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Building Science

A PhD and an Architect Build a Net-Zero Home

The North Carolina residents explain how they did it and what they would do differently next time

Matt and Amy's net zero energy home in Asheville, North Carolina.
Image Credit: Vandemusser Design
View Gallery 6 images
Matt and Amy's net zero energy home in Asheville, North Carolina.
Image Credit: Vandemusser Design
Amy and Matt's living room in their net zero energy home
Image Credit: Vandemusser Design
Matt Vande is a treehugger!
Image Credit: Vandemusser Design
Slab and Superior Wall foundation walls in Amy and Matt's net zero home.
Image Credit: Vandemusser Design
Installing a rafter with no thermal bridge
Image Credit: Vandemusser Design
Even though it's cavernous compared to what you find in many homes, the mechanical room proved to be too small. They had to install some of the equipment other places.
Image Credit: Vandemusser Design

Amy Musser has a PhD in Architectural Engineering and, like me, used to be a college professor. Her husband, Matthew Vande, is an architect with an MS in Architectural Engineering. He is also a treehugger (see the black and white photo below). Together, they founded Vandemusser Design, a firm that provides green design, certification, and consulting. They walk the talk, having designed, built, and moved into their net zero energy home in Asheville, North Carolina.

They gave an evening presentation on their home at this year’s RESNET conference, and this net zero home is impressive. I’ll give you an overview here, and you can get more detail from their presentation, which you can download in pdf form from RESNET’s site. If you know about HERS ratings and the HERS Index, you may be surprised about one aspect of this home.

I travel up to the North Carolina mountains frequently and hope to visit their place on one of my trips this year. (Of course, this was news to them when I first posted the article in the Energy Vanguard Blog, since I hadn’t asked them yet. Nothing like publicly inviting yourself to someone’s home, right? Fortunately, they responded positively!) The photo below shows the front of their home in its natural habitat. (As far as I know, it never leaves that habitat, but with a net zero home, you just never know.)

About their home

They use the building both as their home and their office. They live upstairs and the office is in the basement. It’s 3,100 square feet total. Other features include:

  • All electric
  • HERS Index 44 before solar
  • HERS Index 12 with solar
  • IECC climate zone 4
  • True net zero performance (see next section)
  • Multiple certifications (which we sometimes call a NASCAR house): LEED for Homes Platinum, ENERGY STAR, EPA Indoor AirPlus, NC Healthy Built Platinum.

I’ll talk about the energy features in the section on getting to net zero. Not all of them were energy savers, and even some that were didn’t have favorable cost-effectiveness. Keep reading.

What is a net zero energy home?

The term “net zero energy home” can mean different things to different people. Basically, it means that you’re producing as much energy on site as you’re using over the course of a year. The controversy comes in when homes that use natural gas, propane, or other non-electric fuels exempt those other fuels from the calculations. They focus only on the electricity because that’s what they can produce on site, typically with photovoltaic (PV) modules mounted to the roof.

In my opinion, net zero energy should include all fuels used on-site. Using natural gas or diesel? Calculate how many kilowatt-hours (kWh) those BTUs add up to and make sure your PV system produces enough to cancel them out.

One thing a net zero energy home does NOT have to be is off the grid. In the early days of PV systems, you had to be off-grid because the electric utilities didn’t want hippies sending their homemade electricity into their beautiful grid of shiny corporate electricity. But the hippies wouldn’t go away, and the engineers figured out how to make hippie electricity look just like the shiny corporate stuff while also keeping the linemen safe.

That’s how we got to where we are now, with all the interest in net zero. You don’t have to be a hippie to have PV on your house nowadays. In fact, some of the big production home builders are even building net zero spec homes. Yeah, it’s mostly in California, I think because of all the hippies … uh, I mean, incentives there, but this thing is catching on.

So, back to Amy and Matt’s home now.

How they got to net zero (and other energy features)

Did I mention that Amy and Matt are pretty smart and they have lots of college degrees? And that they’re deeply involved with green building and high performance homes? Well, as you might expect, they went into this process knowing way more about how to do it right than I did when I built a green home.

Here are some of the nice features in their home:

Highly insulated basement walls. They used the Superior Wall system, as you see in Image #4 below. They’ve got R-12.5 continuous plus R-19 cavity insulation. Nice!

