passivhaus in Austin, Texas

Wandered over to the eastside today to visit with Nicholas Koch. A trailblazer, Nicholas is building the first passivhaus in Texas (that's "passive house" in Texan) and one of the first few passivhäuser (the proper plural of passivhaus) in the hot and humid southern states. A work and experiment in progress, Nicholass project is technically a remodel, but its one serious remodel: most of the house was taken down to its bones and redone. Originally, Nicholas planned on razing the house and building a completely new one, but asbestos issues made it more cost effective to cocoon the afflicted siding untouched within the walls of the new house. As a bonus, it added (a little) R-value!

The walls of his haus are quite thick. From the inside out: ¾ inch sheetrock, 2x4 wall filled with cellulose insulation, 3 inch gap filled with cellulose for a thermal break, OSB plywood for a vapor barrier, another 2x4 wall filled with cellulose insulation, ¾ inch plywood or real wood, asbestos siding, ¾ inch XPS foam, and cement board. Eventually the cement board will be covered with stucco.

The place seems to be working quite well. I walked in as the day started flirting with 95 degrees Fahrenheit, and the house was comfortable and cool. The house also sports a heat-sink water heater (helps to cool the house in the summer), an energy recovery ventilator (UltimateAir RecoupAerator), and a ¾ ton mini-split to cool 1,800 square feet. Nicholas is still experimenting with distributing air in the house: push or pull? Push the air into the bedrooms or pull air out via the ventilator and let cool air flow in.

Nicholas has a highly insulated floor (R30) because his place is pier and beam. Another house hes helping to design will have a concrete foundation and no need for crazy-thick insulation, which works against cooling the home in our getting-more-brutal-by-the-moment central Texas climate. In colder climates, passivhäuser are real touchy about breaking the thermal envelope and letting heat go. In hot and humid climates, you have to get the heat and moisture out. Along those lines, Nicholas vents his bathrooms (with timers) and his stovetop to the outdoors. He also plans to put in an outdoor kitchen (to control heat generation inside the home) and an outdoor washer and dryer (dryers suck up a bunch of your preciously cooled air, heats it, and then chucks it out the exhaust). For windows, he used Inline fiberglass with doublepanes from Canada, in large part because they test their products so you know how energy efficient they really are. Theyre nice and solid (and energy efficient). Hes now selling them locally. Nicholas is still working on the house, with plans for front and back porches and a 5,000 gallon rainwater harvesting system in addition to the stucco.

In addition to the home tour, Nicholas also gave me a quick tour through the Passivhaus Institutes three-dimensional-heat-transfer-modeling-in-a-spreadsheet spreadsheet as well as a one-dimensional heat and moisture cladding simulator called WUFI. Darn cool stuff about hot stuff!

Nicholas, via his company Equitable Green Group, offers passivhaus design consultation, passivhausish design consultation (hes not so orthodox as to hoot and holler at your home plans Passivhaus or Nohaus!!!), construction consultation (say on overseeing and QAing the building envelope), or full-blown general contracting. And hes a nice and knowledgeable dude to boot.

notes on the austin passivhaus:

· Built by Nicholas Blaise Koch, Equitable Green Group

· 1,800 square feet

· Inline fiberglass windows, double-paned (Canadian)

· Insulated with cellulose, used OSB plywood for vapor barrier

· 2x4 wall, then 3 inches, then another 2x4 wall

· 5/8 inch sheetrock

· ¾-inch XPS foam on outside

· R-30 for floor (pier and beam) and ceiling, R-60 for the walls

· UltimateAir RecoupAerator energy-recovery ventilator

· 3/4-ton mini-split (oversized)

· hybrid heat pump for water heating

· $80 to $100 per square foot

· bathrooms and range vented to the outside

· Dryer planned for outdoor back porch

ACHy breaky heart: Air changes per hour and understanding leakage language

ACH stands for Air Changes per Hour and is a measure of ventilation. A house with an ACH of 1.0 means that the amount of new air coming into the home over an hour is equal to the total volume of air in the home (1.0 multiplied by 100 = 100 percent air change per hour). An ACH of 0.5 means that the amount of new air coming into the home over an hour is equal to 50 percent of the total volume of air in the home (0.5 multiplied by 100 = 50 percent air change per hour). Note that an ACH of 1.0 doesnt mean that every molecule of air has been replaced in your home (its difficult to flush out them corners and closets). In a well-ventilated space with an ACH of 1.0, only about 63 percent of the actual air in the home will get flushed and replaced.

