Well over a decade ago, I was sitting in a Building Enclosure Council meeting in Boston and the topic of insulating precast concrete panel veneers came up.

A person from a well-respected engineering company made the statement, “Insulating precast concrete panel veneers is a nightmare”

Of course, this set my mind to work trying to come up with a solution.

“Insulating precast concrete panel veneers is a nightmare”

WUFI® Pro Hygrothermal Modeling Software


I thought about the various assemblies that I had seen and the various insulation materials that I had seen used in the assemblies and I made a list of them. I then sat down in front of my computer and started analyzing them using the WUFI® Pro hygrothermal modeling software.

I re-ran the analyses using WUFI® Pro Version 5.2 for this paper.

  • I selected a North orientation for the simulations as it has the least drying potential to the exterior since it is never exposed to direct sunlight. Therefore, it is the most likely orientation to fail in this climate region.
  • I used the weather file for Boston in a cold climate for the exterior climate and the medium moisture load file for the interior climate.

The developers of WUFI® claim that the program can accurately model moisture transfer due to air flow, but I have my doubts. If the exact flow path and exact amount of the air can be determined, then WUFI® can accurately model moisture transport due to air flow. But I find this virtually impossible to determine accurately in an exterior wall assembly since we can never know how many imperfections that the people that built the wall included in the assembly nor what imperfections are added to the assembly after it is built. Thus air flow due to convection within the assembly was not included in the simulations.

Analysis 1 – Un-faced, Low-density Fiberglass Batts Insulation
The first assembly that I modeled was un-faced, low-density fiberglass batts insulation attached to the back side of the precast panel veneer with stick to clips with light gauge metal framing and gypsum wallboard to the interior of the precast. (Figure 1)

This would be the least costly assembly due to the relatively low cost of furnishing and installing the batts insulation. I assumed that the precast concrete panels with sealant at the panel joints was the air barrier in the assembly.

This assembly failed miserably as the RH levels at the interior face of the precast concrete panels ( monitor position 3 ) hit 100% (Condensation) every Winter (Figure 2). These RH levels combined with below freezing temperatures would result in the growth of ice on the interior face of the precast concrete veneer panel. A condition that I had witnessed first hand on occasion.


Analysis 2 – Faced Low-Density Fiberglass Batts Insulation
I then tried adding a vapor barrier to the interior face of the batts insulation which would simulate foil-face batts insulation (Figure 3). I thought that the vapor barrier would prevent the gaseous moisture ( vapor ) from the interior from migrating to the air within the batts insulation thereby keeping the RH levels of this air below levels that would cause the assembly to fail.

This assembly failed also as the RH levels at monitor position 3 (Figure 4) remained above 80% for most of the simulation period. The steel attachment hardware for the precast concrete panel veneer are located in this area of the assembly and will corrode when subjected to RH levels above 80%. Monitor Position 4 (Figure 4) experienced RH levels above 80% for significant periods of time every Summer and Monitor Position 5 (Figure 5) experienced even higher RH levels and for greater periods of time. In watching the simulation, I saw that the added moisture within the insulation layer was coming from the precast panel as it has the ability to absorb and store moisture.

Analysis 3 – UnFaced Low-Density Fiberglass Batts Insulation with a Vapor Barrier to the Interior Face of the Studs
I thought that if I moved the vapor barrier to the interior face of the studs that the increase in the volume of air between the precast and the vapor barrier would allow for enough hygric redistribution to drop the RH levels in the airspace below 80% (Figure 6). I thought wrong. As you see from (Figures 7), (Figure 8), & (Figure 9), the RH levels were over 80% for various, but significant time periods at Monitor Positions 3 through 8.
Analysis 4 – Rock Wool Insulation
I decided to try substituting rock wool insulation (Figure 10) for the batts insulation since it is a much higher density material. That did not work either. This assembly pretty much had the same results as the assembly with un-faced batts insulation (Figure 11). Due to these results, I did not try adding a vapor barrier to the assembly.

The results of the first four analyses convinced me that the insulation for this assembly needed to be very dense to the degree that the cell structure of the insulation was so small that a water droplet could not form within it.

