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Out of the box – Insulation vol.2

Out of the box – Insulation vol.2

We have already written about the potential of using corrugated cardboard in building construction, as it consists a material which not only can be found in infinite amounts and for free, but it can also be applied through a simple procedure. This article refers to our research about the insulation properties of cardboard, its pros and cons as well as information and details of our prototype.

Different types of cardboard had been used by architects and builders to create various constructions: Shigeru Ban used widely cardboard cubes in construction[1]. (image 1), Ro&Ad Architects used honeycell cardboard for the interior construction of an office[2] , while Hans Ruijssenaars created a theater from cardboard and paper rods[3]. Although the list of people that had experimented with cardboard is long [4], the potential of this material in building construction has not been fully discovered yet.

image 1. Shigeru Ban’s emergency shelters in Kobe, Japan (source: http://popupcity.net/emergency-shelters-made-from-paper/)

Generally, cardboard consists a lightweight, cheap and recyclable material which means that it is suitable for sustainable construction. Nevertheless, in order to use it, we should examine the ways through which, we can expand its lifespan and also make it safer in case of fire, water leaking etc. First of all we examine the properties of cardboard that can lead us to search the conditions that it can be used and the problems that may occur during the installation process.

As regards to its insulation properties, cardboard is actually a great insulator as it has poor thermal conductivity. The definition of conductivity is actually the property of a material to transmit energy. Corrugated cardboards’ conductivity is estimated to be about the ⅓ of water’s conductivity and the same as fiberglass’ which is widely used for insulation. This means that thanks to the holes that are formed inside it, it has the potential to trap the air and deter its exhaustion. Thus, warm air can be kept inside the house, when cold air is prevented from entering inside, and vise versa. A simple example of this can be the temporary covering of a broken window with a piece of cardboard which prevents the cold air from entering inside, or the use of cardboard from homeless people to create a temporary shelter. Except from its satisfying thermal properties, it is also proved that cardboard can efficiently protect our space from noise pollution. The porous composition of cardboard is the reason that makes it an excellent sound-proof material[5]. An excellent proof of that is the use of partitions made of honeycomb cardboard in the office of an advertising company[6], which eventually reduced the sound of the open – plan office while creating small spaces (image 2).

image 2. The interior cardboard - design of an advertising company, by Paul Coudamy (source: http://coudamyarchitectures.com)

Despite its several advantages though, cardboard has also some serious drawbacks which should be taken under consideration when it is used in construction. The most important disadvantage of cardboard is its poor resistance to water.

Actually, water is both a friend and an enemy of cardboard. During its production process a large amount of water is added to the fibre (99% water and 1% fibre) (7).

Although water is one of the substances that creates it, when cardboard comes to contact with water, it loses its properties enormously and becomes vulnerable to mould. Therefore, the use of cardboard should be restricted in interior space, not in immediate contact with water or rain. However, other than repelling water from the exterior, cardboard should be also protected from air moisture. Cardboard, is an hygroscopic material which means that has the ability to absorb moisture from the air and eventually create mould and lose its strength. For a short amount of time, cardboard can efficiently absorb an amount of moist, but if it is going to be used as a construction material in humid places, it should be somehow protected. So the first problem that we have to cope with is: Is it possible to protect our cardboard insulation panels from humidity and make them water resistant?

While examine the different examples of cardboard construction, we found different ways to protect cardboard from water and humidity:

a) Minimize the edges between the cardboard panels. To accomplish this solution, we have to make panels that are big enough to cover a surface(wall or roof) with as less pieces as possible. At the same time we should find a size that is not too big and can help us treat the cardboard easily before installation.

b) Use solid cardboard (or plywood board) for the first and the last layer. As we explain precedently, it is important to seal cardboard panels from water entering from the exterior surface. Corrugated cardboard is consisted of a middle fluted layer covered by two thin and flat paper liner boards. This automatically makes it vulnerable to water and damages, so if we use as a cladding of our panels a solid cardboard layer, we can protect our insulation panels. The solid cardboard can be well protected from moisture and water, as its consistency is dense and does not form open cells. Simultaneously, it can efficiently protect the inner layers from damages as it performs better in load pressure.

c) Cover the cardboard with plastic layers or foils. This solution had been applied in several projects in which the cardboard was used in places that it could come in contact with water or rain[8]Nevertheless this solution can prevent cardboard from being recycled. More specifically, one of the most important benefits of cardboard as a material is that when its life is ended it can be recycled. A coat of plastic adhered on it can difficult the recycling process.

d) Apply a coat of paint, lime or varnish on the last surface. This solution consists the most efficient one as we can use lime plaster to cover the last layer of solid cardboard and then protect it efficiently from water and moisture, without using plastic materials.

