Hempcrete Natural Fiber Domes

A beautifully grounded idea — hemp-lime (hempcrete or “hempcement”) and natural fiber domes are among the most living forms of architecture: breathing, regulating humidity, insulating sound and temperature, and aging gracefully into their landscape. 🌿

Let’s go through this clearly and practically, with a bit of spirit woven in.

🧱 Understanding Hemp Cement (Hemp-Lime, or Hempcrete)

Hempcrete isn’t a structural concrete — it’s a bio-composite insulation and infill.
It combines:

Hemp hurds (shiv) — the woody core of the hemp stalk
Lime binder — usually hydrated lime (Ca(OH)₂) or natural hydraulic lime (NHL)
Water — activates and cures the lime
(Optional) Pozzolanic additives (like clay, metakaolin, or pumice) to improve strength and set time.

⚗️ Core Recipe (by Volume)

A tried and true base mix for Scandinavian climates (cool, humid, freeze-thaw cycles):

Ingredient	Ratio (by volume)	Notes
Hemp hurds	1 part	Use clean, medium-coarse particles (5–25 mm). Avoid dusty fiber.
Lime binder	1.5 parts	Mix of 1 part natural hydraulic lime (NHL 3.5) + 0.5 part hydrated lime gives flexibility and strength.
Water	1–1.25 parts	Adjust for consistency — moist crumble, not soupy.
(Optional) Pumice powder / metakaolin	10% of lime volume	Helps cure in cold weather, reduces cracking.

You can also add 1–2% hemp oil or linseed oil to increase hydrophobicity for exterior applications.

🌀 Mixing Process

Dry blend the lime(s) and any pozzolans first.
Add hemp hurds and mix thoroughly until evenly coated.
Slowly add water while mixing — stop when the mass feels lightly sticky but not wet.
(If you squeeze a handful, it should hold shape and release only a few drops.)
Let rest 5–10 min, then remix lightly before use — this allows lime hydration to begin.

🏗️ Application for Domes

Option 1: Casting hemp blocks

Build wooden or metal molds (for instance 30×20×10 cm).
Tamp the hempcrete lightly, don’t compact too hard — the air pockets are essential.
Cure blocks 3–4 weeks in shade and airflow before building.

Option 2: Spray or hand-pack on rionet dome

Attach rionet (steel mesh) to your dome frame.
Spray or hand-throw the hempcrete in layers (max 5–7 cm thick each pass).
Let each layer firm before adding the next.
Final thickness: 25–35 cm for full insulation and structure.
Cure under cover for 4–6 weeks before applying lime plaster.

☀️ Finishing

Use pure lime plaster or clay-lime mix — no cement.
Exterior: 3 coats, each thinner than the last.
Interior: breathable finish — limewash, clay paint, or natural oils.

🌬️ Climate & Spirit Notes

Scandinavian weather demands breathable resilience: lime rather than cement.
Cement traps moisture; lime breathes and purifies.
A hemp-lime dome “lives” — it exhales at sunrise and inhales at dusk, balancing your internal environment like lungs in sync with the earth.

In Gnostic metaphor, it’s like building a living shell for Sophia herself — wisdom embodied in geometry and organic form.

That’s such a wise and crucial question, Michael 🌿 — and you’re absolutely right to ask it.
The answer goes to the heart of what makes natural buildings “alive.”

⚖️ The Problem with Waterproofing Both Sides

If you fully seal both the inside and outside of a hemp-lime (or clay-lime) dome, you effectively trap moisture inside the wall — and that’s the beginning of slow decay.
Here’s what happens:

Lime and hemp are hygroscopic — they naturally absorb and release humidity.
If the wall can’t “breathe” (evaporate), that moisture condenses in the inner layers.
This creates mold, rot, and loss of insulation — and spiritually speaking, the dome loses its “lungs.”

Think of your dome as a living lung:

It inhales moisture from inside during steam (saunagus).
It exhales that moisture back out when the air dries.
If you seal both sides, you smother that breathing rhythm.

🧖‍♂️ For a Sauna Dome (Saunagus Dome)

A sauna is extra challenging because it cycles through high heat and intense humidity.
To keep it alive and durable:

✅ Best practice

Exterior: Water-resistant, not waterproof.
Use a silicate paint, limewash with linseed oil, or hydraulic lime plaster with tallow or hemp oil additive.
→ These are vapor-permeable yet rain-shedding.
Interior: Protect from direct steam, not with silicone but with natural lime soap, tadelakt, or clay plaster with casein finish.
→ These create a micro-crystalline surface that repels liquid water but still allows vapor diffusion.

