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October-December 1998 Issue


Straw-Based Insulation for Pakistan:
Addressing the Needs of Developing Regions


Three years ago, a team of Energy Laboratory researchers in MIT's Building Technology Program went to northern Pakistan to help villagers determine how to upgrade their homes, schools, and other buildings. Based on surveys of occupants and measurements taken in buildings, the team made a simple recommendation: install insulation. Most of the buildings have walls of uninsulated stone or concrete block--materials that provide little protection against the cold temperatures of winter in this mountainous region. People burn firewood for heat, but wood is scarce and indoor temperatures are often only a few degrees above those outdoors. Indeed, schools become so cold that they are unusable for several months in the winter. Adding insulation would both improve living conditions and conserve scarce resources.

While the researchers' recommendation was straightforward, carrying it out was not. The stone or concrete walls of the local buildings are a single layer with no interior cavity, so using loose fill insulation is not feasible. The only option is rigid insulation that can be fitted or retrofitted on interior walls, where it would be protected from the harsh weather. Rigid expanded polystyrene insulation is available in more developed areas of Pakistan and could be trucked to these remote villages, but the cost would be high.

The researchers--Leon R. Glicksman, Leslie K. Norford, Joseph A. Charlson, Henry S. Harvey, and Gregory P. Sullivan--therefore set out to develop boards or panels that could be made on-site from waste (or near-waste) materials by the local people using simple machinery and little energy. As their primary material, the researchers selected straw, a wheat-threshing by-product that is abundantly available at little cost in Pakistan. Straw has long been used worldwide in construction, generally in compressed form as structural boards. However, making a straw-based board that is both strong and insulating posed new challenges. Straw fibers conduct heat more readily than air does, so insulating value will be highest if the board contains little straw and lots of air, preferably trapped in tiny pockets so it cannot circulate and transfer heat. But the insulating board must also be strong enough to support itself during construction and to resist damage at its surface--requirements best fulfilled by using more straw and less air. The challenge, then, is to create a straw board that has low enough density to be a good insulator but high enough density to be sufficiently strong--all at an acceptable cost.

Guided by theoretical models, the researchers considered various ways of making insulation from straw. One approach is to mix straw with an adhesive (glue) to make a pulp feedstock. The result would be a strong, homogeneous board--but without the optimal distribution of air pockets and straw fibers. The researchers therefore tried shredding the straw in a hammer mill, spraying or foaming adhesive onto it, and then forming the final shape. But when the amount of adhesive used was low enough for costs to be acceptable, the samples produced were fragile and flaky.

The problem was uneven distribution of the adhesive: some areas received clumps of adhesive and other areas none at all. To solve that problem, the researchers turned to a technique developed by ICI Polyurethanes for spreading fine layers of adhesive onto particles. The shredded straw is placed in a large tumbler. A fine nozzle at the center of the tumbler sprays tiny droplets of a high-performance ICI adhesive, methane di-isocyanate (MDI), into the tumbling straw. The straw, now coated with a fine layer of adhesive, is removed from the tumbler, placed into a framework, and pressed while being heated.

Working with ICI personnel, the MIT research team produced samples of straw insulation with varying densities and adhesive contents. Thermal and structural tests showed that samples with densities of 128 kg/m3 and 160 kg/m3 and adhesive contents as low as 2-4% by weight are insulating and strong. The sample boards have insulation ratings of R4 and R3 per inch, respectively--values typical of the better air-based insulations such as expanded polystyrene, fiberglass, and cellulose. (Modern insulation can have R values up to 7 per inch, but they contain a low-conductivity gas rather than air--not an option here.) The sample boards are stronger in compression than widely used boards such as expanded polystyrene and rigid fiberglass; and the higher-density samples have a bending strength equal to that of extruded polystyrene, a material used in forming concrete foundations and placed under footings. Removing fine particles before processing the straw would further increase the compressive and bending strength of both boards without affecting their thermal resistance.

Economic analyses showed that the straw insulation would cost less than half as much as the polystyrene insulation would cost for a given insulating value. Moreover, unlike the polystyrene insulation, the straw insulation could be made in local facilities using local materials. Only the MDI adhesive would have to be imported. Although MDI is a relatively expensive adhesive, the small amount used makes the purchase cost tolerable for many applications. And the shipping cost for only the MDI adhesive will be much lower than that of a finished insulation product such as polystyrene since shipping costs are largely based on volume.

To assess the energy savings from using the insulation, the researchers surveyed and then simulated energy performance at four Pakistani schools with different designs and wall constructions. Energy use was based on a target temperature of 17C (63F). The efficiency of wood stoves was estimated to be 50%; infiltration rates were estimated from measured air tightness and available wind-speed data; and classroom occupancies noted in the site surveys were incorporated into the simulations. The simulations included an uninsulated base case and a series of scenarios in which insulation was placed on various external walls, internal corridor walls, and ceilings.

The figure below shows the estimated energy needs at one building of the Ahmedabad School, located in the Hunza Valley of Pakistan's Northern Areas. The building includes three classrooms, one office, and a hallway. Comparison of the insulation scenarios with the uninsulated base case shows that applying insulation could reduce energy use substantially. Applying insulation (3 inches thick for a total rating of R10) to all the external walls of occupied rooms except the southeast wall would reduce the energy consumed by 30% relative to the uninsulated base case. Insulating all the external walls brings a 44% reduction, and insulating the ceilings as well pushes the total reduction to more than 75%.

The calculated payback period for the straw insulation varies from school to school. Depending on the school and the insulation scenario selected, the cost of buying the necessary insulating board is offset by savings from reduced firewood purchases in 3 to 8 years. The cost of attaching the insulation to the stone or concrete walls and giving it a plaster finish coat--in both cases using special methods developed by the MIT researchers--lengthens the payback period. Considering the cost of the insulation plus labor and materials for installing it, the payback period ranges from 4 to 11 years. Comparable values for the polystyrene insulation are almost twice as long. Comparing the use of straw as insulation with its use as a fuel to heat the building, the payback of the insulation in energy savings is less than one year.

While results thus far are promising, more work is needed to make the straw insulation a practical option for the people of northern Pakistan. Given added financial support, the researchers would next develop a low-cost tumbler and an MDI sprayer that could be used inexpensively and safely in Pakistani villages. They would perform field tests of the new insulation to assess its installed thermal and structural performance and its long-term durability. And they would continue theoretical and experimental work to improve the insulation itself.

In the longer term, the processes used for shredding the straw, applying the binder, and forming the strong, porous boards could be used with other feedstocks--indeed, with whatever low-value fibrous materials are available in a given region. Thus, locally made, inexpensive insulation could become available in other parts of the developing world, improving living standards while reducing energy use and related environmental and climate effects.



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