So, will this double wall have condensation problems?
We specifically put the air barrier in the middle of the wall and we are following the 1/3 inside 2/3 outside ratios.
Building Science Corporation studied a number of wall configurations in the Minneapolis climate zone and published a report. While none of them were exactly the same assembly, they are close enough to use their dew point models as illustrations. Our wall is closest to the Hybrid wall 5 from an air barrier and location of insulation perspective. However, this model used spray foam as an air barrier and we are using plastic/polyiso. They had 3 inches of polyiso as exterior insulation - we have 1 inch of polyiso and a 2x6 wall with mineral wool batts. So, we have more R value outside of the air barrier which further reduces the condensation risk.
Standard 2x6 wall:
We specifically put the air barrier in the middle of the wall and we are following the 1/3 inside 2/3 outside ratios.
Building Science Corporation studied a number of wall configurations in the Minneapolis climate zone and published a report. While none of them were exactly the same assembly, they are close enough to use their dew point models as illustrations. Our wall is closest to the Hybrid wall 5 from an air barrier and location of insulation perspective. However, this model used spray foam as an air barrier and we are using plastic/polyiso. They had 3 inches of polyiso as exterior insulation - we have 1 inch of polyiso and a 2x6 wall with mineral wool batts. So, we have more R value outside of the air barrier which further reduces the condensation risk.
"Figure 5 through Figure 11 show the dew point of the interior air graphed in conjunction with the
surface temperature of the surface within the wall where condensation could occur. For Standard
Wall 1 (Figure 5), the possible condensation plane is the inside face of the OSB. Each hour the
temperature of the inside face of the OSB (orange line) is below the dew point of the interior air
(sinusoidal blue line), a bar is added in blue. The annual total of the bars in blue indicates the
total annual risk of condensation. A summary of the hours of condensation for each modeled
wall can be found in Table 8. For the exterior insulated wall, the condensation surface monitored
was the inside face of the exterior insulation. In the case of the hybrid walls, the inside face of
the ccSPF was monitored.
As the amount of insulation present outside of the possible condensation plane increases, the
associated risk decreases. This is most significant when comparing any one of the hybrid walls to
the standard case.
Table 8 shows a summary of the reductions in condensation potential. In the initial Minneapolis modeling, hybrid wall 1 showed a 95% decrease in risk compared to the standard wall. hybrid wall 3 showed a 98% reduction and hybrid wall 5 showed a 100% decrease and no condensation potential risk. In the hybrid cases, the condensation plane is a material that is not affected by moisture whereas the OSB wall will deteriorate eventually if the wetting is significant and is not balanced with drying. Since drying is minimal during periods of condensation, the severity of condensation increases the further below the dew point line the sheathing temperature falls and the length of time the sheathing temperature is below the interior air dew point line. For condensation to occur on this surface and if the enclosure is air sealed very well, there must also be air leakage to the surface from the interior. However, there is a risk that no moisture will be deposited.
Table 8 shows a summary of the reductions in condensation potential. In the initial Minneapolis modeling, hybrid wall 1 showed a 95% decrease in risk compared to the standard wall. hybrid wall 3 showed a 98% reduction and hybrid wall 5 showed a 100% decrease and no condensation potential risk. In the hybrid cases, the condensation plane is a material that is not affected by moisture whereas the OSB wall will deteriorate eventually if the wetting is significant and is not balanced with drying. Since drying is minimal during periods of condensation, the severity of condensation increases the further below the dew point line the sheathing temperature falls and the length of time the sheathing temperature is below the interior air dew point line. For condensation to occur on this surface and if the enclosure is air sealed very well, there must also be air leakage to the surface from the interior. However, there is a risk that no moisture will be deposited.
