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Producing cardboard insulation

Producing cardboard insulation

Following up on the previous blog entries about research on cardboard insulation, this article describes and evaluates the methodology we developed to finally produce insulation panels on a larger scale. Our goal was to insulate an entire roof, thus applying Critical Concretes prototype on to a real project and aiming to meet basic requirements: Insulation material needs to be protected from water and humidity, fire retardant an easy to install. Furthermore, it should be produced in an eco-friendly and cheap process. Last, this process should be easy to replicate, simple and effective, thus being able to produce enough panels to insulate an entire house or at least a considerable part of it. The method we used will be evaluated by these criteria.

During the soon to be Summer School 2018, the house of a family of three will be renovated completely. During the last weeks, the leaking roof was taken off and made new. We used the opportunity to insulate the new roof, a considerable task for which we needed 73m² of cardboard panels, installed in two layers.

In this article, we will enumerate the employed resources and describe the methodology we used to produce the needed panels, continuing with a detailed discussion of the different challenges we encountered and the main obstacles that are yet to solve. We conclude evaluating our work.


To insulate a surface of 36,5 m² with fire resistant cardboard panels, with a final insulation thickness of 6cm, we produced about 73 m² of cardboard insulation, installed in two layers of overlapping panels. Given this outcome and looking at the processed materials, we can calculate the resources needed per square meter:

  1. Cardboard: For every panel to reach 0,03m of thickness, we glued 5 to 6 pieces together, depending on the thickness of the different cardboard types we collected. This gives us an average of 5,5 layers per m². Regardless of produced leftovers, multiplying our outcome of 73m² of insulation by 5,5 leaves us with 401,5m² of processed cardboard.
  2. Borax: We calculated an average of 0,33 Kg per m², amounting to a total of 24 Kg of Borax.
  3. Starch: We used about 400g for one m² of insulation panels, using a total of 29,2 Kg of starch.

Production Methodology

Before starting the production process, a basic infrastructure is needed. Mainly, these are wooden molds to compress the cardboard pieces into panels and simple wooden structures that work as airers, or driers, for the treated cardboard.

Graphic representation of our production method.

Collecting: The main resource for producing our insulation panel is used and discarded cardboard, mostly found in form of boxes of different sizes on the street, the public garbage collection containers and local businesses such as supermarkets which daily discard packaging material. We went around our neighbourhood every day, collecting cardboard.
Cutting: In order to handle the collected material and to produce regular sized products, we cut the different boxes and cardboard bits into even pieces. To get the most out of the many different sizes we collect, and also to be able to cut in a fast and simple way, we decided to cut them into square pieces of 50cm x 50cm using a circular table saw. In order to reduce unused remnants, we cut the smaller leftovers into 25cm x 50cm pieces, which could later be put together easily when forming the final panel. In this step, we also removed parcel tape and other plastic adhesives from the cardboard.
Borax Treatment (Bathing): Giving the good results of our previous research, which showed that treating cardboard with borax powder makes it resistant to fire, we looked for a way of applying as much borax as possible on the cardboard in order to make it safe. Submerging the cardboard in a solution of warm water with 10% of Borax allows for the Borax to penetrate the cardboard. Limiting the submersion time to a maximum of two minutes avoids the disintegration of the layered cardboard, a specification that must be adapted to the different types and thicknesses of the previously collected cardboard. Thinner cardboard types could only take around 30 seconds of submersion, before they would start to fall apart. You can watch this short clip about it.
Drying: After submersion, the cardboard is not only impregnated with the fire retardant mineral, but also soaking wet and unfit to work with. We built driers, simple wooden structures, which are stackable on top of each other and allow ventilation to the wet cardboard from each side. The dryers can be moved out into the sun and back under a roof depending on the weather.
Gluing: Once the cardboard pieces are completely dry and have regained their stability, they can be glued together, forming the insulation panel. Using the proven self made starch adhesive and applying it with a spatula, five to six cardboard pieces were glued together and placed inside a mold. After a few tests, we realized that the glue is best when stirring water at 85°C into an equivalent amount of starch.
Compressing: To ensure that the starch adhesive can do its work properly, the pieces were then submitted to pressure by clamps, inside a mold. After one to two hours, the mold is opened and the cardboard, now a single panel consisting of 5 to 6 layers of treated cardboard, can be removed. We built several molds, thus being able to compress ten panels at once.

The result of this process is a sturdy 0,03 x 0,5  x 0,5m panel, ready to be installed at the construction site.

Costs and Workforce

To evaluate the total cost of the produced outcome, we summarize material costs, additional costs as well as the employed workforce.

Material cost

  • Cardboard: Collected local leftovers: No cost.
  • Borax: Best price found in Porto was 1,95 Euro per Kilo. So for the consumed 24 Kg we payed 47,00 Euros.
  • Starch: Best price found in Porto was 1,59 Euro per 400g. So for the consumed 73 packs we payed 116,07 Euro.

Total budget spent: 163,07 Euro

Additional costs

Some days we could not gather enough cardboard in the neighborhood, so we used a van to reach bigger supermarkets in other parts of the city, thus adding a cost in diesel. Also, for the glue we heated up water using an electric water boiler. Taken together with the circular saw, electricity was consumed. For heating up water in bigger quantities to dissolve borax, we used a gas cooker.

