What is K-Value in roofing insulation?
This post is part of four-part series exploring energy efficiency values in insulation, such as K-Value, C-Value, R-Value, and U-Value.
K-Value. C-Value. R-Value. U-Value: what do they all mean? What do they have to do with insulation and thermal energy, especially when it comes to roofing insulation?
Let’s start with a simpler question: how does insulation work? The goal of insulation is to create a controlled energy environment by limiting thermal influence. Translation: insulation insulates against temperature changes.
Different insulation types are rated against a series of energy “values” they carry. These values measure how efficiently (their “value”) a particular insulation type performs against external energy influencers, like heat conduction, air flow, moisture invasion, and the more obvious, weather elements.
There are four types of values that insulation types are typically measured against: K-Value, C-Value, U-Value, and R-Value. Over the next four weeks, we will be explaining how each of these values is measured and how they affect different types of insulation. Let’s start with K-Value…
What’s K-Value?
K-Value measures thermal conductivity, which is defined by ASTM International as:
“Thermal conductivity, n: the time rate of steady state heat flow through a unit area of a homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.” (ASTM International C168)
Think of the water pressure in your home. It’s measured at a gallons-per-minute (gpm) flow rate. The K-Value of an insulation type is a similar measurement with the flow rate of energy across a surface over a period of time. In energy terms, that rate is usually measured in Btus per hour.
How insulation surface area influences K-Value
Of course, the influencing factor of the energy impact is how much of a surface area is being affected by the energy flow. For example, a kitchen faucet flowing at 3.5 gpm won’t conduct nearly as much water as a fire hydrant flowing at 3.5 gpm. This is where the “unit area” part of ASTM International’s definition comes into play. The size of the piping is much larger and therefore, more energy (i.e., water) is transferred because of the opening difference. In a similar fashion, the larger surface area covered by a single type of material (“homogeneous material), like insulation, the greater energy influence.
Unit Temperature Flow and K-Value
Energy is constantly in motion, like when it’s 10°F outside in January and you’re trying to keep your house in the mid-60’s inside. Hot area is trying to escape outside and Old Man Winter is trying to fight and claw his way inside. This takes us to the final phrase of ASTM International’s definition of thermal conductivity, “…unit temperature gradient in a direction perpendicular to that unit area.”
In closing, the K-Value of an insulation type can be best described in this manner:
K-Value = how long energy is constantly flowing x surface size covered by a single type of insulation x temperature differences between the insulated and non-insulated contact areas
Clear as mud? We know, we know, as soon as we turned it into a math equation, you had these nightmare flashback to 8th-grade algebra. Don’t worry- you don’t have to be the expert at knowing all the specifications and values of insulation; that’s our job.
It’s also our job to give you the right information to make the best decision for insulating your building. We work to explain the technical details of insulation to help answer everyday, practical questions, such as, “What insulation will help keep my building the most comfortable, no matter what the temperature?” Our R&A Contracting team is here to help you get the answers you need.