Cheapest way to build net zero house?
Hi. I have some questions about building an affordable net zero energy house. I will present a basic outline of my plan, and then ask some questions. In addition, I welcome any comments on any parts of the plan which could use adjustment.
Note: I am not a professional and only know what little I know from browsing these and other websites.
Plan: Northeast Ohio. 30’x50′ exterior. Slab on grade foundation with R-10 rigid foam below slab. ICF stem walls? Double stud walls with R-40 in cellulose. R-60 roof with high heel roof trusses. Simple gable metal roof. Solar panels. Basic interior finishes: nothing custom.
Questions:
Which is more cost effective:
1. Ductless minisplit vs. more solar and electric heat?
2. HRV or ERV vs. more solar? Can I get by with one or two WhisperComforts?
3. Triple pane vs. more solar? If triple pane is indicated, which company is cheapest? Intus?
4. Staying with windows, what dimensions are cheapest? Has anyone had luck calling a company to get custom product which was not sold for cheap(ish) and then designing around those dimensions?
5. I would like to avoid venting my dryer. Do you know the most affordable condensing dryers?
6. Is drain heat recovery worthwhile? Can you use drain heat recovery if you are using on demand water heaters? Are on demand water heaters more cost effective?
7. Is having just one exterior door acceptable? What is the most cost effective door?
Thanks for your consideration!
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Replies
See the zones 4 & 5 rows of Table 2 in this document:
https://buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf
You're in zone 5, but that was written 7-8 years ago, when better class heat pumps has an HSPF of 10 rather than 12 or higher, and typical better-class rooftop PV was 15% efficiency rather than today's ~20%, so you can probably get there with the zone 4 numbers if you optimize it.
Note, those are "whole assembly R" not center-cavity R values, factoring in all the thermal bridging. An R25 wall could be a 2x6/R20 wall with 2" of exterior polyiso foam. An R30 wall would take 3" of exterior foam. A double studwall with a foot of cellulose also comes in the mid 30s, despite a center-cavity R40+.
See case 4 in this document, which is R30 whole-wall, a double studwall with 9.5" of cellulose.
https://buildingscience.com/sites/default/files/migrate/pdf/BA-0903_High-R_Value_Walls_Case_Study_rev_2014.pdf
A cold-climate mini-splits will leverage the heating power use by about 3:1 in your climate if sized correctly, and is a far cheaper than tripling the amount of PV needed for heating with baseboards, and it will provide high efficiency cooling as well. If heating with resistance electricity there may not even be sufficient roof real estate available.
A U0.25 double-pane is probably going to be the sweet-spot for Net Zero in your climate, even though triple panes would be more comfortable on the coldest hours of the coldest days. Avoid west facing windows to keep peak cooling loads bounded, and shade the south facing windows with roof overhang to cut midday gains in summer. To cost & performance optimize all of this it's worth modeling it with BeOpt, a freebie download courtesy of the US D.O.E.:
https://beopt.nrel.gov/downloadBEopt2
Drainwater heat recovery requires at least 4' of vertical drain downstream of the shower, so it doesn't really work for single story slab-on-grade, but is fine for 2-story, provided the shower is upstairs. It can be used with any type of water heater, and is worthwhile if it can be implemented without changing your house design (probably not.)
Electric on-demand water heaters are abusive to the grid, requiring way-upsized grid infrastructure to deliver the intermittent loads. If "demand charges" get applied to your local rate structures the LAST thing you would want is an electric tankless water heater, since a 15 minute shower could add tens of dollars to your bill. This isn't purely theoretical- utilities in areas with high solar market penetration and nearly net-zero electricity customers are resorting to demand charges to cover their sunk infrastructure costs. A plain old electric tank sufficient to fill your biggest tub is probably your best bet, though heat pump water heaters have advantages if there's some place to put it where the noise isn't a problem, and the room is big enough to support it. (You can't just jam them into closets.)
ERV cores are prone to freeze damage in your climate. For a 1500' house with a relatively open floor plan you can probably get by with a single pair of ductless Lunos HRV and a Panasonic WhisperGreen bath fan with the optional humidity/condensation sensor for ventilation.
