howz that energy efficiency coming along?

A reader recently asked how the house is doing energy-wise. Good question! I still think it's premature to pass full judgement on how we did (I feel like we really need a year's worth of data and, given some issues we've had with the HVAC system, perhaps at least two years), but what the heck, let's see how we're doing so far...

conversions

Since we use multiple sources of energy, the first thing we need to do is combine the energy from electricity and natural gas into one number. That requires (in the U S of A, at least) converting energy use into BTUs (British Thermal Units):

1 cubic foot of natural gas equals 1,020 BTUs
1 kilowatt hour of electricity equals 3,412 BTUs

Note that natural gas may be reported as CCF: 1 CCF of natural gas equals 100 cubic feet of natural gas which in turn equals 102,000 BTUs.

our house

We currently have data for a seven month period from July of last year through January of this year. Using the above conversions, calculating an average monthly energy consumption per square foot (based on a 2,281 square-foot house) and then multiplying that by 12 to get a yearly number, we come up with a projected annual energy consumption of 28,876 BTUs per square foot.

how we compare

The plot below from U.S. World News shows millions of BTUs per occupant per year (the blue bars) and thousands of BTUs per square foot per year (red bar) and trends over time in the vintage of the housing stock. Not surprisingly, the trend is down as houses get more efficient. Since a metric based on the square-footage is a better metric of how a house is performing, that's what we'll focus on in this post.

(click on that sucker if you want to see it BIG)

For the most recent home construction (2000 to 2009), homes have had an energy consumption of about 37,000 BTUs per square-foot per year. Our place comes in at about 29,000, about 80 percent of the energy consumption of an average home built last decade. Nothing spectacular, but not bad.

According to the Department of Energy's 2010 Buildings Energy Data Book, an average American home (all home stock) used 94.5 million BTUs in 2005 (Table 2.1.10 of that report). Ours is currently at 66.1 million BTUs, about 70 percent of the average American home. Focusing only on the West South Central part of the country, the average household use was 82.4 million BTUs in 2005 with 56,600 BTUs per square foot. Compared to the average home in our part of the country, we use about half the energy per square foot.

Looking back to our old house built in the 1880s (and doubled in size to 1,100 square feet in the 1910s), it used about 83,000 BTUs per square foot. That means our current per square foot usage is only 35 percent of what we were using before! Despite having a house more than twice the size of our original place, we use about 30 percent less total energy than the old house.

The downtown apartment? We used about 12,000 BTUs per square foot. (Big giant envious) gulp. However, we had neighbors on four of six sides warming and cooling our sides. Nevertheless, apartment/condo living has a much smaller energy footprint.

Although we toyed with trying to build a passivhaus, we ultimately abandoned that (potential) goal. Nevertheless, how do we compare to that Germanic ideal? Annual energy use in the passivhaus standard for heating and cooling cannot exceed 4,755 BTUs per square foot; we are currently at about 11,000. Looks like we missed that one... According to passivhaus standards, total energy usage should be less than 38,000 13,300 BTUs per square foot. [See notes at end. I wrote Passive House US about the mistake on their site. No response... Seems pretty basic to get that number right, doesn't it?] That kinda shocks me and makes me wonder if the hausers made a conversion error somewhere (otherwise, what the hell are those Germans doing behind those thick walls and tiny closed curtains?!?!). I find this hard to believe because that would mean that half of newly built homes in the United States (assuming a normal distribution...) meets this particular passivhaus standard! And ultimately, who gives a hoot on how much energy you use for heating and cooling: Total energy consumption should govern the standard.

Assuming the hausers know what they are doing, we totally miss the passivhaus standard for heating and cooling but totally kill it on total energy consumption. Passivhaus in spirit?

the future

We've had some issues with the HVAC dumping air upstairs that surely caused us to use more energy than we should have (down to 60 degrees in the summer, up to 98 degrees in the winter!), so we're working to address those issues. That should reduce overall energy use. We're also in the process of replacing our halogen bulbs with LEDs (and yes, Green House Lady, we're installing a timer for our front "porch" lights...). We also plan to employ our energy measurer to see where there may be phantom power issues. Our current baseline power use (that is, power use without heating and cooling) is about 700 kilo-watt hours per month whereas it was about 270 in the apartment and 650 to 900 (growing over time...) in the old house.

a vaguely fascinating side point...

