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Class I Vapor Retarder in Cold Climate

entomodonata | Posted in General Questions on

As part of a ‘down to stud’ remodel project on a 1.5 story brick Cape-cod in Zone 5, I am re-insulating. I am furring out existing study bays to accept R23 Rockwool, and and planned to apply continuous interior 1″ foil faced polyiso for additional R value and to reduce thermal bridging. The polyiso would be applied to all surfaces that would be drywalled, including the ceiling, prior to reframing interior walls, effectively creating a seam-taped polyiso ‘hat’ for the house.

Reviewing the data sheets for Dow Super TUFF-R, which is the product I planned to use, I see the vapor permeance is .03, a Class I vapor retarder.

I was under the impression that Class I vapor retarders, such as the dreaded polyethylene, should be avoided in Climate Zone 5 due to in wall condensation concerns.

Is this risk mitigated with polyiso because of the inherent R value?

Are there any other concerns with this approach? The 1st floor of the home has not been reinsulated, so I do wonder if the lack of balance between a lower R value leaky 1st floor, and a higher R value, seam taped air tight 2nd floor could cause any issues?

Thank you for your time.

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  1. ohioandy | | #1

    Is there already insulation in the roof that's staying in place? To clarify, you've fully gutted the second floor of your cape, and you're proposing creating a cathedral ceiling with R23 mineral wool covered by 1" continuous interior polyiso, followed by reframing of kneewalls and ceiling collar ties. The way you worded it sounds like the polyiso is only going on the interior finished surfaces of the room, so kneewall-rafter-collar tie. In any case, you didn't mention your strategy for venting the roof, plus if it's a typical cape you've got board sheathing above which is not airtight. And finally, at an optimistic R30 you're far short of Zone 5 insulation requirements. Condensation danger is on both sides of the taped foam.

    1. entomodonata | | #4

      Hi Andy -- thank you for your reply! The design of the existing finished space already utilized sloped cathedral ceilings, as is typical with 1.5 story Cape Cods. There's about a 4' knee wall, a 4' sloped section, and then a 10' wide ceiling.

      Between the 2x6 ceiling joists, there was a hodge podge of existing insulation (2" Balsamwool, 3" yellow fiberglass, 3" old rockwool) all with many penetrations in the kraft facing for overhead lighting, etc. About 3 years ago, I added considerable R value to this 'attic' space by installing a double layer of R30 fiberglass on top of that existing insulation The intent is reuse that fiberglass on top of the polyiso. I will seam tape the entire polyiso 'shell', and then run strapping and frame my interior walls below it to create as few air penetrations into the 'attic space' as possible.

      The polyiso will be going on the interior side of the studs (kneewall, rafter, collar tie, as well as gable ends), and then gypsum board on top of it.

      The roof is vented. I have DCI SmartVent soffit intake vents, and a ridge vent. I've installed vent baffles in every rafter bay. My rafters are a true 2x6, so I was able to slightly compress the R23 batts into the rafter bays without collapsing the vent baffle channels.

      Yes, board sheathing for the entire home including the roof. Is this an issue for the roof? With the vented design, I figure it will only help increase airflow behind the insulation.

      For the gable walls, I could tape the board sheathing seams before re-insulating. I'd like to understand this better, though. My interior polyiso will be my air barrier. Is the concern here wind washing of the insulation? I think I've been fortunate with this home and it's brick facade in being relatively resistant to air intrusion through the building walls.

      I'm aware I do not meet Zone 5 insulation requirements. If I were to fur out the rafters to have adequate space to meet code minimums, we'd lose too much interior space and the project wouldn't make any sense. The existing insulation in the rafters was perhaps R5 at best, so I do feel comfortable that I'm making a significant improvement in R value, addressing some thermal bridging with the continuous polyiso, and making the space much more air tight.

      Seems there are different opinions on the condensation danger in this situation (see Michael's comments below). I wonder if going to 1.5" or 2" polyiso would reduce this risk?

      Thank you for your time!

  2. Expert Member
    Michael Maines | | #2

    If 1" of polyiso was going on the exterior you would definitely have a condensation problem, but placing it on the interior means you'll have no moisture issues during heating season, because it's on the warm side and the condensing surface (the interior foil face) will be above the dewpoint temperature.