BTW: Going with 2" or thicker reclaimed EPS from salvage & reclamation outfits for the sub-slab foam is a lot cheaper than R10 XPS, and a lot greener too. R10 XPS is blown with high global-warming HFCs, and as it loses the blowing agent over time it's performance drops to ~R8.4, the same as EPS of similar density.
With reclaimed goods 2" of either XPS or EPS is fine, but don't use polyisocyanurate under slabs or in contact with soil, since it's hygroscopic and can take on moisture over time. But 3" reclaimed roofing polyiso on a 2x6/R20 wall is also about R30, and may be cheaper than dense-packing that much cellulose.
There are at least a few outfits handling reclaimed foam in NE OH, if the craigslist listings are any indication:
https://cleveland.craigslist.org/search/sss?query=rigid+insulation
Reclaimed insulation uses no new polymer and no new blowing agent, it only extends the benefit side of the cost^benefit divide.
I vote no on the single exterior door. In a fire, you want two ways out. And getting to the back yard by going out the front door will get old the first week.
We have a Whirlpool heat pump dryer that's ventless and works really well.
We have Intus windows. Each window was custom made, so we sized them to fit our needs. They were installed more or less in the middle of our double stud wall.
Definitely do the minisplit over more pv and baseboard, as Dana points out. He's also right about reclaimed foam.
Cheapest way to net zero? Standard construction + solar panels. The trades (even illegals) just can not compete with the efficiency of automated Chinese solar factories.
My impression from reading a lot of PGH threads is that triple pane is overkill for most houses. Get good double pane windows, and used fixed versus operable where you can. Spend more time and attention to your air-sealing details to get more bang for your buck. A simple roof as you indicate will have a number of benefits, less expensive being one of them. Make sure you have enough overhang...nope, you need more, on all four sides. :)
ANON3, you may not be aware what "standard construction" is in Ohio. Our dated state building code is not enforced in many places, and contractors haven't adopted any energy improvements on their own. It would take a lot of solar panels to zero out a build like that. OP, find a contractor soon that understands what you want and is willing to work with you toward your goals. Many of these early design decisions will have a major impact on final cost (and the number of solar panels you'll need.)
If you save $20,000 going standard construction, you can get a 20kw solar panel setup, that's a lot of electricity. I've seen people spend $200,000 over standard construction, they are out of their minds.
Where in Ohio can you get solar PV installed for a buck a watt?
Regardless, I grew up in the midwest and there's no way I'd like to live there again without AC, which means mini-splits (or window AC units to accompany your baseboard heaters, but in that case, you might as well not bother with the superior construction).
Standard-sized windows can be cheaper than custom sizes--but not always. U-0.25 double-panes will indeed be the sweet spot. Just fine and not too expensive.
If you REALLY wanna save money and energy with the clothes dryer, skip it and use a clothesline. Your clothes will thank you, too. In winter when it's not so sunny, put a portable drying rack underneath a vent or mini-split head. The moisture this will add to the air will be much appreciated, and reduce the penalty for not having an ERV if you opt not to go that route.
User-6877304 (It would be helpful if you told us your name),
1. For more information on drainwater heat recovery devices, see this article: Drainwater Heat Recovery Can Lower Your HERS Score.
2. In his first response, Dana Dorsett wrote, "ERV cores are prone to freeze damage in your climate." Dana Dorsett is rarely wrong, but on this point, he is wrong. For more information on the issue, see HRV or ERV?
Stephen Sheehy is correct that you want two exterior doors. Although I'm not sure where this requirement is found in the building code, I'm fairly certain that two exterior doors are required by building codes for single-family homes. (I may be wrong on this point. Even if they aren't required by code, two doors are a very good idea.)
If you really want to optimize your thermal envelope for net zero, you should probably download a free piece of software called BEopt. It will take you a little while to learn, but it's the software you need for the task you face. Here is the link: BEopt software.
Q. "HRV or ERV vs. more solar?"
A. In your climate zone, "more solar" + a simple ventilation system will be a better investment than an HRV or ERV. (There may be reasons why you prefer the HRV or ERV that have nothing to do with the cost, of course.)