All these solutions about how to cover cardboard and protect it from mould and water, gave us ideas on what information and knowledge we can use on our prototype.

Another major disadvantage of cardboard which is the reason why people don’t prefer it as an insulation material, is its resistance to fire. The most important thing about fire safety in a building is the time that the users have to get to safety. That is why the materials used in construction have to be fire retardant. Generally, cardboard is a paper product so as we can imagine is almost unachievable to transfuse fire resistant properties, no matter how many fire retardants will be placed in order to protect it. On the other hand, Dr Andrew Cripps who participated in the construction of the Cardboard School claims that cardboard it is flammable,  but it does not contribute to the spread of the flame[9].

Nevertheless there are several fire retardant materials that can be used, such as borax or chalk, but we should have in mind that they have the tendency to increase the development of smoke during the fire. Fire retardant cardboard (as well as water resistant), can be found nowadays on the market, but in our case we will search other — cheapest— ways to ensure fire safety.

a) Borax dissolution, the one we used in wool insulation, consists a fire retardant mineral which can also be insect repellent.

b) Thickness of cardboard: as illustrated by tests conducted during the development of Cardboard School in Westborough[10] a thick cardboard piece behave like thick wood pieces which has the tendency to char, protecting itself from burning immediately.

c) Cladding with a protecting board. The covering of the insulation with a plywood board or a solid cardboard can efficiently protect the inner layers from fire. This is because solid cardboard has dense consistency and therefore its behavior to fire is similar to wood. Furthermore, this last layer can be easily treated with a fire retardant coat used for wood.

Collecting all this information and through our experience in experimenting with cardboard and sheep wool insulation prototypes, we will now present you the queries and difficulties we faced, during the process of making our prototype, as well as the ideas and solutions we found in order to succeed.


First of all we need to express our concern about either using shredded cardboard, in a cellulose form, or cutting corrugated cardboard panels using boxes leftovers. We decided not to shred the cardboard because of the added required effort and of the reduction of the insulation value by destroying the air pockets that corrugated cardboard provides.

Thus, our prototype started with the creation of two types of panels, the small ones(22x22x5cm) and the larger ones(50x50x5cm). In order to discover the best way to adhere the cardboard parts together and also to treat them with the borax dissolution, which was the same we used on the sheep wool (20 teaspoons/1l water)[11], we ended up in combining these two steps, and mix borax in the glue we wanted to use. Therefore, we decreased the amount of work needed and this allowed us to develop an efficient process of mass production.

The process of making the glue passed some experiments and tests before deciding what type of glue we will use. Our first idea was to use wood glue, but it was impossible to dissolve the borax in it. After some research we found out about the process that the cardboard industry follows:. Τhe cardboard producing industry is using starch adhesive, from which around the 60% is made of wheat starch. Furthermore, borax is a part of the starch adhesive as it is added to speed up the gelatinisation of the glue.

Thus, we started to produce our own starch adhesive to glue the cardboard layers together and form a panel. Our attempts were made of corn starch and — of course — we added borax, to strengthen the fire retardant properties. To test the adhesive properties, we applied it with a brush on the 22x22x5 cm cardboard layers and clamped all together to small test panels. The result was stable and straight panels, so we decided to proceed with the starch adhesive.

As soon as we made the 50x50x5 cm panels we changed the tool to spread the glue. Using a foam roller turned out to be much faster. During drying, the panels need to be clamped into a simple wooden mould in order to create the right thickness and keep the single layers in position (image 3).

image 3. The wooden mould we made

Consequently, we made a fire test to discover if the treatment process with the borax had any positive impact on the cardboard. The results of the test are being presented in the video below:

As we can see, the treated cardboard needs much more time to start burning and at some point the burning process continues really slow. On the other hand, the untreated cardboard needs little time to start burning, but even then we can notice what was before stated, that the cardboard has similar reaction to wood: its surface is more charring than burning and does not play an important role in the expansion of the fire.


The next step in the process of our prototype included the different ways to install these panels on the construction site. Whether the insulation is meant for the walls or for the roof or ceiling, the solution should be able to refer to either options. The first issue with the inside insulation is the contact of the insulation with the exterior wall (or roof). In case of water leaking or humidity issues, the insulation panels have to be placed in a way that they will prevent a possible condensation of the warm air and therefore the creation of water between the wall and the insulation. Thus, we decided to apply 2 layers of cardboard in a way that we will form breaking joints between them (image 4).

image 4. The attachment of the cardboard layers on the wall.

The second issue we had to solve was the way that we could attach the panels on the wall. For that reason we created a test wall to try different ways to attach the panels and decide which solution is the best in our case. Perhaps the most simple way to mount the panels is using the common plastic dowels, normally used to mount styrofoam panels to a wall. This way does not create a smooth surface so that it has to be covered with plasterboard for example. Another way could be to make a new construction of wooden strips which are screwed to the wall and hold the panels into position. That would mean less screws and a visible secondary structure (image 5).