ACH is generally determined with a blower test. For a blower test, the house is closed up (windows and doors shut) except for one of the doors, which is sealed up with a blower. The blower blows air into or out of the home until the pressure difference between indoors and outdoors is 50 pascals. A pressure of 50 pascals is equivalent to the pressure if a 20 mile per hour wind of the pressure of 0.2 inches of water. In other words, not much, but enough to push air in and around your house.

As mentioned, the pressure difference used in blower door tests is usually 50 pascals; however, different pressure differences are sometimes used. Because air changes per hour depend on the pressure difference, its important to know what the pressure difference is. Along those lines, folks will tack a number to the end of ACH (for example, ACH50 or ACH25) to note the pressure difference used during the test. ACH with no number after it (sometimes referred to as the natural ACH, something Ill refer to as ACHnat) reflects air changes per hour at ambient pressure; however, to confuse things, note that some folks are referring to ACH50 when they mention ACH. To get a rough estimate of ACHnat, divide ACH50 by 20. The actual conversion is not so simple.

Newly constructed homes should have an ACH50 of less than 8.0. Older houses tend to have an ACH50 between 10 and 20 and even higher than 20. A house with an ACH50 less than 5 is considered tight, between 5 and 10 is considered moderately sealed, and greater than 10 is considered leaky. For a 1 to 3 star rated home, the City of Austin expects ACHnat to be less than or equal to 0.65. For a 4 and 5 star rated home, the city wants to see ACHnat less than or equal to 0.5. The city will grant a homeowner/builder extra points in its green rating system if the house achieves an ACHnat no greater than 0.25. Passivhaus standards require ACH50 to be less than 0.6, which is approximately equivalent to an ACHnat of 0.03.

Air changes per hour is a double-edged sword. On one hand, you dont want your ACH too high because your house will leak like a sieve; a big deal if youre paying to heat or cool the place (I had a neighbor once describe their 100 year old house as a giant crack). On the other hand, you dont want your house too tight otherwise air quality will suffer.

One standard for ventilation states that ACHnat + ACHmech should not be lower than 0.35 where ACHmech is mechanically induced ventilation. A more modern standard (ASHRAE 62.2-2007 and 62.2-2010) used by the city of Austin is a function of square footage and number of bedrooms (which is used as a proxy for number of people):

ACHvent = [(total square footage/100] + (number of bedrooms + 1) X 7.5 cfm X 60]/(volume of house)

For a 2,500 square foot home with 10-foot tall ceilings and 4 bedrooms, ACHvent is 0.09. Im sorry, but that seems pretty golldarn low. This calculation is heavily influenced by the number of bedrooms (for a 2,500 square foot 4 bedroom house, the square footage only increases the ventilation requirement by 1.1 percent). Furthermore, ACHvent decreases with increasing volume of the house when it seems like it should be the opposite (note that I modified the standard slightly by converting it to an ACH which introduces the volume term; nonetheless, even with my number molesting, the standard doesnt take into account house volume). A table that ASRAE presents with its standards provides more realistic numbers, such as an ACHvent of 0.23 for a 3,000 square-foot house. I suggest using the table instead of the equation (Google ASHRAE 62.2-2010 to see the standard).

ACH also shows up with respect to bathroom vents. The City of Austin recommends a bathroom vent capable of an ACH of 8.0 to 12.0 and having that vent run for 20 minutes after use of the bathroom to remove heat and humidity.

