Ceiling air / vapor barrier… Is this a bad idea?
I’m still struggling with my ceiling air / vapor seal challenge. I’m adding R50 cellulose above my upstairs ceiling. I’m also trying to get the house air and vapor tight. In Minneapolis it’s always either way too dry in winter or too humid in the summer.
My previous thought was 3/4″ polyiso sandwiched between existing ceiling and a new sheet of drywall. Now I’m wondering if the polyiso is even needed, or if I could get away with just a 6 mil vapor barrier sandwiched between the two drywall sheets (the existing ceiling is in rough shape from trying to remove texture unsuccessfully, so it needs to be covered in new sheetrock anyway. This also would provide good sound damping since we’re under a flight path.
My initial thought was there must be a condensation related reason to avoid this, but being directly under R50, would that be an issue? It sure would be a lot cheaper than polyiso.
Thanks,
Ryan
Climate zone 6
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Replies
Ryan,
In your climate, installing a polyethylene vapor barrier on the inside of your ceiling probably won't cause problems. But it's a little risky, especially if your house has air conditioning.
Interior poly is an out-of-date idea. If you want an interior vapor retarder, all you really need is vapor retarder paint.
The most important way you can keep moisture out of your attic is to create an airtight ceiling. The vapor permeance of the ceiling really isn't that important.
If you plan to install R-50 cellulose, you don't really need interior polyiso.
If you are planning to install new drywall, then the drywall is your air barrier. Make sure that you seal air leaks at all penetrations, and that you install weatherstripping and latches on your attic hatch, and you will be all set.
For more information, see Air Sealing an Attic.
Thanks Martin. One contractor recommended 3" spray foam between existing ceiling joists. If we can hit our R-value with cellulose, and apply the airtight drywall technique with vapor barrier paint, is it safe to say that spray foam below the cellulose would be overkill, or are there still benefits that method?
Thanks,
Ryan
The benefit of the spray foam is that it's an easy but expensive way to ensure the airtight ceiling that Martin recommends. But there's really no need to use sprayfoam to achieve that.
"Moisture Risk in Unvented Attics Due to Air Leakage Paths"
"IBACOS completed an initial analysis of moisture damage potential in an unvented attic
insulated with closed-cell spray polyurethane foam."
Two recent trends in the residential housing industry are to install unvented
attics in new homes and to retrofit existing homes from vented to unvented attics. The purpose of this study was to investigate the moisture risk due to airflow paths out of unvented attics that use ccSPF at the roof deck. The results of the study and answers to the research questions are detailed below.
1.Does air leakage between an attic and the outdoors occur after the application of ccSPF?
The results of this study indicate that there is air leakage from an unvented attic to the
outdoors. The data were collected at three houses that were being retrofitted from vented
to unvented attics.
2.For Question 1, what is the magnitude of air leakage rates?
The average leakage of the three houses retrofitted from vented to unvented attics was
207 CFM at 4 Pa of pressure.
3.If there is air leakage in ccSPF attic assemblies, what is the potential moisture
accumulation on the roof sheathing at possible air leakage pathways?
There is potential moisture accumulation in building materials surrounding air leakage
paths that have low airflow. The results from the hygrothermal analysis indicate that
moisture accumulation is localized to an area 5 in. from the crack and dries before the
next winter. The modeling performed by the research team indicates that for a typical meteorological climate year in Minneapolis, Minnesota, airflow paths with low airflow rates will result in sheathing moisture contents above 20% for most of the winter and spring. This could
result in localized damage of building materials.
Higher airflow rates do not have as much moisture accumulation during winter due to the drying potential of the airflow across the crack actually warming the sheathing and eliminating it as a condensing surface.
"Because ccSPF reduces the airflow from an attic, converting a vented attic to an unvented attic may result in reduced airflow through leakage pathways and could result in localized damage."
http://www.nrel.gov/docs/fy15osti/63048.pdf
Richard,
As far as I know, Ryan is talking about installing insulation on his attic floor. There is no reason to believe that his attic is unvented. I think you are guessing about that.
Ryan, is your attic vented or unvented?
Ryan,
You should know that Richard Beyer is a GBA reader who, in recent weeks, has been posting several comments a day on this site. Almost every comment has the same message: "Spray polyurethane foam is dangerous to your health."
