Shortly after moving in almost five months ago, the owner of a newly completed single-story two-bedroom overlooking Echo Lake, in Morris, Minnesota, began monitoring the energy usage of the house. And while it is too soon for a definitive ruling on the building’s four-season performance, the odds are pretty strong it will be exemplary.
Among the factors supporting that expectation are the home’s well-insulated and nearly airtight shell, its simple, rectangular shape, and its extraordinary HERS Index rating: zero.
The 11 7/8-in.-thick double-stud walls are insulated to R-45 with dense-packed cellulose, and the wood truss roof is insulated to R-60 with 18-in. of blown-in cellulose. The slab, with an insulated concrete form (ICF) perimeter stem wall, is insulated to R-40 with 8 in. of extruded polystyrene.
A ground-source heat pump and a photovoltaic array
The slab also is equipped with an in-floor hydronic heat distribution system. Other HVAC needs are met in part by minimally ducted air conditioning from a ground-source heat pump that has three 180-ft. vertical loops. Limited air conditioning is supplied by a fan-coil unit connected to the heat pump, noted Rachel Wagner of Wagner Zaun Architecture, whose team includes Elden Lindamood, the architect who designed the home. There also is a propane fireplace for a “fast bump” in interior temperature.
In all, the house has 1,596 sq. ft. of conditioned interior space, plus an attached two-car garage.
Blower-door testing showed an airtightness of 0.467 air changes per hour at a 50 Pascal pressure difference. Triple-pane Inline Fiberglass windows, with a high solar heat gain coefficient for those installed on the south-facing wall, were used throughout.
Because the home’s 7.74-kW pole-mounted photovoltaic array is expected to generate as much energy on an annual basis as the house uses, the home’s HERS Index is zero.
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nice job on the blower door
nice job on the blower door test!
pole mounted - because the PV area was too large for roof mounting?
with specs at or nearly at passivhaus (outside of the fireplace...) why not dump the GSHP and put portion of the savings into better windows (and no longer need the fireplace)? (in this case, better means better performance and appearance).
also, 7.73kW seems really large for such a small house - the riverdale net zero (PH+PV) needed 5.6kW for a larger house (~1800sf) - this seems to be a trend of projects i've seen lately where it would have been more cost effective to achieve passivhaus first, then add PV.
Great job! Elden and Rachel
I heard about this house when it was in the planning stage, very impressive specifications and ach50 test results. Please keep us posted on energy use for this home.
Pole mounted PV
Perhaps the pole mounted PV is actually referring to a system that tracks the sun. These are always pole mounted and typically produce about 30% more power than stationary PV panels.
Responses to Mike and Kevin
The pv is pole mounted in large part because the clients wanted a low pitch roof for a single story home that related to the prairie-ish surround. With a 6:12 roof, mounting the pv at optimum angle would have been cumbersome, ugly, and likely presented issues with snow drift. Therefore, pole mount seemed ideal and the rural site made it easy. The 7+ kW size came about to optimize the capacity of two poles and the inverters, and the owner liked the idea of potentially producing even more electricity than they'll use. Yes, 7.7 kW is pretty large, but electricity is used for heating, cooling, and domestic hot water (all tied to the GSHP system). The Riverdale Net Zero project also has a sizable solar thermal array for each home, with a large water storage tank.
Also, while we had great solar potential, the views of the lake are to the north and west; this created a design challenge in terms of passive solar design, and the owners wanted to capture the views and also to enter the building from the south. The clients were very involved in design decisions and also very involved in the selected approaches used to produce, reduce, and manage energy. The ground source heat pump was a high priority for the clients, who had access to electricity and propane as energy sources (other than the sun) and chose to use more electricity but to offset the negative environmental impact with the GSHP and more pv. The pv system doesn't have a tracker. Our consultant recommended more pv as the better investment (no moving parts to break and/or service).
Finally, our design team did suggest alternative strategies, including improving the envelope even more, using more passive solar opportunity, heating the house with a small, modulating propane-fired or electric boiler, and using a mini-split for the low A/C load. I agree that such an approach would have cost less than the mechanical systems used.
rachel, thanks for the
thanks for the response! more than familiar with client's drive/desire for certain things, despite better alternatives. also familiar with the non-tracking pole-mounted PVs from a previous job. plusenergy is a bonus (though more karmic than financial) - and plusenergy gets closer (and may even be) carbon neutral - so that's impressive.
from previous posts, it sounds like you are familiar with PHPP: in slightly milder climates (4500-7000HDD), with non-passive solar orientation (N-S oriented) and right glazing (PH high g-wert/low U-value) - we haven't noticed a substantial penalty for increasing northern glazing.
Can you disclose the cost of building this fine energy efficient home?
It seems to be the perfect house I want to build, would also like to see the lay out of the home.
are there specs on the mechanicals available?
which ERV, heat pump, blower distribution systems used, etc??
response about cost and details
We are not at liberty to disclose cost (yet). We used the RenewAire EV 130 HRV for ventilation. I don't have the full spec written for the GSHP and heat/hot water/cooling distribution systems. We plan to submit the house as a GBA case study once we have a year of actual energy use data. If that happens, much more info will be available.
To understand the true nature of efficiency in the structure, can you provide the HERs score before renewables?
Net Zero home and its true meaning
Here is THE bottom line on net zero homes - ANY home can be a net zero home if one installs a large enough PV (photovoltaic) system or equivalent. Al Cobb has the right question. What is the HERS score WITHOUT any PV? The REAL goal is to design and build a home that is so energy efficient that it needs minimal PV to reach net zero. This makes the most energy efficient sense and provides the ultimate long-term goal that is comfortable to live in in and of itself with indoor air temps constant and even (everywhere) 24/7. Generally speaking, it does not matter the design of the home (as long as it is relatively straight forward and simple) but, what DOES make THE difference is the superiority of, and the integrity of the TOTAL thermal envelope, first and foremost. I believe it is that simple and I also believe I have proven this in Tucson, AZ where temps in the summer typically are triple digit. Currently, The S.E.E.D. Home which is being monitored with real time data 24/7 by DOE Building America Team Davis Energy Group, is achieving major results in superior energy efficiency in and of itself. We run the heat pump reverse cycle chiller between the hours of 3 am and 6 am providing the insulated concrete floor with more than enough coolness to coast all day and night with internal temperature variation of 2 degrees. It is truly amazing to walk in to this home at any time, day or night, and the indoor temperature everywhere is a constant and consistent 73-75. It is the HOME itself that makes a real net zero home.
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