Green heating options for tiny, super insulated cabin in northern MN?
We’re designing a 400 sq ft timber frame and straw bale cabin, off grid, that will have 18″ walls and R64 insulated floor and roof. The layout is 24′ x16′ with a shed roof. Open floor plan with a loft. Location is northern MN. The south facing wall will be 16′ tall and 24′ wide and have lots of triple pane windows with high solar heat gain. There will be no plumbing, but we do plan on solar to power LED lights. I’m hoping that on sunny winter days, we’d get a good amount of heating from the passive solar design, but am also wondering what the greenest and most cost effective options will be for supplemental heating for night time and cloudy days. Any suggestions? Could a solar system with battery power an electric baseboard heater?
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A few options, but which is best depends on what you count as 'green' heating.
First, like you mention, is solar and a battery. Use the batter to run heaters at night. This is honestly a good option.
Another would be a heat pump such an air-source or ground-source pump. That could help maintain a comfortable temp throughout the day.
Debatable on it's green credentials. But a wood burning or pellet burning stove is another option.
Wood burning is green from a carbon perspective, the carbon that you're releasing is from the current geologic era and not fossil carbon. Whether the other components of smoke are a pollution problem really depends on local conditions. If you're off-grid, they probably aren't.
"Wood burning is green from a carbon perspective, the carbon that you're releasing is from the current geologic era and not fossil carbon. "
This is a common argument, but I think the phrasing of it leaves a bit to be desired. The atmosphere doesn't care whether the carbon came from fossil fuels or wood. What matters is whether the gathering of the fuel has increased, decreased, or had no effect on the net carbon sequestration rates.
Fossil fuels can simplistically be considered to have zero effect on sequestration rates since it's all old carbon.
Harvesting trees very much has some effect on net sequestration rates. It is these changes in rates that matter; the fact that the carbon isn't as 'old' doesn't really matter or automatically make it better.
If (and only if) *due to the harvest,* net sequestration rates increase by some amount, then burning wood is better than fossil fuels by that some amount (from a carbon budget perspective).
A small backyard stick pulling operation will look different than a largescale commercial operation. That's not to say largescale operations can't or won't increase net sequestration rates, but it's not universally the case and there are degrees of change. When things get properly wacked, land area often becomes a net *source* of carbon post harvest for some time.
Burning wood is most likely and generally better than burning fossil fuels from a carbon perspective, but it's important to realize which side of the coin makes this true, and that the coin toss happens in the woods at the time of harvest. And it may not be as much better as we sometimes think if we ignore the changes in net sequestration rates of the forests due to the extraction.
User ...987,
I think the list of viable options is shorter than Matty's post suggests: https://www.greenbuildingadvisor.com/article/how-to-design-an-off-grid-house
The problem with off-grid is the same as the problem with passive solar and the same as the problem with thermal solar: in a heating climate, the time when the least sun is available is the time when you need the most heat, and there's no good way of storing it. So you have to either build collectors that are ridiculously oversized for most of the year, which is not cost-effective (nor green) and can cause operational issues. Or you size it for some happy median and figure out a way to burn something for heat in the cold parts of the year.
I hope that before building anything you do some heat load calculations, we can help you with those. Let me throw out a few back-of-the-envelope calculations of the kinds of things you should be looking at.
First thing you need are some climate stats for where you are, which you can find here: https://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/County%20Level%20Design%20Temperature%20Reference%20Guide%20-%202015-06-24.pdf . I don't know what part of MN you are in but I picked Lake of the Woods, which has a design temperature of 82F for cooling and -19F for heating. The reporting weather station is Baudette which has an average January temperature of 4F.
For simplicity, assume your house is a cube that's 10 feet on a side. Walls, ceilings and floors are insulated to R40, you have one door that is 30"x80" and one window that is 36"x24", the door is R5 and the window is R4 (U=0.25). On the design day your heat loss is going to be 1285 BTU/hr through the walls, ceiling and floor, 297 through the door and 133 through the window for a total heat loss of 1714 BTU/hr. (But that's not all, read on for the impact of ventilation air). On an average January day (4F) it's 1272 BTU/hr. That's about 500W, which isn't much in the world of heating but not insignificant for off-grid.
In terms of solar gain, this article:
https://www.greenbuildingadvisor.com/article/a-quantitative-look-at-solar-heat-gain
Has gain values for US cities, including Minneapolis, which it lists as 452 BTU/Square Foot/Day for south facing windows. (The article assumes a solar gain coefficient of 0.5, which may not be realistic for windows sold in the US). So that six-square-foot window gains you an average of 2712 BTU/day. The thermal loss through that window is 2376 BTU/day, so while the window is a net positive, the net is small and isn't going to contribute meaningfully to warming the rest of the house. It also depends quite a bit on the choice of window.
In the summer, I'm presuming you won't have air conditioning off-grid. The traditional way to cool in places where it doesn't get too hot is to open the windows. Basically you have to have enough ventilation to make up for the solar gain through the windows. For example, if it's 82F outside at 86F inside, 200 CFM will give you 650 BTU/hr of cooling, which might make up for the solar gain of that window. That's probably more airflow than you can get without some sort of fan. In the summer, shading and insulation are your friends.
