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Understanding heat energy

Thermal convection occurs when hot molecules move from one location to another.

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June 30, 2015 |
Barbara Shirley

Metal building systems are compatible with a wide range of architectural cladding materials. However, in buildings where energy efficiency is a concern, there is a strong case to be made for an insulated metal panel system such as Star Buildings’ Metl-Span panels.

An insulated metal panel is a sandwich: a steel outer layer is wrapped around a core of insulating foam. The steel at the panel edges is formed into an interlocking pattern that helps seal the panel system against air infiltration. The bends in the steel at the interlocking edges also increase the panel’s structural strength and rigidity. With the addition of butyl caulk in the joint(s), the panel also acts as a weather and vapor barrier.

The advantages of this system, as part of an energy efficient building, are its ability to block the transfer of heat energy from exterior to interior, and vice versa.  Heat energy has only three mechanisms through which it can transfer: thermal convection, thermal conduction, and thermal radiation.

Thermal convection occurs when hot molecules move from one location to another. The tendency of hot air to rise is an engine of natural thermal convection.

Thermal conduction is the transfer of heat energy from one molecule to the next.  Every molecule can stay exactly where it is, but the energy can move from one to another. A hot (excited) molecule can transfer some of its energy if the molecule next to it is less excited. Generally speaking, the more dense the material, the more molecules are in contact with each other, the greater the opportunity for conduction. 

Thermal radiation, or radiant energy, is a form of electromagnetic radiation, closely related to visible light. Infra-red electromagnetic radiation cannot be seen, but it moves in much the same way visible light moves: through the vacuum of space, through the atmosphere, through water, and through some solid substances, including some that are opaque to visible light. The sun heats the earth entirely by radiant energy: no molecules move from sun to earth (e.g. there is no convection), and there is 150 million km (93 million miles) of vacuum preventing conduction. At temperatures greater than absolute zero (–273º C [–460º F]) all matter emits some radiant energy.  

The three mechanisms often work together. For example, air is heated in a furnace by conduction and radiation, carried throughout the building by convection, and then heats cooler objects (e.g. occupants) by conduction and radiation.

Insulated metal panel systems control all three modes of heat transfer. 

The steel is a good heat conductor, but the metal in the panel system is thermally broken between interior and exterior sides, so heat cannot conduct through the steel. The foam core has very low density, so there are very few molecules in contact for conduction.

The light-colored foam is a good reflector of radiant energy. Radiant energy is not absorbed and stored, nor is it transferred. It bounces off.

Neither gasses (like air) nor liquids (like water) can pass through the steel skin of the panels. The interlocking edge system and joint gaskets prevent air or water from moving through the wall between panels. Convection is stopped.

This means that the heat of the sun stays outside while you’re trying to cool the building in the summer. In the winter, the heat you put into the building stays inside, instead of leaking away to the exterior. In concert with energy efficient windows and doors, insulated metal panels systems for walls and roofs can create high-performance, energy efficient buildings quickly and cost-effectively.

Barbara Shirley | Metal Building Trends

Barbara Shirley is PEB Manager for Metl-Span and has been with Metl-Span for 15 years, initially as Customer Relations Manager, and now PEB Manager for the NCI Building Brands supporting their efforts in IMP technical, products, quotes, specification review, application of product and training on same.   When she is not traveling, she lives in East Texas where she and her husband enjoy bass fishing.

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