I was surprised at how high the electric consumption was for December. Our HVAC system uses hybrid heat: electricity for the heat pump as well as gas, and decides which to use. I'm not sure what the algorithm is, but I adjusted the thermostat to only use gas for heat in January. Indeed, electrical consumption was down and gas consumption was up for January resulting in a combined energy bill that was 50 bucks less; however, overall energy consumption (expressed as BTUs from electric and gas) was up! That leads me to think that the algorithm is optimized to minimize energy consumption and not cost.

postnote

March 10, 2014:

Here's a screen capture from the Passive House Institute US:

It appears the total energy usage of no more than 38,000 BTUs per square foot per year is accurate. Holy guacamole!

However, Building Science Corp reports a whole house energy consumption goal of 13,300 BTUs per square foot per year, which makes a lot more sense as a passivhaus goal. In that case, we're a wee bit beyond twice the goal.

The Foam Ranger

Our house, as currently designed, will have two by six exterior walls with spray foam between the studs, and that's it (not that that's shabby: most new homes have two by sixes with fiberglass batts). Having spent a bit of time reading about passivhaus and Build America, we wonder: Is that enough? We asked the architect, and he said that spray foam coupled with great attention to air sealing hits the sweet spot. Nonetheless, he has another house being built that's being clad with two inches of foam. If we wanted to, we could clad our house with 3/4 inch or an inch of foam. If we do that, that'll set us back 75 cents per square-foot of wall and roof area. I guesstimate that we have 8,000 square feet of wall and roof coverage, so that, with the builder's fee, would set us back about $10,000 to $12,000. hmmm...

Austin's building code, based on the 2009 International Building Code (but with a few more-strident amendments), calls for R-15 walls (2009 IBC calls for R-13), R-30 in the attic (but can be R-21on the roof if the mechanical system is in the thermal envelope), and windows at U-0.51 (2009 IBC calls for U-0.65) with SHGC-0.30. John Umphress from the Austin Energy Green Building Program is quoted by Matt Risinger, a local green builder, as saying that R-21 is the point of diminishing return for cooling and R-38 is the point of diminishing return for heating (I assume he means [as quoted] "cooling climate" and "heating climate" otherwise that does't make sense to me...).

This map of climate zones is from a document published by the U.S. Department of Energy. The DoE says that we're in a hot-humid climate (yep) and that Travis County, the county of our lot, is in Climate Zone 2 but right on the edge with Climate Zone 3 (also hot-humid).

Building Science America recommends the following insulation levels for the different climate zones:

For us the Building Science folks recommend walls with R-15, roofs with R 40, and windows with U-0.30  and SHGC <0.30. Given that we're on the edge of Climate Zone 3, we could go a bit better and hit walls with R-20, roofs with R-45, and windows with SHGC <0.25. And here are details of what  the Building Science folks recommend for a house built in Houston.

A dude over at Green Building Advisory recommends walls at 20, roofs at 60, and windows at U-0.33 for zones 1 and 2.

How do all these R values translate to wall construction? Glad you asked!

So here's your standard wall:

2x4 or 2x6 construction, fiberglass batt insulation, exterior sheathing, and housewrap. R value for a 2x4 wall would be about R-10 and for a 2x6 wall would be about R-13.7.

So here's our current wall design:

2x6 construction (this assumes advanced framing), spray foam in the walls, some flavor of sheathing and house wrap. According to Building Science, this wall, using high-density foam (which an R of 5.5 to 6.5 per inch as compared to low density foam at 3.6 per inch), has an overall R value of R-16 (the thermal bridging in the 2x6s take the presumed R from 21 to 16). 

