morso 3142 stove, since its heat output is a close match to the design heat load, and since the stove has a ducted outdoor air kit. We ran the outdoor air duct under the slab. When it came time to hook up the stove, however, we learned that experts have mixed thoughts on ducted outdoor air for wood stoves.

We decided to hook up the stove without the ducted outdoor air supply and see what happened. (First, though, we had a discussion with the owners on the importance of being extremely vigilant while the stove was burning, to watch for back-puffing). At first the stove seemed to burn well, but as the heating season advanced, the owners started to experience some problems, which was not too surprising. They had to crack a window to get the fire going, there was the smell of smoke at one point.

I think the fact that their firewood wasn’t fully seasoned made the problem worse, but regardless of the cause, the stove just wasn’t working right. So we decided to hook up the air supply. It turned that the duct is not directly connected to the firebox; it’s just kind of close. So hooking up the dedicated air supply will be like opening a window: cold air comes into the house, and hot air goes into the stove. That’s definitely not ideal. We’re looking into buying a Rika Vitra stove that is specifically designed for Passivhaus buildings.

We thought about trying to retrofit the Morso stove to make it more airtight, but the design really doesn’t make this possible. Although the wood stove is not a success so far, the owners really like the idea of being able to heat the house when there is no power, and so we’ll keep working on it. Luckily we installed electric heaters throughout the house, so the house can still be heated — just not the way we had hoped.

Pay attention to shading

The PHPP shading calculations turned out to be much more stringent than I thought. We used a Solar Pathfinder to calculate the shading at the site. I figured that bare branches in the winter only provided 25% shade, but we were told to calculate bare branches as 50% shading. I still think 50% is a bit high, but being real about the shading is important.

It’s a good idea to get a Solar Pathfinder before you start your project; you definitely want to be do this if you plan to get your project certified.

The West Paris Passive House

We built the West Paris house on a tight schedule. A couple came to us after their house had burned down. They wanted us to build a Passivhaus where their old house had stood, and they wanted us to start tomorrow.

The design process and the calculation of the construction budget took a couple of months, and then we jumped into construction. Throughout the design process we were only one step ahead of construction, if that. It would have been better to add more time to the design process, but that just wasn’t an option on this project. The owners wanted the house now, and I wasn’t going to let the tight schedule keep me from building the house.

The house is built on a south-facing hill and has a daylight basement. The site used to be an apple orchard, and there is a beautiful view of the mountains. There is very little shade; that is great for heating purposes, but it makes the house very prone to overheating.

Installing rigid foam under the footings

When we designed the house, we calculated the inside foundation measurements and ordered the EPS foam that we intended to install under the slab to fit exactly inside the walls. We also ordered sheets of EPS for the inside and outside of the foundation walls; these were ordered to the right lengths so there was no trimming (except for the need to trim the width of the last sheet on each side of the wall).

We installed 4 inches of high-compression-strength EPS foam under the footings. This seemed a bit counterintuitive, but our engineer confirmed our calculations and gave us the go-ahead. When our concrete contractors heard that we were putting foam under the footings, they shook their heads. When I told them that foam is used as fill under highways and train tracks, they nodded, but still didn’t look convinced.

I was concerned about warm inside air hitting the cooler concrete walls and condensing, so we air sealed the seams in the 4’ x 8’ EPS sheets by taping them with 3M 8067 tape. The perm rating of the EPS is on the high side. The exterior side of the waterproofed foundation has a perm rating of near zero, but I think we’ll be fine. We installed a capillary break (some kind of concrete sealer provided by my concrete contractor) between the footings and the concrete foundation walls to limit the movement of moisture up into the walls.

The foundation walls have 14 inches of rigid foam

We installed 10 inches of EPS on the exterior of the foundation and 4 inches of EPS on the interior. With that much exterior foam, the load-bearing part of the above-grade wall would have been in the wrong location, since we wanted the above-grade wall to be in the same plane as the foundation wall. After trying to come up with a double-stud wall design that would work with a full basement, we changed the original plan.

We decided to build a 2×6 wall sheathed on the exterior with 1/2-inch OSB. Then we added 12-inch-deep TJIs to the exterior of the framed wall. We eliminated the wiring chase on the inside of the wall, since all of the wiring and plumbing can be run in the 2×6 wall, just as it is in a traditional house.

We first installed a 22-inch wide strip of 3/4-inch plywood to the top of the pressure-treated mudsill. This plywood strip became the bottom plate for the TJI wall. The plywood plate helped to create an airtight seal between the EPS on the interior side of the foundation walls and the TJI wall. The plywood plate was also used to connect the TJI wall with the 2×6 wall. We used spray foam to seal the small gap between the top of the EPS foam and the plywood plate.

Vana tape

We sheathed the exterior side of the TJIs with the same type of fiberboard that we used on the Newry Passivhaus. This system worked very well.