Passive solar. South-facing windows with high solar heat gain coefficients (SHGC = 0.53) and low SHGC (0.23) on other sides of house. Slabs for thermal mass in basement and first floor.

Reduced thermal bridging. They paid attention to the details, especially the band joist and the rafters. Remember that article I wrote about a sword through the ribs of the building enclosure? Well, they didn’t do that. Instead they stopped the rafters at the enclosure and then lagged rafter tails onto the outside. Voilà.

Structural insulated panel (SIP) walls and roof. They used urethane foam instead of the more common expanded polystyrene (EPS). It has higher R-values, R-24 in the walls and R-38 in the roof.

Ground-source heat pump. Expensive but offset by lots of incentive money. Capable of distributing heating and cooling either through radiant tubes or through air ducts.

Radiant heat. This didn’t save them any energy but they did it anyway for comfort reasons.

As I mentioned above, the HERS Index for their home is 12. Theoretically, a net zero energy home means that the HERS Index will be zero. Their score of 12 means that they’re 12% less efficient than a net zero home. Or so we’re told anyway.

They may not have reached the theoretical net zero of a zero HERS Index, but in terms of performance, they’re net zero. In their first year in the home, here’s how they did:

  • 5,843 kWh consumed
  • 6,147 kWh produced

In terms of actual performance, they’ve got a HERS Index of less than zero! Of course, let’s step back and remember that a HERS rating is an asset label, which means it doesn’t account for how people live in the home. If Amy and Matt moved out and another family moved in, the amount of energy consumed could be very different.

What they would do differently

In their RESNET presentation, Amy and Matt were honest about what they saw as their mistakes. Having lived in the house for a couple of years now, they see things that didn’t quite work out. They’ve also looked at the numbers and found what wasn’t as cost-effective as they hoped it would be.

  • The mechanical room is too small. This one happens everywhere. I’ve rarely been to a house where the designers allocated enough space to the HVAC and water heating systems. And theirs, as you can see in the photo here, is downright spacious compared to some of those little closets I’ve seen that are jam-packed with equipment and ductwork.
  • SIPs are not cost-effective. They calculated that it cost them an extra $12,000 to use SIPs and netted $5 of annual savings. The simple payback on those numbers yields 2,400 years. (I like SIPs and the payback for the house I built would have been much more favorable if I had calculated it.)
  • The fireplace is not used. They’ve had one fire in two years in their home, “mostly out of guilt,” and found that it overheated the living area. They may have saved that $5,000 if they were doing it over.

By eliminating the SIPs, fireplace, radiant heat, rainwater harvesting, and using an HRV instead of an ERV, they calculate that they’d have saved over $41,000. That would have dropped their square foot costs from $156 to $143 and still gotten the same performance. (Those costs factor in the incentives.)

In their presentation (download pdf here), you can also read about their dual-flush toilet woes, their wine cellar, and the box elder beetles that love their home, too.

What they’re happy with

The things that they really like about the house and that worked out well are:

  • Abundant natural light. Having lived in a passive solar house myself, I know the feeling.
  • Thermal mass floors. They’re green (low VOC, recycled fly ash) and comfy warm!
  • Rainwater harvesting system. Although difficult to justify based on savings since Asheville’s water is so cheap, it’s still a good, green thing to do.
  • Money in their pockets. They offset a good chunk of their costs with rebates and tax incentives. The total was nearly $52,000. Of course, they also save every month when the electric bill comes in.

Is a net zero home in your future?

Amy and Matt have built the house that I would have built if I’d known what I was doing in 2001. I’m in the pre-planning phase for my next house, though, and will definitely draw on their experiences as I plan my own really cool net zero home.

What about you?

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


  1. Expert Member
    Dana Dorsett | | #1

    A bit surprised.
    I found it a bit odd that they hadn't run the heat load numbers (or didn't look at them very carefully) before buying the wood burning insert (commented on in the presentation linked to in the article: ) It doesn't take a very big woodburner to turn a high-R house into a sauna, even at the lowest clean-burn firing rates. A mid to high mass woodburner (even a ceramic or soapstone wood stove) would make it more usable, but even there you'd have to size it appropriately for the actual loads to keep it from overheating the place. The high mass floors are moderating that issue some, but it's better to have a good amount of mass in the woodburner itself when it's grossly oversized for the loads.

