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The Truth About R Value?

If you have arrived at this page, you may be trying to understand more about what R value is, or maybe you want to know about the R value of cellulose vs fiberglass, or maybe you just want to have a more energy efficient home.

The big misconception is that people generally feel that if two insulation products have the same R value, then they must be the same, but they are not.

For example, during sub-zero temperatures, fiberglass insulation loses up to 50% of its R-value, where cellulose insulation gains a little when the temperature drops below zero.
R value is determined by tests done in a lab under a controlled environment.

It measures the conduction, the thermal resistance of heat as it transfers through the insulation product.

This does not include the insulation’s ability to resist air from passing through it.
The intention of the insulation is to stop the transfer of heat.


There are 4 ways that heat transfers
in and out your home.

  • Conduction- as the heat passes through the product (how R-value is figured)
  • Convection – in sub-zero temperatures and humidity changes.
  • Radiation – as the sun’s rays pass through the roof and walls of a home.
  • Air Infiltration – as on a windy day, air is pushed through cracks and leaks in.

Only one of those factors is considered for the purpose of R-value.
That being said, all insulations are not the same.

Make sure that you find out how many air changes per hour your home will have.
To reduce air infiltration, a spray application is the best method because it gets behind outlets and receptacles.

It seals around wires, plumbing, and heat runs. It even sprays behind wall partitions. This creates a seal inside all of the cavities.

To reduce the amount of air changes per hour, the entire envelope of the building needs to be sprayed from the framing above grade all the way to the ceiling.

Another very important step in sealing the walls is to caulk and or foam the cracks between the framing.

May times you will even see day light through those cracks, but even the ones you can’t see through add up to a great amount.

How much? How about leaving a window partially open in your home on a cold windy day. That is the equivalent. So, by caulking these gaps, that greatly helps reduce the transfer of heat through the wall, not just through the product.

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