The last issue we faced was the covering of the panels from the front in order to seal them and protect them from water or damages. Our solution is to use a last layer of solid cardboard to cover the insulation, which will be held into position with the system of the wooden strips. Each wood is placed on the connection of the panels which will be sealed with pieces of woolen strips (image 6). For extra protection of the cardboard and also for aesthetic reasons, we can apply paint, wallpaper, plaster or lime on the last layer of solid cardboard.

Image 5 and 6. Section and facade of our proposal

The next step in our research will be the enlargement of the cardboard panels to 80x80x5 cm or 100x100x5 cm in order to fit them properly on the wooden structure of the walls and possibly combine them with the wool insulation panels.

Last but not least, what was also concerning us was the thickness of our insulation panels. We searched about the minimum thickness of cardboard that it is needed in order to reach a low R-Value — high thermal resistance. In a board that we found here, we calculated that with a 6 cm cardboard insulation we have R-value=0,67 and R-value=0,5 for 8 cm . Eventually we need to decide the thickness of our panels not only with regard to how much R-value we want, but also with reference to the solution that it will be implemented on the construction site.

Thanks to our collaboration with the F.E.U.P., our wool and cardboard panel prototypes will be tested in April. Stay tuned for the upcoming results!

In text references:

[1] From the Cardboard Cathedral in Christchurch, New Zealand, to temporary housing aimed to help disaster survivals in Japan, China, Dominican Republic etc.: http://www.shigerubanarchitects.com/works.html

[2]  https://www.ro-ad.org/projecten/cardboard-office/

[3]  http://www.ruijssenaars.nl/hra-wp/portfolio-items/9220/

[4] You can see more examples here.

[5] According to the research of F.Asdrubalia , A.L. Piselloa,, F. D’Alessandroa , F. Bianchia , M. Cornicchiaa , C. Fabiania, Innovative cardboard based panels with recycled materials from the packaging industry: thermal and acoustic performance analysis, if the layers of cardboard are placed concordantly: “a significantly better insulation behavior is detected in the range between 100 and 600 Hz (peak of 70 dB of TL at 400 Hz, for the maximum thickness analyzed of 66 mm)”. 

Also according to this board, with a corrugated cardboard panel of 40mm=44dB

[6]  Coudamy architects converted an industrial space into an office of an advertising company

[7] van Dooren, E. & van Iersel, T. (2008). “A House of Cardboard”, p.82 in: Cardboard in Architecture, IOS Press, 2008.

[8] In Australia, Adriano Pupilli and his team created a prototype shelter, using PET (Polyethylene Terephthalate)  to cover the cardboard. See other examples in: Cardboard in Architecture, IOS Press, 2008.

[9] Read the analytical report about the cardboard structure in Westborough Primary School.

[10] p.102 in Cardboard in Architecture, IOS Press, 2008.

[11] This proportion of borax/water results to 10% dissolution. It is important to be careful and not inhale the dust created by the borax powder!


[1] Asdrubalia, F., Piselloa, A.L., D’Alessandroa, F.,  Bianchia, F.,  Cornicchiaa, M., Fabiania, C. (2015). “Innovative cardboard based panels with recycled materials from the packaging industry: thermal and acoustic performance analysis”, 6th International Building Physics Conference, IBPC 2015.

[2] Stone, D. (2017).Thermal Properties of Cardboard“. Sciencing.com. [Online] available at: https://sciencing.com/info-8789172-thermal-properties-cardboard.html.

[3] Doll Tolliver, K. (n.d.).DIY Cardboard Boxes for Insulation”. EHow. [Online] available at: https://www.ehow.com/how_6811214_diy-cardboard-boxes-insulation.html.

[4] ArchitectureWeek (n.d.). “House of Card”. [Online] available at: http://www.architectureweek.com/2003/0129/environment_2-2.html.

[5] CardboardInsulation.Tripod (n.d.). “Cardboard4Insulation”. [Online] available at: http://cardboardinsul.tripod.com.

[6] Swanson, J. (n.d.).“Why Is Corrugated Cardboard a Good Insulator?”. EHow. [Online] available at: https://www.ehow.com/about_6469087_corrugated-cardboard-good-insulator_.html.

[7] Fauntleroy, G. (n.d.).“How to make Cardboard Weatherproof”. EHow. [Online] available at: https://www.ehow.com/how_7805328_make-cardboard-weatherproof.html. 

[8] Sobich, N. (2008).Nothing to be sneezed at”. StylePark. [Online] available at: https://www.stylepark.com/en/news/nothing-to-be-sneezed-at.

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