    The typical "heats up to xxxx square feet" numbers used by woodburning appliance vendors is an unfortunate standard of that business, but completely inappropriate for analyzing what works for a Net Zero house. The max BTU numbers are more meaningful. If the max-fire BTU rating is >2x your design condition heating load it's going to be tough to avoid the sauna effect, even with a soapstone stove, let alone a low-mass insert. Most wood burners need to run something between one-third to half-fire to keep the soot emissions in the EPA-tested range- any lower and the emissions soar (as efficiency plummets.)

    I'm also surprised that they would suffer the high global warming impact of high density polyurethane SIPs (at any price), given that PU-SIP manufacturers have yet to move over to the newer low-impact blowing agents. Most high density polyurethane is currenlty blown with HFC245fa. In a high-R structure will have a lifecycle global warming impact of the blowing agents alone is many times greater than the carbon footprint of the lifecycle energy use savings over a code-min fiberglass-insulated tract house. It performs well from a thermal point of view, but it's not exactly "green". (EPS at the same R value would have had about 1/200 the global warming impact, and would break even in fairly short years.)

    If I ran the numbers correctly on their incentives listing page (p43 of 44), they ended up paying a net $8KUSD for their ground source heat pump after getting 30% back from the Feds and another 35% back from the state, and a $1K check from the utility? It seems a somewhat mis-placed set of subsidies. In an Ashville NC climate a Daikin Altherma air-to-water heat pump would run at the same or better average efficiency as the GSHP, but would cost about half as much up front (unsubsidized) , but the multiple subsidies makes the GSHP solution cheaper. Were to skip the radiant floors they could have heated the place at comparable efficiency with mini-splits for ~$8K (unsubsidized) upfront cost. Are the taxpayers/ratepayers really getting full value (or ANY value) out of those subsidy dollars here? It takes fine-tipped pencil accounting and a better thermal performance analysis to know if they didn't get negative-value out of it compared to some other solution on the heating/cooling front.

    Nearly very Net Zero house is a one-off experiment- I commend Amy and Matthew for taking that step, and for being frank in sharing what they learned, stating what they would have done differently were they to start over. It's probably a more comfortable house than most of us will ever live in!

  2. GBA Editor
    Martin Holladay | | #2

    Response to Dana Dorsett
    Good points. I also applaud Amy and Matthew for sharing their "lessons learned" stories. Everyone benefits from reading an account like this.

    It makes perfect sense for Amy and Matthew to have accepted all available incentives. Nevertheless, one can't help wondering whether the $52,000 in incentives -- contributed in one way or another by taxpayers and utility ratepayers -- was a good investment. I'm guessing that most of the money was used to subsidize the PV equipment and the ground-source heat pump. Let's say that the equipment has a lifetime of 30 years; that means that the subsidy from taxpayers and ratepayers amounts to $1,733 per year.

    I'm thinking: I could buy a lot of energy for $1,733 per year. But then again, the energy produced by the PV equipment clearly has a lower carbon footprint than grid-supplied power.

  3. user-1006269 | | #3

    The start of a joke
    So, an Architect and a PhD walk into a bar....

    Impressive project!

    Sorry, I couldn't resist.

  4. Expert Member
    Dana Dorsett | | #4

    The PV subsidy I get- the GSHP, maybe not (at least not here)e
    There was a total of ~$18K in subsidy for the3-ton heat pump, which is more than 1/3 of all incentives taken. That $18K is more than the typical installed cost for an equivalent-output Altherma, a best-in-class air source heat pump that would run at about the same or possibly higher than typical GSHP efficiency that climate. The energy savings of the more expensive GSHP is thus effectively zero, maybe even negative compared against Altherma performance in that climate. And a sub-$10K ductless air-air solution would come in about the same on energy use.

    For the dollar amount the ratepayers & taxpayers could have GIVEN them Altherma, and it would have been the same on energy use, and cost Amy & Matthew would have been $0. Alternatively they could have given them a best-in-class ductless system along with $8-10K in cash just for being nice folks. (I'd sign up for that, provided I could actually meet spec on the "nice" requirements! :-) )

    So, for the same percentage level of subsidy they could have subsidized 2-3 houses of similar load, and gotten 2-3 times the bang per buck.

