Planning an Energy Class A+ Renovation for Long-Term Durability
- Herman Kraut
- Sep 16
- 9 min read
Renovating an old granite house to meet modern energy standards is no small feat. Granite keeps cool in summer and stores heat, but on its own it leaks energy faster than you can stack firewood. Add in drafty windows, thermal bridges, and the wrong plaster, and suddenly your dream home is sweating in August and freezing in January.
The good news. With the right plan, you can lift even a centuries-old stone house into Energy Class A+ territory—without losing its soul. We’re living proof. On our homestead in Portugal, we’ve kept our 50 cm granite walls breathable with lime, while adding thick layers of wood-fibre insulation to the timber upper floor and roof, sealing every gap, and choosing windows and heating that work with the house instead of against it.
This post breaks down what to consider when aiming for the highest energy efficiency class in a granite home renovation. Step by step, we’ll cover walls, windows, airtightness, and heating—always balancing long-term durability with comfort.
Want the quick fixes and troubleshooting answers. Head straight to Herman’s Tough Kraut Fixes at the end of this guide.
Granite is mass, insulation is armor
When you aim for Energy Class A+, the assessor doesn’t just look at wall thickness or window type. The rating measures how much energy your home demands for heating, cooling, hot water, and lighting. In simple terms: how comfortable your house is, and how little it costs to run.
Granite walls give you thermal mass. That means they soak up daytime heat and release it slowly, smoothing temperature swings. But granite is not insulation. A 50 cm stone wall looks strong, yet its thermal resistance is roughly R-1 (m²·K/W)—not even close to what modern standards require. By comparison, 24 cm of wood-fibre insulation delivers around R-6.7 (m²·K/W).
That’s why planning a high-efficiency renovation means combining strengths. Let granite do what it does best—store heat and breathe through lime mortar and plaster—while adding proper insulation where you can, especially in the roof and timber structures. Then tighten up the weak spots: windows, doors, and air gaps.
Durability matters too. Using lime-based materials instead of cement allows your walls to dry out naturally. This keeps the stone healthy for decades while still helping you reach modern comfort levels.
Tough Kraut in Practice
On our own renovation, we left the 50 cm granite walls uninsulated for heritage and breathability. Instead, we stacked 240 mm of wood-fibre into the timber upper floor and roof, and carefully wrapped the ring beam. That’s where most heat loss happens in stone houses — not through the middle of the wall.
Walls and Roof: Get the envelope doing the heavy lifting
When you renovate for Energy Class A+, the walls and roof decide most of your energy demand. Granite walls provide thermal stability but very little resistance to heat flow. That’s why the roof, the upper walls, and every joint between them need to carry most of the insulation.
What to consider:
Granite walls: Keep them breathable. Use lime mortar and lime plaster to allow moisture to move. Avoid cement, which traps damp and weakens stone over time. If you want to insulate, add a vapor-open inner lining carefully detailed at floor and ceiling junctions.
Timber walls: Ideal for insulation. A 240 mm layer of wood-fibre or similar natural insulation brings U-values down to modern standards while storing heat in summer.
Roofs: Always your top priority. More heat escapes through the roof than any other surface. Aim for ≥ 240 mm insulation thickness, uninterrupted across rafters and ceiling junctions.
Thermal bridges: Pay attention at the ring beam, wall-to-roof junction, and around window reveals. Even small gaps can create cold spots and condensation risks.
Moisture safety: Combine insulation with lime plaster or breathable membranes so the structure can always dry out.
Tough Kraut in Practice
On our homestead, we decided not to insulate the granite walls but to wrap the ring beam completely with wood-fibre to block thermal bridging. The entire upper floor and roof were built from timber, allowing us to fit about 240 mm of Steico Flex insulation (120 mm outboard + 120 mm between studs). This way, the bulk of the home’s insulation sits where it works best — in the roof and upper walls — while the granite continues to provide mass, breathability, and durability.
Windows and Doors: Control the weak links
Even if your walls and roof are well-insulated, windows and doors often make or break your energy rating. They are transparent by design, but they shouldn’t be a hole in your envelope.
What to consider:
Whole-window performance: Don’t just look at the glass value (Ug). Check the overall Uw value (frame + glass + spacer). For Energy Class A+ renovations, aim for Uw ≤ 1.3 W/m²·K. If possible, go lower.