I researched and wrote this up after being confused about passivhaus and City of Austin ACH standards. My confusion all boiled down to pressure (natural versus 50 pascals). With everything all cleared up now and Blaise being my homeboy, my heart no longer ACHes








Cool House Tour 2011

The Austin Cool House Tour, hosted by the Texas Solar Energy Society and the Austin Energy Green Building Program, is coming up next weekend, June 5th, from 10 am to 6 pm. Fifteen bucks gets you access to 16 homes with various green features, including solar, rainwater harvesting, advanced insulation, and advanced water conservation. My bride and I have gone on this tour before and thoroughly enjoyed and been inspired by it. Given that our project is getting closer to reality, well have lots of questions


Dollars and Sense: Part 2 of 4: What you can build

Previously, I posted about defining your finances. The second budget perspective you need to investigate is what you can build with your budget. Doing this requires looking at what it will cost to bring your house home. The primary categories here include:

- lot/land

- carrying costs

- loan costs

- soft costs

- hard costs

- landscaping

- gewgaws

- contingency

Land is easy enough: How much did your land/lot cost, including closing costs? or: How much are you budgeting for your land/lot? The other thing to think about is carrying costs for the land. If you financed the purchase of your land, how much will you spend on payments until the house is completed? Also, how much will you be spending on property taxes? If these payments are coming out of your cash reserves, then you need to account for them as well.

The construction loan includes the closing costs and the carrying costs. For closing costs, you can assume 4 percent of your anticipated loan amount. Carrying costs are what you need to pay on the construction loan during the build. Construction loans typically work on the draw: You only pay interest on the money you draw from your loan. Depending on your finances (and your loaning bank), you may be able to carry these costs yourself. However, banks may require that the carrying costs, called the "interest reserve", be built into the loan. To estimate these costs, multiply your construction loan amount by 0.60 (to account for the draws over time) and then by your interest rate.

Hopefully you are planning on and can get a single-close construction loan. With a single-close construction loan, you close once and have the construction loan and the final financing of the house taken care of in one two-birds-with-one-close throw. In a double-close situation, you close on the construction loan and then, when the house is finished, you finance the whole project and close again. If you plan on a double-close, you need also plan for those additional closing costs.

The division between soft costs, hard costs, landscaping, and gewgaws (my term for nonstandard options for the project) will vary depending on who (architect and/or builder) you work with; just make sure you and the folks you work with understand what is where. Soft costs generally include everything that doesn't include actual construction costs of the house such as architecture fees, engineering fees, permitting fees, surveys, geotechnical analyses, and connection fees. You can assume that soft costs are about 10 percent of hard costs.

Speaking of hard costs, these are the actual construction costs of the house. These costs generally include time and materials to build the house, contractor overhead, contractor profit, driveway, attached decks, garage, and walkways. At this early point in budgeting for a house, it's best to assume a price per square foot appropriate for your (hoped for) finish out. If you know what size house you'd like, figuring your hard costs is simply multiplying the price per square foot by the total square footage.

As you saw above, some of the landscaping costs may be built into the hard costs: walkways, driveways, and attached decks. Talk to your builder/architect to be sure. But outside of those costs, turf, fences, beds, trees, shrubbery, unconnected decks, paths, and bird feeders are part of the landscaping budget. Depending on what you are planning, budget 5 to 10 percent of construction (hard) costs on scaping your land.

Gewgaws are what I call those extra items that go above and beyond the costs of a standard build: for example, a pool and/or hot tub, a rainwater harvesting system, solar panels, helipad. You'll just have to list these things out and estimate their cost. Don't forget installation!

Contingency is the planned pot of money you can dip into when your estimates lean in the wrong direction. Based on what I’ve read, 99 out of 100 custom homes go over budget for various reasons, so if your budget is tight, planning for unplanned expenditures is wise. And if it goes unspent by the end of your project, you can use it to upgrade fixtures, buy a sofa, or take your partner out for a cherry limeade. Our architect has pointed out that a contingency, especially if it is large, steals square footage from your project. True dat. Ultimately the size of the contingency is a function of the tightness of your budget and how well you deal with uncertainty. For us, at this point in the project, there’s a good deal of uncertainty. We don’t know how much equity we’ll get out of our current house or “exactly” how much the house will cost to build. In our case, the contingency may decrease as our certainty increases.