To seal the seams in the OSB air barrier, we used Vana tape purchased from 475 Building Supply instead of Zip tape (which requires priming). Using Vana tape saved the cost and labor of applying primer to the OSB sheets. I had my doubts about leaving the Vana tape exposed to the exterior, but my doubts were unfounded. The stuff is amazing. (I patched my 9-year-old son’s snow pants with a ½” x 5” strip of Vana tape in early winter, and it’s still attached.)

The TJIs were fastened with 4-inch GRK screws with attached washers. The window openings were framed with 2x4s that were toe-screwed with 3 1/2-inch GRK screws. We built window bucks out of ½-inch OSB, and reinforced the bucks with 1-by stock where we planned to attach the windows.

This time we ordered roof trusses with 24-inch heels, so we could eliminate the angled ceilings at the wall/ceiling intersection. The raised-heel trusses make the house taller — the south wall is over 30 feet tall — but eliminating the angles was a big time-saver.

The exterior walls and attic floor are insulated with cellulose. Again, we installed 42 inches of cellulose on the attic floor.

Insulating behind the OSB

I was a bit nervous about cutting through the OSB air barrier to access the wall cavity for insulating, but my anxiety was unfounded. The guys from C and C Insulation did an amazing job. We cut holes for them on the interior side of the walls, near the top. They fed their 3-inch hose to to the bottom of the wall cavity and packed it.

Then they would run to the basement to cut another hole to make sure that the cavity was packed to their liking; it always was. During the blower-door test, the walls were inspected with an infrared camera; no problems with the insulation were detected. After the walls were insulated, we covered the holes with scraps of ½-inch OSB attached with screws and construction adhesive.

Danish windows

We specified Ecliptica windows from Denmark. The windows are finished on the interior with a beautiful pine frame (they can provide pretty much any kind of wood), while the exterior parts are made of fiberglass. The windows have extremely skinny (1 ¾ inch) frames, and they look amazing.

Although the thermal performance of the window frame isn’t all that great, the fact that it’s so skinny makes it an amazing product. We paid a bit more for the Ecliptica windows than we did for the Intus windows we used on the previous house, but the Ecliptica windows outperform the Intus windows.

All of the operable windows at the West Paris house are awning windows. (Ecliptica also makes casement windows.) It did take a bit of effort to convince the fire marshal’s office that the windows qualify as egress openings, but we eventually got the OK. (The awning windows don’t have a crank at the bottom of the window to get in the way if you have to crawl through the windows to escape.)

Since the sash size on the awning windows is exactly as wide as the window frame, it’s impossible to “overinsulate” the exterior of the window frame. We had to install metal extension jambs and sills on the exterior of each window opening. The window was then installed in the opening, and the gap between the window and the metal jamb was sealed with expanding bitumen tape. We air-sealed the windows on the interior with Contega SL tape. On the exterior, we sealed the perimeter of each window with spray foam and bitumen tape.

This time, no wood stove

We used the same Zehnder ventilation system as in the Newry house.

After our struggles with the Morso stove at the Newry house, we decided not to install a wood stove in the West Paris house. Space heating and cooling are provided by a Mitsubishi ductless minisplit.

We urged the owners to install exterior window shades, but the owners don’t want them. They will try their luck with interior shades, which should cut down on the solar radiation reaching the mass inside the house. However, I cannot imagine that interior shades will eliminate the overheating.

A solar thermal system for domestic hot water

The owners have five children, so the family’s hot water demand is high. A company called Revision Energy installed a solar thermal system with three flat-plate collectors. The same company also installed a 6.2-kW PV array. The house will not quite be net zero, but it will be pretty close. The annual heat demand is estimated at 3.62 kbtu/(s.f. • year).

For our next project, I will finally get to build on a floating slab. We’ll use stick-framed walls with TJIs bolted onto the exterior. The roof will be framed with trusses, and this time we get to use standing-seam metal roofing.

The West Paris house had close to 50 roof penetrations for the PV and solar hot water systems. With standing-seam metal roofing, the solar equipment can be clamped onto the seams, thereby eliminating almost all penetrations.

On my next project, I hope to get away from HardiePlank siding. I understand the allure of “maintenance-free” siding, but it is really hard to make Hardie siding look good — and as a carpenter, that hurts my pride.

How much did these two houses cost?

The cost of the Newry Passivhaus (excluding the solar hot water system, the PV system, the well, and the septic system) was $140 per square foot — about 6% more than if we had built it as a traditional home.

The West Paris Passivhaus came in at $118 per square foot. These cost figures are based on the gross area. If we calculated the cost based on the Treated Floor Area (the German way of calculating net living area), we end up with a cost of $180 per square foot for the Newry house and $141 per square foot for the West Paris house.

Jesper Kruse is the founder of Maine Passive House, a design/build firm in Greenwood, Maine, and is quite possibly the last person in the universe who still draws plans with pen and paper. He lives in the woods of Greenwood, Maine, with his family and their hamster Buddy.