Total R-Values and Thermal Bridging

• Buildings are rarely built of a single material so to determine the total R-value you need to factor in all the individual components

• Thermal bridging is when the overall R-value is lower than the insulation’s R-value

Adding R-values In Series

• When materials are sandwiched together, or perpendicular to the direction of heat flow it is called “in series”

• Cavity brick wall– Two layers of bring, an air gap and ½” of

plasterboard in a row• Heat must pass fully through one material to

get to the next• Simply add the sum

Adding R-values In Parallel

• When materials are sandwiched in parallel to the direction of heat it is called adding “in parallel”

• Heat being transferred does not need to pass through one fully to get to the next material– Takes the path of least resistance– Standard window in a well-insulated wall

• Adding in parallel means 1/sum of the reciprocals of individual r-values

• Conductive materials can short circuit other insulative materials and make the total R low

Resistance from Air Films and Air Spaces

• On the surface and in between building constructions add insulating properties

• Air films are layers of air that are assumed to be static on each side of a building envelope

• Are spaces are volumes of air within building constructions

Cavities and Air Spaces

• An air space is a planar volume is a planar volume of air contained on two sides by some element of an envelope assembly

• Commonly built into wall constructions to help reduce heat transfer when multiple layers are in series

• Has high resistance to heat conduction but almost none to radiation and little to convection

Cavities and Air Spaces

• When conduction, convection, and radiation occur at the same time the thermal resistance of air spaces becomes virtually independent of gap width

• Resistance of a thick air space can be increased by subdividing it into several thin layers– The whole space is then the sum of the resistances of

the thinner air spaces plus separators– Most effective with low emissivity materials like

aluminum and in series

Thermal Bridging and Thermal Breaks

• Thermal bridge – unwanted path for heat flow that bypasses the main insulation of a building envelope– good conductor put in parallel with insulation– Provides a path for heat flow that bypasses

insulation• Avoided by placing insulation in series with

conductive material• Look at the lowest R-value and improve it

Thermal Bridging

Thermal Breaks

• When an assembly that would normally be a thermal bridge is broken up into separate pieces that are isolated by more insulative material

• Ex. Metal window frames are broken up so that one piece of metal faces the outside of the building, a separate piece faces the inside and in between are pieces of rigid plastic

Thermal Break

Framing Factor

• A percentage of reduction in R-value due to the walls, roof, or floor’s framing

• A wall with R-20 insulation and framing factor of 25% would have an overall insulation of R-15

• The more framing members the higher the factor• Steel studs are 50% and above while wood

framing is closer to 25%• Can be eliminated by placing insulation in series

with the framing