Concrete and Radiant Floor Insulation


	(602) 690-1365 PHYSICS OF HEAT LOSS Two main paradigms of energy conservation are heat gain and heat loss. The three main factors in heat gain and heat loss are: · Conduction · Convection · Radiation Conduction is the direct transfer of energy through contact. Conduction is also the strongest form energy transfer that can occur. Conduction is like when you touch your hand to the burner on the stove, yank it back and yell “ouch” (heat gain). Heat flows from a hot or warm medium to a cold medium in three ways: By conduction through solid or fluid materials By convection which involves the physical movement of air By radiation from a warm surface to a cooler surface through an air space Conduction is the direct flow of heat through a material resulting from physical contact. The transfer of heat by conduction is caused by molecular motion in which molecules transfer their energy to adjoining molecules and increase their temperature. A typical example of conduction would be the heat transferred from hot coffee through the cup to the hand holding the cup. Another example as shown above, the contents of the kettle boils from heat transferred from the burner to the kettle. Also, a poker becomes hot from contact with hot coals. Heat transfer by conduction is governed by the fundamental equation described by Fouier's law: (Rate of heat flow) = k x (Area) x (Temperature Gradient) The factor k is called the thermal conductivity and is a characteristic of the material through which heat is flowing, and it varies with temperature and the degree of compaction or its density. Convection is a vapor characteristic in which warm air rises and cooler air falls. Vapors (gas) like air move in mass, thermal currents (temperature difference) cause a warm body of air to rise (warm air has more thermal energy) while a colder body of air will fall (cold air mass). Convection is like when you are on the plane and you hit turbulence – warm air rising next to cold air falling equals turbulence. Generally speaking convection is the largest source of heat loss in most structures, while the strongest transfer is conduction where the floor is in contact with the ground this accounts for about 30% of the envelopes exposure and the walls and roof account for around 70% of the exposure. For years the building industry has had varying ideas about insulating the walls and roof areas and considered the floor a secondary concern, for this reason the bulk of insulation occurs above ground. A tight vapor barrier will trap a high percentile of the convected energy from radiant floor, forced air and furnace heating systems. Radiation is generally defined as energy emitted in 360 degrees from the source. Usually the source is a chemical reaction like our sun (thermonuclear), fire (combustion) and some other sources like electric lights or heating elements that get hot when electrons are transmitted through them. Generally speaking we think of radiant energy as light photons. Light photons are a very powerful energy source traveling at a high rate of speed (speed of light) and constitute narrow high frequency energy that we call light rays. Most of the radiant energy we are aware of is in the visible spectrum of light. Some of the energy that we don’t really think about is in the infra red and ultra violet spectrum. While it is true that many heat sources such as hydronic heat floors have a component of infrared energy to them this energy is not a large factor as the low energy floor source is not comparable to the sun. The vast majority of the energy emission from a structure comes from conduction and convection. In terms of energy transfer, radiation and convection require an air space or an aerobic environment (presence of air in the space or most likely adjacent to the space). Convection is the warm air rising and is the cool air falling, radiation takes place across an air space. While the definition of radiant energy is: energy emitted in 360 degrees from the source, it is also true that radiant energy is conducted through materials by direct contact. This can be observed in the warmboard application where hot water (fluids) are circulated through a conduit (hydronic tubing) and the energy flow from the water through the tubing into the aluminum sheet pressed into the Warmboard subfloor which conducts the energy across the face of the Warmboard evenly. Subsequently the energy warms the air above the Warmboard (if an air space is present as when wooden runners are utilized in a wood floor application above the Warmboard then radiation occurs across this airspace between the Warmboard and the bottom of the floor, conversely if no airspace is present as in a carpet, tile, wood floor with out runners or concrete installled in direct contact with the Warmboard then conduction occurs) which rises through convection and warms the living spaces. When utilizing night vision infra red type goggles or a scope, what is detected is the temperature differences between the heat source and the environment. In other words, if you were to look a radiant heat floor with an infra red detector you would see energy coming off the floor but it is not really seeing light but indicating the temperature difference between the warm floor and the ground or warm floor and the air above the floor in the form of an invisible wave length of energy. So to summarize, when you touch something that is the strongest type of energy transfer a direct path with no barriers to resist the energy flow between the two objects touching. To prevent Conduction there must be an efficient thermal break between the thing being insulated and the material (soil in the case of a concrete slab on grade which it is in contact), which is highly resistant to the transfer of conducted energy to the environment in which the exposure occurs. Energy always migrates from warm to cold areas. So for instance, when you are thinking that your feet are cold when touching the floor of your home or office while sitting at the computer reading this verbose website, your feet are not gaining cold they are losing heat. Do your hands and feet get cold? Yes, but it is heat loss not a cold gain. When you put your ungloved hand into the snow and your fingers get cold you are not gaining cold, your fingers are losing heat to the environment. That is why when you pull your fingers out of the snow they are wet; the energy lost from your hand has converted the flakes of snow crystals to water. The energy transfer is direct and the result is the immediate loss of energy to the conversion of the solid crystalline structure of snow to a liquid form of the same molecules (heat loss). Insulating from conduction prevents this energy transfer. Since energy will migrate if you do not prevent the transfer then migration is occurring 24 hours a day 7 days a week. Therefore when trying to manage the flux of energy within the envelope the properties of conduction, convection and radiation must be addressed. (602) 690-1365 juangarcia@thebarrierinsulation.com