How to Heat a Castle

In an 18th-century chateau in France, the couple behind a popular YouTube channel turns to technologies old and new to keep warm.
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When you dream about buying an old home, whether it’s a 1920s bungalow in Los Angeles, a colonial farmhouse in New England, or an 18th century chateau in France, those dreams tend to take the form of beautiful wood details, spacious gardens, and perfect facades. They don’t usually include thoughts about thermal heating, wood stoves, or the virtues of stone wool versus wood wool insulation.

Unfortunately, as Philipp and Anna Mayrhofer found out, heating and cooling are just as important to the restoration of an old home as all of those glory fixes. The Mayrhofers are lucky enough to own one of those 18th-century chateaus in France; they’ve been documenting their restoration journey on their YouTube channel, How to Renovate a Chateau. "If you don’t heat a house in this area, in the winter it gets very moist and then you have dry rot, which can be a huge problem," says Philipp.

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"In this area" is important there, as well as the exact age of the house (late 1700s for the main house, early 1800s for an extension). The Mayrhofers’ chateau lies in Normandy, near the coast, about two hours from Paris. Historic homes in that region were designed to work with the seasons, which bring mild summers and wet, though usually above freezing temperatures, winters. The walls are made to resist water intrusion during the winters, but allow any moisture that does get trapped to evaporate in the warmer months, a pattern that some later residents would accidentally disrupt.

The Plaster Walls

"Unfortunately what happened in the 1960s, it happened a lot in Europe, is that they used concrete as the plaster on the outside because they thought it was watertight," says Philipp. Unfortunately, because it actually was watertight, water got trapped inside the walls during the winter, leading to mold and mildew. The Mayrhofers stripped off the concrete from the exterior and went back to the more traditional lime plaster. 

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The Fuel Heater  

Old houses are often built this way: in accordance with what has always worked in the area. In Normandy, that means thick stone walls to maintain a comfortable temperature, whether hot or cold. In other parts of the world it might mean different materials, like masonry, local wood, or even some form of earth. These do not, however, always lend themselves to modern heating and cooling technologies. In the United States, starting in the mid-19th century, many homes were built on what’s called "balloon framing." In this style, says Loren Bottem, a field manager with the Twin Cities-based heating and cooling company Genz-Ryan, "the house exterior was built first, and then the insides were constructed. There is no real way to [add ductwork] nicely unless a contractor gets involved and makes duct chases throughout the home." 

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The Radiators  

It often simply isn’t possible to add central heating and cooling to old homes, whether that’s due to the construction of the home, regulations for old building maintenance, or the basic aesthetics of exposed ductwork. Instead, says Bottem, "Boiler heat, high velocity, or mini split systems were made for these types of homes." The Mayrhofers went one further: they found an operational furnace large enough, at around 45kW, to heat their home, already installed; so they shipped in a whopping two tons of cast iron radiators, two per room, to serve as heating. They’re individually controlled, and, says Philipp, it costs around six to seven thousand euros per year to heat the home, but it was by far the easiest and most affordable option.

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The Windows

What made the biggest difference for the Mayrhofers, and this is echoed by the heating and cooling experts I spoke to, is putting on a home contractor Sherlock Holmes hat and hunting out all possible leaks in the home. In an old home, these are likely due to warped or poorly sealed window frames and door frames, but cracks could be anywhere. Insulating spray foam is your friend here: the fewer drafts in your home, the more efficient it’ll be, and the less you’ll need to expend in heating and cooling. 

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The Roof

But in some homes, like the Mayrhofers’ chateau, insulation options are surprisingly limited; with stone walls, they couldn’t simply knock holes and fill the walls up with fiberglass. Instead, they opted to insulate the roof using wood wool. Wood wool has several advantages over rock wool: it’s slightly more environmentally friendly, doesn’t shrink or clump as rock wool can when it gets wet, and dries out quickly. A one-way permeable membrane allows for any moisture to evaporate.

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The Fireplaces

Another potential stumbling block is the humble fireplace. Prior to the installation of the radiators in the 19th century, the Mayrhofers’ chateau was heated exclusively with open log fires. These are cozy, but, it turns out, wildly inefficient. "Generally, a wood-burning fireplace is a very inefficient way to heat your home. Fireplace drafts can pull the warm air up the chimney, causing other rooms to be cooler," according to a guide from the EPA. There are still fire-based solutions, but efficient ones tend to be smaller, hotter, and enclosed. Wood or pellet stoves look, and frankly operate, a bit like a small metal oven that you can insert directly into an existing fireplace. They’re enclosed, and allow you to burn logs or pellets with much more control, greater warmth, less smoke, and more efficiency. Plus, they’re still very cozy. 

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The Doors  

As for air conditioning, most of this still applies—but not to the chateau. It has no air conditioning at all; this is a cultural difference, and Philipp says it is not especially common for homes in that part of Normandy to have air conditioning. Instead, he opens two large, main doors, one on either side of the home, which provide a cross-breeze, and relies on the insulation provided by the thick walls and wood wool. "Most people [in this area] do not have it; it would be complicated to install it in a historic building without it being ugly," says Philipp. "So we just suffer through the one week, two weeks a year [when it’s hottest]."

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