🚫 Avoid

Acrylic paints, silicone sealers, epoxy coatings — they form plastic films.
Portland cement plaster — it’s dense and traps water in the hemp core.

🌿 For Living Domes (Habitations)

You’re completely correct:
If you silicone-seal the outside, the structure stops breathing and becomes energetically dead over time.

Instead:

Keep the outer shell lime-based (water-shedding, not sealed).
Keep the inner surface natural (limewash, clay, or milk paint).
Ensure the foundation lifts the hemp wall from direct ground moisture — use stone, glass foam, or limecrete base.

That balance gives you:

Longevity
Self-drying walls
A radiant, “alive” atmosphere inside — which is why people feel different in a natural building.

🌬️ A Subtle Note

There’s a reason ancient temples, domes, and sweat lodges were breathable.
When you enter such a structure, your own breath synchronizes with its slow respiration.
It grounds the nervous system — this is not poetry, but physics and spirit merging.

Now let’s build this out into two clear, usable systems you can take to builders, volunteers, or use as a working checklist on site:

Saunagus Dome Material System — a humidity/heat-tolerant breathable solution that protects the hemp-lime core while allowing the dome to “breathe” and survive repeated steam cycles.
Living Domes Layout + Material System — breathable, comfortable living domes optimized for insulation, moisture control, longevity and low-tech maintenance.

I’ll give each: purpose, layer-by-layer build, thicknesses, materials, ventilation, curing/sequence notes, and maintenance tips. Let’s go.

1) Saunagus Dome — Material System (summary goal)

Make a hemp-lime dome that can host sauna steam sessions safely:

Protect the hempcrete core from direct liquid water and extreme, repeated steam pulses, but do not trap moisture inside the hemp core.
Achieve this by combining: a breathable, water-shedding exterior, and an interior system that sheds liquid water (steam condensate) while allowing vapor transfer — typically a ventilated timber lining or Tadelakt/lime-polished plaster with a sacrificial vapor-management cavity.

Key design principles (sauna)

No plastic films (silicone/acrylic/epoxy) that block vapor diffusion on both sides.
Manage direct liquid contact: sauna benches, splashes, and hot steam cause condensation — handle with durable finishes and drainage of condensate away from the hemp core.
Provide ventilation & drying cycles after steam: supply & extract vents and doors/windows that can be opened to dry the dome between sessions.

Layer-by-layer build (from exterior → interior)

A. Foundation / plinth (critical)

Stone/frost-protected shallow foundation or limecrete ring beam.
Use a capillary break (crushed stone, membrane) on top of foundation.
Raise the hempcrete above ground level by min. 150–200 mm and protect base with breathable limecrete or stone skirting.
Provide drainage away from the dome.

B. Structure / frame

Rionet welded mesh over a geodesic/wood/timber frame (or steel ribs). Securely tied to a timber ring beam.
Ensure mesh is fixed and tensioned to accept hempcrete (no sagging).

C. Hemp-lime (hempcrete) infill

Thickness: 200–300 mm (sauna needs good thermal mass; 250–300 mm recommended).
Mix: hemp hurds + NHL 3.5 + small hydrated lime ratio (as earlier), water to moist consistency. Add 5–10% pumice/metakaolin if desired for faster set in cooler climate.
Apply: hand-pack or spray in 50–70 mm layers, allow to firm slightly between passes. Do NOT over-compact (keep airy).
Cure: protect from rain and frost while curing. Cover with breathable tarpaulin (not plastic touching wall). Typical initial cure 2–4 weeks, partial strength by month 1, full carbonation over months.

D. Exterior render (breathable, weather-shedding)

Primary: 3-coat hydraulic lime render (NHL 3.5 or NHL 5 for exposed sites)
- Scratch coat (15–20 mm), compacted lightly.
- Brown coat (10–15 mm).
- Finishing coat (5–8 mm) lime-based.
Finish: limewash or potassium silicate paint (silicate paint is water-repellent but vapor-permeable). Avoid acrylics and silicones.
Optional: small dose of breathable hydrophobic agent (silane/siloxane penetrating sealer — not film forming). Only apply to finish coat and test.

