Free heat is the first budget line
When you are off-grid, every watt matters. Propane costs money. Wood takes labour. Solar panels and batteries carry steep upfront costs. But south-facing glass collects energy for free — no fuel, no moving parts, no maintenance. The catch is that passive solar only works if you design for it from the start. Retrofitting orientation is not possible. Retrofitting thermal mass is expensive. Retrofitting overhangs is awkward at best.
On Denman Island, we get mild winters but heavy cloud cover from November through February. That changes the math compared to the Okanagan or the Prairies. You cannot count on five hours of direct sun every winter day. You design for the sun you actually get — and you make sure the building holds onto it.
At Heartland Acres, the main structure faces 12 degrees west of true south. Not perfect, but close enough. The difference between 0 and 15 degrees off-axis costs you less than 5% of annual solar gain. The difference between 0 and 45 degrees costs you roughly 25%. Orientation is the highest-leverage decision you will make, and it happens on day one.
The five things that have to work together
Passive solar is not one trick. It is five systems that reinforce each other. Get one wrong and the others compensate poorly. Get three wrong and you have a cold, overheating, or uncomfortably swinging house — sometimes all three in the same week.
True south is the target. Magnetic declination on Denman Island is about 16 degrees east, so if you are using a compass, correct for it. Acceptable range is within 15 degrees of true south. Beyond that, you start losing meaningful winter gain. Site constraints like slope, trees, and road access sometimes force compromises — but know the cost before you accept it.
More glass means more solar gain during the day and more heat loss at night. The sweet spot for coastal BC is a south-facing glazing ratio between 7% and 12% of total floor area. Go below 7% and you are leaving free heat on the table. Go above 12% without exceptional insulation and you will overheat in September and freeze in January.
Glass lets energy in. Mass holds it. Concrete slabs, stone veneer, water tubes, or earthen floors absorb heat during sunny hours and release it slowly overnight. Without adequate mass, a passive solar house swings 10-15 degrees between afternoon peak and early morning low. With it, the swing drops to 3-5 degrees. On the coast, where we get intermittent sun between cloud banks, mass smooths out the rapid cycling that would otherwise make the space uncomfortable.
The sun is high in summer and low in winter. A properly sized overhang blocks direct gain when you do not want it and admits it when you do. On Denman Island at 49.5 degrees latitude, a 24-inch overhang on a standard 8-foot wall shades the glass from late April through mid-September while allowing full exposure from October through March. Get the depth wrong by even six inches and you either overheat in shoulder seasons or lose winter collection.
None of the above matters if the envelope leaks. A passive solar house needs above-code insulation — R-30 walls, R-50 roof, R-20 under-slab minimum for coastal BC. Air sealing is arguably more important than R-value. A house that tests at 1.5 ACH50 will outperform one rated R-40 everywhere but leaking at 5 ACH50. On the wet coast, the vapour profile matters too. Dry to the outside. Rain screen on every wall.
Sizing for the wet coast
These are not textbook values. They are adjusted for the 1,500-1,800 heating degree days typical of the southern Gulf Islands, the heavy cloud cover from November to February, and the mild but damp shoulder seasons that make moisture management as important as thermal performance.
| Parameter | Recommended Range | Notes |
|---|---|---|
| South glazing ratio | 7–12% of floor area | Below 7% underperforms; above 12% risks overheating |
| Thermal mass | 3–6 in. concrete or equivalent | Direct-gain floors, Trombe walls, or water tubes |
| Overhang depth (8′ wall) | 22–26 in. | Sized for 49.5°N latitude |
| Wall insulation | R-28 to R-35 | Double-stud or exterior rigid foam |
| Roof insulation | R-50 to R-60 | Raised-heel truss for full depth at eaves |
| Under-slab insulation | R-20 minimum | 2 in. XPS or 4 in. EPS |
| Air tightness target | ≤ 1.5 ACH50 | Blower door tested |
| East/west glazing | Minimize | These orientations gain little in winter and overheat in summer |
How heat moves through a passive solar house
The sequence matters. Solar radiation enters through south glazing, strikes thermal mass surfaces, converts to heat, and radiates back into the living space over the following 6-12 hours. The envelope keeps that heat inside. The overhang prevents the same process from overheating the space in summer.
On cloudy days — which is most of winter here — diffuse radiation still contributes. South-facing glass collects roughly 30-40% of its clear-sky potential under overcast conditions. That is not nothing. Over a six-month heating season, it adds up to meaningful savings. The key is that your thermal mass carries you through the cloudy stretches without needing backup heat for every grey afternoon.
What I am seeing at Heartland Acres
The numbers from the first winter season at Heartland Acres track closely with what the CUBED Passive Solar Calculator predicted. The south-facing glazing ratio landed at about 9% of floor area. Thermal mass is a 4-inch concrete slab on grade with 2 inches of XPS underneath. Walls are double-stud at R-32. Roof is R-55 blown cellulose.
The indoor temperature swing on a sunny December day is about 4 degrees — from 19C at dawn to 23C by mid-afternoon. On fully overcast days, the slab releases enough stored heat to keep the space above 17C through the night without backup. That is the whole point. The mass carries you.
What I see owner-builders get wrong
Three errors come up repeatedly. All of them are avoidable if you run the numbers before you pour concrete.
Owner-builders love big windows. So do I. But south-facing glass above 12% of floor area without proportional thermal mass creates a house that overheats by October and loses heat all night in January. More glass requires more mass and more insulation — the three scale together or not at all.
A wood-frame floor with no slab and no mass wall has nowhere to store daytime gain. The air temperature spikes during sun hours and crashes after sunset. Adding a few hundred pounds of concrete, stone, or water in the direct-gain zone makes a disproportionate difference. This is not optional in a passive solar design.
The third mistake is ignoring east and west glazing. On the coast, west-facing glass creates brutal overheating from May through September — right when you least want it. Minimize east and west glass. Put your view windows on the north if you must, and accept the thermal penalty knowingly.
CUBED Passive Solar Calculator
This is why I built the Passive Solar Calculator into CUBED. It takes your floor area, glazing ratio, insulation values, thermal mass, and latitude and runs the energy balance. You can see exactly how much south glass you need, whether your thermal mass is sufficient, and how deep your overhangs should be — before you commit to a foundation design.
Every formula is visible. Every assumption is exposed. If you disagree with a coefficient, you can see it and adjust. That is the glass-box approach. No black-box software telling you a number with no explanation.
The calculator handles the overhang geometry for your latitude, the heating degree day adjustment for coastal BC, and the thermal mass heat capacity calculation. It shows the daily energy balance — solar gain in versus conductive loss out — so you can see where your design sits before you build it.
If you are planning an off-grid home on the coast, run your numbers through it. The decisions you make at the design stage — orientation, glazing ratio, mass, overhangs — are the ones you cannot change later. Get them right the first time.