Concrete Block

Concrete Block Walls: Low Tech and Durable

Bird's-Eye View

Concrete masonry unit walls are durable and easily erected

Building houses from concrete blocks, also known as concrete masonry units (CMUs), is more common in some parts of the country — Florida, for example — than others. CMU construction has many of the same benefits of other masonry options, including high resistance to fire, insects, and rot. CMUs can be reinforced with steel to form extremely strong wall sections.

Key Materials

Lightweight concrete blocks

One alternative to conventional concrete blocks are blocks made from autoclaved aerated concrete (AAC). Lightweight AAC blocks have a long history in Europe but have made only limited inroads in the United States. Blocks are available in a number of sizes, up to 48 inches long, 8 inches high and between 4 inches and 16 inches thick. They are resistant to fire, insects, and fungal decay.

Precast AAC blocks are set much like ordinary concrete block, although the thinner mortar joint requires care on the part of the mason to keep walls aligned and plumb. The process is further slowed by the need to drill out each course for steel reinforcement linking the footing with a poured concrete bond beam at top-plate height. Steel rebar and threaded rods connect the footing with the bond beam.

Hebel is one well-known brand name. AAC blocks are relatively easy to cut. Grooves in the walls for electrical conduit can be formed with a router or with a circular saw and chisel.

The thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. of AAC walls helps level out temperature fluctuations. The material's R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. is about 1.25 per inch.

Design Notes

It's best to insulate block walls with exterior rigid foam

Concrete is a poor thermal insulator, a fact that has limited a wider acceptance of concrete-block construction in the Snowbelt. Manufacturers, however, now offer a variety of block types that feature improved energy performance, including block with reduced internal webs to reduce thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. and blocks that incorporate foam insulation in the concrete mix. Although foam inserts made to fill block cavities can improve thermal performance slightly, it's best to insulate bock walls with a continuous layer of exterior foam.

An interesting project in Colorado suggests that concrete-block construction could work well outside of its warm-weather stronghold. The 3,176-square-foot, off-grid house was built at an elevation of 9,300 feet on a site with more than 9,600 heating degree-days annually. The R-20 exterior walls were constructed of dry-stacked concrete block wrapped in 5 inches of expanded polystyrene insulation, keeping the thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. on the conditioned side of the building. The passive solar design was innovative on many fronts and became a Building America research house; it showed that concrete block, when integrated into a thoughtful whole-house design, could succeed even in a very cold environment.

Builder Tips

Hebel block uses proprietary thin-set mortar

Masons are accustomed to speedy work with block, but working with AAC can be slow-going. You need special trowels and mortar, and AAC requires precise drilling for the threaded rod that connects the foundation with the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate..

For masons using Hebel block, the first course is set in S-type cement mortar, but subsequent courses are set with a special thin-set mortar provided by the manufacturer. Blocks are laid with no visible joint, so it is crucial that mortar be applied carefully. Hebel sells special trowels designed for the job as well.

Set wiring into the block walls.
Hebel will provide a router to cut channels in the walls for electrical conduit, but some builders think it's easier to use a circular saw to slice grooves in block, then knock out the waste. Tip: Use a cheap saw and a diamond-matrix blade.

Before finishing AAC walls, patch any irregularities with Hebel block patch and use a Hebel toothed rasp to smooth the wall surfaces. Stucco is floated on the exterior walls to make a decorative, one-coat finish that needs no vapor barrier. Interior walls can be plastered without wire lath.

The Code

The code is specific and comprehensive

IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code. section 607 covers construction with unit masonry, including appropriate mortars, reinforcement, joint sizes, and wall ties. Multiple-wythe masonry is covered in section 608, and grouted masonry in section 609. Nearly 12 pages long, the information in these three masonry sections is specific and comprehensive.


Although a concrete block wall can be insulated on either the interior or exterior, the occupants will receive a thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. benefit only with exterior insulation. The best way to insulate a concrete block wall is with a thick layer of exterior rigid foam.


