Total R-Values and Thermal Bridging

1. Total R-Values and Thermal Bridging

2. 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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. Thermal Bridging

10. 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

11. Thermal Break

12. 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