John Semmelhack did the math. Here is the link to the article that discusses the question: Are HRVs Cost-Effective? (Note that since John Semmelhack performed the calculations discussed in that article, the cost of PV has dropped, making it harder to justify an HRV or ERV on cost savings alone.)
Even at a buck a watt PV it usually takes more panel area to hit net zero than you have the roof area, if built to code min to heat with resistance heating. (Ohio is under IRC 2009- you're in zone 5A: https://energycode.pnl.gov/EnergyCodeReqs/?state=Ohio ) Building it out in the yard or a solar carport may be possible you have sufficient unshaded area, but leveraging it with heat pumps is still cheaper.
But PV not a buck a watt (yet), and won't be any time soon. The national average cost of small roofop PV in the US is about $3/watt. https://openpv.nrel.gov/ In Australia its closer to $2/watt. If you can wait another 15-20 years it may hit $1/w though.
The rationale for HRV/ERV vs exhaust-only ventilation may not be rational on energy cost savings alone, but in a tight house any ventilation system needs to have known sources of intake air and not rely on random leakage points alone. Also, depressurizing a house in some areas increases the radon levels. Whichever approach you take, think carefully about where the ventilation air is needed, and where the ventilation air is coming from.
Perhaps not all ERV cores are subject to frost damage, but Panasonic voids the warranty on their FV-04 mini-ERVs for frost damage when installed in a NE OH location. See the map on p7 of the manual:
http://s3.supplyhouse.com/manuals/1282144874966/37200_PROD_FILE.pdf
Dana wrote, "depressurizing a house in some areas increases the radon levels."
As I wrote in Comment #9, Dana Dorsett is rarely wrong. He must be having a bad week.
Exhaust-only ventilation systems do not increase radon levels. For more information, see Exhaust-Only Ventilation Systems and Radon.
Dana,
You're right that "Panasonic voids the warranty on their FV-04 mini-ERVs for frost damage when installed in a NE OH location." What I should have written is that an ERV, installed and operated according to the manufacturer's installation instructions, won't be damaged by frost.
I agree that the Panasonic ERV isn't appropriate for cold climates, as a cursory reading of the installation instructions reveals.
Combining the fact that the WhisperComfort switches to exhaust only mode with Lstiburek's "Exhaust-Only Ventilation Does Not Work", leads me to believe that it isn't a good choice.
I'd say that not having triple pane windows is a matter of turning up the heat on the coldest days and does not require tolerating lower comfort. Ie, this one can be treated as a solely economic issue (cost of window upgrade vs cost of increased energy use).
Look into a frost protected shallow foundation vs stem walls. Double interior doors might be useful for reducing delta-T to rooms without heat sources.
PV solar is less than a buck already
https://www.wholesalesolar.com/solar-panels
around 60 cents actually. It's even cheaper wholesale.
Not only will you be net zero, you'll be making money by going standard construction and put all the savings into PV.
Anon3: you appear to be confusing the retail price of the panels with the installed price of the complete system. That website sells a complete 20 kW system for $31k, not the $20k figure you quoted. And regardless, retail or wholesale prices (panels or full system) are completely irrelevant to this discussion unless the OP is planning to install the PV array himself.
Reply to Jon R.
Windows with a better U rating will not just save energy. There is a definite comfort factor for anyone sitting near a window on a cold day. The warmer the interior glass layer, the more comfortable. Whether better windows are worth the extra money is of course pretty subjective.
The installed price of PV is still ~$3, no matter what the panel price is. There are places in the US where it's sometimes in the low $2, but none of those places are in OH.
Reading the lipstick-math on the mirror...
A tight IRC 2009 code-min 1500' house in NE OH, will use between 8000-10,000 BTU or 2.3- 2.9 kwh per HDD- call it 2.5kwh/HDD. The climate there runs about 6000 HDD, so you're looking at 15,000 kwh /year just for space heating.