Finally, let's look at ventilation. The EPA recommends 15 CFM per person, so for two people that would be 1800 CF/hour. The house is 1000 CF so that's 1.8 ACH, which people here would consider moderately tight. If you build it any tighter you'd need mechanical ventilation which would be a complication off-grid. The point is that since the house is so small you probably don't want to build it super-tight. If you end up heating with combustion you really don't want it tight at all. For that 30 CFM, at the design temp of -19F you need 2883 BTU/hr to warm the replacement. Add that to the 1700 BTU/hr going through the walls and that's 4500 BTU/hr, or about 1300 Watts.
Just realized I screwed up the air-change calculation. Yes, you need 1800 CF/hour, but that's 1.8 ACH0, not ACH50. To get that you're going to need a very leaky house, not a somewhat leaky house.
One great saying to remember is: "The very best window makes a really lousy wall" so don't exchange a lot of R30-50 walls for big R-5-9 windows. Yes, you do get heat gain through those windows during the day but you'll lose that heat through them at night. Also, with a lot of big windows facing south you'll have trouble shading them - it will be like sitting in an oven with a lot of glare all day long.
Shed roofs that angle up towards the south are tricky to design - people always think that all they have to do is build a deep overhang and that will shade the windows. Maybe the upper windows for certain times of the day or at certain times of the year, but not the lower windows. You can do a free overhang shading calculation here.
https://susdesign.com/overhang/
As to heating equipment for off grid, you should consider a Mr. Cool heat pump mini-split.
https://acclimatic.ca/?gclid=Cj0KCQjw7aqkBhDPARIsAKGa0oLq_3c54-YHb4O-DZHSOCWP_5qOKlaItJpxmW1izjYU-Tzp34TbQDkaAlJTEALw_wcB
This system doesn't need to have an installer come out to your site to commission or balance it - the line-sets come pre-charged so it's completely DIY .
Additionally, if you are building a tight house, you will probably want to look at your cooking source - propane stoves pump a lot of water vapor into the air so you'll also have to consider having a kitchen vent hood, possibly an ERV and possibly a dehumidifier - all of which add to your solar PV requirements.
Thanks everyone for the excellent and detailed responses! I'll respond to all individually once I've had a chance to learn more about your suggestions. I've started researching some of the topics raised.
Something else to consider is how often this cabin is occupied. Many options are relying on quite a lot of usage to cover the overhead of their construction/installation.
I lived off-grid for decades. Your choices are few: either a woodstove or a propane space heater with through-the-wall venting. Neither is particularly green.
For more on this tension -- off-grid vs. green -- see this article: "Off-Grid Homeowners Contemplate Our All-Electric Future."
The money quote of the article: "For those who’ve looked into the issue, it’s clear that the off-grid lifestyle—at least the version of the off-grid lifestyle that prevails in North America—is usually more environmentally destructive than the grid-connected lifestyle."
Not only is that true, but somewhat counter-intuitively the least environmentally-destructive way to live is at great density, in cities. It's harder on the local environment, but you impact a lot less of it. And people in cities tend to use a lot less energy for transportation and heating and cooling than people out in the country.
DC,
Off-grid only makes sense where tying to the grid is not an option. That reasoning informs our codes here in Canada, which require you to connect if service is available where you are building.
That's on an individual level. On a societal level we should be doing less to encourage people to live dispersed and more to encourage them to live more densely. For example, in the US we have policies like Rural Electrification and Universal Service that subsidize utilities for people in rural areas, that's the opposite of what we should be doing. In most of the US you can have a well and septic tank if you have two acres, that's actually way too small and areas that are built out that way have serious problems with runoff and groundwater pollution. We should be encouraging more people to live close to city centers on water and sewer lines.
DC,
Policy aside, in large part the reasoning is that building codes now require houses to have certain features - full time mechanical ventilation, wired-in smoke and CO2 alarms, etc. - which can't readily included without being grid-tied.
There is the Hotspot Solar Heat Pump Mini-Split that doesn't require batteries - it just runs off of solar panels although it can be connected to grid power as well.
https://canadianoffgriddepot.com/collections/solar-powered-ac-heat-pump/products/solar-powered-air-conditioner-heater-18-000-btu
https://canadianoffgriddepot.com/blogs/news/solar-powered-heat-pumps-air-conditioning-heating
Rockies,
That's not appropriate for heating an off-grid house. An off-grid house needs heating at night and during the cloudy days of winter, when days are short and sunlight is scarce.
I'd be a little more conditional: if you're going to try to heat an off-grid house with a heat pump, that's the one to use. It's probably not going to be able to provide all your heat but it might be a piece of the puzzle.
DC,
If the sun is shining during the winter, it's heating up your house through your south-facing windows. So you don't need much supplemental space heat on a sunny day. You need heat when the sun goes down, and on cloudy days.