    As of 2010 grid mix in NC about 4x the carbon per megawatt-hour of grid power in VT, more than 2x that of NH, and even 25% more than in previously-dirty MA (which was before the coal-plant closings in MA) according to EIA data. The net benefit of PV is WELL worth the subsidy cost there, especially with the reasonable over of PV peak output to the peak cooling loading of the grid, making the PV output worth more to the utility & ratepayer, since it's offsetting a decent amount of peak-power. Since heating loads peak primarily during off-peak hours, much of the baseload power running their heat pumps is from low-carb legacy nukes, but they'll be using a fair share of the coal-fired stuff too.

    Whether on average the heat pumps without the PV would be lower carb than condensing natural gas depends a lot on what NC and other southeastern state energy policy is going forward, but as of the stated of the 2010 grid, even with an average COP of 4 it wouldn't break even with condensing gas looking at just the raw lbs/mwh averages- you'd need a time-of day load & grid-source tracking to know where it really lives. But with the PV summertime excess offsetting of peaker power it probably makes the whole package a net-carbon win.

  5. user-1134737 | | #5

    exterior foundation insulation
    I'm wondering how the insulation was adhered to the exterior of the foundation and how it was finished on the exterior.


  6. jinmtvt | | #6

    wow you guys down south sure like to SHARE!
    Incentives and subsidies ... aren't you guys going crazy about all that ?
    Who decides the amount and or % accorded to homeowners ??
    Neway ... at least , a part of it gets back in salary to the trades .. let's hope that some of the products used were at least manufactured in USA ...


    i seriously dislike the use of PU SIPS

    The home costs alot for a very simple design/finished house
    ( what's the total cost ? 3100 X 156$ ? )

    "cold" side windows only .33U @ .23 shgc?
    Maybe not worth to get higher performance windows in their zone ?

    i agree with Dana on most other points ...

    Achieving near Net-Zero in Zone 4 is also alot easier,
    they seem to get some "temp spikes" according to weatherspark though ...

    A 1000$ "fake/gaz" fireplace would've achieved the same feeling without the cost and maintenance,
    and no overheating issues.

    Seems like they hired another "first timer " SIP crew
    ( how many time we have read about projects using crews that have no experience in the used construction method?? 100% of time it resulted in increase of labor costs and errors ... )

    the unconditioned wine cellar ... mmm ... i would've think that architects would know better ...

    3000$ for rainwater harvesting ? can't you just use a large vessel + some altitude ??
    What is the water used for ?

    What i like :

    - windows seem to be effectively shaded from high sun
    - looks like plenty of natural lighting = goooood :)
    - rain water harvesting
    - they share their errors with us !!!!!

    this house looks like it was designed by some interested folks, with some good knowledge,
    not by a pair of Architects ( i'm sorry ...don't mean to be disrespectful at all here )
    Some basic maths was wrong, some materials used are not green at all ( PU )
    and they have relied heavily on subsidies and incentives ( seems like they've rolled it all )
    and i don't like it when choices are made solely on the purpose of using as much of those "instant rebates " as possible .

    All that said, it still looks like a nice house to live in, and everybody chooses their style of design,
    I would've like to see a little more from the architects, but at least it achieves their primary goal.
    ( and we all know, me the first , that first time mistakes are part of learning )

  7. user-593033 | | #7

    Foundation insulation
    Ben, the foundation is superior walls. The exterior is 5000 psi concrete the interior is XP Rigid insulation between 2 3/8"concrete studs that are covered in 1"of EPS foam. The blog calls it 12.5 continuous but it is not continuous at that R-value. I recently toured a superior walls plant and got to see these style walls being built.

  8. user-1102836 | | #8

    Net-Zero Energy
    Dr. Bailes states: "In my opinion, net zero energy should include all fuels used on-site. Using natural gas or diesel? Calculate how many kilowatt-hours (kWh) those BTUs add up to and make sure your PV system produces enough to cancel them out."
    This statement would suggest that natural gas is just as flexible an energy source as electricity. Can you run a blender or a microwave oven or a TV with natural gas? Many people consider electricity to be a more valuable and flexible form of energy, but it takes 2 to 4 times as much thermal energy to produce the thermal equivalent amount of electricity. For this reason, many folks try to account for the energy required to produce electricity by tracking energy back to the source (

    The author might mention that "net-zero source energy" homes make sense in many areas, especially where electricity is predominately generated by coal (or nuclear, for folks that do not like the idea of land use loss with decommissioned nuclear plants). He also might be more specific and use the term "net-zero site energy" in describing his personal definition of net-zero energy.