Analysis 5 – Extruded Polystyrene Insulation
This led me to try an assembly with extruded polystyrene insulation (Figure 12). This material is also a vapor retarder and therefore would limit the vapor from the interior air that would diffuse through the material. This performed much better than the assemblies with fibrous insulation, but the RH levels at Monitor Position 3 (Figure 13) were still unacceptable to me. The insulation and the precast could withstand the high RH levels without deterioration or performance issues, but the steel connectors for the precast panels would not tolerate exposure to these conditions well. The corrosion of metal also needs oxygen in order to occur. There is plenty of oxygen in the air surrounding the steel elements that connect the precast panel veneer to the structure because the faces and edges of the extruded polystyrene are very flat and true.

The back side of the precast concrete panels is very rough. Installing the insulation over the back of the precast panel results in a considerable amount of air spaces between the two components.

There are also many paths for warm, humid interior air to infiltrate this space at the joints in the insulation and interface of the insulation with other building components. The warm, humid interior air infiltrating these air spaces could be cooled below its dew point temperature resulting in condensation, frost and/or ice on the back side of the precast concrete veneer panels. If the joints and interfaces of the extruded polystyrene insulation could be sealed, the amount of oxygen in the air of the air spaces would be finite and therefore corrosion of steel components would be limited if not eliminated. Based on decades of my experience in the construction industry, I didn’t think this would be possible because perfection does not exist in construction. This was tried, though.

Back in 2003, a project that I was selling some of the products that I represented had one part of the building that had precast concrete panel veneer. They attached extruded polystyrene to the back side of the panels with stick clips and sealed the joints and junctions with sealant. What made matters worse was that the building was a laboratory and two of the exterior walls with precast panel veneer enclosed was the wash area of the lab. Needless to say, there was high humidity in that area.

A year or so later I e-mailed one of the engineers for the construction manager to ask him if there were any issues with the building. He informed me that the building was leaking at the exterior walls that had precast panel veneers and they could not figure out why.

The walls were not leaking. The warm humid air in the ware wash area was getting to the back side of the precast panels and resulting in the condensation, frost and ice that I mentioned earlier in this paragraph. This ruled out using any rigid board stock insulation for me.


Analysis 6 – The Solution – 2# Density Spray Polyurethane Foam Insulation
I came to the conclusion that in order for this assembly to work it needed and non-permeable insulation material that has a tiny cell structure and could conform to the profile of the back face of the precast concrete veneer panel. The answer? Two-pound density spray polyurethane foam insulation (Figure 14). The RH levels at Monitor Position 3 (Figure 15) were above 80% for most of the analysis period, but the spray foam would be adhered to the back face of the precast concrete panel veneer and the adjacent components including the steel attachment components for the precast veneer panels. With no oxygen containing air in this area of high humidity, no corrosion would occur.


Now armed with a solution, I reached out to design professionals to show them what I came up with and offered them a solution. They accepted and used my proposed solution and have used it on dozens of projects in the Boston area and many more beyond Boston.
Does ½# Density Open Cell Spray Polyurethane Foam Work too?
Just for the heck of it, I tried substituting half pound density open cell spray polyurethane foam insulation for the two-pound density insulation (Figure 16). As expected, it resulted in 100% relative humidity ( condensation ) on the back face of the precast concrete panel veneer. (Figure 17) Failed.
Some of the projects that have been occupied for over ten years and have not experienced any problems with the exterior wall assembly are…
Featured Projects
Successful Exterior Wall Assembly
2# Density Spray Polyurethane Foam Insulation

Kendall Residences, Cambridge, MA

Neponset Landing, Quincy, MA

Marriott Renaissance Hotel Boston, MA


As an added bonus, two-pound density spray polyurethane foam insulation has the highest R-Value per inch thickness of any insulation type on the market at this time. Therefore, the required R-Value of the exterior wall assembly can be achieved with the least amount of encroachment into the exterior space when using this type of insulation.
In watching the installation of spray foam on the Kendal Residences project, I realized there was a need for a product to be used as a spray foam stop at the perimeters of door and window openings. That is when I came up with the idea for my Jam-Ex® product.

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