    During times that air conditioning is running, vapor drive is reversed and the relatively warm, moist outdoor air is trying to get into your cool, dry house. If the condensing surface (in this case, the OUTER foil face) drops below the dewpoint temperature you could get condensation. But even poly sheeting is only a risk in relatively extreme (for zone 5) conditions; adding R-5 or so should keep the condensing surface more than warm enough to prevent condensation. I've renovated a couple of homes here in zone 6 with that assembly, built in the 1980s, and saw no evidence of moisture problems.

    As Andy mentions, there are other details such as continuity and venting at the roof.

    1. entomodonata | | #5

      Hi Michael. Thank you for your reply and details on your experiences with this approach. I hope I've addressed Andy's questions in my reply to him above.

    2. ohioandy | | #7

      I appreciate Micheal weighing in on this one, and thanks to OP for explaining more detail in comment #4. I just completed a similar project, but followed a different strategy, and I'm still trying to figure out the best approach.

      So, with interior seam-taped polyiso, I'm assuming that OP is also applying an air barrier to the back of the kneewall, right? It's not a critical detail, or even code-mandated, but it does increase the effectiveness of the insulation in the kneewall. As for the upper attic, as long as there's a nice thick layer of fluffy, this is a great ceiling.

      My concern is in the sloped ceiling portion, where space is limited to a standard rafter bay baffle and and R23 batt of mineral wool. Setting aside code requirements for R-value, I'm worried about airtightness here. Michael, how relevant is the fact that the roof is board-sheathed and presumably air-leaky? Do the baffles establish a de facto pretty good air barrier? Ideally, I think, the kneewall's outside air barrier needs to extend upwards into the rafter bay to the underside of the baffles. Am I overstating the importance of an outside air barrier in the presence of a well-detailed interior air barrier?

      I'm also wondering how the answers would differ if OP's project did not include the installation of a reasonably effective soffit to ridge venting system.

      1. entomodonata | | #8

        Hi Andy. Thanks for putting more thought into this.

        I should have specified in my original post — the cavity behind the knee wall is conditioned space. I had utility company subsidized insulation work performed about three years ago and they installed baffles down to the soffit, filled the remaining cavity space with R13 fiberglass batts (Rockwool or higher performing fiberglass wasn’t an option at the time) and then 2” of foil faced, seam taped polyiso across the rafters. They also made sure the polyiso extended below the floor joists and down to the top plate, all joints air sealed with expanding foam.

        I plan to tie the new ‘above knee wall’ sloped ceiling polyiso in to the existing ‘below knee wall’ sloped ceiling polyiso using expanding foam.

        I’m interested in the discussion of external air tightness requirements on a board sheathed house. I could understand the argument to seam tape board sheathed seams on vertical walls, however on a vented roof, it doesn’t make much sense to me. The entire point of installing the baffles is to ensure we have adequate air flow underneath the sheathing to remove any moisture that works its way into that cavity. Wouldn’t the gaps in the board sheathing just increase that air flow?

        From what I’ve read, wind washing effects on insulation R value are negligible, so as long as my interior air seal is tight, does it really matter?

  3. braun247 | | #3

    Just a note here, are you in Zone 5A or 5C. It does make a difference.

    Here is a link from a past post:

    The three main moisture divisions are:

    Moist (A). This is designated by the letter A after the climate zone number. Here in Atlanta, we’re in climate zone 3A. The primary factor is precipitation. If it doesn’t meet the dry climate definition below, it may be moist. The other necessary condition is that it’s got to fall outside the marine climate conditions.
    Dry (B). This is based on the amount of precipitation and the annual mean temperature. The calculation is 0.44 x (TF – 19.5), where TF is the annual mean temperature in degrees Fahrenheit. If the annual precipitation is less than the number you get, it’s a dry climate and the zone number has a B after it. El Paso, Texas, for example, is in climate zone 3B.
    Marine (C). This is the Goldilocks climate, in a way. It’s not too hot in the summer (warmest month mean temperature < 72° F), not too cold or too warm in winter (between 27 and 65° F), has at least four months with mean temperatures above 50° F, and has its dry season in the summer. We’re talking Santa Barbara (3C), Portland (4C), and Seattle (4C).

    1. entomodonata | | #6

      Hi Joe. My apologies -- I am in 5A.

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