  9. GBA Editor
    Allison A. Bailes III, PhD | | #9

    Response to Lee Dodge
    Good point! I didn't include the site-vs-source energy issue in my discussion, but it's certainly an important one. I don't think I was suggesting that natural gas is just as flexible as electricity, just that all fuels should be counted when you're doing the calculation. But if we're going to talk about accounting for site vs. source energy, then 1 kWh of PV electricity offsets about 4 kWh of primary energy, and yes, when those kWh of primary energy come predominantly from coal, every kWh of PV electricity makes a big difference.

    I just wanted to give a brief discussion of the net zero definition controversy in this article and focus on Matt and Amy's house. Look for an article some time in the future with more depth on the controversy.

  10. vandemusser | | #10

    Some responses...
    Glad to see that Allison's article has generated what appears to be some passionate dialogue about the design decisions that were made in our house. I thought I would try and clear up a few questions that people had:

    1. Yup, a lot of the decisions we made were specifically because the various incentives made them more cost-effective than other options. A good example of this was the GSHP. Because we wanted the flexibility of both forced air and radiant options, the WaterFurnace Synergy was the best option considering the incentives available. As we discussed in our presentation at RESNET this year, if we had to do it over again, we wouldn't probably do a radiant heat system. And then if there were also no incentives in place, we would have simply picked a high-efficiency air-to-air heat pump.
    2. The solar thermal system was definitely another example of this. It was an amazingly quick payback, but if that additional rebate had not been available from our local utility, we probably would have shifted to simply adding some more PV and installed a heat pump water heater instead. The upside to this being that I would have ducted the cool air by-product into the passive wine cellar to assist the humidity and temperature control issues.
    3. The fireplace was partially an aesthetic thing, but also part practicality... we had experienced numerous power outages in our previous house in town (one was almost a week in length) and wanted to have a heating option in case we had an extended power outage again. The requirements for some of the green certifications we were going after stipulated some fairly strict fresh air intake requirements for wood-burning fireplaces. A gas fireplace was out of the question, since we natural gas was not available in our neighborhood and also because we had no desire to pump a flammable substance into our house. Based on those options, this was about as small and cost-effective a fireplace as we were willing to install. Do we use it very often? Hell, no. Will I appreciate it if we ever have a long-term power outage in the middle of winter? You bet I will.
    4. The discussion on what are "good value" state / federal energy-efficiency incentives is a valid one. If the incentives were structured differently, we would have chosen differently.
    5. The discussion about polyurethane SIPs is also a good discussion. A lot of design decisions we made were due to the fact that our house was also functioning as our professional office. Because of this, it also tends to function as a showroom so that we can discuss different types of technology with clients who come to our office. We do see quite a bit of SIP construction in our area and while we had also contemplated both conventional framing as well as a 2nd story of Superior Wall, we thought there was some value to being able to show people how SIPs can be integrated into what we think is a fairly conventional-looking house. As for EPS vs. polyurethane, there are certainly pros and cons to either system, "greenness" being one of them, but EPS certainly also has limitations... light a section of the EPS SIP on fire and let me know if you still think they are the better selection or not
    6. As for Jin's comments... sigh. Sorry you don't think it looks like an architect designed it. Not much I can say there... aesthetics are pretty subjective. We focused on performance more than making it look architect-y. Still don't regret it.
    7. The argument about cost per square foot is a little like saying people in Hawaii are crazy for spending $4.50 a gallon for gas when you can buy it so much cheaper in Kansas. Asheville is not a cheap place to build. The bigger question is how we did relative to local construction costs. And we did quite well, about in line with what a higher-end home in Asheville costs to build, even with our mistakes. Could it have been built less expensively? Yup. We talked about that in the presentation.
    8. You are correct about the non-solar side windows... In general terms, better thermal values don't usually make sense in climate zone 4. The caveat to that being we would probably have upgraded to triple-pane windows simply because of some minor condensation issues on cold mornings.
    9. I would agree to a certain extent about the labor cost associated with building with SIP's. Our builder did an excellent job on our house, but there was definitely a learning curve to it. That being said, it doesn't explain all of the rate of payback issues we saw in the analysis. If we backed out ALL of the labor overruns, we still were looking at a rate of payback for SIP's in terms of centuries, not years or even decades.
    10. The unconditioned wine cellar was definitely an experiment, and a worthwhile one. I think with a few adjustments, we will see that we have a decent performing wine cellar. Not necessarily keeping it at 55 degrees, but probably in the low 60's. Humidity control is probably the trickier issue, but still manageable with the right piece of equipment. As stated before, I do wish we had elected to go with the heat pump water heater... a lot of our issues would have been mitigated right off the bat.
    11. The $3000 rainwater system is essentially because we were using it both indoors (toilets) and outdoors (outdoor irrigation). The local inspectors require quite a bit of backflow prevention and we also integrated a series of solenoid valves to allow it to shift to city water when power went out or when the cistern went dry. As we discussed in the presentation, the system was installed primarily to have the discussion of rainwater usage with clients who built in areas where access to well water was problematic, not so much for our own use. City water in our area is actually quite cheap. If we didn't do what we did as our business, we probably would have gone to a simper gravity-fed system like you describe. It's just a very different system than what we installed.
    12. Still not sure what basic maths (sic) was (sic) wrong... it's pretty impressive to ascertain incorrect mathematical calculations from what was supposed to be a general discussion of net-zero energy. I'm fairly positive that the PhD / engineer was dead on with her calcs, though.

