A Practical Approach to Passive House

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A Practical Approach to Passive House

An architect’s ‘best work to date’ is a super-efficient home that you could build

Posted on Nov 10 2016 by Steve Baczek
prime

I began my career in architecture nearly 17 years ago after spending many years as a contractor. My background has given me a strong appreciation for and understanding of people who design and build homes. I’ve designed more than 30 zero-energy homes, six deep-energy retrofits, and numerous high-performance houses. In truth, the path to optimum performance and durability hasn’t always been easy.

In many ways, the knowledge that I’ve gained over the years has culminated in the design and building of this home — my first certified Passive HouseA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates., in the coastal community of Falmouth, Mass. — which I confidently consider my best work to date. Not only am I proud of what we accomplished in this project, but I’m also proud of how we did it. My informed, organized team and I were able to build a comfortable and exceptionally durable Passive House with standard building materials and practical construction techniques — an approach that’s replicable for any homeowner, builder, or architect looking to build a cutting-edge house of comparable performance.

Chasing the plaque

Passive House standards are the strictest residential-building parameters we have in this country, and hitting their performance target is a challenge.

Passive House isn’t for everyone, and there are some caveats worth considering. Passive House standards are performance-based, with no relevance to cost or aesthetics. Because of this, material and product options can be limited when compared to, say, an Energy StarLabeling system sponsored by the Environmental Protection Agency and the US Department of Energy for labeling the most energy-efficient products on the market; applies to a wide range of products, from computers and office equipment to refrigerators and air conditioners. home. For example, we wanted to use insulated, triple-glazed wood windows on this house because of the way they looked, but when we modeled them with the Passive House software, we realized that they wouldn’t perform well enough. We opted instead for fiberglass Thermotech windows that met Passive House standard but were different from what the homeowner had originally wanted.

Chasing the Passive House plaque is a goal that should be considered carefully and talked about throughout the design phase to avoid too much compromise. Initially, I think Passive House certification should be a goal written in pencil. If the client is interested solely in a very energy-efficient house and not necessarily the plaque on the wall, a house that reaches 80% or 90% of the standard is less costly to build and still far better performing than any typical code-built house in the United States. It’s even better than those houses that are built to other green building standards.

We were committed to the standard and understood that due to the limited number of comparable, exemplary projects, there was limited knowledge within the industry on how to construct a Passive House. Because of that shortcoming, our approach to the project had to be planned carefully.

Passive House in detail
Because Passive House standards are so well defined and because Passive House modeling software — known as the Passive House Planning Package — predicts home performance, the path to certification is clear. Meet the benchmarks, and you’ll obtain the certification. The standards focus on three building attributes: airtightness, BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. consumption, and energy usage.

AIRTIGHTNESS
The Passive House standard requires the home to be tested at or below 0.60 air changes per hour (ACH) at 50 pascals (Pa) — 122 cfm at 50 Pa in this house. That’s tight, considering the typical code-built house might have a tightness range of 3.0 to 7.0 ACH at 50 Pa.

BTU CONSUMPTION
Passive House standards require the annual heating and cooling consumption to be below 4755 Btu per sq. ft. annually. A code-built house can have a consumption rate nearly 10 times that amount.

ENERGY USAGE
The maximum energy use in a Passive House, not counting any photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. offset, must not be greater than 11.1 kwh per sq. ft. Building codes don’t have energy-use provisions, but it’s estimated that the average American house consumes electricity at a rate of 958 kwh per month, or 11,496 kwh per year.

Organized from the start

I’ve developed a simple two-phase philosophy when it comes to designing high-performance houses: I aim to design a house that, when built correctly, converts energy as inexpensively as possible and holds on to that energy for as long as possible.

While the goal is always that simple, the process never is. Building a Passive House is demanding. Planning is imperative, so we chose to use a design/build methodology. The homeowner, the architect, the energy consultant, the general contractor, and the subs all need to have a vested interest in the project. This interest translates to responsibility. In a Passive House project — where each detail, from the window selection to the continuity of a bead of caulk beneath a wall’s bottom plate, matters greatly — that sense of responsibility results in a successful project.

Because we wanted to build that sense of responsibility and because none of the parties had previously constructed a Passive House, we held a number of preconstruction get-togethers that we called “trade-day meetings.” The general contractor and I sat with each subcontractor to define the goals of the project, to outline that subcontractor’s specific responsibilities, and to ask about any concerns or bits of advice that might benefit the project. This planning helped all involved to develop a clear understanding of the project and the effect their work would have on it. For example, the framers knew that if the house didn’t pass its first test for airtightness, which occurs when the house is really just a big plywood box, then they would be held accountable. Good subs welcomed the challenge and learning experience, and they delivered. That first test showed an air exchange rate of 32 cfm at 50 Pa, well under our goal of 60 cfm at 50 Pa.

