Smart vapor retarder (ie: MemBrain) installation details
I’m currently in the final estimation phase of my upcoming new build and we’re coming in a little tight – so every little bit needs to be justified.
I’m located near Ottawa, Ontario where unfortunately poly vapor barrier is still required in the builders and inspector’s eyes. Our walls will feature 1″ foil-faced polyiso on the outside and R-22 of rock wool on the inside. The exterior foam has very low water permeance and consequently I want to avoid trapping moisture – thus the Certainteed Membrain. I had our GC price it out with the insulation sub and it came back with an upcharge of nearly $2000. This is a 2800sf, 2-storey house with full basement. If my calculations are correct, we’re talking less than 6000sf of wall and ceiling area. Finding pricing on the Membrain is a little hard in my area, so based on US pricing (Menards) 800sf of Membrain is $110 as opposed to $42 for the regular 6mil poly. Since this doesn’t add up to the $2000 up charge… my question is: Is there a significant amount more detail work that is required when working with the smart vapor retarder?
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Replies
Mike,
First of all, I recommend that you increase the R-value of your exterior rigid foam, based on the recommendations in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
You are in Ottawa, which has a climate zone that is similar to U.S. climate zone 6 (according to a comment by Christopher Solar, a resident of Ottawa, on this page: All About Climate Zones). If you are using 2x6 wall framing, then the minimum R-value of your exterior foam should be R-11.25. That means that you need 2 inches of polyiso, not 1 inch.
Second, if you are using MemBrain as a vapor retarder rather than an air barrier, there is no reason whatsoever that it should require more installation labor than polyethylene.
Mike,
Would you be able to do the insulation and installation yourself? I've worked with Roxul in an attic and it is really easy to use (very itchy though, maybe not so annoying as fiberglass, but it's close.) Also if you wisely decide to use MemBrain, you could air seal it so it does function as a secondary air barrier.
I will be using Intello Plus for the house I am getting ready to build and I plan on installing it myself. I think that air barrier, air sealing are best done by people really invested in doing it right.
Also if you take Martin up on his suggestion of 2 inches of polyiso, I would suggest using 2 different layers of polyiso so you can stagger the seams. Dr. Joe Lstiburek talks about how foam shrinks in this article: http://www.buildingscience.com/documents/published-articles/pa-foam-shrinks/view?searchterm=foam%20shrinks
Here is the pdf on installing MemBrain http://www.certainteed.com/resources/30-28-137.pdf
General discussion about MemBrain and ease of installation http://www.toolbase.org/Technology-Inventory/walls/smart-vapor-retarders
Here is a GBA question about using air barriers like MemBrain, answer #6 talks about the owner installing it himself. https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/27416/interior-air-barrier
Sounds like you are going to build a nice house. Good luck.
Lucy and Mike,
We recommend to install INTELLO Plus as the primairy airbarrier. It is on the interior, protected and designed for that purpose. Combined with the Pro Clima tapes that I offer for high performance construction in North America (www.foursevenfive.com), it makes creating a very airtight house - with good vapor control - straight forward and verifiable (visual and with blowerdoor test).
We offer solutions for all critical conditions. Window connections, pipes, ducts gaskets, adhesives for connecting to concrete, etc. Please see the attached installation guides and this blogpost regarding a window installation,
And yes, self installation by home-owners has been done quite a number of times. Most people can work with a staple gun, and applying the tape is not rocket science. The best result come from working neatly and with care.
Thanks for the responses everyone.
Martin - Unfortunately, like I said, the budget is quite tight and adding a 2nd layer of foam plus the extra detail work (or simply going with 2" of foam) is simply not in the books. I realize the dew point will be somewhere on the inside of the foam, therefore it accentuates the fact that I need to be able to dry to the inside.
Mike,
Your plan is riskier than you imply, since your plan more or less guarantees that you will have soggy sheathing. I would borrow the money to avoid sheathing rot if I were you.
Of course, you can always keep an eagle eye on your indoor relative humidity all winter long. But that is an unsatisfying solution to a building specification error.
I understand the point that insufficient exterior foam will leave the dew point somewhere inside of the wall cavity. However, a smart vapor retarder on the warm side will prevent outward vapor drive by diffusion and by airflow. So why does it matter if the exterior foam allows the dew point to be inside of the wall cavity?
Ron,
Even with MemBrain, the ability of the wall to dry to the interior is not unlimited. The guidelines provided by the building code (and reiterated in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing), are based on the assumption that the interior finishes of the wall are vapor-open.
