3.4 Moisture Design Criteria for Assemblies and Details – Above Green

3.4.1 Minimum Interior Surface Temperature for Thermally-Bridged Construction Details

Sometimes, if a thermal bridge is significant enough, PHIUS may ask that an interior surface mold risk analysis be completed according to ISO 13788. The calculator can be downloaded from the Calculators and Protocols page on the PHIUS website.

One of the “hard requirements” for certification pertains to avoiding mold growth on interior surfaces caused by thermal bridges. Even if a thermal bridge is tolerable in terms its impact on the space conditioning loads and demands, it is not tolerable if it can lead to mold growth on the inside. The protocol follows ISO 13788, and one of our calculator tools follows its methods. Just as in calculating the energy impact of a thermal bridge, the detail is modeled in THERM. But instead of calculating the extra energy loss, the critical result is the point of lowest temperature on the inside surface, and the criterion is that at that point, the interior air, when chilled down to that temperature, should be at less than 80% relative humidity.

ISO 13788 addresses how to determine the appropriate boundary conditions – the outside temperature and the indoor relative humidity. This is based on consideration of the monthly average outside temperature and humidity for the climate. The outdoor humidity is added to an indoor source that depends on one of five building humidity classes from low to high.

For each month, a psychometric calculation is then done to determine a minimum inside surface temperature needed to keep the RH at the surface below 80%.

The critical month is the one in which that minimum surface temperature is farthest from the outside temperature and closest to the inside temperature, because that requires the detail to be the most “insulating.” This “surface temperature factor” (fRsi) of the building element is defined mathematically as:

fRsi = (𝐼𝑛𝑠𝑖𝑑𝑒 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑡𝑒𝑚𝑝−𝑂𝑢𝑡𝑠𝑖𝑑𝑒 𝑡𝑒𝑚𝑝) (𝐼𝑛𝑠𝑖𝑑𝑒 𝑡𝑒𝑚𝑝−𝑂𝑢𝑡𝑠𝑖𝑑𝑒 𝑡𝑒𝑚𝑝)

with a surface resistance at the inside surface of Rsi.

(Usually the critical month is also the coldest month - but not always. Depending on the climate it might be in October, for example.)

3.4.2 Window Condensation Resistance

ISO 13788 also addresses assessment of condensation on “low thermal inertia” elements such as windows and doors, using a similar procedure, but with some differences: instead of keeping the RH below 80%, the goal is to avoid outright condensation (RH=100%). This is because windows and doors have impermeable surfaces that aren’t as subject to mold, but vulnerable to rot and corrosion if outright wet. But it calls for a more sever outside design temperature - instead of a monthly average, the calculator uses for the ASHRAE 99% design temperature.

Currently, it is recommended to do a window condensation check when any of these risk factors are present:

Window U-value significantly above the comfort requirement.
Frame U-value significantly above the glass U-value.
Presence of aluminum spacers.
Low-e coating on the inside surface of the glass.

The current passing criterion is that:

1-D calculations on the surface temperatures, OR
fRsi of the frame and the glass, OR
AAMA CRF rating should meet the ISO 13788 minimums at the ASHRAE 99% design temperature for the climate, with some safety margin, or that a CSA I-value meets it without a safety margin.

Exception 1: Dog doors are not required to pass the condensation resistance test.

Exception 2: The whole door U-value may be used for the condensation risk assessment for exterior doors that are required to be ADA compliant, egress rated, fire rated, etc., instead of requiring that each of the individual elements pass (glazing and frame).

PHIUS has developed a Window Comfort & Condensation Risk Assessment Calculator to determine if a window is at risk of condensation.

Example: With an interior RH of 48% in the coldest month, the dew point of the interior air is 47.7 F, so the inside surface must be warmer than that.

The calculator does a one-dimensional calculation with the frame U-value, for example 0.28, to determine if this is the case. Instead of the lowest daily mean temperature, use (for convenience) the ASHRAE 99% design temperature. Suppose this is 13.8 F.

With an interior temperature of 68 F, and an inside film resistance of 0.74 h.ft2 .F/Btu, the inside surface temperature then is 68-(0.29*0.74)*(68-13.8) = 56.7 F, that is, 9 degrees above the dew point.

Of course, this does ignore the fact that the surface temperature could be lower right in the corner where the frame meets the glass, because of the conductivity of the spacer, but 9 F provides a comfortable margin. ISO 13788 does caution that one-dimensional calculations aren’t generally good enough, but it is a place to start.

