When you renovate an old building, you know this moment: You enter a room on the ground floor that hasn’t been properly heated or ventilated for years. It smells earthy, slightly musty, and the cold literally creeps up your legs from below. During my restoration project, an old farmhouse from 1888 in Transylvania, I faced exactly this fundamental problem. Many living rooms, as well as old workshops and storage rooms, were often built directly on the earth in the past. A cellar is completely missing there. The floor consists of compacted rammed earth, which permanently releases moisture from the ground upwards into the room.
When we talk about the topic of rising damp in old buildings, exactly this type of floor construction is one of the main causes for later building damage – but usually only when incorrectly renovated. If you ask craftsmen, architects, or construction companies for a solution today, the standard answer is almost universally the same: “Everything has to come out. Then a clean foil goes in, we pour a proper concrete slab, torch a bitumen welding membrane on top, and then pack insulation and cement screed on top of that.”
This approach complies with modern DIN standards for new buildings and has become second nature to craftsmen. But what is correct in a modern building with a ventilation system and complete insulation turns out to be the surest way to sustainably destroy the historic building fabric in old building renovations. I want to explain to you in detail from practical experience why modern sealing techniques provoke massive and expensive building damage in old buildings. And I will show you how you can instead create a permanently dry, healthy room climate that respects the building physics of your house by using capillary-breaking foam glass, heavy lime fillings, and a massive, vapor-permeable lime screed.
The Fundamental Difference: Open Systems vs. Closed Systems
To understand why historic buildings react so drastically differently to modern building materials, we need to take a deep look at building physics. A modern house today is designed as a self-contained, almost hermetically sealed system. From the floor slab made of water-impermeable concrete (WU concrete) to vapor-proof foils in the walls and a highly dense roof, the building completely isolates itself from the environment. Any moisture generated inside by breathing, cooking, or showering must necessarily be exhausted outside through complex technical ventilation systems or strict shock ventilation. The house itself no longer absorbs moisture.
An old building from the 19th century or older, on the other hand, functions as an open, breathing system. The building was mostly constructed without horizontal moisture barriers like roofing felt in the masonry joints. The building materials used – rubble stone, fieldstone, traditional solid bricks, lime mortar, and loam – were highly capillary-active and vapor-permeable (diffusion-open). This means that moisture from the ground could certainly penetrate into the walls and the floor in small amounts. However, this moisture did not accumulate. Through the chimney effect of old stoves, natural drafts at leaky wooden windows, and the large, vapor-permeable surface of the rammed earth floor, the moisture was permanently released back into the room air and ventilated outside. There was a dynamic, physical equilibrium that kept the house intact for centuries.
The Concrete Trap: Why Sealing Destroys the Walls
What happens now if you disrupt this centuries-old equilibrium with modern methods? Imagine you excavate the old rammed earth floor in the basement-less area. The earth underneath carries a natural, permanent base moisture. Now you pour a 20-centimeter thick concrete slab and weld bituminous barrier membranes (often called elephant skin) over it. You have now sealed the floor 100 percent. No moisture can enter the room anymore. Goal achieved?
No, quite the opposite. The water in the earth under your house is still present. The hydrostatic pressure and the capillary suction of the surrounding masonry continue unabated. Since the moisture can no longer evaporate upwards over the formerly large surface of the floor, it inevitably seeks another path. And in building physics, water always follows the path of least resistance.
Exactly at this point, the often-dreaded rising damp in old buildings emerges in its most destructive form. The consequences of this craftsman’s mistake rarely show up in the first few weeks, but after two to three years the damage pattern is clear and devastating:
- Massive plaster spalling: The pressure of the upward-pushing moisture and the crystallizing building-damaging salts (like nitrates and sulfates, often called saltpeter) blast the plaster off the walls over large areas.
- Loss of thermal insulation: A damp wall loses its already low insulation capacity almost completely. Wet stones conduct the heating warmth to the outside excellently. The room becomes permanently cold at floor level and feels unpleasantly clammy.
- Dangerous mold growth: Exactly where the warm, humid room air hits the wet and cold wall surface, the water from the air condenses. Black mold inevitably forms, representing a massive health hazard.
- Structural damage to the masonry: In severe frost, the drawn-up moisture in the outer masonry area can freeze. Because water expands when it freezes, it slowly blasts the joint network of the historic lime mortar.
With the supposedly safe concrete slab, you have not defeated the water; you have merely diverted it from the floor into your load-bearing walls. The house is literally renovated to death.
The Solution for Floors on the Ground: Foam Glass and Lime
For rooms standing directly on the ground, the structural challenge is: We must imperatively prevent liquid water (capillary rising damp) and the cold from the deep earth from entering the living space. At the same time, however, we must not hermetically seal the floor. The system must remain vapor-permeable towards the bottom to allow water vapor to pass through without forcing liquid water into the walls.
For this, foam glass gravel (cellular glass) is the perfect solution. It consists 100 percent of recycled waste glass that is expanded and baked. This capillary-breaking material absorbs no water. Once compacted, it completely blocks the wick effect from below, insulates excellently against the cold, and forms an extremely pressure-stable base.
This is followed by a massive lime screed. Naturally hydraulic lime (NHL) binds not only with water but hardens by absorbing CO2 from the air. It lets the floor breathe, stores heat, and effortlessly buffers moisture peaks from the room air. In addition, lime is naturally highly alkaline (high pH value) and offers mold fungi no breeding ground.
The Special Case: The Old Oak Beam Ceiling over the Rubble Stone Cellar
Now we come to a completely different, incredibly exciting situation in terms of building physics. Besides the unexcavated areas, old buildings often have rooms resting on an old cellar. The separation between the cellar and the living floor is mostly a classic wooden beam ceiling, often made of massive oak beams.
