An Insulation for San Pablo

Problem Statement

Throughout the year, the residents of San Pablo are plagued by cold weather conditions. Due to the local construction practices, the interior temperature of the homes is below a comfortable living temperature, and heat loss through the roof of the structure is a major concern for the residents. Consisting of a single sheet of corrugated steel, the roof fails to provide adequate mass and insulation to maintain heat in the homes throughout the night.

San Pablo, Guatemala

The climate in San Pablo is climate, with a dry humid season and cold, wet season. This climate presents a challenge to our group to develop an insulation system that is waterproof and durable in a humid climate.

There are several local materials available in San Pablo, including:
– adobe
– wood
– bamboo
– volcanic ash, pozzolana
– native plants
– corrugated metal
– recycled water bottles
– stone


Our goal is to create a new roofing system that will provide better insulation properties than that of corrugated steel. This material should be:

  • inexpensive
  • easy to construct
  • fabricated from local materials
  • supportive of the local economy
  • light-weight
  • environmentally friendly
  • durable

Possible Solutions

One of the possible solutions we’re considering is the use of pumicrete. Pumice is low density volcanic rock that, when utilized in concrete construction, creates a lightweight concrete that has an insulation value above that of steel.
Other solutions we would like to test include adding available recyclable materials such as styrene to the concrete to further reduce its weight, creating a cob or slip straw mixture made with straw, clay, and water, as well as filling an air cavity between two sheets of corrugated steel with salvaged plastic bottles or fabric.

Decision Matrix

Category Weight No insulation Normal Pumicrete High Pumice Pumicrete Polystyrene Pumicrete Cob Shredded Plastic Plastic Bottles Salvage Fabric Slip Straw
Insulation 10 1 5 7 8 5 7 8 7 5
Weight 8 10 2 4 6 4 10 9 6 8
Constructability 6 10 7 7 6 3 4 6 10 10
Initial Cost 7 10 5 5 4 8 8 10 10 10
Maintenance 3 8 10 10 8 7 6 6 6 6
Durability 9 5 10 10 10 7 5 4 4 5
Moisture-Proof 10 3 10 9 8 6 3 3 2 3
Local Availability 6 8 6 6 5 9 9 9 10 9
Environmental Impact 6 5 4 4 3 10 9 9 10 10
Total 397 423 449 434 414 431 450 442 451

Testing Method

We are testing our materials by creating an insulated box with a heat source, such as boiling water, located at the base of the apparatus. The sample is placed on a corrugate metal sheet suspended above the heat source, and styrene walls prevent loss of heat to the environment from the sides of our apparatus. Using a temperature gauge, we then test the change in temperature of a volume of water placed above our sample. This should create an accurate depiction of the relative insulation properties of our samples.
external image moz-screenshot.png

Corey mixing the styrene pumicrete sample


Corey, Michael, and Alese making the testing apparatus
Michael testing a sample with our temperature gage

Current Activities

Our Samples:

High Pumice Pumicrete
Pumicrete with Styrene
Salvage Fabric
Plastic Bottles and Bags
Shredded Plastic
Slip Straw

Our Progress:

March 14 – After assessing our testing results, we decided that slip straw insulation would be the most appropriate technology to improve the insulation of San Pablo’s roofs.

March 7 – This week we tested our final sample, the slip straw, and calculated the embodied energy and carbon footprint of our system.

February 14 – This week we made and tested our new samples: slip straw, shredded plastic, shredded fabric, and plastic bottles stacked with plastic bags filling the cavities.

February 7 – After all of the testing that we completed last week, we now have a better idea of what characteristics we are looking for in our insulating material. After researching different systems and reviewing different case studies, we decided to try a more natural, re-use- and salvage-oriented approach to insulating San Pablo’s roofs. We decided to test four new combinations: slip straw, shredded plastic, shredded fabric, and plastic bottles stacked with plastic bags filling the cavities. We hope that these systems not only provide insulation, but help to manage some of San Pablo’s waste from plastic bottles and bags.

January 31 – This week we poured all four of our samples and tested their insulation properties using our testing apparatus. Please see the Results section for our testing values!

January 24 – After our test pour, we’re now planning on setting up a form work that allows us to pour four different types of test material. We chose to create a typical pumice-crete mix, a mix with more pumice, a mix with polystyrene added, and a final mix of cob.

January 17 – Our group is currently gathering up the materials necessary to create the pumice-crete. We are also working on an apparatus that will allow us to test the pumice-crete and compare its insulative properties to that of corrugated steel.

Measure of Insulation

R-value is an indication of the thermal resistance of different materials often used in the building and construction industry. It is the ratio of the temperature change across an insulator to the heat transfer per unit area. We are determining the effectiveness of our samples by comparing their calculated relative R-values.

Equation to calculate R-Value:

R = ∆T/Q




Our Proposed Solution

We suggest that San Pablo improve the insulation of its houses by infilling its roofing system with slip straw. The slip straw would be stuffed between roof framing members, underneath the existing corrugated metal roof. Chicken Slip straw, is a natural building material that is made of loose straw coated minimally in clay slip, which is clay diluted with water to the consistency of a milkshake. Slip-straw is used as an infill panel built between framing members; it is tamped by hand or with a stick between “slip forms” and finished with plaster, which would act as a fire and moisture barrier.

Slip straw stuffed between metal roofing and wire mesh.


Environmental and Cost-Benefit Analysis

As there is no environmental impact from our current insulation method, we compared the carbon footprint of the production of different insulation methods, including western methods, which include fiberglass insulation. There is no carbon footprint generated by using our insulation system.



Throughout the quarter, our group has been consulting Troy Peters, an Architecture professor at Cal Poly to better understand R-value as a measurement of insulation and its implications for our project. According to Peters, given the Guatemalan climate and thick-walled houses of San Pablo, an R-value of about 6 would be required to achieve a comfortable indoor environment 92% of the time.

In addition, our group is currently in contact with Niker Cifuentes from San Pablo, Guatemala. So far we have updated Cifuentes on our progress and described the different systems we have been testing, and suggested the slip straw system as a way of improving insulation. According to Cifuentes, clay and straw is easily accessible and inexpensive in Guatemala, and slip straw seems like an effective way to improve the insulation of roofs. Further correspondence would be helpful in determining the framing system that is currently supporting the roofs and developing a slip straw construction method that is appropriate for the existing roof framing.

Future Directions

As this project progresses, there is a huge potential for improvements and future development. Some possible improvements include:

  • Using a more precise method of measuring, such as thermocouples, to determine the temperature change across the thickness of our samples. This would provide more precise data sets for a more accurate analysis and calculation of R-value or other insulation properties.
  • Trying other samples or combinations, such as working with air-entraining concrete, making different slip straw with different amounts of clay slip, or combining pumice rock with straw.

Our Group

(left to right): Corey, Charis, Michael, Alese

Alese Ashuckian

Cory Kawamoto

Michael Meizen

Charis Wu