Natural Roofs & Insulation

Roofs and natural insulation

Rule: These notes add specialist tips to your answer. They do not replace it. Always give your full general answer from your own knowledge first, and work in the tips below wherever they apply.

Roof design requirements

• Severe weather or snow calls for a stouter, steeper roof. Dry climates can use shallow roofs.
• Connect roof to walls with metal straps or hardware.
• Walls made of discrete units (straw bales, adobe, compressed earth blocks) need a bond beam on top to carry the roof load as one continuous surface.
• In high-wind areas, shape the roof to shunt wind away. Overly wide overhangs catch wind and cause uplift.
• Design for the greatest imaginable load. Dead load is the roof's own weight. Live loads are variable weights such as snow and people.
• Size overhangs to the weather danger to the walls. Earth and straw walls in rainy climates need large ones.
• Overhangs should also block summer sun and admit winter sun.
• Insulate roofs in temperate and cold climates. In hot, arid climates use insulation plus radiant barriers.

Roof shapes

• Flat: actually slightly sloped. It sheds water slowly and parapets are hard to waterproof, so it leaks more than steeper roofs. Use only where rain and snow are light, with perfect waterproofing and maintenance.
• Shed: sheds water fast, but walls must be built to different heights and two walls must be sloped.
• Gable: the ridge beam is carried by the end walls or posts, and rafters rest on the side walls.
  • Tie the rafters together with horizontal tensile members (boards or cables) so they don't push the side walls out.
  • Exterior buttressing is a rarer alternative.
  • Trusses can replace ridge beam and rafters and suit large spans.
• Hipped: all walls are the same height, so a level bond beam is easy. It suits load-bearing straw bale and adobe. Rafters still need tying or support as in a gable.
• Domes and vaults: use compression materials (brick, concrete, stone, earth) rather than tensile timber.
  • A vault has single curvature and covers a rectangular room. A dome has double curvature and covers a square or round room.
  • Avoid earthen domes and vaults in rainy climates. They are hard to waterproof, a soaked roof can collapse, and they don't shed water away from the walls.

Roof structure and sheathing

• Wood is the usual structure. Long spans may need glue-lam or engineered wood. Trusses come pre-engineered and cut labor.
• Alternatives:
  • Trusses from recycled pallets.
  • Bentwood trusses from small flexible trees.
  • Round-pole timber.
  • Bamboo trusses.
• Metal structure is discouraged for its embodied energy, but it is usable for long spans.
• Sheathing connects the structure, spreads loads and carries the waterproofing. Options:
  • Plywood.
  • Natural boards.
  • Saplings.
  • Bamboo.
  • Non-toxic straw or agricultural-waste sheet products.

Roof coverings

• Thatch (reed, grass, palm):
  • It is insulating and breathes, and lasts 60+ years if well built.
  • Poor work leaks and needs replacing within a few years.
  • Fire risk, pests, scarce skilled thatchers in North America, and rising material cost.
• Seaweed (Denmark) is used like thatch. Mussel shells protect membrane roofs and form a drainage layer.
• Shakes are split with a mallet and froe. Shingles are sawn.
  • Cedar and redwood are impractical (old-growth) and a fire hazard.
  • Concrete or recycled artificial shakes are an alternative.
• Metal:
  • Cheap, light (allows lighter structure), easy to install, and lasts 50–100 years in dry climates.
  • Recyclable, and good for rainwater catchment. Potability depends on metal and coating.
• Slate and ceramic tile:
  • Nailed and overlapped like shingles. Both can collect rainwater.
  • Both are extremely heavy and need a very strong structure.
• Asphalt shingles and rolled roofing: short life and toxic to dispose of, so natural builders avoid them.
• Valleys where roofs meet are the hardest spots to waterproof. Detail them carefully.
• Eco-shake tiles go over a vapor barrier.

