# Another wall question

| Posted in General Questions on

I am beginning the planning stage on my new home and want to get some conformation on the wall assembly I am thinking of using.  I am in CZ 7A and am looking for a R-40 plus wall.  Here is what I am thinking:

R9 Zip R Sheathing
2″ Closed cell foam in the bays
Remainder of stud bay filled with mineral wool insulation.

I have never used the ZipR sheathing before and am not sure if I am creating a dew point in the middle of the wall or is it safe as the sheathing is on the exterior of the foam?

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### Replies

1. | | #1

I would reference you to two of Martin's blogs on exterior insulation and dew points: here and here.

Here is my take... Though your sheathing is on the outside, there is still the possibility of the rigid foam on the inside of the sheathing being below the dew point and thus allowing moisture accumulation. Using this example from the second link, you can quickly calculate how much spray foam you will need to make sure you will have no moisture problems:
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- Determine the delta-T (Î”T) — that is, the difference between the outdoor temperature (30.3°F in our example) and the indoor temperature (70°F).
- Calculate the percentage of the insulation that is on the interior side of the sheathing by dividing the R-value of the cavity insulation (the insulation between the studs) by the total R-value of the wall.
- Now calculate how cold the sheathing gets, using this formula: Temperature of the sheathing = Indoor temperature – (Delta-T * Percentage of the insulation that is on the interior side of the sheathing)

In our example, the delta-T is 39.7 F°. The percentage of the insulation that is on the interior side of the sheathing is R-19/R-24 = 0.79. The temperature of the sheathing is 70°F – (39.7F° * 0.79) = 70°F – 31.4°F = 38.6°F .

If you follow Lstiburek’s advice and use his indoor conditions — 70°F and 35% relative humidity — you don’t need to look up the dew point in a psychrometric chart, because Lstiburek tells you that the dew point for these conditions is 40°F. So the sheathing in the Boston house is below the dew point — making the wall assembly risky. This wall needs thicker exterior foam to keep the OSB above the dew point.
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2. GBA Editor
| | #2

Mark,
I disagree with Norman. The closed-cell spray foam prevents any interior moisture from reaching the Huber Zip R sheathing, either by diffusion or by air movement, since closed-cell spray foam is an air barrier and a vapor barrier.

For an R-40 wall in Zone 7, you need at least 43% of the wall's R-value to come from the foam layers. (For more on this issue, see "Combining Exterior Rigid Foam With Fluffy Insulation.")

R-9 in the Zip sheathing plus R-13 in the closed-cell spray foam totals R-22, and R-22 is 55% of R-40, so you are OK.

That said, with the Zip R approach, you still end up with cold sheathing. I don't really think that's a problem, though.

It still seems like an expensive way to get to R-40. For more information, see "How to Design a Wall."

3. | | #3

I am not sure we disagree, Martin. I was just saying he needs to do the calculation to be sure there is no problem. You provided the needed calculation, which says he is fine.

I am wondering if you took the example I posted from your article as my content.

4. GBA Editor
| | #4

Norman,
I was reacting to your comment, "there is still the possibility of the rigid foam on the inside of the sheathing being below the dew point and thus allowing moisture accumulation."

The rigid foam would never be the first condensing surface. The first condensing surface would be in the interior side of the cured spray foam.

5. | | #5

Ah, absolutely. What I was trying to get at was that Zip-R has the foam to interior of the sheathing and the articles I linked had it the opposite, but the dew point issue was still valid and needed to be explored.

6. Expert Member
| | #6

A terminology issue that may have some misconceptions:

"....not sure if I am creating a dew point in the middle of the wall ...."

A dew point is a TEMPERATURE, not a location within an assembly. The dew point temperature is the temperature at which a body of air is 100% saturated with moisture, and cooling the air further would cause fog to form. This varies by quite a bit- right after a shower the dew point of the air in the bathroom is often above the temperature of the walls, ergo condensation forms on the walls & winter.

The relevant dew point in this discussion would be the AVERAGE wintertime dew point of the conditioned space air indoors. The sections within the assembly with an average wintertime temperature below the average indoor dew point temperature can be susceptible to moisture accumulation if there isn't an adequate air barrier & vapor retarder between the indoors and that part of the assembly. In addition to it's insulative properties closed cell foam is fairly vapor retardent at thicknesses of an inch or more, and can be used to provide that protection. As long as the warm-in-winter side of the closed cell foam has an average wintertime temperature warmer than the average indoor air dew point there isn't much vapor diffusion drive drawing moisture into the wall cavites, and it doesn't take a strong interior side vapor retarder to prevent moisture from building up- standard interior latex on wallboard is enough.

7. | | #7

Look into whether you can use that thick of Zip-R with 24" centers of advanced framing. Seems like the lack of racking resistance would be a problem.

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