Workforce and Time

Work was fastest and easiest with four people working: Cutting, bathing, gluing and making the glue. Being that the cutting is the fastest task, that person can regularly pause to lay the wet cardboard on to the dryers in the open, allowing the person that is submerging the cardboard to work without interruption. To sum up the working hours needed to complete the task is somewhat difficult given that a continuous workflow was only possible after many interruptions due to experimenting, failing and finding solutions, as well as not finding enough cardboard to process.

Once the cardboard collection is coordinated with the local supermarkets, one can use an estimated net working time as a rule of thumb to plan the process. Calculating with 22 cut pieces of cardboard as layers for one square meter, we can add the processing time per piece and production step.

  • Cutting: Considering 30 seconds to cut every piece, it takes 11 minutes to cut pieces for one square meter.
  • Bathing: Considering an average of 1,5 minutes of submersion per piece, and given that we can submerge two pieces at the same time, it takes 16,5 minutes to treat one square meter with borax.
  • Making Glue: Considering 4 minutes to mix and stir the amount of starch needed to glue the pieces of one panel together, adding the time to heat up water on to the precise temperature needed, we can estimate the time of making glue for one square meter at 20 minutes.
  • Gluing: Applying the glue to each piece takes about 30 seconds, thus adding up to 11 minutes for one square meter.

In total, and without considering the drying time which will be discussed below, it takes an estimated 58,5 working minutes to produce one square meter of our cardboard insulation.

Problems and benefits

This production process, as described above, proved to have three main disadvantages as well as significant merits, which will be specified below.

Main Disadvantages

  • Drying time

First, drying the cardboard after the borax-bath limited the daily production. This step of the production process did not only take the longest, it was also very dependable of the weather. The unpredictable weather in Porto during May and June affected the process heavily, sometimes prolonging the duration of the drying process over two entire days. Also, the cardboard dries faster when spread out and exposed directly to the sun, which eventually takes up a lot of space. In our case, space and time needed for drying limited the production, on ideal weather conditions, to about 32 panels a day. The space available allows us to dry up to 85 pieces of cardboard, enough to make 17 panels. On a hot sunny day, the pieces would dry up fast enough to make a second round on the afternoon.

Drying the panels after submersion under the sun.


  • Borax dissolution

Second, the borax treatment by submersion into a water-borax dissolution can only deliver reliable and evenly results if the borax is completely dissolved into the water, thus being able to penetrate and impregnate the cardboard. However, dissolving 6 Kg of Borax into 60L of water for a longer period of time was challenging, and the mineral would gather on the ground of the water recipient after only a few minutes. A complete dissolution was only possible with heated water and constant stirring, a state that we could hardly keep up for long.

  • Glue quality

Third, maintaining a stable quality of glue was difficult and error prone. Following the example of the cardboard industry as well as our previous tests, we made starch adhesive. We made the glue by hand, mixing water at 85°C with starch, which then would lose its adhesive qualities while it cools down. This affected the sturdiness of some panels. Also, to ensure a continuous flow of good quality glue, thus allowing for a complete panel to be made and pressed together while the glue is still on its strongest level, one person was occupied making glue, while another person would apply it.


  • Resistance to fire

The panels were submitted to a qualified fire test and passed it  successfully. As can be seen in this video, the treated cardboard panel did neither hold or propagate the flame, while untreated cardboard did.

  • Thermal resistance

Thanks to our partnership with the engineering faculty of Porto University, FEUP, our panel was tested in their laboratory. The result of the standard test showed a thermal conductivity of l = 0,068 W/m.ºC, a value similar to that of other insulation products such as glass wool. With only 6 cm of thickness, these panels significantly improve insulation without taking in too much space.

  • Tested on the worksite

The panels proved to be easy to install on the roof, efficiently covering the whole surface in two overlapping layers. After this experience, we can consider our panels to be handy at the construction site.

Installation of insulation panels in two overlapping layers, on the roof of our 2018 Summer School working site.

Conclusion / Evaluation

Can the cardboard panel compete with conventional insulation? If we look at the bare material cost, we spent 2,23 Euro per m² (four panels of 0,03×0,5×0,5m). Doubling the panels to achieve 6 cm of insulation thickness, one square meter of fire resistant cardboard insulation costs 4,46 Euro, without the workforce.

A main factor that is not being considered in the cost calculation is the time we spent working on the panels during this first production process. Since this process followed the principle of learning by doing, a lot of time went into figuring out the best way to treat the cardboard and assemble the panels. Our next challenge is to better organize the collection of the cardboard and increase the available space to dry the panels by improving the airers.

The principal advantage of cardboard insulation lies in its reuse of discarded material – from the ecological point of view, it just makes sense! Although there is still much room for improvement in our method, the basic satisfaction of turning discarded material into a functional product, which improves a homes quality, is granted.

During the soon to be summer school, we plan to produce more panels to continue insulating the house. Stay tuned for our next article, in which we will discuss further experiences in this process.

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