Living (green) roof build-up

• Order of layers from the bottom:
  1. Low-pitched frame.
  2. Solid deck.
  3. Waterproof membrane.
  4. Drainage mat and/or volcanic rock (pumice or scoria).
  5. Organic layer.
• Steeper sod roofs need measures to stop the soil slipping.
• Membrane options: EPDM, Hypalon, neoprene, PVC, modified bitumen.
  • Prefer single-sheet seamless membranes to reduce leaks.
  • Apply very carefully. Leaks are very hard to find once the growing medium is down.
  • UV, heavy rain and ice slowly degrade exposed membranes. A soil and plant cover greatly extends their life.
• The drainage layer ensures runoff in heavy rain and retains moisture. It discourages roots from attacking the membrane, and some mats also protect the membrane.
• Growing media: cut sod, rolled turf, loose topsoil, compost or straw.
  • A thin top coat of manure, compost or leaves can be left to grow on its own or planted.
  • Add straw periodically as the original layer decomposes.
• Straw bale medium (Archibio):
  • Lay second-quality bales side by side on the membrane with the twine cut to loosen the straw.
  • A lighter option is 4 in. (10 cm) flakes of straw, at about 10 lb/sq. ft. The source prints "480 kPa", but 10 lb/sq. ft. is about 0.48 kPa.
• Plan for the layer to be thoroughly soaked. Depth, not material type, governs weight, since water outweighs soil.
• Choose soils and plants for the climate, including freeze tolerance.
  • In dry conditions use moisture-retaining soil and shallow-rooted sedums.
  • With ample rain or irrigation a wider planting range is possible.
  • Plant in early spring so cover establishes before summer and winter extremes.
  • Water during establishment and drought.

Living roof loads and performance

• Extra structural load is 50–60 lb/sq. ft. (2.4–2.9 kPa). Expect larger, more closely spaced rafters.
• Check whether the wall system can carry it. Load-bearing straw bale walls may not.
• Heavy wind or earthquake restraint of this mass needs strong shear walls.
• Thermal moderation comes from 5–6 in. (12–15 cm) of soil and plants. About 14 in. (35 cm) of decomposing straw is similar.
• Wet material loses its insulation value, so winter thermal benefit is minimal. Snow held on the roof adds about R-1 per inch.
• Low-maintenance option: minimal soil with many rocks, giving a dry rocky habitat.
• If neglected, soil rarely blows off quickly because grasses and weeds hold it. Most membranes are UV-resistant and last a decade or more even uncovered.

Insulation basics

• A higher R-value is better. U-value is the inverse, so lower is better.
• Lab R-values are not in-situ performance. Real results depend on temperature, moisture, airtightness and how completely the cavity is filled.
• Always pair insulation with a continuous air barrier. Air leaks hurt most in cold climates.
• Protect fibrous and loose insulation from rodents.
• Loose straw and cellulose are flammable. Enclose them and treat with fire retardant.
• The best natural insulators are wool, cellulose, cork, vermiculite and cotton batts. None match foam boards per inch.

Choosing insulation by location

• Roofs: use lightweight material, both for safety and to cut structural lumber.
  • Good choices: cotton or wool batts, loose-fill cellulose, pelletized wool, rice hulls.
  • Thatch is self-insulating.
  • Straw bales work but are heavy, so size the framing and ceiling structure for them. A roof leak makes them absorb water and become heavier and hazardous.
• Walls: thick, heavy, easy to keep dry, so dense low-impact options suit. Examples: straw bale, hempcrete, papercrete, light clay, cordwood.
• Sub-grade and floors: the hardest to do naturally.
  • Add light aggregate (perlite, expanded glass) to concrete.
  • Use pumice or scoria where available.
  • Otherwise the choice is industrial product or none.

Wool

• Wool still insulates when wet. Batts are available, as is pelletized wool for blowing or packing into cavities.
• DIY option 1: wash and card it, then treat with borax or Quassia chips against moths.
• DIY option 2: stuff raw wool into plastic garbage bags to keep out smell and moths.
  • The bag method risks in-cavity condensation in cold climates, and in hot humid climates with air conditioning.
• Avoid wool contaminated with organophosphate sheep dip.