Here's a wall design with foam on the outside:

2x6 (advanced framing) construction, cellulose in the walls, XPS foam sheathing. This wall has an overall R value of R-20 (1 inch of XPS) to R-34 (4-inches of XPS).

Building Science America recommends that the roof be R-40 to 45 for a compact roof. The handy plot below shows the diminishing returns of increasing R values on energy use (at least for Phoenix) and the logic in choosing R-45 for Climate Zone 3.

It's harder to tell what's what on roof construction with this report.  Building Science Corp recommends 12 or 20 inch SIPs (structurally insulated panels) for Rs between 44 or 74 or lots of insulation in a 10-inch engineering truss and XPS (9.25 inches of cellulose and 6 inches of XPS) for an R of around 60.

One thing I don't get with these studies is why the increased insulation values of the envelope don't show up as energy savings:

(figure from this report on a house in New Orleans)

What's up with that? Windows, air sealing, ductwork, and AC have the biggest bang (which explains why Architect 2d is focused on air sealing). The increased insulation seems to have no effect! (Although the benchmark condition was [oddly] not identified: Perhaps they didn't up the R?] A case study for Houston shows a very similar figure but then states in the text that the higher insulation saves 20 percent of the energy budget. Why doesn't it show up in the parametric analysis?

And then there's Peter Pfeiffer, a well known local green architect with Barley-Pfeiffer (sounds like a very special brew or brew pub...), quoted as saying that "once you hit about R-13 [in Texas], you're really reaching a point of diminishing returns" although he goes on to recommend 3/4 inch insulation on the outside of the exterior sheathing in Dallas. Also pointed out was that windows tend to mess up a whole house R value (if your walls are R-40 but your windows are U-0.50 [that is, R-2] that's like having a nice boiling pot with a hole in the bottom).

So what's a couple concerned about insulation to do?

Since I could't tell what was what, I went ahead and worked up some equations to be able to plot heat flow for different construction assumptions for Austin's climate. The equations look like so (in case you're dying to know...):

The top equation calculates total heat flux based on the thermal properties and areas of the walls, roof, and windows and the temperature difference between the indoors and outdoors. In the spring and fall, when your windows and doors are open, the temperature difference is zero, so no heat flow! This equation also shows why folks-in-the-know recommend greater amounts of insulation in the north than in the south: The temperature difference is greater up north. Here in Central Texas, it gets up around 100 F in the heat of the day in the summer and 40 F in the cool of the night in the winter. Assuming the interior is kept at a comfy 70 degrees, that's a delta T of 30 degrees in either direction. Up north, where the winter temps can be at 10 or 20 F (or lower), you're talking a delta T of 50 to 60 (or higher). The greater the delta T, the more insulation you (probably) need.

The bottom equation shows you how to calculate a whole house R value, interesting for seeing how much your windows screwed up your fancy walls (as I've said before: Vampires have the most energy efficient houses!).

Based on the cost estimate from the architect, I figure we have about 450 square-feet of windows, 6,000 square-feet of walls, and 2,000 square-feet of roof. If we build to the city of Austin's standards (where I think we kinda are at the moment), our total BTU (British thermal unit) heat transfer during a 30-degree delta day is 21,607: 6,750 BTU from the windows, 12,000 BTU from the walls, and 2,857 BTU from the roof. (Note that these are heat transfers due to temperature differences from the outside to the inside and does not consider heat transferred via radiation [the sun shining in a window or heating up your black roof] or advection [air gaps in your house or leaving the front door open].)

I plotted the top equation up for different R-values for the wall to see if we could see the "point of diminishing returns". I think you can see that the curve bends over pretty good at about R-10 to R-15. You lower your btu with more insulation (as the passivhausers will point out), but the real bang for the BTU is up front.

For grins, let's plot this stuff up for different temperature differences:

Just as we suspected, there's greater heat flow for greater temperature differences, and the "bang-for-the-btu" inflection point moves toward the right toward higher Rs.