Glazing choice: Double glazing with low-e coating and argon gas fill is the current minimum. In cold climates or if you want the highest efficiency, triple glazing is worth the investment.
Frames: Timber, alu-clad timber, and high-quality uPVC can all perform well. Durability and maintenance differ, but performance is what matters most.
Sealing and installation: Even the best window leaks heat if poorly installed. Insist on airtight tapes or gaskets between frame and wall.
Shading: Exterior shutters or blinds reduce summer overheating and add night-time insulation. Especially important for south, west, and roof windows.
Doors: Like windows, choose insulated, airtight models with certified U-values. Large glass doors should have at least the same performance as your windows.
Tough Kraut in Practice
In our renovation, we combined new and second-hand solutions. We installed five roof windows (Velux 78×118 cm) plus a window with argon-filled glazing, certified A+ for insulation. We also sourced a balcony door and three double-glazed timber windows second hand from Austria. Buying used saved us money and gave high-quality timber frames another life, but it meant we had to pay extra attention to the installation: sealing every frame-to-wall junction with airtight tapes and filling gaps carefully. For summer, exterior shading helps us prevent overheating — a must in Portugal’s Mediterranean climate.
Airtightness, Ventilation, and Heating: Quietly decisive
Insulation alone won’t get you to Energy Class A+. If air leaks out through cracks, or if fresh air is brought in without control, your heating bills climb and comfort falls. Airtightness and smart ventilation are what turn insulation into real performance.
What to consider:
Airtightness layer: Choose one continuous air barrier on the warm side of the insulation. This might be a membrane, OSB board, or taped panel joints. Seal every junction, pipe, and cable penetration.
Testing: A blower-door test identifies hidden leaks. Aim for ≤ 1.0 ACH@50 Pa. If you manage ≤ 0.6, you are close to Passivhaus standards.
Ventilation: In an airtight house, you need planned ventilation. A balanced mechanical ventilation system with heat recovery (MVHR) is the gold standard, reclaiming up to 90% of outgoing heat while keeping air fresh. In milder climates, you can combine passive strategies with trickle vents, but beware of humidity build-up.
Heating choice: After you’ve cut demand with insulation and airtightness, size your heating to the new lower load. Too big a system wastes energy and money. Wood stoves, pellet stoves, or heat pumps all work when matched properly to the envelope.
Durability: Airtightness tapes and gaskets should be UV-stable and flexible to last decades. Cheap tapes may peel within years, undoing your hard work.
Tough Kraut in Practice
On our homestead, we sealed every gap in the timber structure and filled all interior joints in the granite with lime, creating a tight envelope. Instead of a large system, we installed a Nordica Isetta.16 cast-iron wood burner, sized for steady, efficient heating. Combined with solar power and our own well water, this keeps us independent while cutting energy demand.
Our roof and façade both have ventilated air layers — between the insulation panels and the roof tiles, and behind the untreated larch cladding. This keeps the insulation dry and prevents summer overheating. For airflow inside, we’ll use ceiling fans on the second floor: in summer to move air downward and cool, in winter to pull warm air back through the open staircase. On the ground floor bathroom, which has no windows, we’re adding a simple exhaust fan to remove humidity at the source. It’s a straightforward, low-energy setup that balances comfort, moisture control, and off-grid independence.
Durable comfort without losing the stone
Renovating a granite house for Energy Class A+ is less about covering the walls in foam and more about combining strengths. Granite gives you mass and breathability. Wood-fibre and airtight layers cut heat loss. Lime keeps the structure healthy. Windows and doors bring in light without letting warmth slip away. Fans and simple exhausts move air where it’s needed, while a modest wood stove matches the lower demand.
Do these things step by step and you’ll end up with a home that’s warm in winter, cool in summer, cheap to run, and built to last — all while keeping the soul of the stone intact.
Want ongoing tips and stories from the land. Join the Kraut Crew — our community of self-sufficient learners and doers — and grow your resilience with us.
Herman’s Tough Kraut Fixes: Common Energy Class A+ Renovation Challenges
Renovating a granite home comes with unique challenges, and mistakes can get expensive fast. That’s why we’ve put together this FAQ and troubleshooting guide for anyone aiming for the highest energy classes. These are the questions readers ask most often — with fixes we’ve tested and learned on our Tough Kraut journey.