Drafting practical plans

One of the biggest problems I see in the design and construction industry is that architects and designers want to design homes and then apply energy-efficiency features or durability features to the design instead of making them an integral part of the house. High R-values are made possible by the way this house was framed. There was no need to apply systems to the home to compensate for an inefficient or flawed design.

In creating construction drawings, I carefully drafted two identical sets of wall-section details. One set delivered the rough framing dimensions and air-sealing details. The companion set delivered the exterior and interior finishes. The construction details were driven by my desire to have them be recognizable to the crews working on the house. The goal was not to overburden the subcontractors with information that they may not need. This allowed the framer, for example, to focus clearly on framing and the associated air-sealing. Likewise, the siding contractor did not need to have the air-sealing information of the wood frame.

The field of building science (see The Trouble With Building Science) is saturated with complicated construction details that are sometimes difficult to implement accurately. That doesn’t have to be the case. For example, many Passive Houses have a large amount of rigid foam beneath the slab, well over a foot in most cases, and around the slab’s edge. This approach not only is costly, but it also makes detailing the transition between the slab edge and the wall plane cumbersome to even the savviest contractor. In the initial design meetings, our energy consultant wanted to see a minimum of 8 in. of rigid foam around the edge of the slab. Looking for a better and easier solution, I decided to move the slab outside the thermal envelope, to frame a more conventional floor over two layers of rigid foam, and to insulate the floor to R-76 with cellulose. Essentially, I took our wall assembly and laid it on its side, a concept our framers could implement easily and correctly.

It seems that many in our industry are searching for a silver bullet or new material that solves all our problems. This plan reinforces the idea that we can build better-performing homes with the building materials that are currently available and construction techniques that are commonplace.

Holding on to energy

At its core, this house is designed and constructed to satisfy the simple desire to convert energy as inexpensively as possible and to keep it contained for as long as possible. This home does both extremely well.

The solar gain that comes through predominantly south-facing windows meets 55% of the home’s heating demand, helping to reduce its cold-season operating cost considerably. Because this house is so airtight and well insulated — the floor is R-76, the walls are R-60, and the roof is R-105 — the heating and cooling loads are low. Supplemental space heating, as well as cooling and dehumidification, is accomplished with two high-efficiency ductless minisplits. These Daikin units have exceptional dehumidification performance, which is important because this home is a stone’s throw from saltwater. An UltimateAir energy-recovery ventilator (ERV(ERV). The part of a balanced ventilation system that captures water vapor and heat from one airstream to condition another. In cold climates, water vapor captured from the outgoing airstream by ERVs can humidify incoming air. In hot-humid climates, ERVs can help maintain (but not reduce) the interior relative humidity as outside air is conditioned by the ERV.), chosen because it is well insulated and electrically efficient, helps to meet Passive House heating, cooling, and primary-energy criteria. The ventilation system was commissioned, meaning the intake and exhaust airflows were balanced.

To ensure comfort, the supply ventilation air is distributed to the bedroom closets first. The tempered air then is distributed to living spaces through louvered closet doors. This prevents drafts of incoming air from cooling conditioned air in the living spaces.

We estimate that the electricity consumed by the mechanical systems — as well as by the LED lights, energy-efficient appliances, and electric water heater — would have cost $930 annually. However, we also installed a 2.8-kw photovoltaic array on the roof, which will lower the home’s annual net operating cost to around $280.

Making it last

As the architect, I believe that this home should have a level of durability proportional to its energy efficiency. The Galvalume metal roof will outlast most other roofing products, and its deep overhangs will help to protect the home’s exterior. The wall claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. , a mix of cedar shakes and clapboards, was installed over a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. and left unfinished. The home will weather to the silvery-gray tone common to the beach homes in the region, and because there isn’t any finish to maintain, the homeowner will never have to worry about maintenance. The result of all this work is an ultra-high-performance home that’s not only practical to build but also practical to live in.

CONSTRUCTION PLANS IN DETAIL

(Click images to enlarge)

BUILD TWO CONVENTIONAL WALLS
Of the various wall assemblies, a wood-frame double wall is the easiest way to achieve high R-values. This home has a conventionally framed 2x6 outer wall with a 2x4 inner wall mirroring it. Framers can build these walls easily. The assembly allows a high degree of flexibility in width, which accommodates various R-value targets easily.

Because of the floor design, the 2x6 walls were framed first, followed by the second floor and the roof, which allowed the home to be dried in quickly.