If Mike installs MemBrain, then his walls will be vapor-open; that part is good. However, he isn't following the guideline on the minimum R-value of the exterior rigid foam. That's risky.
If Mike installs MemBrain it's only vapor open when there is a sufficiently high moisture level, but only as vapor-open as the finish paint (3-5 perms, if standard latex. When the cavity behind the MemBrain is dry it's <1 perm, a class-ii vapor barrier. while insulating sheathing would allow it to be 3+ perms all the time, using membrain isn't exactly "risky"- does what needs do when it:
During the winter the air in the wall cavity is dry- even if it's only dry because the moisture is being stored in in the cold sheathing. That dryness still limits the rate at which further moisture is passing through the MemBrain into the cavity. During the spring warm-up when the sheathing is releasing the accumulated moisture it becomes more vapor open during warm part of the day as the air humidity inside the cavity rises, but still shuts down becoming low-perm when the sheathing drops below the dew point of the conditioned space air- it's something of a moisture-diode.
During the summer the vapor permeance of the MemBrain will vary all over the place, but it doesn't matter- it still shuts down whenever the sheathing temp drops below the dew point of the conditioned space air, so overall it's drying function actually performs BETTER than having a constant 3-5 perms (like paint), since it's always goes low perm whenever the sheathing is adsorbing moisture, and higher-perm when the sheathing is releasing moisture.
It's not as sure-fire as simply having sufficient exterior-R for dew point control, but it's still not particularly high-risk, even with a low permeance exterior. But skipping the foil facers and using a higher permeance rigid insulation such as EPS or rigid rock wool would be FAR preferable, since that would also allow SUBSTANTIAL drying capacity toward the exterior.
Any chance you can change that inch of foil-face to an inch or two of EPS or rock wool? A couple inches of EPS is much higher R than an inch of iso at the temperatures that matter, and usually comparable in price, but rather than being 0.002 perms due to double-foil facers it'll be more like 2.0 perms. Alternatively, an inch of fiber faced roofing iso is usually 0.4-0.8perms, still more than an order of magnitude better exterior drying capacity than foil faced goods.
BTW: "dew point" is a temperature (the temperature at which moisture in the air begins to condense), not a location inside the wall.
Dana,
Regarding your comment:
"BTW: "dew point" is a temperature (the temperature at which moisture in the air begins to condense), not a location inside the wall."
Dew point is a temperature, but isn't that place where the dew point temperature is encountered a location inside the wall?
The temperatures within the walls are changing all the time, even if you keep the interior temps rock-solid at a single temperature & humidity. The location within the wall where the temperature corresponds to the dew point of the interior air is therefore not a single layer- it varies with outdoor temperature and the time delays of the thermal mass of the wall's layers.
For purposes mold/rot control one can run calculations to estimate at which layer in the wall will be susceptible to accumulating moisture in a given climate, given some boundary conditions. It will be approximately the layer where the binned hourly wintertime temperature average will correspond to whatever interior air dew point is presumed. But that location within the wall is not called a "dew point", any more than it is called a "condensing plane", which is a related but not identical concept.
The condensing plane is an air impermeable layer on the exterior side of a fiber layer, which will be where the moisture in the cavity aggregates via condensation or condensation when it's cold outside. The dew point of the entrained air in air permeable layer the wall cavity will track that of the condensing plane fairly closely with falling outdoor temperatures, as long as the air isn't constantly being replenished by air leaks from the interior. The layer that corresponds to the dew point of the conditioned space air isn't (usually_ the layer where the moisture accumulates during cold outdoor temps- the condensing plane is where the action is, since that's where the condensing or adsorption is taking place.
Dana,
Just to clarify:
In post 6, I said this:
“I understand the point that insufficient exterior foam will leave the dew point somewhere inside of the wall cavity.”
In post 8, you said this:
“BTW: "dew point" is a temperature (the temperature at which moisture in the air begins to condense), not a location inside the wall.”
Maybe you were not responding to what I had said in post 6, but I did associate dew point with a “location” when I referred to it being somewhere inside of the wall cavity.
But in any case, in post 10 where you respond to my question in post 9, you said this: “The location within the wall where the temperature corresponds to the dew point of the interior air is therefore not a single layer- it varies with outdoor temperature and the time delays of the thermal mass of the wall's layers.”
You use the term, “location” at the start of that sentence to refer to the location of the dew point. That is how I implied a dew point “location” in post 6.