We can also use this equation (assuming walls at R-15) to figure out how changes in roof insulation and window U-values help or hinder (and how much):

Increasing the R of the roof has diminishing returns (in this particular example) beyond R-30 or so. In fact, taking the windows from R-0.5 to R-0.35 has a better overall benefit than tripling your roof insulation! This is pretty cool because you can start to balance cost with benefit here. Does is make more financial sense to thicken up the attic insulation or get better windows? Hopefully your architect has figured out all this hookamaloo out (as it appears ours has).

So where does that leave us? Good question. Although we've focused on R values here, there are other considerations such as air sealing (all these calculations above assume perfect air sealing...), the efficiency of the HVAC, and how good the relatives are at keeping the doors closed during the heat of the summer (ours aren't very good at that...). Our guys (the architect and builder) seem to know what they're doing (they're both very focused on air sealing), so we'll let 'em have their space and ask questions (hopefully not too irritating [clients don't tend to ask about this stuff {and you don't wanna ask too much because architect's time = $}]) to better understand where they're going and why.

first Passive House Alliance--Austin Chapter meeting this Monday!

From Nicholas Koch, President of the Passive House Alliance--Austin Chapter:

This is a free informational meeting for anyone and everyone interested in Passive House hosted at the office of E Green Group.  This informal meeting will consist of a brief presentation, time for questions and discussion, and a social portion with snacks and refreshments.

Monday's presentation will cover:

An introduction to the Passive House building system

Challenges and Benefits of the Passive House Building system in Austin's hot southern climate

Cost Comparisons of Passive House vs. the competition

Join us at 7:30 at 2415 E 5th St. Bldg E.

Thank you,

Nicholas Blaise Koch
President, PHA - Austin Chapter
Owner, E Green Group

Nicholas is the real deal, having built the first passive house in Austin and Texas and one of the first (second?) passive house in the south.

I'll huff and I'll puff and I'll blow your wall down!

The passivhaus movement has gotten me interested in energy efficient wall and roof design. As it turns out, there’s more to a wall than nailing a few two by fours together and covering them with Hardie board. Ultimately, a wall needs to be designed to address expected weather conditions. A poorly designed wall will cost you money in increased heating and cooling costs and could, in wet and humid climates, reward you with a serious mold problem. Interestingly, there is a push and pull between designing walls for cold versus hot climates: design choices for hot sometimes work against choices you would make for cold and vice versa.

Down here in the hot and humid south, we have to mostly worry about heat and moisture. That makes me a little suspicious of the passivhaus standards which are strongly influenced by the climate in their German birthplace. Nonetheless, the goals of passivhaus are admirable but perhaps need to be adjusted for our southern climate.

A great resource I’ve found on wall systems is the Building Science Corporation website. They have a number of free PDFs about various building science topics, including those specific to various building climate zones across the United States. And they have quite a collection of articles on wall systems. They also have a nicely written general article about highly insulated wall systems for the United States. Building America, a program by the U.S. Department of Energy, also provides energy efficient building design suggestions, although they generally have less ambitious goals than other programs. Nonetheless, they have climate-specific resources, case studies, and economic analyses that are quite useful.

So what do these folks suggest for houses built in Austin, Texas?

We are building in Climate Zone 2. For this zone, the Building Science Corp recommends walls of R-15 (R refers to the amount of insulation; the higher the number, the higher the amount of insulation; 3.5 inches of your standard fiberglass batt insulation has an R-value of 11), windows with a U of 0.35 and an SHGC (solar heat gain coefficient) less than 0.25, a roof of R-40, a slab edge of R-5, and nothing for under the slab. A different article about a house built near Houston that strove to meet these standards passed on the slab edge insulation to avoid creating a potential pathway for termites. These R-values are probably lower than those of passivhaus standards, “probably” because passivhaus is about reaching certain energy use goals—the R and U values to get there depend on your house as whole. For comparison, the Louisiana passivhaus has walls of R-28, a roof of R-55, a crawlspace of R 16.5, and windows with U’s in the neighborhood of 0.20. The Austin passivhaus has walls of R-60, a roof of R-30, a crawlspace of R-30, and windows around U-0.25 to 0.30.

