What are Insulation R-Values?

Carol Francois
Carol Francois
Thickness of insulation determines its R-value.
Thickness of insulation determines its R-value.

Insulation R-values are numerical values that indicate the level of thermal resistance in insulation. Thermal resistance is the ability of a material to reduce the heat loss of a wall. In the building and construction industry, the insulation R-value is a measure of the effectiveness of insulation products.

The insulation R-value of a material is determined under very specific test conditions, designed to mimic the standard home. Insulation is used to stop the loss of heat through convection, conduction and radiation. In homes and buildings, most of the air in a room is lost through natural convection to the outside.

Insulation works as a barrier to heat loss by acting just like trapped air. The insulation fills up the space between the outer wall and the inner walls of a room. The higher the insulation R-value, the closer it is to the thermal conductivity of trapped air. Air cannot move through air, and so the insulation eliminates heat loss through natural convection by forming an insulation barrier.

Insulation R-values are measured in K·m²/W or kelvin meters squared per watt. As a rule of thumb, the greater the value, the greater the level of insulation provided. The purpose of insulation values is to provide homeowners and consumes one comparable value to assist in the decision making process.

The highest insulation R-values are for Vacuum insulated panels at R-45. Flat, aerogel has a rating of R-10, with isocyanurate foam at R-8.3 and phenolic foam at R-7. Fiberglass blown or loose cellulose both have insulation R-values of R-3.

In the US, there were many cases of deceptive advertising claims on insulation R-values. In response to this the Federal Trade Commission (FTC) issued the R-Value Rule to stop industry wide unfair or deceptive acts. A insulation R-values fact sheet must be provided to customers before the product is purchased and the insulation R-values of the product must be clearly listed on the product label.

Insulation R-values are calculated based on a specific product thickness. Doubling the thickness does not necessarily increase the effectiveness of the insulation and may in fact have the opposite effect. For this reason, insulation R-values are used instead of inches.

The heat loss value of a wall must take into consideration the losses due to windows, wall studs and other conduits through the wall. All of these factors impact on the heat loss of a wall. To maintain a consistently low heat loss values, investigate the insulation R-value of the windows installed in the room.

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Discussion Comments


@jmc88 - The R value for various materials really just comes down to the quality and how it is being used. Like the article says, a lot of the time the R value of something like blown or rolled insulation will just depend on thickness.

I think the more important thing to consider when you are getting insulation is the dollars per R value or something along those lines. If you are going to get the same performance (same R value) with rolled fiberglass, blown, or boards, why not just buy the one that is going to be the least expensive?

That is assuming all of them are a valid option, which is definitely not always the case, but I think you probably get my point. You should consider things like installation cost and effectiveness, too, but those can all be factored into the R/dollar figure.


@kentuckycat - I was wondering about the same thing, so I just now looked it up. It was passed in 2005. From what I can tell, the reasoning behind it is that most of the time you can't inspect the insulation packaging before you buy it, so stores have to provide you with the information beforehand, so everyone can make a more informed decision.

It sounds like before, companies were making claims about their R values that were not completely true or might have been under special circumstances. I thought most people knew what R values were, but maybe not.

What I was also wondering about was whether anyone knows the normal R value differences between batting versus blown insulation.


@jcraig - I actually just bought some double pane windows, and what they use is called a U value. It is basically the opposite of the R value. I'm not sure I completely understand how both of the vales relate to each other or how a U value is calculated, but I know you want a lower value for your windows.

Does anyone know anything more about the R value Rule? I had never heard of this before reading the article. What exactly does it do, and when was it passed? I bought insulation for my attic a few years ago, and I don't remember getting any sort of handout about what type of insulation I was buying. I'm just curious if it might have been a more recent thing that I missed out on.


Wow, I didn't know there was so much that went into the R values on insulation. I thought it was interesting that the article mentioned that thickness wasn't a good predictor of R value and that stacking insulation could produce the opposite effect of what you would want. What makes this the case? For any insulation I have seen, the thicker it is, the better, in general. Plus, it seems odd that layering it could actually decrease the insulating ability.

I was wondering, too, do they supply some sort of R value for double or triple pane windows? I know they are becoming a lot more common, and it seems like a lot of people would want to know how well their new windows are going to insulate.

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    • Thickness of insulation determines its R-value.
      By: Cyril Comtat
      Thickness of insulation determines its R-value.