Using tin foil or cardboard can block out light completely, but it isn't a desirable method to use in your room or apartment, and should only really be used as a last resort.
Opaque substances don't allow passage of light. Cardboard, wooden cupboard, metal, cell phone are examples of opaque substances.
UV can easily be shielded by materials such as polycarbonate, metal, cardboard, and wood. Ordinary glass blocks most UV light of wavelengths less than 330 nm but may also transmit most of the UV for longer wavelengths. It should not be relied for UV protection unless UV shielding is verified.
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.
Typical cardboard boxes have low thermal conductivity, meaning they can keep things cool when hot outside and vice versa.
Cover your windows with cardboard to completely block the sun. If you want to block the sun completely and are looking for a quick, cheap option, covering your windows with cardboard is a good temporary fix.
Cardboard's ability to block heat
Materials with a low thermal conductivity like cardboard are perfect for making insulation. These are used to keep heat from transferring between objects or locations. A good example of this is when faced with a window being broken and left without replacements on hand.
Standard cardboard is made of layers of thick paper that have been pressed and dried into stiff sheets. While great for storage purposes, plain cardboard is also highly flammable. According to research by the Smithsonian Institution, cardboard can ignite at temperatures between 425-475°F if exposed to heat.
Cardboard is sometimes recommended to insulate a shed. While it is true that corrugated cardboard can reduce heat loss, the effects will be minimal in a shed. The most difficult difficulty is that cardboard is porous and absorbs water quickly throughout the winter.
In both cases, consistent exposure to sunlight can lead to damage to the physical structure of the box as well as the fading of colors. This is known as photodegradation, and is less so the sunlight but the ultraviolet light present within the sunlight.
Cardboard is made from cellulose, a plant-based material that many pests find appetizing. Certain bugs, such as silverfish, cockroaches, and termites, are drawn to cellulose as a food source. These pests can chew through cardboard, causing damage to both the boxes and the items stored inside.
Chromogenic groups in paper absorb UV light, generating free radicals, which initiate chain reactions resulting in the formation of carboxyl groups and the breaking of the cellulose main chain.
UV is easily shielded by opaque materials such as metal, wood, and cardboard. Polycarbonate material is also a good UV shield. Some types of clear glass may transmit significant amounts of UV-A radiation and should not be relied on for UV protection unless UV shielding is verified.
Opaque objects block light and do not allow it to travel through them. Most of the light that passes through opaque objects is either reflected by the object or absorbed and converted to thermal energy. For example, wood, stone, and metals are opaque to visible light.
While it's easy to ignite and may seem like a good way to help get a fire started, burning cardboard can be dangerous. Because cardboard is treated with chemicals and often contains inks, it can release them into the air when burned. You don't want to breathe those in!
Cardboard burns at approximately 450 degrees Fahrenheit. Since the general consensus for reheating pizza in the oven is to keep the temperature set to 350 degrees Fahrenheit it should be safe to leave your pizza in the box, however, you must be extremely vigilant while it is baking.
Untreated paper and cardboard are easily flammable materials. Flammability limits a product's use as construction material, for example. Flame retardant treatment improves an end product's fire-resistance rating, which can open up entirely new applications and even entire markets.
In applications such as space flight or balloon-borne instruments, where weight is a primary concern, aluminum heat shields are favored. While copper and aluminum both have high thermal conductivities, copper has traditionally been favored because of the ease of joining copper to copper and copper to stainless steel.
Shea Butter
Shea butter has natural thermal conductivity properties, so it's a natural replacement for the silicones used in some heat protectants. Silicones coat the hair and protect it from the heat of your styling tools. Shea butter has similar properties, so it's ideal as a natural heat protectant.
Increase the Shade
You'll want to do what you can to block UV rays from entering a sun-facing room. Adding window treatments is one of the easiest ways to keep sunlight from heating your living space. Curtains or drapes with a reflective or light-colored backing would do the best job since they won't absorb extra heat.
Applying weatherstripping around your windows is one of the quickest, easiest, and most cost-effective solutions to insulating windows and preventing heat from entering. Windows often have gaps that are common points for air leaks in homes.
The ignition temperature of cardboard is over 400°C. If it gets that hot using it as an enclosure, you have other problems. It just doesn't look as cool as a metal or plastic.