Assuming a 15% capacity factor (may be a bit high for AC output) that'll be about 1.3kwh/year per watt of panel, so it'll take 11-12,000 watts of panel just to cover the heating energy use. At about 10 watts per square foot you're looking at 1200 square feet just to cover the heating energy use, and for a 1500' house there is probably AT BEST only 1000 square feet of south facing roof pitch (unless it's one big shed roof, not a gable with an east-west ridge line.)
Most homes will run through another 4000-5000 kwh/year or more for all plug loads including some air condition & ventilation, and if heating water with a plain old electric tank, another 4000-5000 kwh of water heating. Going all-in on energy efficient equipment & low-flow shower heads, drying clothes outdoors etc that can probably be cost effectively reduced to 7000 kwh/year, which would take ~5400 watts of panel, or 540 square feet of panel.
Add them together and it's 1740 square feet of panel- bigger than the footprint of the house, but sorta-do-able in theory if it's just one big shed roof with big overhangs, or you're willing to build out a solar carport or ground-mounted solar.
Heating with mini-splits drops the 1200' of panel needed for space heating down 400-600 square feet, so with the 540' of panel for other uses it might be sorta-squeezable onto an optimally pitched south facing pitch of symmetric gable, but not really.
But at a $3/watt the $18000+ in PV savings buys more than enough mini-split, and is still cash positive on simple 30 year analysis even if the mini-splits are assumed to be replaced at year 15. (In a net present value analysis it's an absolute no brainer.)
Cutting the heat load by between a third to a half below a code-min house (following the zone 4 or zone 5 guidelines in Table-2 the BA-1005 document linked to at the beginning of the first response) makes it's a heluva lot easier to fit the array onto the roof. The lifecycle of the higher-R building envelope is longer than the lifecycle of PV, the house is more comfortable, and the heat pumps can be smaller (and quieter), with at least some dollar savings on the mechanicals.
Thanks so much everybody!
To summarize what I am hearing so far...
Addressing the questions I asked:
1. Ductless minisplits are definitely worth it over electrical resistance heating.
2. Exhaust only ventilation system would get the job done, but will likely result in inferior air quality. There is some debate about the need for passive air inlets. Another question: Would undercutting all the doors help with air quality if I go this route? What are the major downsides to doing this?
3. Good double pane windows seem to be adequate in my climate, although their may be a small comfort penalty on the coldest days.
4. Maybe getting some "stanard size" windows is cheaper. My thought was that I might be able to find good surplus windows in a weird size if I called around? I will probably just have to try this and see...
5. Clothesline! Why did I not think of this? Surely a more resilient technology...
6. Drain heat recovery is not doable given slab on grade, which is what I want. On demand water heaters generate large demand over short periods, and should be avoided.
7. Probably worth it to get two exterior doors from a safety perspective. Might be code.
Other suggestions I didn't ask about, but got good feedback on:
a. Buy reclaimed foam for underslab, and possibly exterior wall, insulation. @DanaDorsett: What wall assembly are you proposing exactly? 2x6 studs, cavities filled with (cellulose? fiberglass?), and 2 inches of external reclaimed polyiso?
b. Model everything using BEopt! Very cool resource I didn't know about.
c. I like ANON3's suggestion of installing my own solar panels. I will look into how difficult this would be. However, Dana's analysis seems pretty convincing that building code minimum+ a buttload of PV is probably not the most cost effective way to get to net zero. In any case, I do want a "pretty good house", and I think one built with an eye towards air sealing details, etc, will probably outlive a code standard house by a long margin. Solar panels will need to be replaced, but I want the "bones" of the house to be good.
Additional questions:
This house will be located on a 5 acre permaculture nut and fruit farm which I am starting (trees are planted, irrigation system in, not much else currently). Part of my aim is to use the house as basically an adult dorm for myself, my wife, and a bunch of roommates who are also interested in regenerative agriculture. I suspect that I will have 4 bedrooms, each occupied with either single individuals or couples. So let's estimate that there will be 6 adults living in the house. I understand that I should try to aim for 6*7.5 +1*15 = 60 cfm according to ASHRAE? I am concerned about humidity with so many breathing people in such a small space. Also would the body heat of this many people adjust projected heating/cooling needs at all?
