Can Naiades: first winter in our Passivhaus…warmth, comfort and tiny energy bills

ideadmin on 19 May 2026

First winter in our Passivhaus: The house has felt warm, calm and incredibly comfortable throughout the winter.

Can Naiades: first winter in our Passivhaus…warmth, comfort and tiny energy bills

By Oliver Style, Praxis CEO

I’ve lived in a few cool-temperate climates over the years – the UK, Switzerland, northern France – places where you’d expect winter to feel like winter. But, weirdly enough, I don’t think I’d ever really felt the cold at home as much as when I first moved to Barcelona in 2010 and installed myself in a flat in the historic city centre. Not because Barcelona is especially cold. It isn’t at all. But because many homes aren’t fit for the (thankfully!) short Mediterranean winter, and- in the case of the flat I moved in to- didn’t have the luxuries of central heating.

So, having got through our first summer in Can Naiades — our new Passivhaus — with flying colours, the next big question was: what would winter be like?

Would the house stay warm? Would the real energy consumption match the PHPP model? Would the solar PV and battery still pull their weight at the time of year when solar generation is at its lowest and energy demand is at its highest? Let’s have a look…

Nice and toasty

The short version is: it has been absolutely lovely.

The house has felt warm, calm and incredibly comfortable throughout the winter. Not “warm” in that slightly aggressive way you get when a radiator is blasting away in one corner of the room while your feet are still cold. Just evenly, quietly, consistently warm.

Temperatures have been stable and homogeneous throughout the house, with no noticeable cold spots, no draughts, and no cold air pouring in around windows, doors or junctions. That might sound like a small thing, but when you’ve lived in enough leaky buildings, the absence of discomfort becomes a kind of luxury.

There’s also something very particular about the quality of the air in a Passivhaus in winter. Because the house is airtight and ventilated continuously with heat recovery, we’re not relying on random infiltration through cracks and gaps to provide “fresh” air. The ventilation system just gets on with it: extracting stale, humid air from the kitchen and bathrooms, supplying filtered fresh air to the bedrooms, office and living spaces, and recovering heat from the outgoing air in the process.

The result is a house that feels fresh without feeling cold. That’s the magic trick. And then there’s the other great thing: no mould and no condensation. None. Zero.

This is the first home I’ve lived in, in Catalonia, where winter has not meant some combination of wet window frames, condensation on glass, suspiciously dark corners, or the occasional nasty surprise behind a wardrobe. In previous homes, condensation and mould felt like something you just had to manage: ventilate more, heat more, wipe it down, paint it over, move the furniture away from the wall, hope for the best.

Here, it just hasn’t happened. That’s not an accident. Warm internal surface temperatures, good insulation, reduced thermal bridges, airtightness, and continuous mechanical ventilation all work together. In practice, the experience is wonderfully uneventful: walls stay warm, indoor humidity is controlled, the air stays fresh, and nothing goes furry.

Fantastic!

The numbers

Of course, comfort is the most important thing. Buildings are for people, not spreadsheets. But the numbers matter too — especially if we want to show that low-energy, high-comfort buildings work not just in theory, but in real life, with real families, real weather, real cooking, real showers, real washing machines, and real life generally getting in the way.

So how did Can Naiades perform?

Really well. Over the winter period, the total measured energy consumption of the house was only 3% higher than the values predicted by the PHPP energy model. That’s a pretty remarkable result, given that PHPP is a design tool and real life is…well, real life, and consistently unpredictable.

There are always differences between modelling and measured performance: occupant behaviour, set-point temperatures, appliance use, hot water consumption, weather variations, commissioning, controls, and the million small things that happen once a building is occupied. So, to be within 3% of the predicted value is a very good sign that the design assumptions, construction quality and installed systems are all broadly doing what they were supposed to do.

Even better, despite winter being the worst time of year for solar generation — short days, lower sun angles, more cloud, and higher household demand — we were still 68% self-sufficient between November and February, using electricity generated by our solar PV panels and stored in the battery.

That really changes your relationship with energy. You become much more aware of when the sun is shining, when the battery is full, when it makes sense to run the washing machine, and how little energy the house actually needs to stay comfortable. It’s not about living with less comfort. It’s about getting more comfort from much less energy.

Compared with our previous flat, the difference is stark. This winter, we spent 79% less on energy bills and used 93% less energy.

Ninety-three percent less energy!

That number still makes me stop and look at it twice. Because this isn’t a smaller, colder, more miserable house. It’s the opposite: it’s bigger, warmer, healthier, quieter and more comfortable. We’re not saving energy by putting up with discomfort. We’re saving energy because the building fabric does most of the work before the mechanical systems even need to get involved.

That, for me, is the key point.

The house doesn’t need much heating because it doesn’t lose much heat and because the sun does most of the heating. The windows don’t feel cold because they’re high-performance and properly installed. The air doesn’t feel stale because the ventilation system is doing its job. The indoor temperature doesn’t swing all over the place because the envelope is insulated, airtight and carefully designed. The systems can be small because the demand is small.

It’s all very boring, really. And that’s exactly how it should be.

Can Naiades: First Winter PHPP vs. measured energy comsumption

Can Naiades: First Winter. Measured solar PV production

Can Naiades: First Winter. Real solor PV generation vs. real energy consumption

The best kWh…

After our first summer, I wrote that living in a Passivhaus in a Mediterranean heat wave felt like a dream come true. After our first winter, I’d say the same thing again — only with a jumper I didn’t really need.

Can Naiades has been warm, fresh, dry and comfortable, while using a tiny amount of energy. The PHPP model has proven to be very close to measured reality. The PV and battery have provided a surprisingly high level of self-sufficiency, even in winter. And our energy bills have dropped dramatically compared with our previous home.

But beyond the technical satisfaction, there’s a broader reflection.

We often talk about energy in terms of production: more renewables, more generation, more infrastructure, more supply. And of course, we need all of that. But living in this house is a daily reminder that the best kWh is still the one you don’t consume.

That truth resonates every time another war breaks out, another geopolitical crisis sends energy prices sky high, or another family has to choose between heating their home properly and paying the rest of the bills.

Reducing demand is not boring. It’s resilience. It’s comfort. It’s climate action. It’s protection against volatile energy prices. And, at the most basic level, it means living in a home that feels good.

After one summer and one winter in Can Naiades, I can confirm: Passivhaus works. And it works beautifully.