Damp and Humidity in Spanish Coastal Homes — What’s Causing It and What You Can Do

If you’ve moved to the Costa Blanca from somewhere inland, or even from the UK, the humidity here can come as a shock. Not just to you personally, but to your home. Humidity on the coast regularly hits 80–90%, and if your house isn’t built to handle it — which, honestly, many of them aren’t — you’ll start to notice the signs fairly quickly.

Clothes that won’t dry. A faint musty smell you can’t quite place. Bedding that feels slightly damp. Mould appearing in the corners of rooms, or inside wardrobes. These aren’t just annoyances — they’re signs that moisture is building up somewhere it shouldn’t be, and it’s worth taking seriously.

Why ventilation alone often isn’t enough

The instinct is to open the windows and let the air through. And yes, that helps. But if the house was designed without proper ventilation in mind — no airflow paths, no extraction points, no thought given to how air actually moves through the space — then opening a window might just be letting more humid air in.

This is more common than you’d think. A lot of properties here, old and new, were built without any real ventilation plan. The result is that moisture accumulates over time, with nowhere to go. You can do everything right as a homeowner and still end up with a damp problem, simply because of how the place was built.

Where the moisture is actually coming from

For houses specifically, the source of damp is usually one of four places.

The terrace-to-wall junction

Where your outdoor terrace meets the exterior wall of the house is one of the most common entry points for water. In a properly built terrace, there should be waterproofing underneath — under the screed, under the tiles — along with proper drainage so water doesn’t sit and soak. The junction between terrace and wall should be sealed and waterproofed too.

In practice, we almost never see this done. What we do see is cracked tiles, damaged grout lines, and that telltale damp patch on the interior wall just inside the terrace door. The water has been slowly finding its way in every time it rains, or even just when dew settles overnight.

Balconies

Same principle as terraces. Balconies need waterproofing underneath the finish and they need to slope correctly so water drains away rather than pooling. A flat balcony with no drainage is essentially a shallow tray that slowly feeds moisture into the structure of the building every time it rains.

The roof

An older roof, or one that’s been patched rather than properly repaired, can let in water in ways that aren’t obvious until you’ve got damp patches on the ceiling or mould growing in the top floor rooms. If you haven’t had your roof inspected and it’s more than a few years old, it’s worth doing — especially before the rainy season. If you’re putting on a new roof, waterproofing is non-negotiable.

The foundations

This one is harder to fix after the fact. In some parts of the Costa Blanca you’re building on rock, which is generally fine. But in others, groundwater is closer to the surface than you’d expect. If the original builders didn’t properly waterproof and drain the foundations, moisture can come up from below — literally through the base of the house. Unfortunately there’s not much you can do about that structurally once it’s built. The best you can manage is improving internal ventilation and looking at internal waterproofing options, but it won’t solve the underlying issue.

The bigger picture

The common thread in all of this is that waterproofing and drainage simply weren’t taken seriously during construction — and this isn’t just a problem with old houses. We see it in relatively recent builds too. It’s just not something that’s been consistently prioritised here, and the people who end up living in these homes are the ones who deal with the consequences.

The good news is that most of these issues can be addressed when you’re doing renovation work. A terrace redo, a roof repair, a bathroom refurb — these are all opportunities to do things properly and protect the house against moisture for the long term. The key is knowing what to look for and not cutting corners on the parts that aren’t visible once the work is done.

As always, the calculator gives you a ballpark. The real figure comes after someone’s had a proper look at the job.