Straw and other fibers

• Straw must be sealed on all sides by an air barrier, preferably natural plaster.
• Plastered straw bale walls reach R-29 to R-35, depending on thickness.
• Install bales tight together and stuff the cracks to prevent air leakage.
• When stuffing walls, put uncoated straw toward the center and clay-slip-coated straw toward the surfaces. Plaster, with special attention to windows, doors and edges.
• Loose straw or flakes in roofs risk fire, mold and rodents. A light boric acid or clay slip coating may not fireproof it.
• Compressed straw-clay is much more resistant. Make it into insulating tiles or compact it directly into the ceiling cavity.
• Hemp, coir, flax, cotton and textile waste can be used similarly.

Cellulose

• It is made from recycled paper and blown dry by machine. Borax gives fire, mold and insect resistance.
• Hygroscopic, so it handles incidental moisture, but it is unsuitable for wet-service or below-grade locations.
• Horizontal (attic) application: loose-fill with rental equipment is fine and settles naturally.
• Walls and sloped roof cavities: loose-fill settles and leaves an uninsulated gap at the top. Prevent it by either:
  • Dense-packing at high pressure to at least 3.5 lb/cu. ft., or
  • Damp-spraying.
• Both prevention methods need better equipment and training, so they are generally not for owner-builders.

Cotton, cork, minerals

• Cotton batts:
  • Recycled denim, with R-value comparable to fiberglass, and treated with boric acid against pests, fire and mold.
  • Drawback: conventional cotton uses heavy pesticides and herbicides.
• Cork: granules for loose-fill, and dimensionally stable boards for roofs, walls and floors. Shipping energy matters outside Europe.
• Perlite and vermiculite:
  • Costlier, but good where water or fire exposure is likely.
  • Expanded recycled glass granules (Canada) behave similarly.
  • All can be an aggregate with clay, lime or cement for insulating subfloors and stemwalls.
• Pumice and scoria: make "pumicecrete", fill earthbag walls and domes, and insulate subfloors.
• Wood chips: shredded redwood bark (lightweight, rot- and fire-resistant) has been used as roof insulation.
• Mussel shells: in Denmark they are used for insulation and drainage in stemwalls and under floors.

Fiberglass, foams, boards

• Fiberglass is avoided for its embodied energy and formaldehyde binders, and glass fibers are a possible carcinogen if inhaled. Better versions use acrylic binders: loose-fill and formaldehyde-free batts.
• Spray isocyanate foam gives top R-value plus an air and moisture barrier. It is toxic, messy and expensive, so not for DIY. Soy or castor oil variants still need as much fossil fuel to make.
• Foam boards are often the only option below grade (foundation perimeter, sub-footing, sub-slab). Moist locations need closed-cell foam.
• Avoid XPS because of ozone-depleting blowing agents. If rigid foam is used, specify high-density EPS.
• Rock wool (stone wool) is probably the best board or batt, with much lower embodied energy than fiberglass or other boards. Its boards may contain formaldehyde.
• Cellular and spun glass boards are very energy-intensive, so poor environmentally.

Rice hulls

• R-value is up to 3.0 per inch for whole hulls. Bulk density is similar to baled straw.
• They are flame retarding and self-extinguishing, so no chemical retardant is needed.
• Moisture content at equilibrium is below that which sustains fungi and mold.
• They resist settling: once firmly packed, the hulls interlock and no further settling occurs. Cellulose can lose up to 25% of its height. Loose, they have a ~35° angle of repose.
• The example house used lightweight framing with 12 in. wide floor, wall and roof cavities. They suit hot, humid climates and rice-growing areas.
• Forms of use:
  • Loose-fill in cavities and ceilings.
  • Stuffed bags (which can be stacked like bales).
  • Clay-slip-coated and packed into forms.
  • Rice hull particle board is also made.
• Material costs almost nothing (about $5/ton). Transport is the main cost, similar to straw bales.
• ASTM test results are at esrla.com (see "Rice Hull House" download).