Let’s take a closer look at the cellar in my project: It has thick, unplastered rubble stone walls. Due to their rough, irregular structure, they have a gigantic surface area through which they continuously release moisture from the ground into the cellar air. The floor in the cellar is also an open rammed earth floor. The climate down here is permanently humid, but the temperature never drops below 0 degrees Celsius even in the harshest Transylvanian winter.
What does this mean for the living space above? Since the cellar is frost-free, we don’t need high-performance thermal insulation on the oak beam ceiling like in the earth-touching area. The main problem is the enormous vapor pressure. The warm air from the cellar, highly saturated with water vapor, inevitably pushes upwards. The classic mistake now would be to screw tight OSB panels onto the old wooden ceiling or to lay out a PE foil. The rising water vapor would condense on the underside of the foil or the glued OSB panel, permanently soak the oak beams, and cause them to rot within a few years.
The Ideal Fill: Lime Chippings Instead of Foam Glass
Historically, there was a heavy loam fill on the false floor (the wooden insertion between the oak beams). It provided weight for sound insulation (mass insulates low frequencies) and regulated moisture to protect the wood. Since pure building loam is hardly available in good quality in Transylvania today, I had to find an equivalent replacement. Foam glass gravel is not the right material here! It is too light (poor sound insulation) and its sharp edges can cut the building paper during the minimal vibrations of the wooden ceiling.
My tip: The physically and historically perfect alternative to the loam fill is a lime chippings fill or a dry expanded clay fill mixed with pure lime (NHL). Limestone brings the urgently needed heavy mass for impact sound insulation back to the wooden ceiling. Even more important, however, is the chemical aspect: Thanks to its high alkalinity, the lime preserves the oak wood perfectly. The fill is absolutely vapor-permeable. The water vapor from the damp rubble stone cellar can easily penetrate the wood and the lime fill without precipitating as liquid water.
The Construction: Step by Step to a Vapor-Permeable Floor
Regardless of whether you are working on the ground or over the damp cellar, the execution must be precise. Here both paths are explained in detail.
Area 1: The Basement-less Floor on the Ground
- Excavation and subgrade: Excavate the old floor, but never dig deeper than the bottom edge of the existing foundation! Level the earth to an even subgrade and compact it slightly.
- Lay geotextile (separating fleece): Lay out a robust road construction fleece (class 3 or 4) over the entire area and pull it up the surrounding walls to the planned top edge of the floor. The fleece cleanly separates the earth from the gravel but remains permeable to water vapor.
- Apply foam glass: Pour in the foam glass gravel (with approx. 30 percent excess height for compaction). Compact the material layer by layer with a light vibratory plate (approx. 70-100 kg) until it engages with a hard sound. The goal is at least 20 centimeters of compacted layer.
Area 2: The Oak Beam Ceiling over the Cellar
- Prepare the false floor: Clean the false floor (wooden insertion) from old debris. Line the spaces with a breathable building paper (Kraft paper) as a trickle protection so that the fine material does not trickle into the cellar.
- Apply lime fill: Fill the cavities between the oak beams with dry lime chippings or fine expanded clay. This layer is never compacted with a machine! Simply level it flush with the top edge of the oak beams using a board. This brings the mass back into the ceiling.
Common Steps: The Lime Screed for the Entire Floor
As soon as the substructure (whether foam glass or lime fill) is prepared, the screed construction follows for all rooms equally:
- Edge insulation strip and trickle protection: Place an ecological edge insulation strip (made of cork or wood fiber) around all walls. Then lay a full-surface layer of Kraft paper as a separating layer over the foam glass gravel or over the oak beams and the lime fill. This prevents wet mortar water from seeping into the insulation or the wood.
- Mix and install lime screed: Mix naturally hydraulic lime (NHL 5) with washed screed sand in a ratio of 1:3. The mortar must absolutely be mixed earth-moist (semi-dry) – never liquid! Apply the screed in a thickness of about 5 to 8 centimeters, level it horizontally over screed rails, and rub it intensively with a float until the surface is closed and hard. On the wooden beam ceiling, it acts as a massive, floating screed.
- The aftercare: Lime needs time. Protect the screed from direct sun and drafts in the first few days. Keep it slightly moist (e.g., with a fine water mist) so that it can optimally carbonate and harden.
The Final Floor Covering: Remaining Consistently Breathable
When the lime screed is light-colored and thoroughly dried out after a few weeks, the covering follows. Do not make the mistake now of sealing your open system with plastic! Untreated terracotta tiles (Cotto) can traditionally be laid in a lime mortar bed and subsequently oiled. If you prefer solid wood floorboards, you would have already had to press conical sleepers in during the installation of the moist screed. Oak, pine, or larch boards can then be concealed-screwed onto them and treated with pure linseed oil varnish.
Further Well-Founded Sources of Information
The renovation of old walls is a complex field. To deepen your knowledge regarding building protection, I can highly recommend the publications of the Fraunhofer IRB (Information Center for Regional Planning and Building Construction) and the leaflets of the WTA (Scientific-Technical Working Group for Building Preservation and Monument Conservation). Technical data on foam glass fills are provided by manufacturers like Glapor or Geocell.
To renovate a historic building correctly in terms of building physics means showing respect for the old craftsmanship. By relying on capillary-breaking foam glass in the earth area and protecting the wooden beams above the damp cellar with heavy lime fills, you preserve the house from rising damp in old buildings and create a healthy room climate for generations.