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Passive Solar Garage

Greenovision’s new website is pretty much complete now so we can get back to the fun stuff, creating new content about what we are doing and thinking about.

In Montana we have long winters (stating the obvious) and we have cold cars and trucks to get into every day.   Not only are cold vehicles uncomfortable to get into, it’s bad for the vehicles to cold start everyday.  The car’s lifeblood (its oil and coolant) are liable to gel and not flow correctly, which is very hard on the engine and ignition system. Gaskets and all rubber bushings, etc get stiff when they get cold (meaning below freezing) and function poorly.

More importantly, it’s not fun or enjoyable getting into a frozen icebox.  So with comfort and function in mind our garages need to maintain above freezing temperatures when housing them.  This not an incredible feat to accomplish in a new garage design; heating a garage is the issue.  The hurdle is that most people don’t really want to invest the money upfront to have their garage insulated and heated.

Let’s return to old man winter- it’s a cold sunny Montana day, the sun angles are low and cast long shadows, but the rays penetrate deeply into any south-facing window or door that has glass.  Just because it’s winter that doesn’t mean our projects in life stop.  We have utility needs, like where can I comfortably wax my skis without bringing toxic fumes into my home. We might need to have a bench for simple, yet possibly dirty hobbies, woodworking, repair work, etc.  It is always better to be doing such projects at temperatures that are liveable without gloves, hat, and a jacket.  What I am getting at is most of us have domestic needs for a heated accessory space as well as the functional goals of keeping our vehicle happily warm, allowing it to melt down overnight.

Most modern garages are insulated in the walls and ceiling which is great at keeping the interior temperature stable.  The weak link in insulating a garage is the concrete slab itself.  Often concrete work for garages omits placing a rigid foam insulative thermal break to the ground that the concrete slab sits on.  If the garage is not designed and detailed, as such, corners are cut. Another such detail left out are floor drains.  Knowing that your car will warm up and snow will melt from it is a no brainer, drains need to be installed and the concrete slab given the correct pitch to those drains to capture such vehicular runoff.

What this means is that the earth is convecting heat or cold directly into your garage as concrete is a strong thermal conductor.  This is where the rigid  foam insulation comes in.   It sets up a thermal break because the rigid foam is a poor conductor.  In Greenovision designs, we use a minimum of 2″-4″ of SPF or EPS rigid foam, which stops the ground’s temperature from migrating through the concrete slab.

Heating a garage is commonly done with added fuel-dependent heaters.  If a home is designed with a radiant floor heating system, it makes sense to also add tubing to the garage concrete slab as a separate zone with a thermostat set accordingly to how you end up using your garage.  For example, one of our clients houses his dogs in his garage at night, so he keeps the garage thermostat set around high fifties.

Another heating solution comes in the way of passive solar heating.  In order to bring the sun’s radiation into the garage, there are a variety of garage door glazing options that allow the door to harvest the solar radiation and natural light.  We commonly use a polycarbonate glazed door panel constructed from aluminum rectangular metal frame.  Polycarbonate cellular panels actually have an insulative value due to the cell’s air voids.  This type of panel has a  coating that reflects ultraviolet radiation.  UV light is not good for a vehicle’s paint job and plastic exterior finishes.


While you have driven away for the day, the sun’s energy charges the concrete slab, assisting in heating the garage.  On cold winter days when it is cloudy, the garage is containing the heat that was previously gained during sunny days.  This relies on the insulation and the seals of the garage doors and whatever windows.  The temperature is assuredly going to drop when the garage doors are opened and in general, the garage doors themselves have higher conductive heating loss than a insulated wall.

This is where added heating may be necessary if the occupant has needs like the pet-owning client mentioned earlier. Or if one finds themselves working in their garage a lot and have the need to keep it at human occupancy temperatures.

The huge added benefit of getting ample natural light into a garage makes the space much easier to work in, to see in your car when you get in, and for doing those various household chores impractical to accomplish inside the house. However, as with all passive solar designs, the garage has to follow basic passive solar direct gain principles: south facing garage doors and proper roof overhangs over the garage doors to keep summer overheating from happening by shielding the summer’s high sun-angled rays.  Such overhangs have the added benefit of  protecting the threshold of those ports of entry.


Another benefit for simple shed style garage roofs is a large surface for solar panels to be mounted on, as in the Hawk Ridge Home garage, which also has a radiant concrete slab run by a air to water source heat pump as a separate heating zone.  The electric heat pump is powered by the solar panels.  This is a simplification obviously of the system, but you get the point.  There is obvious added infrastructure needed like a PV inverter, grid tied power hook up, meters, etc for such a system to operate. This makes sense if such a system is built for the primary household heating requirements.

There are other methods of additional heating ranging from a simple wood stove to a modified wood stove with a heating coil like in the WerkHaus shop in which the sauna wood stove heats a water heating jacket, which in turn heats the concrete radiant slab.