INSULATE ABOVE THE SLAB
Instead of more than a foot of rigid foam insulating the slab from below, this house has a raised-frame floor built above it. This floor system is recognizable to most builders, it makes plumbing installation easier, it costs less to insulate, and it enables the conventional installation of solid-plank wood flooring.
This approach eliminates the mass of the slab, which many Passive house designers use as a heat sinkWhere heat is dumped by an air conditioner or by a heat pump used in cooling mode; usually the outdoor air or ground. See air-source heat pump and ground-source heat pump. to regulate interior temperatures. However, the energy penalty of excluding the mass of the slab from this assembly was minor, especially when weighed against the loads on this house and the ease of construction.

BREAK THE THERMAL BRIDGE AT THE SECOND FLOOR
Floor trusses make up the second-floor framing on this house. Detailing the intersection of the floor and walls has to be considered carefully to eliminate air leaks and thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. .

USE A VENTED TRUSS ROOF
Cost, airtightness, and speed and ease of installation were the motivating factors in the decision to build a truss roof with an insulated attic floor. Because the interior walls had not been framed when the drywall was hung on the second-floor ceiling, hanging large sheets was easier, and many of the seams that could lead to air leakage were eliminated.

The attic isn’t used for storage or to house mechanicals, so an interior attic hatch — which is notoriously leaky — was unnecessary. Instead, a small doorway was placed in a gable end of the roof and positioned above the insulation level, so its leakiness has no bearing on the thermal envelope’s performance. The door is accessed by ladder.

TWO WAYS TO DETAIL WINDOWS IN THICK WALLS
Windows and doors are the Achilles' heel of superinsulated wall assemblies. Prior to the installation of the windows and doors, this home had been tested for airtightness twice. The first test, when the home was a plywood shell, resulted in an air-leakage rate of 32 cfm at 50 Pa. The second test, which took place after a 2-in.-thick coat of spray foam was applied to the back of the exterior wall, resulted in an air-leakage rate of 25 cfm at 50 Pa. Finally, after the doors and the windows were installed, the air-leakage rate jumped up to 125 cfm at 50 Pa. The airtightness target for this Passive house was 122 cfm at 50 Pa. The final test, done just prior to occupancy, registered 116 cfm at 50 Pa.

With so much performance riding on the windows and their installation, getting the details correct was critical. This home has windows placed in two positions within its 17-in.-thick walls. the first floor has outies — that is, windows flush with the outer plane of the wall. The second floor has innies, or windows that are set into the wall.

Innies: Windows set into the wall are better protected from the elements, but they require detailed attention to the exterior sill, which covers part of the exterior wall. Also, because these windows are flangeless, they’re more difficult to air-seal. Flangeless units are suspended within the rough opening, making the perimeter connection with the air-barrier sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. a soft joint, which challenges the concept of continuity.

Outies: Placing windows in line with the wall sheathing allows flanged units to be used. It’s easier to get an airtight installation with a flanged window than with a flangeless unit because the flange is part of the window frame and bridges the gap across the rough opening. This makes the connection to the exterior sheathing, which is the air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. on this home, more seamless.

Steven Baczek is an architect in Reading, Mass. Illustrations by John Hartman.

From Fine Homebuilding No. 227 — Download a PDF of this article.


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  1. John Moore

1.
Nov 10, 2016 10:33 AM ET

Very nice drawings. I am
by Jon R

Very nice drawings. I am interested in data supporting the moisture performance of such a wall.


2.
Nov 10, 2016 10:53 AM ET

Edited Nov 10, 2016 12:22 PM ET.

Response to Jon R
by Martin Holladay

Jon,
I'm not sure whether this house has any monitoring equipment installed in the walls to keep track of moisture levels. I doubt that it does.

I will say this: Installing 2 inches of closed-cell spray foam on the exterior side of blown-in fiberglass insulation in a double-stud wall breaks most conventional rules for wall design. This type of wall can't dry to the exterior -- the closed-cell foam blocks outward drying -- and the closed-cell spray foam isn't thick enough to keep the interior surface of the closed-cell foam above the dew point during the winter. I don't recommend that you copy Steve Baczek's approach.

Everyone has opinions. Here are links to two articles that provide my advice:

How to Design a Wall

Exterior Rigid Foam on Double-Stud Walls Is a No-No


3.
Nov 10, 2016 12:00 PM ET

The wall is indeed a bit suspicious, I agree.
by Dana Dorsett

Falmouth MA is on the warm edge of zone 5A, and the stated R value of that wall is R60, but there is only ~ R12 of closed cell foam to the exterior of presumably ~R48 fiberglass. R12/R60 is a ratio of 20%, which is well below the IRC prescriptives for dew point control at the foam/fiber boundary on thinner walls in zone 5A.

