How much forest does one need to keep his home heated.

April 24, 2006 11:41 AM

How many forested acres would it take to heat a modest midwestern home in perpetuity?

Say you've got a house in, I dunno, central Kansas, that you will heat only with wood from the surrounding forest. For argument's sake, the home is well-insulated (but not exceptionally so), and you need to heat 12,000 cubic feet, or a 1500 sqft home with 8 foot ceilings to a cozy 68 Fahrenheit degrees. The surrounding forest is moderately wooded, consisting chiefly of deciduous trees. You cannot plant trees-- you must wait for nature to replace the trees that you have cleared. You may not burn brush or grass to heat your home.

Assuming a typical natural rate of forest succession, how many acres will you need so that you always have enough wood to heat the house?

posted by Kwantsar to science & nature (15 comments total)

I can't give you any actual numbers, but what you're talking about exists and is often called a woodlot.
posted by driveler at 12:11 PM on April 24, 2006

This is hard to answer, obviously. I remember from a forestry class I took in college that the professor claimed that a 10 acre woodlot would supply enough for 1-2 cords of wood per year. But this was a managed woodlot, which matters quite a bit. Here is an old but decent take on practical forestry that is short but lays out several of the issues to consider. This article from the Virginia Coop Extension service talks about calculating how much wood you might need, which would have to be increased for Kansas.

I'd just suggest that harvesting wood becomes a kind of de facto forest management, so the strictures on your thought experiment might not make that much sense. In other word, once you start harvesting, unless you just take deadfall, there is no "natural forest" anymore.
posted by OmieWise at 12:20 PM on April 24, 2006

Central Kansas has no trees.
posted by geoff. at 12:20 PM on April 24, 2006

Tough question. I would imagine there's another variable to consider - is there ductwork and a fan to move the warm air about the house, and is it two storeys or a bungalow?

I found a site at Natural Resources Canada that looks quite relevant though it doesn't answer the question as it doesn't deal with the issue of maintaining the woodlot itself.

As well, the other thing that comes to mind is that if the woodlot is forested with hardwood now, when it is culled the replacement trees will likely first be softwood (poplar and other fast-growing trees), which are decidedly less efficient.

Then there is the issue of drying the wood - is there an option to have a kiln or something to dry the wood more quickly, or will you rely on air drying?
posted by mikel at 12:27 PM on April 24, 2006

My forestry degree brother-in-law figures he can sustain a harvest of 1/2 cord per acre of high quality upland New Hampshire oak/maple/beech/birch forest per year. He's planning to heat his house entirely from a 10 acre woodlot. Based on what he paid for the land and pays in taxes and the time it takes to cut trees and haul and stack wood and the space to store the wood (two seasons to cure) and splitting logs, etc., etc., etc., I don't think it's a very good deal.
posted by TimeFactor at 12:35 PM on April 24, 2006

Great answers. As I think about it, the curing/drying issue ought not effect the throughput, but it would have a drastic effect on cycle time that would need to be worked around.

Also, it never occurred to me that Hickory and Oak boast nearly twice the energy content of poplar. Interesting. Thanks, everyone (except geoff.)
posted by Kwantsar at 12:51 PM on April 24, 2006

One nice thing about your brother-in-law's plans to use only wood for heating is this: if it turns out not to be enough, he can look into augmentation with a solar system -- so still "off-the-grid", as it were.
posted by davejay at 2:28 PM on April 24, 2006

To address the question on a larger scale: There are 730 million acres of hardwood forest in the US, but they contain 40% evergreen trees you don't want to burn for heat. So effectively, 292 million acres. If the average house would use 5 cords, which sounds about right, at the half-cord per acre sustainable yields that would heat 29 million homes. But we'd have to quit making furniture.

Nobody would use a kiln to dry out their firewood, by the way.
posted by beagle at 2:38 PM on April 24, 2006

Cabin in the Adirondacks (near Saranac Lake - often one of the coldest places in the nation).
Heated exclusively through a centrally located wood stove(potbelly type) for almost 25 years now.
The woodlot is about 3-4 acres, mostly we use downed trees but not always from the woodlot, sometimes they come from the main property.

Things to consider -
It's a cabin, so we mostly heat the one room(dining/kitchen), the upstairs gets residual heat, but is much cooler than the downstairs.
Comfort level, we rarely take it above 65 degrees, keeping it above 70 takes _a lot_ more heat. Bedrooms are probably below 60.
Although it's cold, we're tucked in a bit of a hollow, so we don't get that Kansas winter wind I remember so well.
Heating exclusively through wood (if you do all the work yourself) is a bit of work. We dedicate probably a week or two in the summer just to replenishing the stockpile.
Buy (or rent) a log splitter. Doing it by hand builds character, but log splitters are just fun.
We burn every bit of the tree in one form or another. Don't waste it, and don't let it rot.
posted by madajb at 2:41 PM on April 24, 2006

I just spoke to someone this weekend about heating using corn and a quick googling turns up a lot of corn stoves. If you're interested in comparing options for growing your own heat you might want to look into it as well.
posted by phearlez at 3:09 PM on April 24, 2006

The most important thing to consider here is not how much to heat it, but how you can use insulation and building design to lessen the need for burning a bunch of wood.