The bank, recognizing the fickleness of best laid plans, may require a contingency when agreeing to give you a construction loan. In this case, they will add 5 to 10 percent to the construction loan amount to ensure you don’t run out of money. But remember, if you spend this money, you’ve increased the total cost of the project and will have a higher house payment. Nonetheless, it’s good to have that money there.

There are other funds that can be used as a financial fallback if things start to go awry. For example, landscaping and gewgaws can be jettisoned from the budget later in the building process to finish the house. It’s good to know what Plan B (and C) are when building.

Because so many of these costs are dependent on what the hard costs are, I've put together a spreadsheet to calculate, ultimately, how many square feet your future house could have. What’s cool about this spreadsheet is that you can see how different decisions impact the size of your house. You want a pool? Estimate the cost, add it to the GewGaw list, and watch your square footage evaporate. Aunt Mary leave you 50 grand in her will? You can quickly see how a financial windfall, however sadly obtained, increases your square footage.

Note: If you use this spreadsheet to make decisions, use it at your own risk. I don't guarantee its accuracy, although I sure hope it's accurate because we're using it to make decisions. I'll keep tinkering with it to increase it's usefulness and usability and will post comments below about any upgrades or errors. If you find something wrong (or have a suggestion), please let me know!

Coming soon:

Part 3: What your location will support

Part 4: What the bank will give you



The passivhaus movement started in Germany in the late 1980s with the goal of building hyper energy-efficient buildings. The thermal image above (from Wikicommons) shows the results. The house on the left, leaking heat like a sinking Titanic heat island, is a standard built structure while the house on the right follows the tenants of passivhaus. A passivhaus tends to have (1) good insulation (U-value lower than 0.15 watts per square meter per Kelvin degree [R-value higher than 44.5 feet squared degree Fahrenheit per btu]; see note at end of post on conversions and use), (2) southern orientation and shade considerations, (3) energy efficient windows (U-factor lower than 0.80 [R-value higher than 7]), (4) tight building envelope (air changes per hour at 50 pascals less than 0.6) but forced ventilation (of at least 40 percent of the haus volume per hour) for air quality, (5) passive pre-heating of fresh air, (6) a heat recovery ventilation system, (7) hot water from solar or heat pumps, and (8) energy efficient appliances. Overall, the haus must not use more than 15 kilowatt hours per square meter per year (1.4 kilowatt hours per square foot per year) for heating and have total energy consumption less than 120 kilowatt hours per square meter per year (11.1 kilowatt hours per square foot per year).

Probably the most atypical aspect of a passivhaus is its aggressive insulation. The walls and roofs are thick with insulation and even the slab or basement is heavily insulated. Consideration of passive solar is taken seriously. The windows are highly energy efficient and often triple paned (although double-paned will work). And the haus is nearly airtight.

A home typically has 10 air changes per hour with a 50 pascals pressure difference between the indoors and outdoors. Under normal pressure differences, this amount to half the air being changed every hour. A passivhaus has an air change per hour with a 50 pascal pressure difference of 0.6, 94 percent less than a typical home. Under ambient pressure conditions, a passivhaus, without mechanical ventilation, will only change out 3 percent of its internal air per hour. If the air change per hour falls below 0.4, air quality in the home suffers (the air becomes stale). Therefore, a passivhaus will vent out internal air and bring in external air to achieve an air change per hour of 0.4. You may be thinking to yourself: Say what?!?!!? I just made my house super airtight and now Im going to leak it!!! However, unlike a normal home where that fresh air is coming from wherever the homes perimeter has been compromised, a passivhaus knows where the air is coming from and can filter and pre-condition it.
When a passivhaus exhales, it can harvest the heat from that air and feed it back into the home. A passivhaus in Germany is typically just heated by body heat and heat from lights and appliances. Thats crazy cool (or hot [or something]).

