Unconventional. Home.

The major twist of the Passive House concept is the absence of a thermal bridge. By partially quoting Wikipedia, this could be defined by any structural component with a higher thermal conductivity than the surrounding materials that can create a path of least resistance for heat transfer. In layman’s terms, it’s a thermal portal that needs to be broken. In order to achieve this, the entire house envelope must be sealed and air-tight from the ground up.

It’s all about insulation

Below are a thermally broken foundation and basement wall detail for the NOK Passive House project. As shown, an insulation layer should run continuously under footings, walls, and basement slab. The amount of insulation is calculated based on the size of the house and the local climate. The type of insulation is limited to sturdy 25-40 PSI foam since it needs to support the distributed weight of the entire house. In this case, it is 10” of EPS foam on the walls, 8” under the entire footing perimeter, and about 10” under the basement slab.

And yes, of course, I was worried about the weight. Really worried …until I saw an excavator lively crossing over one of those foam slabs barely doing any damage. My peace of mind was fully restored that day.

There were also some concerns about material sourcing and the price tag. Fortunately, it was somewhat easier than I thought. There were local suppliers in the area willing to work with contractors, builders, and homeowners alike. The prices were marginally different, so the winner was the company with the lowest delivery cost. The slabs usually come fresh off the production line, very well packed, cleanly cut, and ready to go.

The S/SE facade is designed to have the most amount of glass surfaces to capture and conserve solar energy. This, among other benefits, helps regulate interior temperatures and significantly lowers your electric bills. Large glass area is one of the main requirements of the Passive House concept but there is one caveat: the bigger the house, the harder it is to manage temperature fluctuations and keep the interior from overheating during summer. To mitigate possible overheating, we built a 3-foot overhang structure and a 5-foot cantilever around the largest windows. They help the house to stay cooler when the sun is high but still capture a lot of daylight in the winter months.

The quest for the right windows

The windows for a Passive House, a marvel in itself, are triple pane blocks of glass sitting inside the wood, plastic, fiberglass, or aluminum frame. While all the aforementioned materials are acceptable choices for passive house construction, the price quotes varied vastly with PVC still being the cheapest option.

Since the majority of residential builders do not deal with this type of construction, I began my own fenestration research. I quickly realized that the  U.S. lagged badly in quality window manufacturing with triple-glazed production being a VERY niche market. At the time of my research, Marvin Windows was arguably the only mainstream company producing stunning frames that met project requirements. Their high quality, of course, came with a forbidden price tag for my project. Thankfully, there was an abundance of custom window manufacturers in the U.S. and Canada to fit the budget.

…Or so I thought

To my dismay, about a third of the local market was reselling Canadian fiberglass profiles. Domestically-made PVC frames either did not meet PH standards or minimalistic aesthetics I was after. Another third were high-quality custom shops whose names can be often found while fanning through architectural magazines like Dwell or AD. The rest were dealers of meticulously produced European-made windows that one can rarely look at before buying. The temptation of the latter was too overwhelming to harness… Six months and about a dozen price quotes later, I finally took a risk and placed an order for German-made tilt-and-turn Passive House certified windows in wood with aluminum cladding. They took about 4 months to arrive but were totally worth the wait.

Added forced ventilation (energy recovery ventilation or ERV) is another attribute of a passive house, thanks to extreme air-tightness. ERV transfers about 50% of humidity between fresh and stale air streams circulating in your house at any given time. This means less dry indoor air conditions during the winter months. It also helps transfer humid air back to the exterior in the summer easing AC loads. Air volume is based on house envelope size, shape, number of corners, open spaces, etc. If ventilation system design is well calculated and done right, installation becomes a very straight-forward process.

Efficiency above all:
choosing the right ventilation system

There are quite a few brands to choose from nowadays, from world-known companies to niche market manufacturers. The prices vary considerably, therefore no matter what local dealers tell you, prepare to do your own research to estimate your system needs and installation costs. German Zehnder is always on top of every PH consultant’s list. It is a beautifully designed system and “just works” but it came with a forbidden price tag at the time of construction. But Ultimate Air, Lennox, Panasonic, Honeywell, Aprilaire, Fantech, just to name a few, are all worthy choices. I assume they would all work fine for the NOK project given they have enough power to supply airflow for longer runs.

At the end of our research, we zeroed in on Ohio-based Ultimate Air for overall energy efficiency vs value ratio. It features a rotating drum system with washable pie filters and works well for a fairly large house. Keep in mind the system is not whisper-quiet. Some low operational fan noise from the unit is audible, therefore it is advised to install the units away from the bedrooms or recreational areas of the house.

Well, this is, without a doubt, one of the best features we had thought of during the design stage. Paying homage to the mid-century iconic structure, it has become rather a convenience element than a design option. It serves many purposes keeping your cars dry, creating a sheltered passage from the car to the house entrance, and making a nice wind tunnel to keep you cool outside in the summer. (ok, the last one may or may not be completely made up). Since it’s a carport for a Passive House, it did have its challenges during construction but we manage to overcome those with the proper insulation.