Q: My granite walls are 50 cm thick. Do I really need extra insulation?
A: Granite walls provide thermal mass, not real insulation. A 50 cm stone wall only achieves about R-1, while modern insulation layers achieve R-6 or higher. For heritage reasons, you may leave granite bare, but then you must strengthen other parts of the envelope: roof, timber walls, airtightness, and windows.
Q: How do I avoid damp problems when renovating?
A: Use lime mortars and plasters (like NHL 3.5) instead of cement. Lime stays vapor-open and flexible, so your walls can dry naturally. Cement traps moisture inside the stone, causing cracks, salts, and decay. Always combine insulation with breathable finishes.
Q: Where do stone houses usually fail energy tests?
A: Thermal bridges at the ring beam, roof junctions, window surrounds, and floor edges. These areas bleed heat even if the main walls are thick. Fixes include wrapping the ring beam in insulation, carrying roof insulation across rafters, and taping window frames to the wall.
Q: I can’t afford triple glazing right now. What’s the best stop-gap?
A: Start by sealing your existing window frames with airtight tapes and gaskets. Add exterior shutters or blinds for night-time insulation and summer shading. If drafts are severe, consider temporary interior storm panels. Upgrade to higher-spec glazing when the budget allows.
Q: Do I need a fancy ventilation system like MVHR?
A: Only if your house is extremely airtight. For many granite renovations, a mix of natural ventilation, ceiling fans, and targeted exhaust fans is enough. On our homestead, we added an exhaust fan in the bathroom (no windows) to remove humidity, plus ceiling fans upstairs to recirculate warm and cool air seasonally.
Q: How do I size the heating system?
A: First cut demand with insulation and airtightness. Then size heat to the reduced load. Oversized stoves or boilers cycle inefficiently and waste energy. A smaller, well-run stove often outperforms a large one. Our Nordica Isetta.16 cast-iron stove is sized for steady heating, not oversized blasts.
Q: What documentation do I need for the energy assessor?
A: Collect everything in one folder:
Insulation datasheets with λ-values
Window and door certificates with Uw values
Ventilation system or fan specs
Heating system specs
Blower-door test results (if available)
The clearer your evidence, the smoother your path to an A+ rating.
Recommended Books & Resources
Books
The Passivhaus Handbook by Cotterell & Dadeby
Clear, beginner-friendly guide to ultra-low energy retrofits. Great for understanding airtightness, U-values, and staged upgrades that still play nice with old fabric.
EnerPHit: A Step-by-Step Guide to Low Energy Retrofit by James Traynor
Focuses on the Passivhaus retrofit standard for existing buildings. Strong on thermal bridges, sequencing work, and setting targets that are realistic for old houses. Available in print and Kindle.
Building with Lime: A Practical Introduction by Stafford Holmes & Michael Wingate
Definitive primer on lime mortars and plasters. Ideal for breathable granite walls, pointing, and moisture-safe finishes.
Water in Buildings: An Architect’s Guide to Moisture and Mold by William B. Rose
Essential building-science on condensation, vapor, and drying. Helps you avoid hidden moisture traps during insulation and airtightness upgrades.
Resources
FLIR ONE Gen 3 Thermal Camera (smartphone)
Find cold bridges, missing insulation, and damp-related cool spots in minutes. Available for iPhone Lightning or USB-C, plus Android.
Smoke Pencil / Smoke Pen (air-leak detector)
A simple way to “see” drafts at window frames, beam pockets, and service penetrations. Perfect before and after air-sealing work.
Panasonic WhisperCeiling Bathroom Exhaust Fan (FV-0511VQ1)
Quiet, efficient exhaust for windowless baths. Helps control humidity at the source, which protects lime finishes and timber. 50–80–110 CFM settings ≈ 85–136–187 m³/h.
Tough Kraut Resources
Our curated collection for granite house retrofits that aim high on efficiency and last for decades. From lime and wood-fibre know-how to airtightness tapes, leak-finding tools, fans, and smart shading. Tested on our own build so you can plan once, spend less, and stay comfortable.
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