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R-Value of Grout-Filled Concrete Block (CMU) Insulation

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You can increase the R-value of your concrete block insulation by filling the hollows with grout. There are many grout options to choose from—learn about several here.

You can increase the R-value of your concrete block insulation by filling the hollows with grout. There are many grout options to choose from—learn about several here.

What to Know About Cinder Block Insulation

Before we can begin to talk about insulating a home by filling in standard concrete blocks with filler materials, we have to discuss the terminology for insulation. This terminology is called R-value.

What Is R-Value?

R-value is defined as a material’s resistance to heat flow. The higher the R-value of a material, the better it is at resisting heat flow, which makes it a better insulator.

While R-value is often discussed in terms of fiberglass insulation that has been added to a structure, the actual materials that comprise the structure have R-values of their own. Using a highly heat-flow-resistant material for your foundation and walls can significantly reduce your expenditure on heating and cooling for the finished building.

What Is the R-Value of a CMU?

For the purposes of clarity and having a basic unit of comparison, we will use a concrete masonry unit (CMU) as our basic block. There are three sizes: 4-inch, 8-inch, and 12-inch. The standard in the United States is the 8-inch variety, so this is the one we will be using for our comparisons.

The R-value of an unfilled 8x8x16-inch CMU is 1.11. This includes two 4x4x8-inch spaces on the interior of the block. The R-value of this ambient space is 1.00 (any space from ½ inch to four inches has a value of 1.00). This means that the R-value of the actual concrete block is a paltry 0.11.

Can You Fill in the Center With Cement to Raise the R-Value?

A common misconception is that adding more cement in the center of the block will increase the R-value of the block; in fact, the opposite is true. Concrete transfers heat faster than open air. A concrete-filled block has an R-value of .19.

What Fill Material Should You Use?

Given the initial R-value of 1.11 and a space of four inches of fill to deal with, we can begin to calculate the value of the block when filled with various materials.

There are several materials that can be used to fill concrete block. Some of the most popular fills are cellulose, cotton batts, denim batts, fiberglass (both loose-fill and batting), insulating foam, phenolic foam spray insulation, polyurethane foam, and silica aerogel.

The following list gives the R-value for each of these materials per inch. Keep in mind that the fill space is four inches, so to determine the R-value of a block filled with each material, you would need to multiply the number in the table by four.

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MaterialR-Value per Inch



Cotton Batts


Denim Batts


Loose Fiberglass


Fiberglass Batts


Insulating Foam


Phenolic Foam Spray Insulation


Polyurethane Foam


Silica Aerogel


Which Filler Is Best for Green Building?

When making a green choice in insulation, one of the best options is a denim batt. These are usually made from recycled jeans and have the same R-value as traditional fiberglass. Not only is the material saved from becoming landfill, but it also keeps your home insulated.

Some other good green materials for filling CMUs are cotton balls, shredded clothing (cotton-based), and even straw. Each of these materials has an R-value that's higher than an empty block alone.

How to Calculate the R-Value of a CMU Based on the Fill Material

To find out the R-value of your grout-filled block, take the R-value of the fill material and multiply it by four, then add 0.11 for the cement block. So, a Silica Aerogel–filled block would have an R-value of 40.11, where a loose fiberglass–filled block would come in at 14.91, as you can see in the examples below:

Silica Aerogel: 10 × 4 = 40; 40 + .11 = 40.11

Loose Fiberglass 3.7 × 4 = 14.8; 14.8 + .11 = 14.91

Savings per R-Value Point

There are several online calculators that can help you determine the savings made by installing a higher-rated insulation. The standard rule is that for every 1-point increase in R-value, there is a 1% savings in overall heating and cooling costs.

Is a Higher R-Value Worth the Cost?

This depends on the cost of heating and cooling your home. For example, if you project a $37.00/year savings and the cost of the insulation is $740, it will take 20 years to recoup your initial investment.

Green Building Is Important Today and Tomorrow

With the recent movement toward a leaner, greener world, it makes sense that the topic of R-value and house insulation would come into play. To be precise about R-value, we have to understand that no insulation will prevent heat transfer—it just slows it down. The amount by which the transfer is slowed directly affects the need to heat or cool the interior of the building. This means less energy is used for these processes.

What to Remember About R-Value and Filled Cinder Blocks

The R-value of a grout-filled block is determined by the material used to fill it. A concrete-filled block is a poor insulator with a value of 0.19, whereas one with a foam or gel insert can range all the way up to 40.11.

If you are interested in saving money in the long term, a high R-value is essential. This is also essential in an area where there are extremes in temperature. A good R-value will prevent either warm or cool air from leaking out and costing you more money.


Kruger, A. & Seville, C. (2012). Green Building: Principles and Practices in Residential Construction (Go Green with Renewable Energy Resources) (1st ed.). Cengage Learning.

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