Avoiding thermal bridging around carport

The carport is located under the master suite and also has an adjacent wall with the workshop. The workshop itself is not a part of the Passive House envelope, so some design decisions had to be made to properly isolate it from the rest of the structure. Since it is directly under a part of a Passive House, it is still insulated on all sides, including the ceiling, making it a semi-conditioned space. To avoid thermal bridging through the master suite floor, the space between the master suite and carport ceiling was double-insulated. First, the floor joist cavity was dense-packed with cellulose. Then we added a 5-inch deep outer “cage” to the carport ceiling and packed it with Rockwool COMFORTBATT® Thermal Batt Insulation. This roughly added R23 to the total insulation value of around R55.

The workshop (that also doubles for a one-car garage) is the only structure that is not a part of the Passive House envelope. It has been designed to structurally support a master suite on the second floor but had to be thermally broken from it by a foot of blown-in cellulose insulation and five inches of Rockwool COMFORTBATT® Thermal Batt Insulation on the exterior wall.

Although technically it is a separate structure, the workshop is treated as a semi-conditioned space and does not require a lot of extra energy to keep constant temperatures throughout the four seasons. It also has an adjacent wall with the carport. The carport itself sits directly under a part of a Passive House. It had to be insulated on all sides, including the ceiling to create a thermal break from the master suite above.

The rural aesthetics

The black siding all around the workshop is actually white pine boards treated with organic black pine tar and linseed oil mix. It has a thick texture with great coating properties. It penetrates the softwood nicely and creates a durable low-maintenance coating. The siding has remained untouched by elements so far. I expect it to keep its look for at least 5 years without extra oil treatment.

Passive House wall construction is not, by all means, an exotic concept. It dwells on an idea of efficiency and energy conservation but great results can be achieved with common construction materials. The walls can be anything from the conventional wood frame to cinderblock to ICF (insulated concrete form). The choices are limitless and will be based on the construction budget. The main requirements are continuous insulation barrier wrapping the house from the ground up and high-performance windows.

It’s in the walls

We opted out for a 2×6 wood frame for the interior wall filled with Rockwool COMFORTBATT® Thermal Batt Insulation. We used Huber ZipSystem® for the exterior sheathing and taped all seams and corners. Another “system” was built on top of the sheathing to hold an exterior insulation. It involved an improvised Larsen Wall construction and was filled with blown-in cellulose.

Breathable concept

The entire exterior was then wrapped in a breathable membrane by SIGA®. In this wall construction concept the ZipSystem® wall sheathing serves as an ultimate moisture barrier between two insulation layers. Any potential condensation inside the interior wall during cold weather periods evaporates into interior where it gets picked up by the whole house ventilation system. By the same logic, any moisture trapped inside the exterior insulation material evaporates out via a breathable membrane and gets picked up by the rainscreen air channel.

To keep the “boxed” integrity of the concept, the flat roof was designed with a slight pitch towards the back of the house. Because of the total pitch run of 40 feet, the parapet walls had to be raised up to 48 inches at the end of the run. But the back of the roof was kept completely open allowing rainwater to run freely into the gutter system.

“Hot roof” insulation

The flat roof construction is a set of pitched rafters. They are sealed with a special breathable lining by SIGA® on the interior and filled with dense-packed cellulose. The lining keeps blown-in insulation inside the rafter bays from falling down. It also prevents moisture accumulation inside the bays since any moisture can escape through the breathable surface to the interior. On the exterior, the rafters are covered with a more conventional “hot roof” sandwich: roof sheathing + 5” of ISO insulation + commercial-grade TPO layer. The total average R-value of the roof is R80. And while it seems like overkill for our climate, knowing ISO’s subpar performance in colder outdoor conditions the R-80 sounded just about right.

With the airtight house envelope and exterior insulation specified well beyond building codes, the walls should still be able to “breathe” to the exterior. One of the siding installation methods that work in this case is a rainscreen wall. The rainscreen siding is installed over a system of wooden boards or metal studs built around the exterior. The very thickness of the installed boards creates an air gap separating wall sheathing (with or without exterior insulation) from the siding. Any outside moisture that gets through the siding quickly dries out inside this air channel. By the same logic, if exterior insulation is used, any potential moisture trapped inside the insulation material evaporates out via a breathable membrane and gets picked up by the same air channel.

Pressure-treated wood vs. metal

After a short debate with my PH consultant, we decided to go with the wood option for the rainscreen support system. The 1×4 pressure-treated boards are a considerably cheaper choice than metal and faster to install. Keep in mind once PT wood dries out, it will “play” a bit. Thanks to its 3/4″ thickness, the boards might also shrink and buckle. These dimensional deformations are known properties and should be taken into consideration when choosing siding materials and installation techniques. For instance, long cedar or fiber cement planks will hold up just fine with slight structural movement behind them. Large fiber cement panels might crack at the corners or between mounting holes if not installed properly.