    We definitely made some mistakes along the way, but we certainly learned a lot from the process and definitely didn't want to cover them up. Honestly, the biggest positive response we received from the presentation was just how refreshing it was to see someone talk about their mistakes. Glad we could bare our souls for you all - I'll be happy to answer any other questions people have...

  11. jinmtvt | | #11

    Matthew Vande:
    About the

    Matthew Vande:

    About the maths, i was mainly referring to the GSHP VS other choices, the fireplace overheating etc..
    please read Dana's post #1 and 4

    And i also again i applause you for being open about errors and sharing them,
    something we don't see often ( even here ).

    I don't agree with you about the PU sips.
    -current PU blowing is not green and wrong
    - SIPs need to be covered with a flame barrier aka Gyspum board,
    if the heat is strong enough to reach flash point of EPS within the SIP then everything else in the house was carcinated before that ...
    -PU is not reusable once it has seen high temps ( 300f+ ?? need to verify the temp ) so even if it still stands, it is dead .
    - PU is much more toxic when burning than EPS for the same volume
    ( can't find my damn source ... well anyone care to chime in on that ? )
    - EPS SIPS are usually cheaper at the same R value

    About the design, i simply find it a little "simple ", the article title pulled me in looking for a little bit more, or maybe something different than what i found ( but i also found more in the means of your
    report on the project !!! :)
    But hey, it is your house, your design and i should not be criticizing the "looks" on this"performance" forum, so i apologize for the comments.

    Congrats on the new house!

  12. Ted Clifton | | #12

    Simple design
    When you are going for net-zero-energy, simple architecture is the best architecture. Nice job on the design!

    I have a problem understanding your comments regarding the cost of SIPS. I have found that we can build with SIPS at a lower cost than stick-framing and then insulating and air sealing to the same level. The site-labor just eats up all the savings, and the finished results are never as good.

  13. watercop | | #13

    Is there any way you could have
    ...buried the rainwater tank?

  14. vandemusser | | #14

    More responses
    Ted, I think the issue regarding real cost of building with SIPs comes down to what you are paying for materials. When we looked at the cost of 2x4 walls with continuous R5 sheathing and blown-in-batt fiberglass in the cavities, it was significantly cheaper material-wise than the SIPs. The time to frame and insulate that system versus the SIPs was pretty neck and neck (maybe these guys just frame a house quickly?). We determined that the energy savings to go to SIPs was around $5 per year. It's really hard for me to make the argument that SIPs made sense financially. And that's not even taking into account the numerous hours I had in coordinating window locations and electrical chases...

