08 January 2015

Total R-Values and Thermal Bridging

One of the main goals for this tiny house is high thermal performance. In order to achieve this you need to understand R-values, see my previous post. Not only this but how R-values of different assemblies like windows and walls for example react with each other. New Zealand and many other nations set minimum requirements for building elements like floors and roofs. Our goal is to blow these out of the water!

Total R-Values and Thermal Bridging 

To work out the thermal performance of a building you must calculate the R-values for all assemblies like walls, roofs, floors and glazing. The total R-value for the building may be higher or lower than the R-value for the insulation. Thermal bridging is when the overall R-value is lower than the insulation's R-value. Buildings are generally made from multiple materials, so to determine the total R-value you need to factor in all the individual assemblies.

Adding R-values in Series

When materials are sandwiched together, perpendicular to the direction of heat flow, it is called adding "in series". The heat must pass all the way through one material before it gets to the next material, so any heat flow blocked by one material is blocked the rest of the way. Mathematically, adding in series is easy: simply sum all thermal resistances (R-values).

Adding R-values in Parallel 

When materials are sandwiched parallel to the direction of heat flow, it is called adding "in parallel". The heat being transferred does not need to pass all the way through one material before it gets to the next material; instead, it can take the path of least resistance. Therefore a highly conductive material can completely short-circuit other insulative materials and cause the total R-value to be low.

Calculating the Total R-Value

The total insulation of an assembly includes all of the resistances of its individual materials, whether in series or in parallel or both. If some materials are in parallel while others are in series, each section of materials in parallel should be treated as a layer, and its overall R-value calculated. Then all layers can be summed for the total R-value.

Thermal Bridging and Thermal Breaks 

Thermal bridging cures when the heat flow bypasses the main insulation and travels though the path of least resistance. This happens when a good conductor is put in parallel with the insulation, like a window frame.

Thermal bridging can be avoided by placing insulation in series with conductive material, rather than in parallel. For instance, you can place insulation outside a stud wall instead of only between the studs. This is sometimes called "exsulation" as opposed to "insulation". Thermal bridging can also be avoided by looking for the lowest R-value in an assembly and improving it. For instance, replacing metal window frames with fiberglass frames.

Thermal Breaks 

A thermal break is when an assembly that would normally be a thermal bridge is broken up into separate pieces that are isolated by a more insulative material. For example, many metal window frames are broken up so that one piece of metal faces the outside of the building, a separate piece of metal faces the inside of the building, and in between are pieces of rigid plastic.

House insulation requirements New Zealand

In the following post I will show now to calculate R-values in the different house assemblies in our tiny house - stay tuned! 


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