How does air insulate

As heat energy flows from the hot object to the cold object, the heat energy from your hot water will be transferred to the surrounding cold air inside the fridge once you put the glass jars inside. The most significant mechanism of heat transfer in this case is convection, which means that the air just next to the hot jar is warmed up by the hot water. Then, the warm air is replaced with cold air, which is also warmed up.

At the same time, the cold air cools down the water inside the jar. The heat of the hot water is transported away by the flow of cold air around the cup. If you left the jars in the fridge long enough, you might have observed that the temperature changes until the hot water reaches the temperature inside the fridge.

Without a temperature difference between the water and the fridge, the heat transfer will stop. Heat from the water is also lost through conduction: the transfer of heat through the material, which is dependent on the thermal conductivity of the material itself. The glass jar can conduct heat relatively well. You notice that when you touch the glass jar with the hot water the glass feels hot as well. What effect did the different wrapping materials have? You should have noticed that with wrapping materials, the temperature of the water after 10 minutes inside the fridge was higher compared to the unwrapped control.

Wrapping the glass jar reduces the heat transfer from the hot water to the cold air inside the fridge. Using wrapping materials that have a very low thermal conductivity reduces the heat loss through conduction. At the same time the insulator can also disrupt or reduce the flow of cold air around the glass jar, which results in less heat loss via convection.

One way of reducing convection is creating air pockets around the jar, for example, by using insulators such as bubble wrap, fabric or wool, which have a lot of air pockets. Air in general is a good thermal insulator, but it can transmit heat through convection.

However, if the air pockets inside the insulating material are separated from each other, heat flow from one air pocket to another cannot happen easily. This is the reason why you should have measured the highest temperature in the bubble-wrapped jar and fabric-wrapped jar. This also explains why most of our clothes are made of fabrics and why you stay warmer when you put on an extra jacket.

Paper and foil make it easier for the heat to escape because they don't have many air pockets. This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Discover World-Changing Science. Key concepts Physics Heat transfer Insulation Material science Introduction What do you do when it gets very cold in winter?

Materials Five glass jars with lids Scissors and adult to help with cutting Tape Aluminum foil Bubble wrap Wool scarf or other wool clothes Paper Hot tap water Thermometer Fridge Timer Paper for writing Pen or pencil Preparation Cut a piece of the aluminum foil, the bubble wrap and the paper have an adult help if necessary. Each piece should be large enough to fit three times around the sides of the glass jar.

Take the piece of aluminum foil and wrap it around the sides of one of the jars. You should have three layers of foil around the glass jar. Use the tape to attach the foil to the jar. Next, wrap another jar with the bubble wrap so that the glass is also covered in three layers. Make sure to tape the bubble wrap onto the jar. Use the cut paper to wrap a third jar in three layers of paper.

Once again, attach the paper to the glass jar. Take another glass jar and wrap the scarf or other wool fabric around the jar. Only make three layers of wrapping and make sure that the scarf stays attached to the jar. Leave the last jar without any wrapping. Scientists depend on empirical evidence to produce scientific knowledge. Add to collection. Nature of science Science is based on and derived from observations of the world around us from which interpretations are made.

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Additionally, reference should also be made to BR Conventions for U-value calculations edition which sets conventions for and gives guidance on the calculation of U-values. Generally, the Combined Method is suitable for most elements of construction except where there are metal repeating thermal bridges in the insulation layer. If the example above is taken to be a timber frame panel, then it becomes apparent that the insulation is bridged by the timber studs. In these circumstances, the combined method is appropriate.

When the combined U value method is applied to the calculation of the U-value of this construction it becomes 0. This proportion is identified as the default timber fraction in BR, and a level 0 correction for air gaps in the insulation layer has been applied because mineral wool is deemed to be cut with a positive tolerance so that it has to be compressed between the timber studs to be fitted and cross joints are compressed together.

If a rigid foam board has been used, it might be considered necessary to apply a level 1 air gap correction because the board has to be cut with a negative tolerance to enable fitting and there may well be air gaps greater than 5mm in width.

Clearly the accurate calculation of U-values requires detailed knowledge of product characteristics, calculation methodologies and standards, and construction techniques. The accurate calculation of U-values is a fundamental building block in the development of whole building energy models and Building Regulations submissions. Skip to main content. Home The Science of Insulation Explained. The Science of Insulation Explained. Energy Efficiency.

Ask an expert. View products. View downloads. The Science of Insulation Explained How heat flows To understand how insulation works, it is first necessary to explain the different ways heat flows through a construction. Conduction Conduction is the transmission of heat through a material, or from one material to another, through direct contact.

Convection Convection occurs in gases and liquids. How to reduce heat flow by the use of insulating materials in buildings In order to perform effectively, an insulation material must reduce heat flow. How conduction is reduced To reduce heat transfer by conduction, an insulating material should have a very small amount of solid material in relation to void. How convection is reduced To reduce heat transfer by convection, an insulating material should contain small voids or air pockets within which air movement is minimised.

How radiation is reduced The transmission of heat by radiation is stopped when it is absorbed into the surface of a material, this results in a rise in temperature of the material. Thermal conductivity by material type The graph below shows the classical curve type associated with the thermal conductivity performance of traditional bulk insulation materials.