Most of the 25,000 houses that meet passivhaus standards are built in Germany and Scandinavia with only a couple dozen in the United States. Note the northern climatic bias. So how do you build a passivhaus in a hot and humid climate like Austin where cooling is more of an issue? Instead of a heat recovery ventilation system, warmer climates can use an energy recovery ventilation system. This system removes heat and humidity from the incoming air in the warm season and then acts as a heat recovery ventilation system during the cool season. The energy recovery ventilation system doesnt cool the incoming airit preconditions it so the house can get by with a smaller air conditioning system. A passivhaus has been built in Louisiana, and a passivhaus remodel/rebuild is being finished in Austin (see links below). The Passivhaus Institute US has a spreadsheet (for a less than nominal fee) that a designer can use to calculate the overall efficiency of a potential hans.

Proponents suggest that building to passivhaus standards only adds 10 to 15 percent to standard building costs but with a 90 percent reduction in energy use. Hmmm. That doesnt sound too bad. Interestingly, many examples of passivhau have lots of windows, so such a house doesnt have to be a cave. And the results are quite impressive:

(chart from the Passivhaus Institute)

Some links:

Some building suppliers:

Other links:

Note: R-value is the inverse of U-value; however, both values, although generally reported without their units, are unit dependent with U-values in metric units (watts per square meter per Kelvin degree) and R-values in English units (feet squared degree Fahrenheit per btu). To convert U-values to R-values, take the inverse of the U-value and multiply by 5.68. Your metric U has now been English Red!


Dollars and sense: Part 1 of 4

A critical first step in building a house is building a budget. A budget helps you answer the question: What can I afford to build? You need to look at your budget from four perspectives:  Your finances, what you can build, what your location will support, and what the bank will give you. Yes, these four perspectives intertwine, but it's important to consider them individually as one perspective may be whispering "Yes! Yes!" while another is shouting "No!!!"

Part 1: Your finances

The financial perspective is simply answering the questions: How much can you spend? and What financial fallbacks do you have if everything goes to Hades in a hammer holster? On what you can spend, you need to identify what cash you are willing to bring to the table(saw), what equity, if any, you have, and how much of a loan you can afford or are willing to apply for. The cash you can bring to the table is simply what you have saved that you are willing to allocate to the project. If you have nothing, you are already in trouble. You are going to need lots of cash for a down payment on land, to pay up front pre-loan costs, and to support an efficient and timely build.

On estimating equity in your current home, be conservative and be sure to subtract off the closing costs and any other costs associated with selling. You may even want to factor in moving costs if you are so (ahem) moved. When estimating equity, you can look to see what your tax appraiser thinks your property is worth (generally a conservative estimate), peruse the paper or open houses for comparables or, if you already have an agent, get actual comparable data. If you plan on selling later in the process, closer to the completion of the new house, you'll also need to consider what the market might look like then, generally a year down the road or more.

I'm generally shocked at how much a bank will loan you based on your salary (seems to be about three times your annual pay). If you are young and/or expect your salary to grow, you may want to stretch a bit on the loan. If not, think carefully about how the house payment impacts your lifestyle. And if you are in a high property tax state like Texas, be sure to also factor in prop taxes which can easily add 500 to 1,000 dollars a month (!!!) to your monthly home costs.

It's also a good idea to consider your financial fallbacks in case things go awry during your build, such as unexpected increased costs. These fallbacks may be other savings you are not willing to put toward the project (but could if forced) and retirement savings in 401Ks and whatnot. Sounds scary (I would never recommend robbing your retirement), but you need to know. Having no financial fallback is scarier.

So your financial perspective looks like this:

- cash to apply to the project,
- equity you can apply to the project,
- loan amount you are willing to pursue, and
- amount of financial fallback funds.

The first three items added together is your project budget. The fourth item is insurance.