Kind regards,
Steven
p.s. Sorry for my user name, I think something went weird when I tried to sign up for my free trial...
"1. Ductless minisplits are definitely worth it over electrical resistance heating."
Or ducted mini-splits.
With plenum trusses (see: https://www.greenbuildingadvisor.com/blogs/dept/building-science/use-plenum-trusses-keep-ducts-out-your-attic ) it's pretty easy to duct heating/cooling to every room to fix distribution issues with doored off rooms, and can make it easier to meet code for minimum room temps at the 99% outside design temp. A 1500' high-R house with a simple shape will usually be within the capacity range of a single 1.5 ton Fujitsu -18RLFCD slim-ducted mini-split at your +5F-ish 99% outside design temperature, and may even be cheaper & more efficient than doing it with a couple of ductless heads.
There are some trade offs to be made here, but don't rule it in or out until you've done some BeOpt tweaking and have reasonably accurate load numbers. But with four doored off bedrooms you may need to go to the smallest legal-egress sized triple-panes in those bedrooms to be able to heat them adequately with the doors closed using a ductless head in the common area. A ductless head per bedroom would be ridiculously oversized and cycling on/off at low efficiency, whereas a ducted solution right-sized for the whole house load can run continuously at high efficiency.
"4. Maybe getting some "stanard size" windows is cheaper. My thought was that I might be able to find good surplus windows in a weird size if I called around? I will probably just have to try this and see..."
Finding standard sized surplus windows under U0.28 is going to be tough, and you will probably need that window performance.
"@DanaDorsett: What wall assembly are you proposing exactly? 2x6 studs, cavities filled with (cellulose? fiberglass?), and 2 inches of external reclaimed polyiso?"
That's pretty much it. Building 2x6 framing 24" on center and taking advantage of at least some advanced framing techniques is easy to do in a simple rectangle footprint. With 24" o.c. stud spacing a single rather than doubled-up top plate is allowed if installing the trusses 24" o.c. directly above the studs. Spacing the windows & doors optimally can reduce the additional framing of narrower random-width stud bays too. Caulk the studs to the sheathing inside every stud bay, and use an EPDM sill gasket under the bottom plate. For an idea of what optimized value /advanced framing is about see this:
https://buildingscience.com/documents/insights/bsi-030-advanced-framing
With air tight sheathing and very regular stud bays and minimal framing fraction R23 rock wool batts are easy to install to near-perfection, and a measurable improvement over a code min 2x6/R20 with 16" o.c. spacing. Damp sprayed cellulose or dense packed fiberglass would work too, but are probably more expensive than R23 rock wool, and would be slightly lower performance.
Designing for advanced framing reduces the total material cost, including a much reduced scrap pile, and is in keeping with the title of this thread.
At reclaimed foam prices it's worth going fatter than 2" on the exterior foam. You'll pay a bit more for the foam and for the longer timber screws, but 3" or 3.5" roofing polyiso is still pretty easy to find, and still pretty easy to assemble. It gets to be more awkward at 5" & up, and not necessarily worth it.
"c. I like ANON3's suggestion of installing my own solar panels. I will look into how difficult this would be."
Tread very carefully here- there are a lot of regulatory hoops to jump through, and they vary by local codes, and by the local utility. An experienced local installer will have already starred in that movie, and is more likely to get the immediate approval from the local regulatory & utility officials. Even if you do it perfectly (and assuming it's even legal for DIY grid attached solar without the necessary licenses & credentials), you will receive more scrutiny than the contractor, and are likely to be waiting in longer queues to be allowed to hook up.
Also, not all PV panels, inverters & racking are made equal, and some perfectly legal products are also perfect junk, unlikely to go the distance. If you really scrounge and study up on the local regulatory landscape you might get your total material & permitting & connection costs down to $1.50/watt with decent quality components. But you'll be the one responsible for the design, construction, and performance of the system. This is WAY more than just another appliance installation job. A solar contractor buys in much higher quantities and can get the high quality goods at a better price point than you can for a one-off house, and the design/installation usually comes with a warranty. In my area it's a competitive business, and you can look through the proposals to see what they're actually going to install, and do your own research on the quality & reputation of the component vendors.