Building Science Corp attempts to balance cost with performance (and hand-in-hand with cost is the familiarity of the trades with high-efficient wall systems), hence the overall lower R values and higher U values than passivhaus. Furthermore, the Corp’s guidelines are independent of whole-house performance. As far as a wall is concerned, the amount of window coverage has a huge impact on the overall performance of the wall. For example, the wall for the Austin passivhaus is R-60, but those windows are R-4, greatly reducing the efficiency of the wall and the house as a whole (vampires probably have the most energy efficient homes around…).

I’ve also been following a blog by Gregory La Vardera who has recently been writing about wall systems. Something intriguing that he proposes is horizontally furring the interior side of exterior walls with two by twos. This gives a pathway for pipes and wires without compromising the insulation, and hence the R value, of the outside wall. Apparently, electricians and plumbers are notorious for hacking away at wall systems to get their job done, thereby compromising the energy efficiency of the design. Maintaining the integrity of the wall system is doubly important for homes that require some flavor of vapor barrier toward the inside of the house (typical for colder climates). A vapor barrier with lots of holes in it is hardly a vapor barrier. Fortunately, in hot and humid climates, the vapor barrier is typically toward the outside of the wall system.

I don’t know the gory details of what The Architect has planned for our walls at this point. The outer walls are two by sixes, and we’ve talked about closed-cell spray foam, although sprayed-in cellulose seems to be the preferred choice for greenies (and it’s less expensive than foam). We plan to use advanced framing, which minimizes thermal bridging (pathways for heat to move into or out of your house, usually through the framing). One builder we talked to felt that the roof was the most important feature in our climate and suggested a metal roof painted white to reflect the brutal Texas sun as much as possible. Passivhausers as well as others work to include thermal breaks in wall and roof design, especially in hot climates. Any hunk of wood or metal that extends from the exterior of the home to the interior provides a pathway for heat. A thermal break is generally accomplished by covering the exterior with non-structural insulation sheathing (this can also be accomplished with structural insulation such as structural insulation panels, what the cool kids call “SIPs”).

So, here is what I’ve learned about what should be done in our hot and humid climate:

From Building America (a Department of Energy program that strives to produce houses 30 to 50 percent more efficient than those that meet the 1993 Model Energy Code):

- Slab: Should have 6-mil polyethylene sheeting directly under it to control vapor and provide a capillary break (to prevent water wicking through the slab and into the house).

- Slab: Sub slab drainage should consist of a gravel capillary break beneath the vapor retarder.

- Slab: Don’t use sand anywhere; address differential drying and cracking through low water-to-concrete ratio and a wetted burlap covering during the initial drying.

- Walls: No vinyl wallpaper on the interior of exterior walls; wall coverings need to be breathable.

- Walls: No vapor barriers on the inside of outside walls

- Walls: Backprime all wood used on outside of house

- Walls: Create an airspace behind every outside surface and provide for drainage. This is especially important for brick and masonry.

- Walls: Use either interior gypsum board, exterior sheathing, or both as air flow retarders.

- Walls: Ensure that plumbing, showers, and tubs on exterior walls are properly sealed: These are often where the largest penetrations in the house occur.

- Ceilings/roof: No recessed lighting into unconditioned spaces.

- Walls: No need for a rain screen, but a need for a drainage plane/drainage space.

From Building Science Corporation on building in hot and humid climates:

- Walls: Need to dry to both the interior and exterior.

- Walls: Bricks are “reservoir claddings”: They store moisture. Sun that comes out after a storm can drive that moisture to the interior.

- Walls: The drying potential of a wall decreases with the level of insulation and increases with the rate of airflow.

- Walls: Clad and tape exterior walls with 1 to 4 inches (2 inches recommended for hot and humid climates) of non-structural exterior insulation sheathing (such as expanded polystyrene [EPS], extruded polystyrene [XPS], or polyisocyanurate [polyiso], rigid fiberglass, or mineral wool)

- Roof: 4 inches of nonstructural exterior sheeting, preferably in 2-inch lifts where the lateral seams between sheets are staggered between the lifts.

