The Phius Certification Guidebook contains a lot of valuable information, but Appendix B is arguably the most important to ensure long-term durability of the passive building enclosure.

Appendix B is also one of the most often referenced during Phius Certification reviews and is often one of the most difficult issues to resolve if a design is far along and the moisture control requirements were overlooked.

This Appendix is titled “Moisture Control Guidelines for Opaque Assemblies” and the guidance on walls and roofs is almost entirely excerpted from building scientist John Straube’s  book titled High Performance Enclosures.

The goal of this Appendix is to provide a prescriptive and digestible guideline for vapor control in passive building enclosure design as a streamlined compliance path for Phius Certification. It also outlines alternative paths should your project’s assembly not meet the prescriptive requirements.

The beginning of the content starts with general guidelines, including the paths for compliance with Phius Certification. Those are:

Option 1: Follow the guidelines in Appendix B
Option 2:  Meet performance criteria by WUFI hygrothermal analysis (according to Phius' specific component simulation protocol)
Option 3:  Present a moisture-engineered design by a qualified licensed professional engineer.

Next, it lists a few commonly made mistakes to avoid, which often lead to risky outcomes (from a hygrothermal perspective).

The following three subsections, B-2, B-3, and B-4 run through Option 1 prescriptions for walls, roofs, and floors, respectively.

B-2: Walls

This section is broken down into above and below grade walls. The above-grade wall types are broken into 4 different categories, based on the insulation types and configuration. 

Many of the walls we see in Phius Certification fall into category B-2 (b) – “Framed assemblies with some insulation value outside of the framing or structure” aka a two-by-x with continuous exterior insulation. With this wall configuration, the important metric to consider is the sheathing to cavity R-value ratio, which is the R-value of the exterior insulation divided by the R-value of the cavity insulation. For example, in Chicago (ASHRAE Climate Zone 5A) the required minimum sheathing-to-cavity ratio is 0.35. 

For a 2x6 cellulose filled frame wall (R-3.2/in) with 2” of EPS (R-4/in), the math would be:

Cavity Insulation: 5.5” x R-3.2/in= R-17.6
Sheathing (Exterior Insulation): 2” x R-4/in = R-8
Sheathing/Cavity Ratio = 8/17.6 = 0.45 (pass!)

You may notice, the R-value of the cavity insulation is calculated as a homogenous value and does not account for framing. From a hygrothermal perspective, this is the more conservative approach.

This ratio increases as the climate gets colder to maintain a desirable temperature at the interior face of the exterior, vapor-closed insulation.  

We have also seen a fair amount of double-stud walls, and this section touches on those. The short story is: exterior load-bearing double-stud walls are discouraged in climate zones 3 and higher because they tend to be too vapor-closed on the outside for the climate. Interior load-bearing double-stud can be OK in some climates with the right vapor control. Refer to the Guidebook for more details.

Following this, there is a bit of additional guidance for below-grade walls, mostly related to placement of Class I vapor control layers, as well as a few links to articles related to interior insulation retrofits on masonry walls. 

B-3: Roofs

Similar to the guidance on walls, the section on roofs is broken into four categories based on the insulation types and configuration. 

The most common roof types we see in Phius Certification are:

b) Vented, framed roof assemblies with all or most of the insulation value installed between the framing as vapor permeable (more than 10 perm) insulation AND

c) Unvented framed roof assemblies with some insulation provided by air impermeable insulation. 

For type ‘b’, the most common design flaw is including a ventilation gap that is not wide enough (the requirement is an open air space of 1.5” or more). 

For type ‘c’, roofs that are composed of both vapor permeable and vapor impermeable insulation, there are again required ratios of those two insulation types per climate zone. Like the walls, there are ratios for exterior insulation but they are slightly different than walls - it is a required “Outer air-impermeable insulation value,” stated as a percentage of the total assembly R-value. Notice this is a different kind of ratio than the one used for walls.

For example, in Chicago (ASHRAE Climate Zone 5A) the required outer air-impermeable insulation value must be >35% of the total R-value. 

For a roof with a 12” interior framed truss filled with cellulose (R-3.2/in) and 4” of exterior EPS (R-4/in), the math would be:

Cavity Insulation: 12” x R-3.2/in= R-38.4
Sheathing (Exterior Insulation): 4” x R-4/in = R-16
Total Assembly R-value (clear section) = R-54.4
Outer air-impermeable insulation value = 16/54.4 = 29% (fail!)

This assembly didn’t quite pass the required metric, so, in this case the exterior insulation would need to be increased or the interior insulation decreased in order to meet the 35%.

Please note in the case of a flat roof with tapered insulation above the roof deck and cavity insulation below, the minimum exterior insulation thickness should be used when calculating the ratios noted above. 

B-4: Floors

The section on floors is not an excerpt from John Straube’s book, which did not provide guidance on floors. This section was written by Phius and is based on cases we have seen in project certification and in the supporting hygrothermal analyses we have performed. 

Most floor assemblies we see in Phius Certification are composed of all vapor impermeable insulation outside the structure with Class I vapor control between the structure and insulation –  that is an example of the perfect floor. However, when air-permeable insulation is introduced, there are more factors to consider. The guidance differentiates between floors that are vulnerable to bulk water events (like in kitchens and bathrooms) versus floors that aren’t. 

Generally Phius recommends against using more than 2.5” on air permeable insulation in floors subject to bulk water event risk. Review the guidebook for more details and special considerations. 

Overall, we highly recommend that project teams design the enclosure to follow the prescriptive guidelines in this Appendix to ensure a healthy, durable enclosure, which we believe is possible with all new construction projects. We understand that retrofits or existing conditions may hinder the ability to do that, and in those cases, Options 2 or 3 may be more appropriate pathways for compliance with Phius’ moisture control guidelines.