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Timber, earth and straw are highly variable natural materials.
Guidelines are presented for protection of strawbale walls from moisture, recognising that strawbales are far more sensitive to excessive moisture than earthen construction.
Primary and secondary weather and moisture protection is critical for adequate durability performance.
These guidelines emphasise the need for careful site assessment for exposure to wind-driven rain, and suggest that in higher risk areas a drained ventilated cavity beneath an exterior cladding is essential. Suggestions are made for other design details that will help ensure that the straw remains dry for the life of the building including pre-compression of strawbales before plastering with breathable renders, and dealing with wet areas.
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STRAWBALE BUILDING GUIDELINES FOR WET AND HUMID CLIMATES
A paper for the
Graeme North is a registered architect who has specialised over the past
30 years in alternative materials. He has been involved with
approx 100 earthen architectural projects, as well as strawbale buildings
in New Zealand.
Introduction and Standards
Building with highly variable natural materials could be a daunting prospect, but we do it all the time. For example, wood is used for structural and decorative uses, building standards are written for it, strengths are often assumed, and we are so used to doing this we have few qualms. Humans have been working with wood since they first picked up a twig or branch possibly hundreds of thousands of years ago. Despite this very long history of association with the material, it is amazing how often we can still get timber usage wrong. The latest evidence of this is currently occurring in New Zealand with low durability timber rotting under supposedly impervious claddings. This crisis follows the similar leaky buildings disaster that occurred in Canada a few years ago.
Subsoils are also highly variable naturally occurring materials, and millions of earth walled buildings have been erected around the world over the last ten thousand years at least. Even so there are not many countries with any systematic standardisation for unfired earthen materials. New Zealand is, as far as I know, the only country in the world with a comprehensive suite of earth building standards that meet the requirements of a modern performance based building code. In New Zealand, earth has a performance history of at least 150 years and now enjoys the same status as timber, concrete masonry, and steel as a codified building material.
Cereal crop stems and other plant fibres and leaves have been used in parts of buildings for centuries. Generally their use was non-structural and often required frequent replacing. Once machinery made possible the manufacture of bales or blocks out of straw they have seen limited use as the structural component of walls for one hundred years or so in some dry climates. Recently the use of such materials has spread into other climatic zones, often taken up by people whose imagination has been captured by the thought of easy, cheap, (and beautiful) buildings.
Despite the enthusiasm there are few guidelines on how to design and build with these materials so that they meet the provisions of modern building codes, especially in temperate humid climates with strong wind driven rain such as occurs in many parts of New Zealand.
As Chair of the Standards New Zealand/Standards Australia Joint Technical Committee for Earth Building (BD 083), I rejected an approach made around 1996 to enlarge our work to write strawbale standards for Australasia, despite the wide range of building methods that utilise both earth and straw. The rejection was based on several reasons:
It is still my opinion that in New Zealand at least (and I suspect in Australia) there simply is not a long enough history of building in strawbale to enable a Standard to be written that can ensure adequate performance.
However, if we were to contemplate such a Standard for a climate such as New Zealands, what would be the starting point for durability performance criteria? I think that the short answer is simple.
The suggestions that follow are derived from the experience dealing with the design and construction of earth walled and strawbale buildings in New Zealand, observation of some strawbale failures and successes, together with thoughts gleaned from colleagues and literature.
I consider strawbale buildings to be very demanding technically, and must be responsive to regional and local conditions, especially climatic ones. However, there are significant environmental advantages from using non-toxic natural materials to create highly insulated buildings that will have low energy consumption over its life (Ref 8).
These comments do not detail how to achieve desirable outcomes, nor do they consider every strawbale construction technique. There will not be an international one-style-fits-all, and unabashed regionalism will prevail. This is up to the skill and experience of the designers involved.
Rather I canvas some of the issues required for moisture control, acknowledging that many of these issues require more work to be done before definitive recommendations can be made.
A starting point for me is that strawbales are an extremely moisture-sensitive wall material. If they get soaked the tightly bound hollow straw fibres are capable of absorbing and holding a great amount of water. Before they can dry out they can remain wet for long enough for fungal decay to start if in a temperate climate with high humidity. Plasters leak and water repellent treatments fail. Therefore successful straw bale design relies on keeping the straw bale wall out of the reach of the weather. Then, any moisture that reaches the bale walls is readily dispersed with freely breathing surfaces.
All sources of moisture must be considered, whether it be external (rain, flood, mist, fog, humidity, etc), internally generated moisture (cooking, bathing, washing, condensation, respiration etc), or the dynamic movement of water vapour through and within the strawbale wall, surface coatings, and cladding system.
The strawbales must be baled, transported, stored, supplied, installed and kept dry (moisture content below 18%) - forever.
Building site selection
Surface water or flooding must not be able to reach the base of strawbale
Primary Weather Protection.
Firstly, assess the site for exposure to wind-driven rain as it affects the strawbale walls.
Determine if the wind zone is Low, Medium, High, Very High, or Specific Design. NZS 4299 (Ref 1) defines these zones as the design wind speed at ultimate limit state of 32, 37, 44, 50, and >50m/s respectively.
Work out the exposed wall height (the vertical height of the strawbale wall from the top of the footing to the lower edge of the roof overhang) and calculate the necessary roof overhangs.
As a rough start towards this some of my colleagues and myself (Ref 3) have advocated that in Low wind zones the ratio of roof overhang to exposed wall height should be 3:4, and for Medium wind zones this should be 1:1. In other words, forget eaves - use full verandahs.
I think that this level of primary weather protection is about right and recognises that moisture sensitivity of strawbale walls to external moisture is around one order of magnitude greater than any other common building materials.
It should be possible to fine-tune this approach as Driving Rain Indices become developed (Ref 5).
