Newsletter Article

Unique "Post-Tensioned" Insulated Block Wall System

By: J. D. Ned Nisson, Editor

From Energy Design Update, Vol. 14, No. 11
Cutter Information Corp
Arlington, MA

The Integra wall system is the most promising energy efficient masonry wall system on the market.  Developed by Superlite Block of Phoenix, Arizona, it uses a unique "post-tensioning" system to create a block wall with greater strength and three times the R-value of conventional insulated block walls.
More than 8,000 homes have been built using the system and it recently received approval from the International Conference of Building Officials (ICBO).  Oldcastle Architectural Products Group, a nationwide organization of masonry producers, has just adopted the system and will distribute it from seven sites across the US.  Given its performance, practicality, and extensive track record, the Integra Wall System deserves serious consideration for both basements and above-grade walls.

"Post-tensioning" versus "rebar and grout"

The Integra system consists of specially designed concrete blocks with site-applied polyurethane foam insulation.  The unique aspect of this system is that it is the only commercialized masonry block system on the market that uses "post-tensioned" construction.

Post-tensioning is a process whereby a wall is "squeezed" using threaded steel rods.  It is similar in principle to "pre-stressed" concrete, except that instead of stretching a cable through the wall prior to assembly as is done with pre-stressed systems, post-tensioning uses threaded steel rods to compress the wall after it is built.

Post-tensioning should not be confused with conventional reinforced concrete, which uses reinforcement bar-embedded in grout to provide strength.  Post-tensioning employs a completely different engineering principle.  The purpose of the tensioning bars is to compress the blocks.  Once compressed, the inner and outer block faces carry the full load.

Threaded steel rods are embedded in the footing using special anchor bolts.  The rods extend up through the block cavities without grout.  At the top of the wall, the rods extend through steel bearing plates.  Threaded nuts are then screwed onto the rod to create vertical compression.  Special "indicator washers" show when the required 7,400 pounds of compressive pressure has been reached.

The National Concrete Masonry Association (NCMA) has tested the Integra system for structural integrity.  NCMA spokesperson Steve Szoke told EDU that Integra walls are as strong or stronger than conventional block walls.

From a thermal standpoint, the advantage of the post-tensioned system is that the internal webs in the block can be significantly reduced in size and number.  The basic Integra 8-inch block has a single small web connecting inner and outer faces.  The result is significantly less thermal bridging through the block and more room for insulation.  Also, unlike conventional reinforced concrete, post-tensioned systems do not require grout around the steel rods.

Non-CFC foam - R-16 to R-20 in an 8-inch wall

Integra is sold as a complete system, including foam insulation.  Superlite is using polyurethane foam that is made with HCFC-14lb - a non-CFC blowing agent.  The New England Integra distributor, FOAM-TECH Inc. of North Thetford, Vermont, uses its "SUPERGREEN" chlorine-free polyurethane foam (see EDU, May 1993).  It is not yet certain what the "aged" R-value of polyurethane foam made with those blowing agent is.  Estimates range from R-6.0 to R-7.6 per inch.  Within that range, the overall R-value of  8-inch Ingegra walls should be between R-16 and R-20.

Availability and cost

The Integra wall system will be produced by seven block manufacturers, all of which are independent members of the Oldcastle Architectural Products Group.  Manufacturing facilities are located in New York, Virginia, North Carolina, Texas, Idaho, Utah and Arizona.

According to Ed Freyermuth, president of Superlite, the total intalled cost is between $4.00 and $4.50 per square foot.  With FOAM-TECH's SUPERGREEN FOAM, the cost is slightly higher.