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FIVE TIPS FOR KEEPING STRAWBALE WALLS DRY IN A WET CLIMATE
an edited version of this article was published in The Last Straw Journal issue

1. protect wall base
2. window & door detailing
3. roof eave detailing
4. avoid metal inside bale walls
5. breathable finishes
related articles

The big question concerning strawbale construction that I get here in the Mid-Atlantic and Northeast, is "What about moisture?"

By far, the biggest concern with strawbale walls, as with most materials in a wet or humid climate, is moisture. The bad new is that high moisture content enables fungi to grow and leads to rotting in cellulose-based materials, such as wood and straw. The good news is that there are easy methods to "slay" the moisture "dragon". I will outline some of the key places that moisture can accumulate in strawbale walls and provide simple ways to prevent this from happening.

There are two main ways that moisture can get into walls: it can flow in as a liquid or it can infiltrate via airborne vapor. Liquid water can be prevented from entering a wall through careful detailing at wall penetrations and horizontal surfaces. Airborne vapor is a concern only if it becomes trapped. To prevent water vapor from getting trapped in walls, vapor barriers should be eliminated and finishes should be selected for their ability to breathe.

A)  KEEP OUT LIQUID WATER

1.  PROTECT WALL BASE

PROBLEM: When it rains, water can splash off the ground and onto the base of the wall. The result is a concentration of water, and thus a greater risk that moisture will creep into the strawbales.

SOLUTION: Keep bales well above grade (18 to 24 inches in wet climates) so that splashing hits the wall below the level of the strawbales. It is also possible to use a vapor barrier on the exterior bottom 24 inches of the wall. However, since any water inside the bales tends to fall to the base (due to gravity), you could end up with a situation where water gets concentrated without being able to evaporate back out of the wall (because it cannot penetrate the vapor barrier).

PROBLEM: Moisture can creep through a concrete slab from below and cause a wet spot at the base of the strawbale wall.

SOLUTION: When using a slab-on-grade foundation: place a concrete-rated moisture barrier under the slab, place a waterproofing membrane on the slab below the bale wall (either with sill seal or 30-LB roofing felt), and place the bales up on a platform to separate them physically from the concrete. When creating a platform for the bales, be sure to insulate the gap, or you have a thermal break in the wall. I recommend either a non-compressible insulation or recycled cotton batt insulation, as these options will not lose their insulating value if water is present. I do not recommend gravel in a humid or wet climate, as this creates a thermal break as well as an artificial dew point for any airborne moisture to condense on. (The gravel can have a colder surface temperature than the surrounding materials, and therefore water will condense on it.) See Base of Wall Details for Concrete Slab A and B.

2.  WINDOW & DOOR DETAILING

PROBLEM: Window and door openings are particularly susceptible to moisture penetration in any type of construction. Water can migrate horizontally and even uphill through surface tension. As a result, water can travel behind molding and sills and into the wall beyond. The most common place for water to work its way into a wall is through a poorly waterproofed windowsill.

SOLUTION: When designing waterproofing details, try to create a detail that physically diverts water instead of relying on caulking. (This doesn't mean "don't caulk", it just means "don't RELY on caulk".) At window and door head (top) molding install "drip flashing" (or "Z-flashing"), and at windowsills create a drip groove on the underside of the finish sill material. Drip edges force water to fall before it can work its way back into the wall.

At windowsills, take additional precautions by creating a system of redundant waterproofing. The first defense is a flexible (preferably self-healing) waterproofing membrane that runs down the windowsill framing, over the bales on the outside, and down 4" of the exterior wall. When the window is installed, the integral flange on the bottom covers the upper edge of this membrane. Any water that works its way behind the windowsill will be diverted down and out to the exterior of the wall. The second defense is a layer of 30-LB felt installed between the bales and the window framing at the sill. (This means that window frames are secured through tops and sides only.) Window Sill Detail.

3.  ROOF EAVE DETAILING

PROBLEM: A roof leak or an ice-dam can result in hidden water penetrating into strawbale walls without ever giving a signal that there is a problem until it is too late.

SOLUTION: Create a back-up system for shedding water on top of the strawbale wall just in case the roof leaks. The idea is to divert water so it will be visible, not hidden inside walls. I prefer to use a piece of plywood on top of roof trusses and install waterproofing (such as 15-LB roofing felt) on top of the plywood. The plywood provides a flat surface for the waterproofing so that no puddles will form, and so that unintentional tearing during installation and other construction are less likely. In addition, it is recommended to provide ample roof overhang (typically 18 to 24") to physically protect exterior walls from driving rain.

B)  MANAGE WATER VAPOR

4.  AVOID METAL INSIDE BALE WALLS

PROBLEM: Metal, especially steel, will tend to have a lower surface temperature than straw or wood inside a wall. As a result, airborne moisture will tend to condense on the surface of the metal, similar to water condensing on a cold glass in the summer. This creates a point in the wall where water can congregate, eventually rotting the straw around the metal. The greater the surface area of the metal, the greater the concentration of moisture.

SOLUTION: Use bamboo pins (1/2" to 3/4") instead of rebar to pin bale walls together. Bamboo is a low embodied energy material, it is renewable and biodegradable, and it is often locally available. (With over 1,000 species, bamboo can survive in nearly every climate. You can often find someone nearby who will be more than happy to unload some bamboo that has invaded their property.) If you must use rebar for some reason, it is recommended to use an external pinning method to keep the metal outside the wall. (This method is also effective with bamboo, and is recommended if the available bamboo is too thick to drive through the strawbales.) In addition, it is preferable to use bales that are tied with polypropylene or hemp twine (over bales tied with metal wire). Any metal structural elements should be kept out of the interior of bale walls.

5.  BREATHABLE FINISHES

PROBLEM: Standard construction calls for a vapor barrier on the warm side of a wall. The problem in many regions is that the warm side varies depending on the season. If the vapor barrier is on the cold side of the wall, airborne vapor will travel through from the warm side (warm air holds more moisture than cold air) until it reaches a condensation point somewhere inside the wall. At this point, the vapor barrier serves to trap the moisture inside, causing potential moisture accumulation above 20%.

SOLUTION: It is unlikely for vapor to accumulate to concentrations above 20% moisture content unless it becomes trapped. I therefore recommend eliminating vapor barriers in humid or wet climates. Finish strawbale walls with water-resistant but breathable finishes on the exterior, such as lime plaster, and with breathable finishes on the interior, such as natural earth or lime plaster. Do not use cement-based finishes in humid or wet climates. Cement-based finishes are extremely brittle and eventually form cracks that allow water penetration. Since cement finishes are not breathable, they do not allow this moisture to transpire back out at the same rate, causing water to build up inside the walls. Lime-based and earthen-based finishes are plastic and lend themselves to resurfacing, thereby providing a water-resistant, but breathable, finish that is more suitable to strawbale construction. Paints and coatings should also be breathable, such as limewash, casein paint, or linseed oil (earthen walls only).

Note that these drawing details are primarily for strawbale infill, however, they can easily be adapted to loadbearing walls as well. Down to Earth creates custom details on a project-by project basis.

RELATED ARTICLES

Overview of Strawbale Construction

Tips on Building Permits for Strawbale

Down to Earth Design
Sigi Koko, principal
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