LEEDLeadership in Energy and Environmental Design. LEED for Homes is the residential green building program from the United States Green Building Council (USGBC). While this program is primarily designed for and applicable to new home projects, major gut rehabs can qualify. -H Wall system influences thermal performance, potentially affecting 3 to 5 points in EA1 (Energy & Atmosphere), EA2, and/or EA3. CMUs are not considered environmentally preferable per MR2.2 (Materials & Resources) unless they are made with fly ashFine particulates consisting primarily of silica, alumina, and iron that are collected from flue gases during coal combustion. Flyash is employed as a substitute for some of the portland cement used in the making of concrete, producing a denser, stronger, and slower-setting material while eliminating a portion of the energy-intensive cement required. More info or slag instead of portland cement.

NGBSNational Green Building Standard Based on the NAHB Model Green Home Building Guidelines and passed through ANSI. This standard can be applied to both new homes, remodeling projects, and additions. Under Chapter 7, "Energy Efficiency": Concrete block can contribute up to 4 points as part of thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. in a passive solar design (704.3.1.3 - .4). Under Chapter 6, "Resource Efficiency": up to 4 points for recycled content (fly ash or slag instead of portland cement in block mix) (604.1).


A tried-and-true system

Concrete masonry units (CMUs) are the standard hollow concrete blocks; the product has been around for more than a century. CMU use is more common in the southeastern U.S. than in cold-weather regions. CMUs are resistant to fire and insects, and, with adequate steel reinforcement, also offer high wind resistance. Walls can be insulated on the inside or the outside.

Fluffier concrete
An only slightly younger cousin to CMUs are aerated autoclaved concrete (AAC) blocks. Sold under several brand names, AAC blocks are made with a mixture of portland cement, lime, silica sand, water, and aluminum powder. A reaction between the cement and aluminum creates tiny bubbles of hydrogen that increase the mixture’s volume fivefold.

Interior Insulation Retrofits of Load-Bearing Masonry Walls in Cold Climates

Image Credits:

  1. Steve Culpepper/Fine Homebuilding #111
  2. Scott Gibson/Fine Homebuilding #123
  3. Charles Miller/Fine Homebuilding #171
  4. Kathleen Jardine and James Cameron/Fine Homebuilding #123
Tags: ,
Sep 4, 2012 4:05 AM ET

Edited Sep 4, 2012 4:06 AM ET.

Response to Simon Berry
by Martin Holladay, GBA Advisor

I don't know what "DPC" is, but I'm guessing it's polyethyhlene.

If you are building a garden shed, it hardly matters how you build it. There won't be any heat or air conditioning, right?

The main problem with an unplastered concrete-block wall is that it is ugly. If you don't mind looking at it, you'll be fine.

Of course, this depends on having a roof with adequate overhangs so that the walls don't get too wet. What's your climate?

Sep 3, 2012 3:25 PM ET

concrete block walls
by simon berry

new user and learning techniques. Would anyone tell me that by doing solid conc block exterior wall to a garden building without a render or sealant be?? ok as long as we have dpc under concrete slab? The internal wall is going to be plaster board on timber frame with a mulitfoil insulation layer between the timber and internal walls. Do I need to leave a gap? Thank you in advance!

Nov 16, 2010 9:10 PM ET

Response to Doug
by Martin Holladay, GBA Advisor

Q. "Can we glue 3/4" styform to the inside walls to increase the R value of the wall?"

A. Yes.

Q. "Now will the foil side of the styrofoam face the outside or inside?"

A. The foil facing will not provide any thermal benefit unless it faces an air space. If you glue one side to the concrete block, don't glue the side with the foil. Face the foil side toward the inside; then install wood strapping, 16 inches on center, on top of the foam. You can attach your drywall to the strapping.

Nov 16, 2010 8:44 PM ET

Concrete block homes in Florida
by Doug Haisch

Can we glue 3/4" styform to the inside walls to increase the R value of the wall?
Then finish with paneling or a laminatred panel to finish the wall. Now will the foil side of the styaform face the out side or inside?
Thank you for reading any your suggestions

Jul 4, 2010 4:39 PM ET

Autoclaved Concrete
by Stephen Colley

I would like to see more third party information and commentary regarding building with aerated autoclaved concrete (AAC). I designed a 2-story 2600sf house out of AAC almost ten years ago, and the owners are very happy with the decision to use it. I'm in a hot-humid climate. I'm assuming there's little on the subject primarily due to so few homes and businesses building with it.

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