Installing fiber cement siding

To minimize siding maintenance, we decided to go with fiber cement for our rainscreen. As mentioned earlier, the air gap is usually achieved by mounting 1×4-inch pressure-treated spacer boards directly to the wall sheathing. Since we used exterior insulation for our NOK project as well, an extra framing had to be built to “cage” it in. The spacer boards were placed vertically 16” OC around the entire perimeter from top to bottom. Then the siding went directly on top of the spacers using more conventional installation methods.

There were two different fiber cement products used for the project. The “wood” planks are 7-1/2” stained boards by Allura. They accept nails (galvanized or SS roof nails work best) and can be installed by one person with the right equipment. The large grey panels are 4×8-foot sheets of color-through fiber cement that we had to order overseas. The panels proved to be rather heavy at 92 pounds each and needed three people to install. We used scrap wood boards to create temporary supports and used pilot screws to keep the panels in place. One of the crucial parts of the process was to find the right fasteners for the job. Multiple online resources lead me to SFS Intec. Their products are specifically designed for concrete and cement board fastening, they are self-tapping stainless screws with a great “bite”. They can also be ordered with painted heads to match your siding color.

Important considerations

When installing large fiber cement panels, make sure your 1×4 structural boards are covered with a layer of EPDM-like underlayment strips. They help a lot with the panel movement over time and minimize pressure points. For the same purposes, make sure your mounting holes are about 1/16″ larger than the screw thread diameter. I know it can be very tempting at times but DO NOT OVERTIGHTEN THE SCREWS! Set your screwdriver at different power settings and try it on a couple of mounting samples before the real installation.

Not all flat roofs are created equal. We did not want to risk waterproofing properties of ours by creating back scoopers to direct rainwater into the downspouts. Instead, we went with the open concept by eliminating back parapets and letting the rainwater pour into the conventional gutter system and then channeled into the underground drain. The number and size of the downspouts were calculated based on the roof’s area and pitch. Here is a closer look at the Passive House roof construction.

Helpful rainwater draining considerations

Unfortunately, periodical gutter clogging from nearby maple trees made rainwater spill over the gutters during a hard rain. After unsuccessfully trying out all available and more trivial solutions, I came across the downspout extension by InvisaFlow. Instead of blocking leaves and any tree debris at the downspout entry points (our main reason for clogging), it lets it go through downspouts freely and has a catch-all filter at the bottom. This allows easy access for occasional cleaning and saves me from unnecessary trips to the roof. It also keeps the gutters relatively clean.

Going “passive” always means choosing the best energy-efficient equipment possible. And the larger the house, the more complicated this task gets. Mechanical system calculation often ends up being either not efficient enough or too expensive to install. A suitable compromise is a mini-split system that can be programmed for each room or number of rooms that get heated or cooled off of the same unit.

Considering Mini-split system

For our house envelope, we ended up using two units, both by Mitsubishi: the maximum capacity multi-zone model that pretty much covers our main 40×40-foot open-space cube and the smallest available wall-mounted head for the master bedroom. For the latter, the decision was made to split the service due to the bedroom’s semi-autonomous location with extended duct runs and considerable heat loss. In the end, this solution worked out great and did not draw a lot of extra power. As a bonus, mini-splits can run in “Drying” mode when outside temperatures get lower but the house still retains higher RH values. This setting still dehumidifies the air and lowers the indoor temperature just a bit but the fans are running at much lower speeds reducing electrical bills even further.

In conclusion, it has been a few years since the house was occupied, and the system has performed exceptionally well so far. Considering a fairly larger house envelope comparing to our previous dwelling and electricity as the only energy source, our average monthly electric bills are still almost twice a low.

With the Passive House house envelope being completely air-tight, any wall or roof surface penetrations become less than desirable due to a possible thermal bridging or an air leak. But that is not to say that a chimney is out of the question, for instance. With certain precautions and extra insulation, some necessary openings to the exterior can be achieved without compromising Passive House efficiency.

Sealing all points of entry

Our ventilation system required two sets of air intake and exhaust. There are two plumbing vent stacks and one chimney flue coming through the roof. And finally, there is laundry. For the latter, we really tried to go with a very efficient self-condensing unit that does not require exhaust but some well-known drawbacks (mainly the time it takes to wash and dry along with the average unit capacity) took it out of consideration. So we ended up going with a more conventional double-unit with 4” exhaust pipe to the exterior. We pack extra insulation around the in-wall portion of the laundry exhaust to keep condensation away from the walls.

In addition, there were rather small openings for electrical wiring, plumbing, and HVAC equipment. With all wall penetrations taped up tightly with SIGA® adhesive tape, the door-blowing test did not show any significant air leakages at those entry points.

Keep in mind that the number of wall penetrations will vary depending on the dwelling’s square footage, the number of ventilation units, and one’s desire to sacrifice laundry drying time for efficiency.