Heating a Homestead: Firewood, Greenhouses, and What the Numbers Say
How much firewood does your home need? What size heater does your greenhouse require? Real math for staying warm on the homestead.
Two of the most common homestead planning conversations are "how much firewood do I need?" and "what size heater do I need for my greenhouse?" They seem like separate questions, but they're really the same question: how do I size a heating system for a specific load in my specific climate? The math is the same. The variables are just different.
Home Heating with Wood: What the Numbers Actually Mean
A cord of firewood is a precisely defined unit: a stack 4 feet wide, 4 feet tall, and 8 feet long — 128 cubic feet of stacked wood including the air gaps. When people talk about burning "a couple cords" per winter they're using a real, measurable quantity. The problem is that one cord of oak and one cord of cottonwood are very different amounts of heat — the BTU content per cord varies by nearly 3× between species.
| Wood Species | BTU per Cord (millions) | Relative to Oak | Notes |
|---|---|---|---|
| Black Locust | 26.8M | Excellent | Dense and long-burning; excellent coals |
| Hickory | 27.7M | Excellent | Best BTU of common species; great for smoking too |
| Oak (White/Red) | 29.1M | Baseline | The standard for firewood quality |
| Hard Maple | 25.5M | Good | Burns clean with good heat output |
| Cherry | 20.4M | Moderate | Pleasant aroma; lower heat output |
| Ash | 23.6M | Good | Burns well even when slightly green |
| White Birch | 20.3M | Moderate | Burns fast; popular but not efficient |
| Pine | 17.1M | Lower | Burns fast with more creosote; OK for kindling |
| Poplar / Cottonwood | 13.5M | Low | High moisture when green; poor heat output |
This matters practically. If you heat primarily with oak and you suddenly have access to a lot of cottonwood, you need approximately 2× the volume to get the same heat output. A neighbor who heats their similar house with 4 cords of poplar isn't telling you that you need 4 cords of oak.
How Many Cords Do You Actually Need?
The variables that determine seasonal firewood needs are: heated square footage, insulation quality, local climate severity, and wood species BTU content. A well-insulated 1,500 square foot home in Tennessee might need 2 to 3 cords of oak per winter. The same home in Maine needs 4 to 6 cords. A poorly insulated 2,000 square foot farmhouse in Wisconsin can burn 8 to 10 cords.
A rough starting point for a moderately insulated home using oak as your reference wood:
| Home Size | Cold Climate (Zone 3–4) | Moderate (Zone 5–6) | Mild (Zone 7–8) |
|---|---|---|---|
| Under 1,000 sq ft | 3–5 cords | 2–3 cords | 1–2 cords |
| 1,000–1,500 sq ft | 4–7 cords | 3–4 cords | 2–3 cords |
| 1,500–2,000 sq ft | 6–9 cords | 4–6 cords | 3–4 cords |
| 2,000–3,000 sq ft | 8–12 cords | 5–8 cords | 4–5 cords |
Greenhouse Heating: The Same Math, Different Scale
A greenhouse heater sizing problem is structurally identical to a home heating problem. You have an enclosed space losing heat through its walls at a rate determined by the area of those walls, their insulation value (R-value), and the temperature difference between inside and outside. The formula is:
BTU/hr needed = (Surface Area ÷ R-value) × Temperature Difference × Safety Factor
What changes between houses and greenhouses is that greenhouse glazing materials have much lower R-values than insulated walls. A double-pane glass wall might be R-2. An insulated stud wall is R-13 to R-21. This is why greenhouses lose heat so much faster per square foot than houses — the glazing is thin, and thin glazing means low insulation.
Glazing Material Makes an Enormous Difference
Your choice of covering material affects both your upfront cost and your ongoing heating cost more than almost any other greenhouse decision.
| Covering Material | R-Value | Light Transmission | Relative Cost |
|---|---|---|---|
| Single-layer poly film | 0.83 | Excellent | Lowest |
| Double-layer inflated poly | 1.7 | Very good | Low |
| Twin-wall polycarbonate | 1.6 | Very good | Moderate |
| Triple-wall polycarbonate | 2.5 | Good | Higher |
| Single glass | 1.0 | Excellent | High |
| Double-pane glass | 2.0 | Excellent | Highest |
Upgrading from single-layer poly (R-0.83) to double-layer inflated poly (R-1.7) cuts your heating load nearly in half. Upgrading from double-layer poly to triple-wall polycarbonate (R-2.5) reduces it by another 30%. Before buying a bigger heater, always evaluate whether upgrading your glazing would solve the problem more economically.
Thermal Mass: The Free Heating Upgrade
Thermal mass — dense material that absorbs heat during the day and releases it at night — is one of the most cost-effective greenhouse heating strategies available. Black-painted water barrels are the standard tool: they absorb solar energy during daylight hours and release it slowly overnight, reducing the temperature swing and cutting overnight heating demand by 20 to 40% in well-designed installations.
The rule of thumb is 2 to 3 gallons of water per square foot of south-facing glazing. A 12×20 greenhouse with 240 square feet of south-facing glass wants 480 to 720 gallons of thermal mass — roughly 4 to 6 55-gallon drums. Painted black on the outside, placed along the north wall to maximize solar exposure without shading plants, they work passively with no operating cost.
The Off-Grid Connection
If you're heating your greenhouse with propane or natural gas, you're buying fuel. If you're heating with electric and you're off-grid or trying to reduce grid dependence, your greenhouse heater becomes a significant part of your solar and battery bank sizing calculation. A 15,000 BTU electric heater running for 8 hours overnight is about 35 kWh — more than most homestead solar systems can provide from a single day's generation.
This is why the firewood and greenhouse heating decisions and the solar system sizing decision are often best made together. The energy demands of your heating loads directly affect how large a battery bank you need to handle overnight draw.