Yet, in warmer climates-and often on the west and east sides of our houses in more northern climates-we want windows to block the entry of unwanted solar gain. On the south side of our homes, especially in colder climates, windows are often expected to transmit beneficial solar heat, helping keep us warm. They block the escape of heat and prevent condensation on the glass. This focus has reminded me just how much we expect of our windows and what an amazing job they do.įirst, let's look at everything that windows are expected to do: They provide us with views to the outdoors-whether of drifting snow on a frigid winter morning or kids playing in the backyard in mid-summer.
#Plate glass windows r value series
I've been working for the past couple weeks on a report on windows-the latest in BuildingGreen's series of special reports on green building (the last one covering insulation). R val = Δ T ϕ q is the apparent thermal resistivity of the material ( K⋅ m/ W).Īn apparent R-value quantifies the physical quantity called thermal insulance.This Marvin Tri-pane window has three panes of glass, creating two dead air spaces. Here is the technical expression of that, using kelvins (K) instead of degrees Celsius (☌): But for that material alone, 450 W is going out, and can be replaced with a 450 W heater inside, to maintain the inside temperature. There will be other losses through the floor, windows, ventilation slots, etc. If the material has an R-value of 4, it will lose 0.25 W/(☌⋅m 2). In its most practical form it must be inverted, so an R-value of 4 becomes 1 / 4, and that is 0.25 watts being lost, for each degree Celsius temperature difference, through each square metre of area.įor example, in winter it might be 2 ☌ outside and 20 ☌ inside, making a temperature difference of 18 ☌.
This relates to the technical/constructional value. 16.2.1 Non-reflective surface R-values for air films.13.3 The limitations of R-values in evaluating radiant barriers.13.2 Surface temperature in relationship to mode of heat transfer.13.1 Thermal conductivity versus apparent thermal conductivity.It is intended as a simple, non-technical rating which consumers can understand. Insulating camping products such as sleeping mats and foam mattresses are often given a rating such as R-7 or R24, which may be called "the R Value" but is not the same. They are useful as it is a way of predicting the composite behaviour of an entire building element rather than relying on the properties of individual materials. A low U-value, or conversely a high R-Value usually indicates high levels of insulation. The higher the U-value, the lower the ability of the building envelope to resist heat transfer. It is expressed in watts per square metre kelvin (W/m 2⋅K). The elements are commonly assemblies of many layers of materials, such as those that make up the building envelope. It is a property that describes how well building elements conduct heat per unit area across a temperature gradient. The U-factor or U-value is the overall heat transfer coefficient and can be found by taking the inverse of the R-value. R-values are additive for layers of materials, and the higher the R-value the better the performance. In the case of materials, it is often expressed in terms of R-value per metre. for polyethylene foam), or for an assembly of materials (e.g. Īn R-value can be given for a material (e.g. The R-value is the building industry term for thermal resistance "per unit area." It is sometimes denoted RSI-value if the SI units are used. R-value is the temperature difference per unit of heat flux needed to sustain one unit of heat flux between the warmer surface and colder surface of a barrier under steady-state conditions. In the context of construction, the R-value is a measure of how well a two-dimensional barrier, such as a layer of insulation, a window or a complete wall or ceiling, resists the conductive flow of heat. Installed faced fiberglass batt insulation with its R-value visible (R-21)