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R-value (insulation) - Wikipedia, the free encyclopedia
Primary role
The R-value is a measure of an insulation sample's ability to reduce the rate of heat flow under specified test conditions. The primary mode of heat tras*fer impeded by insulation is conduction, but insulation also reduces heat loss by all three heat tras*fer modes: conduction, convection, and radiation. The primary means of heat loss across an uninsulated air-filled space is natural convection, which occurs because of changes in air density with temperature. Insulation greatly retards natural convection making the primary mode of heat tras*fer conduction. Porous insulations accomplish this by trapping air so that significant convective heat loss is eliminated, leaving only conduction and minor radiation tras*fer. The primary role of such insulation is to make the thermal conductivity of the insulation that of trapped, stagnant air. However this cannot be realized fully because the glass wool or foam needed to prevent convection increases the heat conduction compared to that of still air. The minor radiative heat tras*fer is minimized by having many surfaces interrupting a "clear view" between the inner and outer surfaces of the insulation much as visible light is interrupted from passing through porous materials. Such multiple surfaces are abundant in batting and porous foam. Radiation is also minimized by low emissivity (highly reflective) exterior surfaces such as aluminum foil. Lower thermal conductivity, or higher R-values, can be achieved by replacing air with argon when practical such as within special closed-pore foam insulation because argon has a lower thermal conductivity than air.
http://www.physics.ohio-state.edu/~wilkins/energy/Companion/E09.4.pdf.xpdf
Dry air does not conduct very well. Most insulation works by trapping air so that it
cannot move. Dry, relatively still air an inch thick has an R-value of about 1. Completely
motionless air an inch thick has an R-value of 3 to 4.
If the air is not still, the thermal energy loss through the window will be much higher. That’s why windows feel
so cold on the inside when we are near them on a windy day. Adding storm windows
serves two purposes—it will add a space with still air, and, more important, more layers
of air at each window surface.
Low-e windows
Low-emissivity windows, mentioned in the Chapter, are windows with a plastic coating
that reflects thermal energy while letting most visible light through. Windows treated with
low-e film help in both the winter and summer, with little leakage of thermal energy to the
inside on a hot summer day, or little leakage to the outside in winter. The windows are
usually also double-paned, and may have a filling of an inert gas such as argon or nitrogen.
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