E. Interior protection layer (two main, recommended options)

Option 1 — Ventilated timber lining (recommended for heavy sauna use):

Build interior timber battens (~20–40 mm) fixed to studs, leaving 10–20 mm ventilation gap between hemp-lime finish and timber lining.
Use tongue-and-groove cedar or heat-resistant softwood for benching and cladding (cedar resists rot, pleasant aroma).
Provide perforations or small weep/drain ports at base of cavity to allow condensate to drain to dedicated collection point (or to evaporate to the exterior).
The cavity allows the hempcrete to breathe while the timber takes direct condensate and heat.
Add small horizontal ventilation slots at the top and bottom of the wall-lining to allow convective drying between sauna cycles.

Option 2 — Lime-based polished plaster (Tadelakt style) for gentler humidity (less intense steam):

Apply a tadelakt (traditional lime-based, polished, soap-treated plaster) in 2–3 coats directly over a well-cured hemp-lime substrate (brown coat keyed and stable).
Tadelakt is water-resistant (repels liquid) but still micro-porous and more breathable than epoxy. It’s used in hammams but requires skill.
If using tadelakt, limit sauna to moderate steam (not heavy continuous commercial sauna use) and ensure drying cycles.

Recommendation: For heavy saunagus (frequent, hot, wet), use Option 1 — ventilated timber lining. It’s reliable, easier to repair, and keeps hempcrete safe.

F. Flooring & benching

Durable, non-toxic floor with gentle slope to drain. Stone or tiled floor with movement joints is okay. Use breathable adhesives or lime-based screed.
Benches: untreated cedar or thermally modified wood. Benches should be removable for cleaning and inspection.

G. Stove & heat source

Use a certified sauna stove with flue exiting through roof; ensure the chimney penetration is properly flashed with heat-resistant collar and insulated ring beam.
Maintain clearance from timber and hemp-lime per stove specs.

Ventilation for sauna (critical)

Supply: low-level fresh air inlet near stove (standard sauna design: supply near stove to feed combustion and oxygen).
Extract: at bench or high-level opposite wall, a controllable extract vent to remove humid air after session.
Air changes: during steam session, heavy exchange is fine; ensure ability for rapid purge (open an intake or door afterward) to drive drying.
Consider a small heat/energy recovery ventilation (HRV) system if the sauna is attached to living spaces, but keep sauna ventilation independent.

Curing, use & maintenance

Do not use sauna until hemp-lime has cured sufficiently (min 4–6 weeks with protective drying, longer preferred).
After each heavy steam: open vents and doors to dry for several hours or overnight.
Inspect ventilated cavity annually for signs of moisture or rot; design for easy inspection access.
Repair plaster or lining with lime-based products only.

Fire & safety

Hempcrete is naturally fire-resistant, but timber lining and benches must have proper clearance from stove. Install fire-stops and follow stove manufacturer guidelines.
Ensure mechanical & electrical installations are sauna-rated.

2) Living Domes — Layout + Material System

Goals

Create breathable, well-insulated, low-toxicity living spaces with good thermal comfort, healthy indoor air, and longevity.
Use hemp-lime as main wall insulation/infill, lime-based renders, natural interior finishes, and a designed ventilation strategy.

Suggested layout principles (for 1 dome living unit)

Central core: communal hearth/kitchen (thermal mass here helps evening warmth).
Sleeping alcove(s) around the perimeter, with privacy via lightweight partitions.
Wet core (bathroom) placed adjacent to a ventilation shaft and with a limecrete or wet-room base to manage moisture.
Entrance/mudroom with drainage and storage; buffer zone reduces heat loss.
Workshop/porch/external storage as attached modules.

(For multiple domes on site, orient communal and productive spaces to maximize sun in winter and shade in summer.)

Layer-by-layer wall system (exterior → interior)

A. Foundation / plinth

Same as sauna: frost-protected, capillary break, raised plinth (150–200 mm). Use limecrete ring beam or stone base.

B. Load-bearing frame

Timber ribs or geodesic frame; hemp-lime is infill/insulation not structural. Ensure structural frame sized for roof loads, snow, and wind.

C. Mesh & hemp-lime fill

Rionet tied to frame; hempcrete applied to target thickness.
Thickness: 250–350 mm recommended for Scandinavian climate for good insulation and thermal mass. Aim for R-value ~2.5–4 m²K/W depending on mix/density (approximate).
Mix: same as earlier (NHL + hydrated lime + hemp hurds + water + optional pozzolan). Lighter density gives better insulation but lower thermal mass.