Most GBA readers have read this message from Richard several times, but he never tires of repeating it. You should make your own judgment concerning whether spray foam is dangerous to your health.
To answer your question: spray polyurethane foam can be used to seal air leaks in a ceiling, but it is not the only method of sealing air leaks. Many builders and homeowners seal air leaks with caulk and high-quality European tape. Whatever method you use, sealing air leaks is an important first step that must be performed before you insulate your attic floor. For more information on this topic, see Air Sealing an Attic.
Martin,
Do you really think it's necessary to insult me when the data posted belongs to paid research teams?
Did I say somewhere here that this research belongs to me and is of my opinion? I think not! Are you feeling threatened by University research and now this is your attempt at discrediting me for something I did not author. Is it threatening your credentials? Unlike you, I give credit where credit is deserved and I've done this many times in favor of your personal opinions.
In my opinion, your picking an argument and attempting to discredit the wrong person.
My argument is and has been, if you breach a tested system by piecemealing the assembly your asking for trouble. You disagree with this as you voiced in another thread that it's okay to skip a step.
The research reports posted confirm failure for skipping step(s). Including the report posted before your defaming insults.
Now check this out Martin!
"Application of Spray Foam Insulation Under Plywood and OSB Roof Sheathing"
"Conclusions"
Based on this modeling, there are no known risks with using SPF insulation
under plywood and OSB roof decks "IF THE FOLLOWING REQUIREMENTS ARE MET":
•
The installation complies with the 2012 International Residential Code.
•
A fully adhered leak-free roof membrane is installed.
•
The roof sheathing and framing are dry below 18% before SPF installation.
•
When using open cell SPF, a low-perm Class II vapor retarder is installed
where required
http://www.nrel.gov/docs/fy14osti/60432.pdf
Good day Sir!
If you use caulk etc and do a good job with sealing your old and or new drywall that is the first step and then the R 50 cellulose will do a better job. The attic should still be vented but not over vented. Control and know your indoor moisture and humidity. Be careful with humidifiers most times there should not be a humidifier. That said they install them around my area in all new homes almost. Some day we will know if for example a huge group of townhouses have issues with homeowner's using there duct system humidifiers.
Richard, the information in your last post is noteworthy. You have an option to hit the edit button and remove all the non information. Everyone at this site loves Martin. That is not going to change with you flipping out at him or me or anyone who is tired of your constant warnings of what next is about to kill us. I'm here still alive and so at least one person has lived through all and I mean all of your warnings including decades of cutting and installing pressure treated woods, spray foaming windows, hundreds of them... having homes sprayed with Icynene open cell and standing in the home for much of the work.... gloves? no.. no gloves... masks... I do now when I sand exotic hardwoods.... can't tell you how much fuel has been spilled on my hands.. thousands of times now in a life time of filling small engines... just counted up my small engines the other day... lost count past 30... alive Richard, risks are there we all agree,...
anyway.... clean up your posts so it sounds like you like and respect Martin, myself and others,.. we'll return the favor!! And maybe post more good assembly advice like your last post but make sure it is relevant which your last post is to do with an unvented cathedral style assembly not an attic floor type simply insulated and vented assembly.
You said earlier something about not being in a street fight... good... seems the last post was more like a street fight.
We all need to be nice here or just not post including me. That's what it says at the top of this page. If you can't be polite with Martin, what next?
Peace bro.... I am going to delete most of my post and leave the info, how about you?
Richard Beyer: I'm not sure what the relevance of unvented roof assembly requirements are to this discussion(?). While those are interesting reads, there is nothing to indicated this is an unvented assembly with the insulation at the roof deck.
My reading of Ryan's - "I'm adding R50 cellulose above my upstairs ceiling."- is that he is insulating at the attic floor (of a presumably vented attic), not at the roof deck. (This was apparently Martin's reading of that verbiage too.)
In a vented unconditioned attic air tightness alone is sufficient. The ~5 perm vapor retardency of standard latex ceiling paint would be more than sufficient. If the attic has no venting to the outdoors, it may be prudent to add some, but would be even more critical to include if Ryan were to install interior-side polyethylene.
There is no point to making the house vapor tight- air tight is good enough. If you air condition the place a poly vapor retarder could even be a problem on some of the muggier humid summer days. If the attic isn't showing signs of moisture problems in it's current not-so-air tight case, it will not have a problem if you simply air-seal the ceiling and add R50 cellulose.