What is a K-Value in construction?

Written by LetsBuild

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The K-value is basically a shortened phrase for thermal conductivity, which is the time rate that heat steadily flows through a unit that is induced by the unit temperature in a direction that is perpendicular to the area.  Okay, so that sounds really complicated, but honestly, it isn’t.

Start to consider the amount of heat that flows when you are running hot water and you basically know what thermal conductivity is and how it works.


K-values are important, because they allow you to assess what the heat transfer is between the outside and inside of a building.  Since you are using many different materials, your K-values can be different, so this is something that truly is important to know.

As soon as you have fully researched the definition of a K-value, you will see that it is units of Btu per inch per hour per square foot per Fahrenheit degree for the temperature.

Once you know the K-values for the building you are working on, you can easily use that information to determine the other values that you will need to know.


A C-value is different than a K-value, because it focuses on thermal conductance.  This value depends on the thickness of the material, whereas the K-value does not care how thick the material is.  The C-value is determined by Btu per hour per square foot per Fahrenheit degree for the temperature.

When you are looking at the C-value of a building, you will instantly notice that the C-value is going to be cut in half if you use a one inch thick insulation board instead of a two inch thick piece of the same board.


An R-value is considered thermal resistance between two defined surfaces of material.  It can also be any type of construction that induces unit heat flow through any area.

To determine the R-value of any item, you must divide one by the C-value or divide the thickness by the K-value.  Therefore, you can see that a C-value of 0.5 will have an R-value of 2.0, while the R-value is 4 when the K-value is 0.25 and the thickness of the material is 1.


U-values are basically thermal transmittance and it is where the heat transmission goes through material construction and the boundary air firms according to the environment on each side of the building.  The lower the U-value is, the lower the rate of heat flow, and that means heat is not escaping and the building is staying warm.  Of course, higher U-values usually pop up in poorly insulated buildings and in those scenarios, the heat will be on much more.

It is a lot of work to determine the U-value for any building, but when you do, you must know these things first:

  • The C-value of both the indoor and outdoor air film
  • K-value of the 3 and ½ inch wide wood studs
  • The spacing between the studs
  • The K-value of the fiberglass insulation batts and the thickness of them
  • The K-value, as well as the thickness of the wood siding material

The Rules of These Values

The lower the K-value is, the greater the insulating value for the given thickness and the set of conditions.

The better the performance of insulation means a greater R-value and a lower C-value.

The better the insulation is, the lower the U-value will be.

All these values are used to create energy savings, protect those who are in the building, and control condensation.

Therefore, it is important to know all these values inside any building that you construct.  The K-values are necessary so you can figure out all the rest, so do not try to take shortcuts.  The results of a shortcut can be skewed numbers that will not truly represent what the value really is and that can create more problems in the future.