- Roof/ceiling: Use gypsum board as air barrier; however, that means that there shouldn’t be any ceiling penetrations.

Images respectfully cribbed from Building Science Corporation.

the appliance from hell: the lowly clothes dryer

One thing that’s neat about passivhaus is the unholy fixation practitioners have on heat sources and sinks. Stuff you (or at least I…) don’t think about. Like the thermal impact of flushing a toilet. While that impact is (presumably?) minor, some thermal impacts are not. The one impact that has kept me up at night sweating and shivering in the corner is the appliance from hell, the lowly clothes dryer.

Before considering the lowly clothes dryer in the context of passivhaus, I thought of that white jiggly box in our house as something to (ahem) dry our clothes. You put clothes in, you turn it on, you come back later: dry clothes! Instead, I now see it for what it is: A well camouflaged thief—an embezzler, really—who skims off the top and steals your money.

The crime the dryer commits is multi-faceted. First, it pilfers your cool air. Just plumb takes it. I was enjoying that cool air, fer cry eye! And I paid to cool it! That's right, the dryer pulls air from its surroundings to do its deed. Second, it then uses valuable electricity to warm that cool air to drying temperature. Begin sobbing here. And third, it ejects that air to the outdoors, creating a negative pressure in the house through which hot air seeps back in as make-up air from Gawd knows where, which I then have to cool so the dryer can warm it up again. It’s a miracle I’ve gotten any sleep at all lately!

Amazingly, there isn’t a dryer on the market (that I am aware of) that uses outdoor air to dry your clothes unless your dryer is already outdoors such as in the garage or on an unconditioned back porch. The dryer in our current house is on our back porch. I figured we were low-rent: turns out we are passivhaus pioneers! Tellingly, the local passivhaus dude plans to put his dryer on a back porch.

There are options. Such as the helpful suggestion by the passivhaus creator, Dr. Wolfgang Feist, to dry your clothes on a clothesline. And while I find drying clothes on a clothesline vaguely romantic (we indeed do it from time to time), it’s not a good all-the-time option. There is also something called a “drying closet” which is essentially an enhanced indoor clothesline in a rather large box. There are also condensing dryers, built for cases where there is no place for an exhaust. However, reviews of these critters are mixed with none other than the Canadian government advising against them.

Our current dryer had to get serviced recently, and I took the opportunity to gaze at its innards to see if I could connect hosing to supply it outside air. The repairman thought I was nuts (“Dude: It doesn’t use that much air!” “But, dude, have you seen that sucker blow! It‘s 200 cfm!!!”). I’m thinking we’ll chose our next dryer based on whether or not we can make this modification.

Given that it seems to be getting hotter around here (two days of record-setting temps at 105 degrees…), I’m hoping that manufacturers will start making appliances for different climates. For example, having a fridge with a condenser that could be placed outside (we don’t put our AC condensers in the house, now do we?). Until that time, it appears you’ll see me stumbling about town with dark bags under my eyes (and higher electric bills).

the windows are the windows to your soul

Been thinking about and struggling to understand energy efficiency and windows, which ties into broader subjects related to insulation, energy efficiency, and climate (more on that later…). In short, as far as I can tell, insulation, window characteristics, and wall (and roof) design depends on where you are at climatically. At first blush, this seems obvious, but it becomes less so when you wade into the details. For example, one would think that whatever is good for keeping heat inside in a cold climate would work just as well for keeping heat out. But that ain’t so. As near as I can tell, it all boils down to the (apparent) fact that it is easier to control heat than it is to control cool. But I’m still thinking about that…

My handwringing over this comes from our starting-point decision to use storefront aluminum windows for the house. Storefront aluminum windows are sleek and durable, and they sure look mighty fine in a modern house. However, passivhausers would rather poke their eye out with leaky ductwork than put aluminum windows in their houses. And for good reason: Aluminum is a great conductor of heat, even better than steel. Thermally broken aluminum windows are available, but even these windows don’t approach the overall energy-efficiency of wood, fiberglass, and vinyl windows.