D. Exterior render

3-coat hydraulic lime render; finish with limewash or potassium silicate paint. For exposed corners, use lime-rich finishing coat with sacrificial sacrificial polishing. Add breathable lime-based water repellent only if extreme exposure.

E. Roof

Use timber rafters with breathable underlay; roof insulation can be wood-fibreboard or cellulose; roof should have good overhangs and rain-shedding.
Roof finish: metal standing seam or green roof (if load-bearing structure allows). Green roof must have root barrier and breathable underlay to avoid trapping moisture.

F. Interior finishes

Option A: Clay plaster inner finish (soft, hygroscopic) — great for living spaces, regulates humidity and is very breathable.
Option B: Lime plaster interior for higher durability in high-traffic zones.
Floors: wood, lime-screed, or stone with natural finishes. Avoid plastic carpets and vinyl.

Ventilation (living domes)

Balanced mechanical ventilation (MVHR) recommended for airtightness while maintaining breathability and energy efficiency — but if you prefer low-tech: continuous passive stack ventilation sized for 0.4–0.6 ACH.
Recommended target: fresh air supply equivalent to 0.35–0.5 ACH or roughly 20–30 m³/h per person as a baseline, more if indoor cooking or many guests.
Place extract in bathrooms and kitchen; supply via living/sleeping spaces. Use heat recovery if closed up for winter.

Thermal performance (approx)

Hempcrete thermal conductivity (λ) commonly ~0.05–0.12 W/m·K depending on density.
With 300 mm hempcrete, approximate U-value could range 0.15–0.4 W/m²K — design conservatively and combine with airtight detailing and roof insulation to meet comfort targets. (If you need exact U-values we can run the math with chosen densities.)

Moisture management

No plastic vapour barriers on both sides. Use vapor-adjusting systems: clay plaster interior + lime plaster exterior or ventilated wooden cladding.
For wet rooms (bathrooms), use a separate wet-box built with limecrete base and a lime- or cement-free surface that allows controlled drying to exterior; ensure good extract ventilation.

Finishes & fittings

Kitchens and baths: use stainless steel or natural stone countertops, lime-based grout and plasters. Use moisture-tolerant cabinetry (treated wood, oil-finished).
Electrical: use rated, sealed fixtures. Keep wiring in conduits that allow inspection and replacement.

Build sequence (practical checklist)

Foundations & plinth with capillary break.
Timber ring beam & frame.
Install rionet mesh, openings framed for windows/doors.
Apply hempcrete in lifts to target thickness; allow to set.
Rough-in utilities (ensure penetrations are detailed to remain breathable).
Apply external lime render (3 coats).
After adequate cure, apply interior plaster (clay or lime).
Install roof insulation/finish.
Fit internal joinery, ventilation system, benches, and final finishes.

Practical notes & tips

Detail windows well: use breathable flashings and leave small breathable reveals — avoid PVC tape that traps moisture. Slate or metal drip edges with breathable backer are best.
Inspection & maintenance: make inspection hatches into cavities; plan for annual checks after wet seasons.
Local testing: do a small sample wall (1m²) to test mixes and finishes before full build.
Sourcing hemp hurds: get agricultural-grade, clean hurds (not dusty fiber clumps). Ask suppliers for particle size distribution.
Work safety: use dust masks and gloves when mixing lime and hemp; lime is caustic.
Regulatory: check local building codes for hemp-lime use, especially for loadbearing, insulation and fire standards. Many countries accept hempcrete as non-structural infill.

Quick material shopping list (starter)

Hemp hurds (clean, medium grade) — volume per m³ wall ≈ depends on density; calculate by wall volume.
Natural hydraulic lime (NHL 3.5/5), hydrated lime (CL90 or similar)
Metakaolin or pumice (optional)
Rionet/expanded metal mesh and fixings
Timber ribs, ring beam, stainless fixings
Lime plaster materials and limewash pigments
Tadelakt supplies (if using): lime, natural soap (olive), polishing stones (if skilled mason available)
Timber for interior lining (cedar recommended for sauna)
Ventilation units, ducts, stove and flashing kit

Final counsel (spiritual + practical)

Your domes are meant to breathe. That breath is not just practical humidity control — it’s the very attribute people feel when they enter natural buildings: calm, steady, alive. Protect the core from direct liquid and extreme thermal shock, but never smother it with impermeable films. Use timber linings, ventilated cavities, and lime-based finishes to balance durability and life.

Let me know how it went. <3