3.4.2.1 Other Condensation Resistance Ratings

We have been asked whether we can specify an NFRC Condensation Resistance rating (CR). The AAMA published a good summary paper (AAMA CRS-15) that explains the differences between NFRC’s Condensation Resistance (CR), AAMA’s Condensation Resistance Factor (CRF), and the Canadian temperature Index or I-value, per CSA A440.2. All of these are 0-100% higher-is-better ratings, but they are not directly comparable to each other.

From that paper it is clear that the CRF and the I-value are similar to what ISO 13788 calls fRsi – ratios that indicate how far some critical inside surface temperature is towards the inside air temperature. Therefore, if that data is available for a window of interest, those ratings could be compared directly to the required fRsi from a 13788 calculation for “low thermal inertia elements” for an indication as to whether a window is good enough in the climate location of interest.

The AAMA white paper indicates that the I-value is generally more conservative/stringent than the CRF due to differences in the temperature sensor placements. Both of these are physical tests.

AAMA provides an online calculator that takes a given outdoor temperature, indoor temperature, and relative humidity, and computes the dew point and the required CRF, so it is making the same kind of calculation as called for in ISO 13788. (The disclaimer for it makes many valid points.)

The NFRC Condensation Resistance rating is more complicated and harder to interpret, except as a relative ranking. It is basically the percentage of the window frame, glass, or edge-of-glass area (whichever is worst) that is below dew point under the standard test condition temperatures, averaged over interior RH levels of 30, 50, and 70%. It is based on modeling rather than a physical test.

3.4.3 Limiting Moisture Risk in Assemblies

3.4.3.1 Climate-Appropriate Wall and Roof Assemblies

It’s important to avoid risks related to mold and moisture. Following the prescriptive guidelines in Appendix B will generally allow PHIUS to accept or “green light” an assembly in certification. Assemblies that do not comply are given a “yellow light”. The next step is usually a request for revision of the assembly to follow the prescriptive guidelines. If revision is not possible or desired, next is a WUFI hygrothermal analysis. Sometimes a WUFI analysis will turn a yellow light green, but sometimes it will turn it red, in which case, we would require a letter from a qualified, licensed professional engineer, to the effect that the mold / moisture risk to the assembly is acceptably low.

In general, we recommend against using wet spray cellulose.

3.4.3.2 Masonry Walls and Freeze-Thaw

Interior insulation retrofits of masonry walls in cold climates can cause durability problems. Please review the information in these articles: Certification staff may require a “hold harmless” agreement.

3.4.3.3 Floor/Slab Components

Fluffy Floors

Floors are subject to “bulk water events” from above, and have trouble drying out.
Framed floors with fluffy insulation and rigid on the bottom are riskier than all-fluff or allrigid.
For fluffy floors without rigid on the bottom, over crawlspace or cantilevered over ambient conditions, hydrophobic insulation such as fiberglass or most preferably mineral wool, is preferable to cellulose (drain-down strategy instead of buffering.)
Don’t do: slab, with rigid on top, and then a framed floor with fluffy insulation on top of that. Such floors should be almost all rigid insulation with only a little fluffy insulation on top for sound deadening.

Framed Floors with Rigid and Fluffy Insulation

Floors are preferably insulated with all rigid insulation, except for at most a thin layer of fluffy insulation near the inside for acoustic reasons.
Not Certifiable: Mostly-fluffy floors directly on slabs, or with a thin layer of rigid on the bottom and slab or ground directly below. The risk of never drying out from a bulk water event is too high - repair would be very expensive, could entail removal of interior walls.
Certifiable: Partly fluffy floors over crawlspace, over unheated basement, or cantilevered above ambient conditions with 50% or more insulation value from rigid on the bottom. The floor structure is protected from humid conditions below, is accessible from below for repair in the event of water from above, and the cold side of the fluffy insulation stays warm enough to be out of trouble with mold under normal conditions.
Conditionally Certifiable: Partly-fluffy floors over crawlspace or unheated basement with less than 50% insulation value from outboard rigid, are acceptable if WUFI Bio analysis, assuming outside climate without rain or sun (stress case for cold-season moisture accumulation), shows green light at the outboard side of the fluffy insulation in the years after the initial dry-out.