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Community and Q&A

Taping Foam Board Insulation

NormanWB | Posted in Energy Efficiency and Durability on

Greenville, SC
CZ 3A (mixed humid)

I am finalizing my wall design which current works like this from the sheathing out:

– 1/2″ OSB with taped seams and Knauf Eco-Seal Plus on the interior for air barrier (will do air-tight drywall inside for a belt and suspenders approach).
– Tyvek DrainWrap with taped seams (WRB and drain-plane)
– 1/2″ foli face polyiso R-3.6.(to make sure I move the dew point outside of the sheathing)
– air gap for radiant barrier to work (the foil on the poly iso is “free” so whiy not get another R-3?)
– siding (brick and stone veneer and some Hardie plank (or equivalent)

Dupont says to not tape foam board when placed over Tyvek since:
– The board will shrink and expand stressing the tape to eventual failure
– Leaving the tape off allows the wall assembly to dry faster

However, I am not sure I agree with Dupont…

From what I understand, some tapes (including Tyvek tape) hold up pretty well on foamboard. I would prefer the tape to be foil faced, but it is more important that it hold up “forever”. Also, while the Tyvek will serve as the primary drain plane, would it hurt to use the taped foam board as one as well?

However, foil faced polyiso only has a perm rating of <.3, vs 30-50 for Tyvek, so will Tyvek DrainWrap be able to “vent” properly? I want the radiant barrier properties, but not at the cost of the sheathing and wall having moisture issues. So, maybe I do agree with Dupont.., Your thoughts?

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  1. Expert Member

    Taping the seams helps to stop air movement, but has next to no effect on the foam's role as a vapour barrier. So whether you tape, or whether the tape falls off makes no difference to the cavity's ability to dry.

  2. NormanWB | | #2

    Thanks, Malcolm.

    Air movement should not be an issue with the other steps I plan to take in place, so I guess I could save the labor and material cost of taping the polyiso.

    I expect this assembly to dry to the interior from the sheathing (a la The Perfect Wall figure 9, so will the low permeability of the foam board cause an issue if un-taped?

  3. Jon_R | | #3

    IMO, the most robust partitions can breath to both sides. Ie, avoid foil facing.

  4. JC72 | | #4

    No rain screen gap?

    IMO DuPont is correct in that taped foam will inhibit drying to the exterior of the wall however that's different from saying that walls MUST be able to dry to both the interior and exterior.

    Exterior walls which "breathe" still function like a two-way street. Afterall why would we want water vapor to enter the home to being with? Especially this time of year when it's really humid and you're getting the morning sun starts to drive that moisture through the brick/hardi board into the house.

    My two cents.

  5. Expert Member
    Dana Dorsett | | #5

    "- 1/2" foli face polyiso R-3.6.(to make sure I move the dew point outside of the sheathing)"

    Dew point is a temperature, not a location within the assembly. (The temperature at which the air is at 100% moisture saturation and further cooling causes liquid water to form in the air or on cool surfaces.) The dew point of the air changes with the weather, and the temperature of the materials is constantly changing. The notion that there is some magic "point" within the assembly where the dew point of the air and the temperature of the materials coincide is silly. It's the seasonal averages that matter, and in a zone 3A location the seasonal average temperature of all materials in a wall assembly is higher than the dew point of typical indoor air.

    Tape the seams of the foil faced foam with a high quality temperature rated foil tape, even if there is SOME potential for shrinkage over time that would cause the tape to fail.The drying potential of a few less-sealed seams is miniscule & irrelevant, but the difference in air leakage may be relevant. Detail the OSB as the primary air barrier, but seal other layers as well.

  6. Jon_R | | #6

    My guess is that highly stretchable tapes (like 3M 8067) provide better long term air sealing performance than non-stretchable aluminum tape. Similar for highly flexible caulks/sealants.

  7. NormanWB | | #7

    John Clark: Yes, there is a rain gap between the polyiso and the siding, plus there is a builtin gap behind the polyiso due to the wrinkly surface of the DrainWrap, so I think this assembly should not hold appreciable water. Am I missing something?

    Dana: Bad phrasing on my part re: dew point. The "point" I am trying to address is making sure there is no expected (design) temperature where the water vapor would "condense" on surfaces that would be degraded by the moisture (wood, fibrous insulation, drywall), especially in relatively high humidity areas such as bathrooms and laundry rooms. These will be vented, of course, but sometimes people forget. :)

    Jon R.: Great idea. Do you know any that are foil faced? I am looking at the 3M Venture line, but there are so many choices...

  8. JC72 | | #8


    You're not missing anything. I just didn't see rain screen gap mentioned.

  9. Expert Member
    Dana Dorsett | | #10

    "The "point" I am trying to address is making sure there is no expected (design) temperature where the water vapor would "condense" on surfaces that would be degraded by the moisture (wood, fibrous insulation, drywall), especially in relatively high humidity areas such as bathrooms and laundry rooms. "

    Condensation/adsorption in some internal layers is normal & expected at the 99% outside design temperature or colder, but the amount of moisture is minimal, and the amount of time is brief. Even the paint facing the interior of the bathroom will experience condensation/adsorption from the room air when someone is showering even with reasonable bath fan ventilation. (Do you expect to NEVER see condensation on the bathroom mirror?) But the duty cycle is low, and the moisture won't accumulate inside the walls, as long as the AVERAGE dew point of the bathroom air is reasonable. A bath fan operated with a humidity control would be one way to guarantee that, continuous low cfm ventilation with dry air would be another.

    As long as the walls are reasonably air tight to the interior the sheathing won't accumulate enough moisture over a zone 3A climate winter to ever matter. the moisture drive off brick cladding in and air conditioned house would be a more significant issue, especially very low permeance interior finishes such as vinyl or foil wallpapers are used. But the foil facers on your polyiso are protective enough.

  10. Horence | | #11

    After reading the whole post, I have learned more about this topic. Thank you so much.

  11. Ccap | | #12

    I know it's been a while but I'm responding anyway.
    Actually polyio has an "R" rating of 6-6.5/inch. However, unlike XPS and EPS, it's effectiveness or actual "R" (resistance) value drops dramatically and eventually exponentially. After 59°f (15°c) is what I it's performance begins to reduce at -4f (-20c) is ability to resist the cold from penetrating is nearly gone completely. MDI and polyol (blowing agent gases that fill the cells) begin to diffuse out as plain atmospheric air diffuses in.
    Taping it actually has a backwards affect on ISO than in virtually any other route of ridgid foam board by trapping moisture and air in as it and will penetrate. The blowing agents, during there outward diffusion, push plain air in and won't allow it to escape until complete diffusion after that time, there is nothing left to build the cells in place and the material becomes soft and will crumble when touched.

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