It's doable, but study hard and think about it before diving in.
> There is a definite comfort factor for anyone sitting near a window on a cold day.
Which, within reasonable ranges, can be compensated for by turning up the heat. Perhaps to the point where the entire building, with higher U-factor windows, becomes more comfortable than one with lower U-factors (at a lower temperature).
I usually solve the comfort problem on sub-zero days by sitting somewhere other than right next to a window. :-)
A U0.25-ish window with an insulated glass unit low-E coatings on surfaces 2 & 4 (eg Cardinal's LoE-i89 glass) can still be pretty comfortable standing next to it, right up to the point where condensation begins to form on the surface, raising the emissivity of surface #4. Even though it has a lower surface temperature than if the second low-E coating was on surface #3, it feels better because it is reflecting body heat (and room heat) back at the human emitter. In colder climates the condensation events would be frequent, but in NE OH it would be rare enough that it'll be just fine. It won't end up with puddles on the sills or constantly hazed-over windows the way it might in much colder climates.
More info from one vendor of surfaces 2 & 4 double-pane glass: http://www.cardinalcorp.com/products/coated-glass/loe-i89/
Those windows do look pretty neat. Do you know the price/sqft?
Another question: would it substantially save costs two build 2 floors and reduce the footprint? Cutting the size of foundation and roof in half is my thinking, but it also makes construction somewhat more complicated I would think. I am not too concerned about loosing roofstop solar: I have enough land to put up as many ground mounted units as I need.
Thanks,
Steve
Steven, I'm researching drainwater heat recovery systems myself, and its my understanding that one advantage to you doing two stories would be that this system could then be installed below the upstairs shower (and laundry if you put the washer up there too).
There's little to no value putting drain water heat exchangers on a residential laundry. Clothes washers, like bathtubs are batch draws. The heat exchange can only happen if the water is being drawn while hot/warm water is going down the drain simultaneously. Thus it works well for showers, but not at all for tubs or washers.
For large commercial laundries with dozens of machines running at once there is some benefit, since there is guaranteed to have some overlap of machines draining while others are filling.
Having two floors reduces the heat load due to the lower surface area per square foot of conditioned space, but the stairways reduce the amount of usable space a bit, and the smaller footprint also reduces the amount of roof area to exploit for PV. Framing a 2-story with advance framing techniques is also a bit more complicated than with 1-story, and it will require a minimum of two heating/cooling zones ( by floor) to stay comfortable. A simple shed roof or assymetric gable can usually buy back anything lost in roof area compared to a gable roof design, if that's needed.
Covering a yard with PV adds it's own visual clutter, but makes it more accessible for maintenance or snow removal. (I personally prefer the solar carport approach if it's not going to be on the house. YMMV.)
Many major window companies use Cardinal's LoE-i89 insulated glass in various gas fill options, an different low-E options for the surface 2 coating. (The surface 2 coating has a significant affect on the SGHC, so think about it, model it in BeOpt before specifying the glass. You may opt for different surface 2 options depending on which side of the house it is.) The price of course depends on the window manufacturer & model- I'm not sure what the cost of the glass units themselves cost. I may find out within the next year or two. I have some large Pella's with clear glass double panes in a family room addition that I've been meaning to re-glaze with surface 2 & 4 argon double panes, but it's not at the top of the list at my house just yet. (Houses are never really done- you know that going in, right? :-) Mine has been under construction since 1923... ) Like others, Pella offers LoE-i89 glass on many or most of their current windows, and should be able to cross reference it to the existing glazing units.
Depending on what is required for permits and the involvement of an electrician, and how good of a bargain hunter you are, it might be possible to install your own PV system for around $1 a watt. A friend has done this earlier this year, and his cost included hiring an electrician and a lift to place panels on the roof, and also includes the 30% Federal tax benefit. Some description and pics of his project is here: http://300mpg.org/2017/02/09/garage-update-feb-9-2017/
I've installed all of the PVs and the wind turbine at our home, made much easier by an interested building inspector and the classes that I had taken at the MREA over the years.