    Curt, you definitely can design a rainwater tank to be installed underground, but not the kind you see in the photos. It would collapse the tank. We originally wanted to bury the tank 3 feet in the ground so that the plumbing line coming out of it would be below the frost line, but the geothermal lines were fighting for the same space to come into the mechanical room. Because of this, we kept it all above grade and built a raised section to artificially keep the plumbing line below ground. The main reason we didn't bury ours was to make it very visible for our clients to see. That was really the only reason. We have seen several people locally bury their rainwater tanks by using daisy-chained septic tanks, which are built locally. The downside with that setup is that the septic tanks tend to be smaller capacity. Other than that, they appear to work quite nicely.

  15. Expert Member
    Dana Dorsett | | #15

    Response to Matthew
    "As for EPS vs. polyurethane, there are certainly pros and cons to either system, "greenness" being one of them, but EPS certainly also has limitations... light a section of the EPS SIP on fire and let me know if you still think they are the better selection or not."

    You're talking about somewhat orthogonal aspects of the material, but no foam is completely green, and to the extent that can be reasonably/economically designed out, there's are good arguments that it should be.

    It matters that even at fairly moderate R values PU foam blown with high GWP blowing agents becomes net-negative for the environment even on a whole lifecycle basis. But neither am I a fan of EPS SIPs for a variety of reasons. (Insect infestation problems & flammability/toxic-smoke issues among them.) SFAIK nobody is making a polyisocyanurate SIP, but nailbase polyiso panel systems exist, and like EPS is also blown with low-GWP pentane. But like polyurethane iso is much harder to light-off than EPS, and chars rather than melting, even when actively burning. But at the ~R25 (whole-wall) in this design there is a lot of merit to going with cellulose-insulated 2x6 studwalls plus 2" of polyiso sheathing in terms of overall build-ability, cost, & resilience, with far less global warming potential than any SIP or insulated wall-panel system.

    Better than wall systems the best application of EPS is probably the exterior sides of foundations (particularly under slabs) where it has superior long-term resistance to water absorption and far lower GWP than XPS, usually blown with HFC134a, or a blend of HFCs, always many times the GWP of pentane. (In Europe XPS is blown with CO2, but has the same R/inch as EPS of similar density.) If the dirt under or adjacent to your foundation has reached the ignition temp of EPS, the odds that your house hasn't already blown away as smoke are pretty low. But high-density rigid rock wool panels rated for ground contact (eg Roxul Drainboard) may be worth the upcharge on foundation walls particularly in termite country, on both insect resilience & flammability grounds.

  16. jinmtvt | | #16

    Dana ..
    Again i agree with you , but fire rated EPS is harder than one might think to light up
    ( i've done it quite often while welding my open web steel joists in my ICF house where MIG blewoffs fell in the foam ( ~3000f ) melting through it. lighting a local flame that usually stops within 1-2 seconds... )

    Then again, with drywall hang inside, what is the probability than the wall structure light a fire with current electrical codes ?? without knowing the data, i'd bet that most residential fires in recent years come from inside the rooms ( occupants, appliances etc.. )

    Until we get Co2 blown XPS , let's stuck with rockwoll, eps and iso.

  17. user-4405197 | | #17

    Superior Walls under-slab insulation
    Hoping someone reads this - I realize this is an old thread. How did you deal with the underslab insulation for the basement? I was under the impression from the Superior Walls Builder's Guidebook...

    ...that you couldn't place the foam up against the bottom footer beam, that you had to leave room so at least 2 inches of the footer beam would be in contact with the slab. Superior Walls says you can pour the concrete so it overflows the footer beam into the cavities - is that what you did on your house?

    I also found this link...

    ...they had for a pdf of how to deal with radiant floor systems. I'm confused as to why they show the beveled gravel base by the wall, and what the plastic shims are used for. Any help explaining this would be appreciated. We're considering using Superior Walls for our walkout basement, and were concerned we wouldn't adequately be able to insulate the slab. Thank you.

  18. GBA Editor
    Martin Holladay | | #18

    Response to Brian W
    I have two suggestions:

    1. Your questions about details for the Superior Walls system should be directed to technical help specialists at Superior Walls. The phone number is 800-452-9255.

    2. If you still have unanswered questions after that phone call, you should post your questions on GBA's Q&A page. (That way, more GBA readers will see your question and be able to offer help.) Here is the link:

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