Growing at Home: HUG’s Passive Solar Residential Greenhouse


by Corrine Bruning • September 2011
Plants in one of HUG’s passive solar greenhouses

Plenty of us have imagined the ideal sustainable community: Greenhouses attached to every house, growing your own fruits and vegetables, knowing our neighbors, and being able to take a short walk to shops and entertainment. Hunt Utilities Group (HUG) in Pine River, MN is trying to make the idea of resilient living a reality with a number of research projects including building a residential greenhouse attached to the south side of a new house on their campus, which allows for passive solar heating, on-site food production, and the comfort of a sunny four-season porch.

HUG started sustainable building research in 2003 after purchasing 65 acres near Pine River, MN. Their first buildings were a main office and workshop insulated with and built out of straw bale and mud. Since then, with each new structure built, including greenhouses (such as hoop houses), manufacturing houses, they have faced challenges, learned lessons, and gained new ideas.

Their near term goal, according to Project Coordinator Paul Hunt, is to “build neighborhoods within walking distance of small downtowns. The HUG houses in these neighborhoods, which should take very little (if any) fuel, would help feed people and recycle nutrients.”

Thus, HUG’s research group set out to test some greenhouse designs that could provide food and energy for the local community.

Project Planning
The goal for the greenhouse was to “stack functions” much like the principles of permaculture. “A greenhouse that is part of a residence seems to have a lot of functions going for it. It gathers huge amounts of solar heat in the winter. Even at night, it buffers the south side of the house from losing heat. It adds wonderful space to a house. It helps clean and humidify the air (saving more energy). It helps feed the occupants, “ says Paul Hunt.

HUG built the research greenhouse using twin polycarbonate walls on the building’s exterior and translucent walls adjacent to the house. Doors on these walls, open and provide more direct heat and sunlight to the house, and can close to regulate heat. Also, air that is heated by outside solar panels is pulled through indoor ducts down 10 miles of tubing snaked under the office/building portion of the structure.

Once the air and heat pushes through this tubing, it is released into the greenhouse. The floor of the greenhouse has a sand mass underneath it to store the remaining heat circulated by the solar panels. Windows provide ventilation and cooling.

They also built the greenhouse with three rooms, each room connected to a room in the house. A network of sensors throughout HUG’s campus collects data that is stored in their computerized creation, HUGnet. These sensors measure important energy and heat inputs such as temperature, heat flow, light, and air quality. They have been able to narrow down the best combinations of room ratio’s and other inputs to best optimize future projects. If you have the right ratio of rooms (greenhouse room to the room in the house it’s connected to) and the rooms are sealed properly, you minimally need an alternate heating source. As HUG believes that others can benefit from their work, they have released all of the hardware and software associated with HUGnet to Open Source/Open Hardware.

Lessons Learned
Greenhouses are generally known for producing plants and food, but HUG’s main interest was using greenhouses as a way to supply food without using much energy. They found that their on-campus hoop houses were the best option for producing food. They were able to start seedlings in the larger greenhouse, but temperatures were too great for plants to survive in the long term. However, the hoop houses do not produce enough food in the winter due to a lack of light. HUG found that the only way to solve this problem is to use more lighting and thus more energy. So, for HUG producing food in the spring and fall is best.

HUG is already moving forward building new structures, including a small three-bedroom, two-bath house without a greenhouse (the Small and Practical Agricultural Reslient Community house, or SPARC), but with lots of south facing windows, to take advantage of passive solar heating. The greenhouse is still functional, as is HUG’s goal to feed itself locally year round. Hunt eluded that it might be more effective if they bring the greenhouse into the central part of a home, and that we might see the research on this soon once another structure is completed.

The CERTS Connection
HUG understands that clean energy research projects require collaboration and community support to move forward. CERTs is one way that HUG connects to other community members. For this particular project, the Central CERT provided a grant of $5,000. HUG has hosted tours for CERT team members, showing them their latest research projects. Greenhouse operator Barb Wagner adds, “CERTS helped us learn by giving us information and connections with like minded people. They helped us organize the thinking on our project, and of course the grant helped us to pull it off financially.”

As HUG continues building structures and testing out other sustainable ideas, they share their research with groups that tour the different HUG buildings. Students come from the area’s schools, engineering students from nearby colleges and abroad, and even a Red Hat Society group toured recently. Marketing occurs through word of mouth, and they are always willing to show new interested individuals their facilities and findings.

To learn more about HUG and its projects, feel free to contact HUG Administrative Assistant Kathy Hoefs at or by phone: (218) 587-5001. Click here to learn more about the SPARC, completed in June 2011.

Project Snapshot

  • Project: Research structures that provide living quarters as well as food production and waste recycling through the use of solar energy.
  • Technology: Passive solar greenhouse affect, sand mass storage and 10 miles of tubing to transfer heat, south-facing windows, and the HUGnet system to monitor energy delivery
  • Grant: Central CERT Grant $5,000

Passive Solar Heating: Passive solar heat directly uses the heat of the sun, rather than converting it to electricity. Passive solar heating can be used to heat air or water through the use of solar thermal panels. Passive solar heat can also mean incorporating south facing windows to capture heat from the sun and warm the room or building.

Permaculture: Sustainable design strategy to provide food, energy, and infrastructure in an ecologically permanent way. Often this design includes stacking functions, such as planing trees with broad shade leaves to protect bushes below them which hold together and protect a vegetable garden that further cleanses a gray water system.

Relates to the following technologies

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Minnesotans building a clean energy future


    CERTs Partners:

Minnesota Department of Commerce University of Minnesota Regional Sustainable Development Partnerships and Extension Great Plains Institute Southwest Regional Development Commission