Let’s talk DOW SIS exterior sheathing (Dewpoint Calcs)
Is it possible to use a 1″ DOW SIS panel as exterior sheathing on a 2×6 wall with cellulose, 1/2″ GWB and latex paint in Denver, Colorado (Climate Zone 5)? Here is my analysis. Please provide your thoughts, analysis, and comments. Thanks!
FACTS:
1.) DOW SIS board is a vapor-impermeable product (Class I vapor retarder, i.e. vapor barrier) at 0.03 perms. The product cutsheet states 0.3 perms but I called DOW and confirmed it was 0.03 perms. Source: http://dow-styrofoam.custhelp.com/app/answers/detail/a_id/10620/%7E/what-is-the-perm-rating-of-styrofoam-sis%E2%84%A2-brand-structural-insulated-sheathing%3F
2.) DOW SIS board is composed (ext. to int.) of polyisocyanurate, foil face, thin layer of fiber-type sheathing.
3.) DOW SIS R value = R5.5
4.) Denver average monthly temperature in December, January, and February is 31.07 degrees F. Source: http://www.crh.noaa.gov/bou/?n=climo
5.) Latex paint is a Class III vapor retarder
6.) Dewpoint Calculator: http://www.lamtec.com/tb_dew_point_calculator.html
7.) Cellulose: assumed at R3.5/inch.
Calculation is based on this article by Joe Lstiburek. Also found in “Builder’s Guide to Cold Climates.” http://www.buildingscience.com/documents/reports/rr-0410-vapor-barriers-and-wall-design
Ts = Ti – ΔT(Rc / Rt)
Ts = 70Ëš – 38.93Ëš( (5.5 x 3.5) / ( (5.5 x 3.5) + 5.5) )
Ts = 70Ëš – 38.93Ëš(19.25 / 24.75)
Ts = 70Ëš – 38.93Ëš(0.78)
Ts = 70Ëš – 30.36Ëš
Ts = 39.64Ëš
Page 23 of the article (Figure 3d.2): “Dewpoint temperature of 70ËšF, 35% RH air is 40ËšF.
Therefore, the temperature of the cavity side of the sheathing (Ts) must be greater than 40ËšF.”
Ts = 39.64Ëš is less than 40Ëš, therefore there is a potential for condensation at the cavity side of
the exterior sheathing during the coldest months of the year.
Also, there are plenty of times when the exterior temperature in Denver drops below 31 degrees F, so I believe that it would be even more likely to condense
With all this said, so if you have a wall already designed as noted above, what’s the fix? (that’s if you agree there is a problem, which I do) I was thinking a 1″ layer of ccSPF (closed cell spray foam at R6.67/inch) applied to the interior face of the DOW SIS; per my calculation that would put the Ts at 48.20 degrees F.
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Replies
Jeremy,
1. Did you read this thread?
https://www.greenbuildingadvisor.com/community/forum/gba-pro-help/22069/advice-insulating-sheathing-climate-zone-5
2, Here's the article with the chart showing minimum R-values for foam sheathing: Calculating the Minimum Thickness of Rigid Foam Sheathing.
In your climate zone, you need foam sheathing with a minimum R-value of R-7.5 for a 2x6 wall.
3. Here's more information on structural insulated sheathing: Styrofoam SIS. The one-inch-thick product has an R-value of R-5.5. That's not thick enough for your climate, so you shouldn't use SIS. You can use either 1.5 inch of XPS, 1.5 inch of polyiso, or 2 inches of EPS.
Thanks Martin. I had previously read the first two threads you are referring, and a few other threads you had posted about the thickness of exterior insulation. The third thread is for GBA Pro members which I am not a member. I understand that the 2007 supplement to the 2006 IRC calls for R7.5 exterior insulation with a 2x6 framed wall, but I wanted to run the calculation to see if Denver's specific climate agreed with the prescriptive requirements for this code section; it appears that the code is correct.
The builder has already chosen to use DOW SIS. The home is almost completely framed. I am trying to figure out a solution to this situation, and the only one I can think of is to spray the interior face of the SIS with ccSPF to raise the surface temperature of the first condensing surface which would be the interior face of the ccSPF. The interior wall would be covered with a Class III vapor retarder (latex paint), so all drying would be to the insider. Any thoughts on this approach?
Jeremy,
It's always possible to add a layer of rigid foam on the outside of the SIS, if you need more R-value. Then you would probably want to install vertical furring strips to create a rainscreen.
As I said before, another option is to add a layer of insulated vinyl siding. Ugly but effective.
Thanks Martin. So you don't like the idea of adding a 1" layer of ccSPF on the interior?
Jeremy,
Your way will also work. Some people who read this website don't like spray foam, but you're welcome to use it if you want. From a building science perspective, your way will work fine. And it will do a good job of air-sealing your wall, if the spray foam operator is conscientious.
Thanks Armando. I am not familiar with Climate Consultant 5. I just downloaded it. I got my temperatures straight from NOAA, so I'm pretty confident they are correct. Where is the chart from? Is that something you created?
Jeremy,
I used Climate Consultant 5 for Denver’s coldest 3-month average is 33°F (almost the same as Santa Fe). Assuming a constant 70°F interior temperature, I get the attached figures; and it appears to me, you should be OK with 1” SIS board.
Problems can result IF the interior temps and/or the RH are higher. In normal circumstances, RH should be ≤35%, however, occupant’s number and behavior, leaky house or crawl space, can change that in a second.
Jeremy,
I changed the chart to reflect different results when the temperature change. This is a chart I created in Excel. All I have to do is change the temperature, insulation amounts and dew points, and it will let me know if a particular assembly works or not. It calculates several assemblies at once.
The beauty of it all is that you can have unlimited wall assemblies and unlimited temperatures (if you make the time to find the dew points). The last column is a simple IF statement.
Actually, the temperature data I used came from the EnergyPlus website, which can be read through CC5. The temperatures I used for Denver were from the Denver International Airport.
Since the DOW SIS structure has an approximate 1/8" layer of fiber type sheathing on the interior, will applying a 1" layer of ccSPF to the interior face of this sheathing create drying problems if it should become wet? Basically will there be a vapor barrier on each side the fiber type sheathing?
From taking a look at a few closed cell spray foam manufacturer's websites:
Demelic at 1.5" thick = 0.79 perms
Certainteed at 1" = 1.51 perms
Certainteed at 2" = .76 perms
The fiber type sheathing could become wet if a flashing tap fails at a window, or something similar.
Jeremy,
I think that's a valid concern about the fiber material. This sounds like using cardboard to brace a structure, and a sensible person would ask, how long will that last?