Vapor barrier or no vapor barrier?
Climate Zone 6 (Maine), Renovating a 2 story cape:
First Floor Wall assembly: wood cladding, 1/2″ rain screen, R12-Zip, Original wood sheathing boards, 2×4 rough cut framing, DP cellulose w/insulweb….
Second Floor wall assembly: wood cladding, 1/2″ rain screen, R12-Zip, 2×6 framing, DP cellulose w/insulweb….
Ceiling: Cathedral, un-vented, standing seam metal roof, HT ice and water shield completely covered, R12-Zip, Advantech roof sheathing, 2×10 rafters, DP cellulose w/insulweb….
Question: Should I install a vapor smart membrane like Intello Plus on the interior of my framing envelope?
The logic: The membrane will “close” up in the winter not allowing vapor diffusion from the warm air inside to penetrate and condense on the insulation/framing during the winter. The membrane will “open” up in the summer to allow any vapor penetration from the outside to dry inwards toward the building?
Intello plus perm ratings: 0.13 – 13+ (Low Class II)
Any advice is appreciated!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I would follow code (eg, R806.5) and the recommendations here, using a smart-retarder where it calls for Class II.
https://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers
Jon R (#1) Thank you for your reply, I made a mistake in my roof assembly, I have Advantech roofing down as structural sheathing first, then the R12-zip on top of that.
I read the 2015 IRC R806.5, if I am reading it correctly, I would fall under 5.1.2. if I used DP cellulose, but would be screwed on meeting the R value (25) for the rigid board on the exterior of the structural sheathing since my R-Zip is only a 12.6.
So in reality using an air-impermeable insulation, like CC spray foam on the roof, is the only way to go, just not he green option I was hoping for....and I guess a vapor smart barrier like Intello at that point would not be necessary?
> I guess a vapor smart barrier like Intello at that point would not be necessary?
You need a Class II vapor retarder (see Lstiburek's Table 2, point 9). There is pretty good evidence for a smart membrane being better, but you can use a Class II paint.
If you decide to use a Class III retarder with minimum or near minimum code amounts of low perm foam, at least consider page 48 here: https://www.nrel.gov/docs/fy13osti/56709.pdf
T.G.E.,
First of all, can you tell us your name? (I'm Martin.)
Above your roof sheathing, your rigid foam layer isn't thick enough. You have R-12 of rigid foam, but the building code requires that your rigid foam layer have a minimum R-value of R-25 in Climate Zone 6.
More information here: "Combining Exterior Rigid Foam With Fluffy Insulation."
Martin,
Sorry, my name is Tyler
The response from David Powers (#9) in the article you linked “combining rigid exterior foam with fluffy insulation”, his concerns noted in his 2x10 roof assembly with DP cellulose and R15 exterior rigid foam resulting in a MC of 30% is quite alarming. I would be curious to know what type of vapor barrier he used, if any? If he would have instead sprayed 2” of CC spray foam first, then added the DP cellulose, followed by a smart vapor membrane; would that have resulted in a lower MC, and also allowing the sheathing to dry?
Jon R
Thank you again for the information in the link, really good information, still wrapping my brain around it all; but I agree with you a class II smart vapor retarder looks to be a must!
Jon and Martin gave me some pretty good information in a thread that drifted to a similar question as this. see:
https://www.greenbuildingadvisor.com/question/below-grade-wall-to-above-grade-wall-transition-gap
I've read the attachments that Jon posted on my thread and this one here so I'm fairly convinced that I need to use a Class II vapor retarder smart membrane inside of my drywall.
Forgive my intrusion into your thread but I still have a few questions on vapor barriers. Are there any downsides to using a class II vapor retarder smart membrane in a wall assembly that is designed to dry to the interior?
Jon mentions Class II paint. I need to familiarize myself with different paint permeabilities. Does anyone have a link detailing the types of paints and their permeabilities?
It seems to me the risk associated with a wall designed to dry to the inside seems to be that in the future (maybe new owners or my alzheimer's) that someone slaps on the wrong type of paint or puts on too many layers of paint or puts up wallpaper thus reducing the permeability to such a level that the wall never dries to the inside.
If repainting a wall assembly like this I assume you have to strip the existing paint rather than painting over what's there?
Steve
Steve
Vapor permeability of paint: you really need to get these numbers from the manufacturer. If they can't or won't provide you with the info, switch to a pain manufacturer who will. And those permeabilities are more like "guidelines" depending on the mil thickness of what they test and what you apply. Do your best to match your mil thickness to what they tested.
What happens if someone changes interior in the future? Well, same thing if someone changes cladding systems if the assembly is designed to dry to the exterior.
The key is to have a homeowner manual that includes issues such as these as part of high performance home building and operation.
Peter
It's a good question. If code allows you to use a Class I, II or III vapor barrier/retarder in a cold climate, which should you use? I'd say that a Class II/III smart-membrane is usually best in such cases. If forced to pick between II and III, I'd pick II. This is supported by various references, including my links above and Table 4 in this one (showing that Class II provides enough inward drying):
http://www.appliedbuildingtech.com/system/files/abtgrr_1501_02_vermont_residential_energy_code.pdf
Table 8 here shows that even a Class I usually outperforms a Class III (both with external insulation).
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1316_Moisture_Management_for%20High_R-Value_Walls_rev.pdf
There is certainly conflicting information out there. Perhaps there are similarly good references that show that Class III is sometimes better.
Classes span a very large (10:1) range. It may not be unreasonable to specify something like "high side of Class II" or "low side of Class III".
Keep in mind that air barriers are more important than vapor retarders. How well they are done, testing, location, redundancy (eg interior *and* exterior sides), durability, etc.