One thing that has made it difficult to achieve an apples-to-apples comparison between different window designs is the lack of comprehensive data. The passivhaus subculture and the window manufacturers that cater to that subculture know how to do it. To do it right, you need to know the properties of the glass as well as the properties of the frame. Pretty basic stuff. However, most window manufacturers don’t report this data. Instead, they report numbers for a complete window of a seemingly random size. I say seemingly random because some manufacturers, if you read the fine print, test a ridiculously oversized window, presumably to minimize the less-than-desirable thermal properties of the frame. One manufacturer appeared to size its test windows to achieve a certain U-value. No apples to apples there even among windows of the same manufacturer (although it creates the mathematical possibility of teasing out an estimate of frame properties…). Some manufacturers only report the numbers for the glass, which is unfortunate and horribly misleading because the frame is often the thermal weak point.

So what’s a tossing-through-the-night-because-I’m-worried-about-them-darn-windows dude or dudette to do? Ultimately, I climbed the mountain to visit the Efficient Windows Collaborative. These fine folks show comparisons of energy costs among windows of different designs in different climatic settings across North America. What I get from poking around this site is that in Austin’s warm climate, having a low solar heat gain coefficient (SHGC; a measure of how much solar radiation comes through the window to warm your house) is the most important factor. In a cool climate, you want a higher SHGC because you want the sun to warm your house. In a warm climate, you want a low SHGC because the solar radiation is your thermal enemy. The U-value is a measure of non-solar radiation heat flow through your window. Having a low U-value appears to be less important in a warm climate than in a cool climate (see my graphic modified from the Collaborative above). A thermally broken metal frame is less efficient, but “only” results in heating and cooling costs some 5 percent higher than non-metal windows for San Antonio (some green-tinged folks would cringe at me using the word “only” here; see my modified graphic above).

The Efficient Windows Collaborative has a window selection tool to help folks evaluate different windows and identify manufacturers that meet your goals. They also recommend checking out some free software from Lawrence Berkeley Labs if you want to do your own thermal modeling. I don’t know about you, but that sounds hot!

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, Nicholas’s project is technically a remodel, but it’s 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 he’s 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. They’re nice and solid (and energy efficient). He’s 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 Institute’s 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 (he’s 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 he’s 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 doesn’t mean that every molecule of air has been replaced in your home (it’s 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, it’s 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 I’ll 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 don’t want your ACH too high because your house will leak like a sieve; a big deal if you’re 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 don’t 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. I’m 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 doesn’t 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…

Sources:

http://www.pct.edu/wtc/docs/articles/Blower-Door-FINAL.pdf

http://www.builditsolar.com/Projects/Conservation/Southface22blowdoor.pdf

http://www.bae.uky.edu/energy/residential/guide/guidehtml/guidep20.htm

http://www.engineeringtoolbox.com/air-change-rate-room-d_867.html

http://www.ashrae.org/technology/page/548

passiv(aggressiv)haus

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 I’m going to leak it!!! However, unlike a normal home where that fresh air is coming from wherever the home’s 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. That’s 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 doesn’t cool the incoming air—it 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 doesn’t sound too bad. Interestingly, many examples of passivhau have lots of windows, so such a house doesn’t have to be a cave. And the results are quite impressive:

(chart from the Passivhaus Institute)

Some links:

House built in New York State (with specs).

Website for said house.

A Habitat for Humanity passivhaus.

A passivhaus in Louisiana (with specs).

Another haus in NY.

A haus in Martha’s Vineyard.

A haus in Portland.

Diagram of a passivhaus.

A certified passivhaus consultant in Texas.

Building the first passivhaus in Texas (yep: it’s in Austin!).

Some building suppliers:

SeriousWindows

Inline windows

UltimateAir

AirXchange

RenewAire

Other links:

Efficient Windows Collaborative

ERVs in hot, humid climates

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 R’ed!