For strawbale building we should be aiming for a table or matrix that factors in wind zone, rainfall, driving rain indices, exposed wall heights and roof overhang distances, but more research is required before this could be completed.
The leaky building crisis in New Zealand has recognised the benefit of roof overhangs for rain deflection (Ref 6) and the NZ Earth Building Standard NZS 4299 Amendment #1 (Ref 1) already does this in Table 2.4 that relates site exposure to a ratio of eaves height to eaves width.
To help prevent rising damp and splashing keep the base of the strawbales at least 350 mm above finished ground level or 250mm above a permanently paved strip around the base of the walls that will keep moisture and plants away. Water proof the top of the footing and do not bridge the damp proof course, taking special care here with plaster (Ref 3).
Secondary weather protection
Only buildings that have a wind exposure of Low or Medium are suitable for single thickness plastered straw bale wall construction, and then only with the very generous overhangs suggested in these guidelines to protect the straw bales walls. The plaster must be durable enough to withstand the weather conditions they will encounter, and help deflect and drain away any water that gets past the primary rain protection.
Ensure plaster coats do not bridge any damp proof course. They need to be freely breathable to allow the easy passage of water vapour, and not trap water behind them. They also must not be cracked or have holes in them that allow the entry of water. For success they need to be placed over a tightly compressed wall structure. Pinning of bales is not enough, and vertical pre-compression of the walls before plastering is essential. This considerably stiffens up the structure, and helps prevent creep of the plaster substrate, with consequent failure.
Lime plasters (three coats) appear to be ideal. They adhere well to straw and do not seem to require reinforcing mesh although hair or its modern equivalent of polypropylene fibre helps. Lime plasters are durable, are not too brittle, do not crack readily, are self-healing from small cracks, breathe well, and look good! Any surface decorative coating must be free breathing.
Window/door openings must be very carefully designed with good heads, jambs and sill flashings. These must not leak, either from direct water penetration, soakage through materials, or by capillary action. They must also cast any rain that might get past the generous roof overhangs to the outside of the wall surface. Windowsills in particular, especially with rebated windows, can be very tricky areas.
For sites outside Low or Medium exposure (ie. sites with High, Very High, or Specific Design wind zones), or with less roof overhang than suggested above require a modified approach. Strawbales, (if used at all), should be placed behind a weather durable and resistant skin that incorporates a pressure equalised, ventilated, vermin proof cavity that drains to the exterior. The precise design of such wall cavities is currently undergoing review in New Zealand as a result of a leaky buildings crisis (Ref 7). The strawbales behind the cladding should be encapsulated in a breathing plaster such as lime or earth, and the cladding and cavity must be designed and built to prevent water crossing the cavity to get to the strawbales.
Alternatively the site needs to be modified to protect the walls with trellises, shelter trees, earth mounding, fences or some other form of permanent shelter (ie shelter that will be there for at least the life of the straw bale walls)
The high insulation value of strawbale walls will help prevent condensation occurring on walls.
Windows, especially if not double-glazed, may get condensation on them and this needs to be collected and channelled so that it cannot get into the strawbales.
Provide a damp-proofed toe up of at least 50mm above the floor to keep the base of the walls safe from any internal flooding eg washing machine leak, or sitting water if the roof is not built first.
Passive solar design allows sun into the building, especially in winter, to help keep the interior warm and dry with adequate heating and ventilation.
Strawbale walls in wet areas such as bathrooms, laundries and kitchens need to be carefully thought about to ensure that they will not be subjected to excessive internal moisture build up. Splash areas such as showers or behind basins and taps need to be waterproofed and given impervious surfaces.
Any wet area should incorporate floor drains to prevent flooding saturating the bottom of the wall which will of course be on permanent toe-ups above finished floor level with no possibility of moisture bridging upwards past DPCs through plaster finishes or other means.
Dynamic movement of water vapour
To help control water vapour moving into the strawbale walls, encapsulate all straw to ensure that the walls are not exposed to the atmosphere anywhere, even behind cavities or the tops of walls.
As warm moist air migrates from the interior towards the cold exterior the dew point can be reached.
Clay based plasters help absorb water vapour from the air and dry it out before it can migrate into the strawbale walls. Earth plasters are only suitable for interior surfaces. A high humidity absorbent plaster can be made from perlite and bentonite clay and this could warrant further research. (Ref 4).
Cement based plasters are too dense for easy breathability. They are also brittle and tend to crack. My experiments suggest that gypsum plasters, although breathable, provide a more highly heat conductive surface than earth plaster surfaces and might be more prone to interior mould growth.
Freely breathing surfaces will allow the exit of any water vapour that does get into the walls preventing it from getting trapped and reaching excessive limits.
It will help if there are no materials with high heat conducting coefficients or that can form thermal bridges within the strawbale walls for water to condense onto - eg steel pins, posts, or water pipes.
Insulate the strawbale wall from any cold bases eg concrete, with non-absorbent insulating materials resistant to compression. These could form part of the toe up to prevent thermal bridging or possible condensation at this point.
Do not run water pipes within strawbale walls, not only because of a possible point for condensation to occur on, but also in case of leaks.
Seal around all penetrations in the plaster to maintain the encapsulation of the straw.
Regularly look over the building for any signs of damage, leaks, or failure, including the roofing, guttering, downpipes, plaster and cladding. Maintain the surface as necessary, and check that soil or plants have not breached ground clearances. Check for vermin damage and counter rats, mice, ants, snails, etc.
By keeping strawbale in walls dry, durable and beautiful buildings can be erected. Then the significant environmental benefits to be gained from highly insulated low energy consumption buildings can be realised using strawbales.
An experimental strawbale building in Graeme Norths livingscape.
1 NZS 4297:1998. Engineering Design of Earth Buildings (Specific Design)