But it has comfortable margin over the IRC prescriptive ratios for zone 4A, which makes me think it just might work in an air-tight wall in Falmouth even with without interior side vapor retarders, given just hot temperate Falmouth is relative to most of zone 5A. I would hope that if the didn't verify that it works with a few WUFI simulations that they used half-perm latex on the interior (which is not specified.)

PassiveHouse designs tend to run higher than typical indoor humidity in winter (unless the ventilation system is run under dehumidistat control), which is another risk factor here. I'm sure it's fine if they keep they hold the line at 30%RH, but it's pretty risky north of 40% unless there is a class-II vapor retarder on the interior.


4.
Nov 11, 2016 8:20 PM ET

Edited Nov 11, 2016 8:21 PM ET.

Crawlspace?
by Malcolm Taylor

I'm interested in the code implications of the main floor assembly. It appears to be a crawlspace filled with blown insulation. If it is a crawlspace, it appears to not allow the required access to the services that are contained in it because a) it is too short and B) it is filled with insulation. If it isn't a crawlspace, what does the code define it as?

That aside, Mr. Baczec offers one of the best reasoned approaches I've seen on whether to attempt to achieve Passive House accreditation, and the design implications of doing so.


5.
Nov 12, 2016 6:38 AM ET

Response to Malcolm Taylor
by Martin Holladay

Malcolm,
I suppose that some building inspectors might consider this to be a crawl space foundation. Others will conclude that it is a slab-on-grade foundation with floor framing above the slab (which, as you point out, may still require access, depending on how you interpret the code). It's certainly an unusual foundation.

.

Baczek foundation.jpg


6.
Nov 12, 2016 10:29 AM ET

Response to Malcom Taylor
by Steve Baczek

Morning Malcom,
Thank You for you alignment in thought. Daily, I am on the fence about "Passive House" but in some ways that's all we have right now, and my approach to all my designs (and future ones) will be to do what we do, just with a more conscious effort than usual. There is some value in using new materials, and trying to build a better wheel. But there is also extreme value (for the masses) if we can achieve extreme performance with the materials and tasks of everyday homebuilding - what a concept hey?

As for the crawlspace, with all my designs I set up a pre construction meeting with the local building inspector to personally review the drawings and details, and to discuss any matters that may seem in question. This question never arose in the conversation and this Is why (I believe). The code requires "Access to all under floor spaces". In this case specifically - there is no under floor space. It is a filled cavity, just like a double wall, or unvented roof assembly. If questioned that would have been my logic. I find it interesting, many people condemn Building inspectors, I actually find them quite genuine and interested when we meet. Giving them a chance to do their job - makes them very reasonable people. It's when you go in there with a bullying attitude - they bully back.

Again thanks for your alignment in thought
Steve


7.
Nov 12, 2016 10:42 AM ET

Martin, Dana, and John,
by Steve Baczek

I agree with your thoughts initially - but I properly vetted the wall section thru numerous colleagues and 3 different WUFI runs by three different individuals. One of which sells VR membranes. Based on their feedback the assembly, although maybe close, did in fact meet their level of scrutiny. The one gent said. "I'd love to sell you a VR, but it doesn't seem to be required here, although I sell it to you anyway". We do have a low-perm paint on the interior wall.

We didn't monitor the interior of the assemblies, but we did monitor the interior conditions for the first year. The RH held in the low 30's pretty consistently.

That being said - the next passive House I did - literally a mile away from this one, we elevated the CCSF to 4" instead of the 2". Both homes have a completely vented 3/4" rainscreen. The above house achieved .55 ACH 50 Pa, and the subsequent house achieved .45 ACH 50 Pa. Both with continuous mechanical ventilation of course.

Our understanding of what we do is an evolution, on the front lines we need to make our best informed decisions because my clients actually want to build something, not talk about it. I greatly appreciate your inquiries as it keeps us all in check.

Steve


8.
Nov 12, 2016 1:48 PM ET

4" spray foam
by Charlie Sullivan

Moving to 4" of spray foam mitigates the moisture issue, but makes the global warming impact worse. If your clients are climate change deniers who care only about the energy cost savings and about avoiding reliance on oil imports, that's an appropriate design, but if they are motivated in part by climate impact, I don't think that's a responsible choice, especially if your clients are not nerdy enough to understand the issues involved in the different blowing agents in different foams.

You could avoid that problem by using the new Lapolla 4G closed cell spray foam that using a new blowing agent without a low global warming impact.