Super-insulate the home, face it south, and build it so that there is plenty of thermal mass to maintain consistency of temperature. Do those, and you'll end up with a cozy home in winter and a cool one in summer.

There's a lot to this, but plenty of it is available on the net. Look up "strawbale building" and "cordwood house" for insulation and thermal mass, and go from there. The forums on Daycreek.com are very useful in this regard and full of people who have built with some or all of these principles.

Myself, I'm planning a cordwood house in the BC interior on 5 acres (3 wooded). I don't think I'll be able to completely support my heating needs on wood alone, but won't be surprised if the building I create makes it much more likely.
posted by Kickstart70 at 3:35 PM on April 24, 2006

0, apparently.

Note that this fits the "modest" constraint--the guy claims that the heating system paid for itself in the first several years.
posted by A dead Quaker at 5:37 PM on April 24, 2006

I heat a 2500 square foot (3 story) log cabin entirely by wood. It takes 10 cords a year to keep it at 63-65 degrees. I would say that 1/2 cord/acre is right for good hardwood.

Cutting that wood and splitting it with a splitter then stacking it in a drying shed takes about 60 hours (clearing brush and making trails for hauling it with a 4 wheeler takes about 1/3 of that time). It is not recommended to cut, haul or split in the rain as it gets very slippery.

Kiln drying wood is bad. The moisture content of good firewood is around 20-25% while kiln dried is around 5-7%. This results in a fire in a woodstove which is very hard to control and chimney fires are a big problem. I get well dried wood by logging and splitting in the early spring, stacking it in a well ventilated shed for summer and fall so that burning can start in November. As expected, split wood dries much faster. You need to leave about 4" between rows of wood and if each piece is laid curved side down onto the row below, then good circulation will dry out the wood quickly. I stack each row 7' high, because it is within sheltering walls. If the wind were able to get at the wood, you shouldn't stack higher than 4'. Even that can blow over in a gust, so each piece of wood must be checked for stability when stacked.

You need to have a woodstove with a forced air heat exchanger. This can almost double the efficiency. The first floor of this cabin is tile and the second is carpeted wood. A central stairwell is the main conduit for hot air. By making a large opening in the drop ceiling above the stove so that the rest of the ceiling becomes a plenum, I can keep the entire house at the same temperature. To enhance circulation in the plenum, I removed ceiling tiles (2' x 4') from each corner of the basement to act as cold air returns. This heats the huge mass of tiles quite evenly and acts as a thermal buffer. The second floor gets stairwell heat and the foors are warmed by the hot air rising in the rooms below. There is only a 2 degree difference between floors. The full basement where the stove is generally runs at about 80 - 85 degrees.

There is a huge difference between wood that has been cut green or dead only one season and wood which has started to get "punky". The latter will contain only a fraction of the energy, so while it may be nice to clean up the dead wood, you might rue that come February when you have used up your winter supply. I have logged in the winter and it is not fun. You also get to burn wet wood, which has much less heat and takes constant minding to keep from starting a chimney fire. That is because the draft setting necessary to sustain a fire in wet wood is much too high once the water boils off.

Finally, you need to budget time to tend the stove. Figure on about 1 hour/day for good wood, double that if the wood is less than ideal. Each stove is different and each type of wood (maple, beech, cherry, hickory, etc) burns differently. Thus, you need to plan to watch the stove closely for the first few weeks that you run it to learn its quirks. (I've had mine go from room temp to 900 degrees in under 10 minutes! The next day it took 2 hours to reach 600. The only difference was the way the wood was stacked inside.)
posted by RMALCOLM at 8:15 PM on April 24, 2006 [2 favorites]

RMALCOLM -- flagged as particularly informative.

As for cutting wood for fuel, I can't say, but I would be surprised if you could sustain oak, hickory, maple, or red cherry cutting 1/2 cord per acre per year. It would have to be beech, poplar, ash, pine or another fast growing species. Red and white oak saplings I remember on my parents property from 25 years ago are now only 6-7 in. in diameter.
posted by mrmojoflying at 8:37 PM on April 24, 2006

Keep in mind that you may need a great deal more land to have the same number of trees in Kansas (as opposed to New Hampshire) -- being that it is prairie land and does not naturally have many trees.
posted by Margalo Epps at 8:42 PM on April 24, 2006