Similarly, substituting EPS for the XPS on the slab would be a good idea.


9.
Nov 12, 2016 6:07 PM ET

Response to Charlie
by Steve Baczek

Charlie, I agree and that is why I was one of the first adopters of the Lapolla 4G product with a GWP of 1. I've been using it exclusively since it was available. I work very closely with my general contractors and the sub contractors and scrutinize all choices, even if we have just finished a project with them. I try to ensure our decisions of date are the latest/best decision available. To further that note, on the project written about above we attempted to use a new soy based product that failed miserably. We were fortunate to catch the failure before we buried it. As for my clients, they are very intelligent, successful people that I work in close collaboration with to identify all aspects of our design to vet the best possible solution at the time - "nerdiness" not required!


10.
Nov 12, 2016 6:33 PM ET

Response to Steve
by Charlie Sullivan

That great to hear. Sorry I assumed otherwise.

If I had had an architect like you I might never have needed to become an energy nerd!


11.
Nov 12, 2016 9:21 PM ET

Response to Charlie
by Steve Baczek

NP Charlie, our industry is a tough business that is challenging everyday. Regardless we have to rely on each other to promote our industry's best effort!!!


12.
Nov 12, 2016 11:52 PM ET

Window flanges as air barrier
by Brian Knight

Great article, comments and answers! Especially appreciate including some of the math for the passive solar influences.

I know that window flanges contribute to airtightness, but feel the industry should view them mainly as bulk water barriers and not air barriers. Most window manufacturers rightfully require the bottom flange be unsealed. This means air has a pathway into the entire cavity around the window between framing and jambs. I think that window air barriers are best handled entirely from the interior. Treating flanges as air barriers will lead to even more bottom flanges getting caulked and taped, something that's already happening much too often.

Curious of your thoughts on this Steve, and please keep sharing your projects with us.


13.
Nov 13, 2016 3:49 PM ET

Response to Brian Knight
by Steve Baczek

Hello Brian, good thoughts man - what's interesting is that I used to be a "flange" fan, but with some extensive use of some of the European models and most recently some absolutely beautiful Menck windows made here in MA, I am really liking the advantages of "flangeless". First they allow me to put the window in the center of the thicker walls. Doing this offers some nice aesthetics from the exterior. it also offers some nice protection for the window head which I believe is the weak link to window success. In most of my work our team is striving for outstanding airtightness. This goal is inherently forcing us to treat windows more as a barrier system, rather than a managed system. I guess it may be more of a hybrid barrier system as we do have some aspects of the install that anticipate water challenges from a management view. Typically I tape the windows inside AND out. The outside tape has a much higher permeance to allow doe some limited drying should some water get in. The trim, sill, exterior finish all have nice easy pathways for water to travel away from and by the window limiting any challenge. A couple other things I like is to protect the windows. If I don't let the window see water then it can't be challenged. The house above has some 23 windows and doors in it with only two od them not under a 24" overhang. To me that's money in the bank!!! and the best approach to water management.
Thanks for the thoughts Brian - enjoy the day
Steve


14.
Jan 26, 2017 3:29 PM ET

Edited Jan 26, 2017 3:30 PM ET.

Floor Design Question
by Gordon Franke

I'm interested in doing a floor similar to the one above but without foam. Will be in a renovation of an 1800's cottage with a "crawlspace" maybe one foot deep. Instead of digging it out into a normal crawlspace and then dealing with sealing it and framing across it, the solution the contractor and engineer came up with was to pull the floor up, lay down a concrete slab, then frame the new floor to sit on the slab.

I was thinking of insulating the framed space between the slab and the subfloor with Roxul batts (trying to avoid using foam so no under-slab insulation).

The question is, without foam, could such a design result in condensation on the slab leading to mold or rot?

I think the answer is no, as long as the foundation walls/slab edges are insulated from the exterior (say with R-12 Roxul board), but I am not sure.

I assume the insulated slab temperature would hover around the average soil temperature for my area which is 56 degrees F (humid zone 4).

If indoor humidity is controlled with a dehumidifier to a max of 40%, in winter if the indoor temp is 70 then the dew point will be 45, which is much lower than the hypothetical slab temperature of 56. Of course the slab might be a little colder that time of year, but the indoor air is also often drier than 40% in winter.

In the summer if the indoor temp is 76 then the dew point will be 50, which is much closer to 56, but if the slab temp varies with the seasons at all it will be on the high side of 56 that time of year.

If the foundation walls/slab edges were not insulated from the exterior then I would expect them to be colder and possibly go lower than the dew point.

Any feedback on this analysis/strategy would be appreciated.


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